3.3.1
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Working with object-oriented software and a relational database can be cumbersome and time consuming in today's enterprise environments. Hibernate is an object/relational mapping tool for Java environments. The term object/relational mapping (ORM) refers to the technique of mapping a data representation from an object model to a relational data model with a SQL-based schema.
Hibernate not only takes care of the mapping from Java classes to database tables (and from Java data types to SQL data types), but also provides data query and retrieval facilities and can significantly reduce development time otherwise spent with manual data handling in SQL and JDBC.
Hibernates goal is to relieve the developer from 95 percent of common data persistence related programming tasks. Hibernate may not be the best solution for data-centric applications that only use stored-procedures to implement the business logic in the database, it is most useful with object-oriented domain models and business logic in the Java-based middle-tier. However, Hibernate can certainly help you to remove or encapsulate vendor-specific SQL code and will help with the common task of result set translation from a tabular representation to a graph of objects.
If you are new to Hibernate and Object/Relational Mapping or even Java, please follow these steps:
Read Chapter 1, Introduction to Hibernate for a tutorial with step-by-step
instructions. The source code for the tutorial is included in the
distribution in the doc/reference/tutorial/
directory.
Read Chapter 2, Architecture to understand the environments where Hibernate can be used.
Have a look at the eg/
directory in the Hibernate
distribution, it contains a simple standalone application. Copy your
JDBC driver to the lib/
directory and edit
etc/hibernate.properties
, specifying correct values for
your database. From a command prompt in the distribution directory,
type ant eg
(using Ant), or under Windows, type
build eg
.
Use this reference documentation as your primary source of information. Consider reading Java Persistence with Hibernate (http://www.manning.com/bauer2) if you need more help with application design or if you prefer a step-by-step tutorial. Also visit http://caveatemptor.hibernate.org and download the example application for Java Persistence with Hibernate.
FAQs are answered on the Hibernate website.
Third party demos, examples, and tutorials are linked on the Hibernate website.
The Community Area on the Hibernate website is a good resource for design patterns and various integration solutions (Tomcat, JBoss AS, Struts, EJB, etc.).
If you have questions, use the user forum linked on the Hibernate website. We also provide a JIRA issue trackings system for bug reports and feature requests. If you are interested in the development of Hibernate, join the developer mailing list. If you are interested in translating this documentation into your language, contact us on the developer mailing list.
Commercial development support, production support, and training for Hibernate is available through JBoss Inc. (see http://www.hibernate.org/SupportTraining/). Hibernate is a Professional Open Source project and a critical component of the JBoss Enterprise Middleware System (JEMS) suite of products.
This chapter is an introduction to Hibernate by way of a tutorial,
intended for new users of Hibernate. We start with a simple
application using an in-memory database. We build the
application in small, easy to understand steps. The tutorial is
based on another, earlier one developed by Michael Gloegl. All
code is contained in the tutorials/web
directory
of the project source.
This tutorial expects the user have knowledge of both Java and SQL. If you are new or uncomfortable with either, it is advised that you start with a good introduction to that technology prior to attempting to learn Hibernate. It will save time and effort in the long run.
There is another tutorial/example application in the
/tutorials/eg
directory of the project source.
That example is console based and as such would not have the
dependency on a servlet container to execute. The basic setup is
the same as the instructions below.
Let's assume we need a small database application that can store events we want to attend, and information about the host(s) of these events. We will use an in-memory, Java database named HSQLDB to avoid describing installation/setup of any particular database servers. Feel free to tweak this tutorial to use whatever database you feel comfortable using.
The first thing we need to do is set up our development environment,
and specifically to setup all the required dependencies to Hibernate
as well as other libraries. Hibernate is built using Maven which
amongst other features provides dependecy management
;
moreover it provides transitive
dependecy management
which simply means that to use
Hibernate we can simply define our dependency on Hibernate, Hibernate
itself defines the dependencies it needs which then become transitive
dependencies of our project.
. <project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd"> ... <dependencies> <dependency> <groupId>${groupId}</groupId> <artifactId>hibernate-core</artifactId> </dependency> <!-- Because this is a web app, we also have a dependency on the servlet api. --> <dependency> <groupId>javax.servlet</groupId> <artifactId>servlet-api</artifactId> </dependency> </dependencies> </project>
Essentially we are describing here the
/tutorials/web/pom.xml
file. See the
Maven site for more information.
While not strictly necessary, most IDEs have integration with Maven to read these POM files and automatically set up a project for you which can save lots of time and effort.
Next we create a class that represents the event we want to store in database.
Our first persistent class is a simple JavaBean class with some properties:
package org.hibernate.tutorial.domain; import java.util.Date; public class Event { private Long id; private String title; private Date date; public Event() {} public Long getId() { return id; } private void setId(Long id) { this.id = id; } public Date getDate() { return date; } public void setDate(Date date) { this.date = date; } public String getTitle() { return title; } public void setTitle(String title) { this.title = title; } }
You can see that this class uses standard JavaBean naming conventions for property getter and setter methods, as well as private visibility for the fields. This is a recommended design - but not required. Hibernate can also access fields directly, the benefit of accessor methods is robustness for refactoring. The no-argument constructor is required to instantiate an object of this class through reflection.
The id
property holds a unique identifier value for a particular event.
All persistent entity classes (there are less important dependent classes as well) will need
such an identifier property if we want to use the full feature set of Hibernate. In fact,
most applications (esp. web applications) need to distinguish objects by identifier, so you
should consider this a feature rather than a limitation. However, we usually don't manipulate
the identity of an object, hence the setter method should be private. Only Hibernate will assign
identifiers when an object is saved. You can see that Hibernate can access public, private,
and protected accessor methods, as well as (public, private, protected) fields directly. The
choice is up to you and you can match it to fit your application design.
The no-argument constructor is a requirement for all persistent classes; Hibernate has to create objects for you, using Java Reflection. The constructor can be private, however, package visibility is required for runtime proxy generation and efficient data retrieval without bytecode instrumentation.
Place this Java source file in a directory called src
in the
development folder, and in its correct package. The directory should now look like this:
. +lib <Hibernate and third-party libraries> +src +events Event.java
In the next step, we tell Hibernate about this persistent class.
Hibernate needs to know how to load and store objects of the persistent class. This is where the Hibernate mapping file comes into play. The mapping file tells Hibernate what table in the database it has to access, and what columns in that table it should use.
The basic structure of a mapping file looks like this:
<?xml version="1.0"?> <!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN" "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd"> <hibernate-mapping> [...] </hibernate-mapping>
Note that the Hibernate DTD is very sophisticated. You can use it for
auto-completion of XML mapping elements and attributes in your editor or
IDE. You also should open up the DTD file in your text editor - it's the
easiest way to get an overview of all elements and attributes and to see
the defaults, as well as some comments. Note that Hibernate will not
load the DTD file from the web, but first look it up from the classpath
of the application. The DTD file is included in hibernate3.jar
as well as in the src/
directory of the Hibernate distribution.
We will omit the DTD declaration in future examples to shorten the code. It is of course not optional.
Between the two hibernate-mapping
tags, include a
class
element. All persistent entity classes (again, there
might be dependent classes later on, which are not first-class entities) need
such a mapping, to a table in the SQL database:
<hibernate-mapping> <class name="events.Event" table="EVENTS"> </class> </hibernate-mapping>
So far we told Hibernate how to persist and load object of class Event
to the table EVENTS
, each instance represented by a row in that table.
Now we continue with a mapping of the unique identifier property to the tables primary key.
In addition, as we don't want to care about handling this identifier, we configure Hibernate's
identifier generation strategy for a surrogate primary key column:
<hibernate-mapping> <class name="events.Event" table="EVENTS"> <id name="id" column="EVENT_ID"> <generator class="native"/> </id> </class> </hibernate-mapping>
The id
element is the declaration of the identifier property,
name="id"
declares the name of the Java property -
Hibernate will use the getter and setter methods to access the property.
The column attribute tells Hibernate which column of the
EVENTS
table we use for this primary key. The nested
generator
element specifies the identifier generation strategy,
in this case we used native
, which picks the best strategy depending
on the configured database (dialect). Hibernate supports database generated, globally
unique, as well as application assigned identifiers (or any strategy you have written
an extension for).
Finally we include declarations for the persistent properties of the class in the mapping file. By default, no properties of the class are considered persistent:
<hibernate-mapping> <class name="events.Event" table="EVENTS"> <id name="id" column="EVENT_ID"> <generator class="native"/> </id> <property name="date" type="timestamp" column="EVENT_DATE"/> <property name="title"/> </class> </hibernate-mapping>
Just as with the id
element, the name
attribute of the property
element tells Hibernate which getter
and setter methods to use. So, in this case, Hibernate will look for
getDate()/setDate()
, as well as getTitle()/setTitle()
.
Why does the date
property mapping include the
column
attribute, but the title
doesn't? Without the column
attribute Hibernate
by default uses the property name as the column name. This works fine for
title
. However, date
is a reserved
keyword in most database, so we better map it to a different name.
The next interesting thing is that the title
mapping also lacks
a type
attribute. The types we declare and use in the mapping
files are not, as you might expect, Java data types. They are also not SQL
database types. These types are so called Hibernate mapping types,
converters which can translate from Java to SQL data types and vice versa. Again,
Hibernate will try to determine the correct conversion and mapping type itself if
the type
attribute is not present in the mapping. In some cases this
automatic detection (using Reflection on the Java class) might not have the default you
expect or need. This is the case with the date
property. Hibernate can't
know if the property (which is of java.util.Date
) should map to a
SQL date
, timestamp
, or time
column.
We preserve full date and time information by mapping the property with a
timestamp
converter.
This mapping file should be saved as Event.hbm.xml
, right in
the directory next to the Event
Java class source file.
The naming of mapping files can be arbitrary, however the hbm.xml
suffix is a convention in the Hibernate developer community. The directory structure
should now look like this:
. +lib <Hibernate and third-party libraries> +src +events Event.java Event.hbm.xml
We continue with the main configuration of Hibernate.
We now have a persistent class and its mapping file in place. It is time to configure
Hibernate. Before we do this, we will need a database. HSQL DB, a java-based SQL DBMS,
can be downloaded from the HSQL DB website(http://hsqldb.org/). Actually, you only need the hsqldb.jar
from this download. Place this file in the lib/
directory of the
development folder.
Create a directory called data
in the root of the development directory -
this is where HSQL DB will store its data files. Now start the database by running
java -classpath ../lib/hsqldb.jar org.hsqldb.Server
in this data directory.
You can see it start up and bind to a TCP/IP socket, this is where our application
will connect later. If you want to start with a fresh database during this tutorial,
shutdown HSQL DB (press CTRL + C
in the window), delete all files in the
data/
directory, and start HSQL DB again.
Hibernate is the layer in your application which connects to this database, so it needs connection information. The connections are made through a JDBC connection pool, which we also have to configure. The Hibernate distribution contains several open source JDBC connection pooling tools, but will use the Hibernate built-in connection pool for this tutorial. Note that you have to copy the required library into your classpath and use different connection pooling settings if you want to use a production-quality third party JDBC pooling software.
For Hibernate's configuration, we can use a simple hibernate.properties
file, a
slightly more sophisticated hibernate.cfg.xml
file, or even complete
programmatic setup. Most users prefer the XML configuration file:
<?xml version='1.0' encoding='utf-8'?> <!DOCTYPE hibernate-configuration PUBLIC "-//Hibernate/Hibernate Configuration DTD 3.0//EN" "http://hibernate.sourceforge.net/hibernate-configuration-3.0.dtd"> <hibernate-configuration> <session-factory> <!-- Database connection settings --> <property name="connection.driver_class">org.hsqldb.jdbcDriver</property> <property name="connection.url">jdbc:hsqldb:hsql://localhost</property> <property name="connection.username">sa</property> <property name="connection.password"></property> <!-- JDBC connection pool (use the built-in) --> <property name="connection.pool_size">1</property> <!-- SQL dialect --> <property name="dialect">org.hibernate.dialect.HSQLDialect</property> <!-- Enable Hibernate's automatic session context management --> <property name="current_session_context_class">thread</property> <!-- Disable the second-level cache --> <property name="cache.provider_class">org.hibernate.cache.NoCacheProvider</property> <!-- Echo all executed SQL to stdout --> <property name="show_sql">true</property> <!-- Drop and re-create the database schema on startup --> <property name="hbm2ddl.auto">create</property> <mapping resource="events/Event.hbm.xml"/> </session-factory> </hibernate-configuration>
Note that this XML configuration uses a different DTD. We configure
Hibernate's SessionFactory
- a global factory responsible
for a particular database. If you have several databases, use several
<session-factory>
configurations, usually in
several configuration files (for easier startup).
The first four property
elements contain the necessary
configuration for the JDBC connection. The dialect property
element specifies the particular SQL variant Hibernate generates.
Hibernate's automatic session management for persistence contexts will
come in handy as you will soon see.
The hbm2ddl.auto
option turns on automatic generation of
database schemas - directly into the database. This can of course also be turned
off (by removing the config option) or redirected to a file with the help of
the SchemaExport
Ant task. Finally, we add the mapping file(s)
for persistent classes to the configuration.
Copy this file into the source directory, so it will end up in the
root of the classpath. Hibernate automatically looks for a file called
hibernate.cfg.xml
in the root of the classpath, on startup.
We'll now build the tutorial with Ant. You will need to have Ant installed - get
it from the Ant download page.
How to install Ant will not be covered here. Please refer to the
Ant manual. After you
have installed Ant, we can start to create the buildfile. It will be called
build.xml
and placed directly in the development directory.
A basic build file looks like this:
<project name="hibernate-tutorial" default="compile"> <property name="sourcedir" value="${basedir}/src"/> <property name="targetdir" value="${basedir}/bin"/> <property name="librarydir" value="${basedir}/lib"/> <path id="libraries"> <fileset dir="${librarydir}"> <include name="*.jar"/> </fileset> </path> <target name="clean"> <delete dir="${targetdir}"/> <mkdir dir="${targetdir}"/> </target> <target name="compile" depends="clean, copy-resources"> <javac srcdir="${sourcedir}" destdir="${targetdir}" classpathref="libraries"/> </target> <target name="copy-resources"> <copy todir="${targetdir}"> <fileset dir="${sourcedir}"> <exclude name="**/*.java"/> </fileset> </copy> </target> </project>
This will tell Ant to add all files in the lib directory ending with .jar
to the classpath used for compilation. It will also copy all non-Java source files to the
target directory, e.g. configuration and Hibernate mapping files. If you now run Ant, you
should get this output:
C:\hibernateTutorial\>ant Buildfile: build.xml copy-resources: [copy] Copying 2 files to C:\hibernateTutorial\bin compile: [javac] Compiling 1 source file to C:\hibernateTutorial\bin BUILD SUCCESSFUL Total time: 1 second
It's time to load and store some Event
objects, but first
we have to complete the setup with some infrastructure code. We have to startup
Hibernate. This startup includes building a global SessionFactory
object and to store it somewhere for easy access in application code.
A SessionFactory
can open up new Session
's.
A Session
represents a single-threaded unit of work, the
SessionFactory
is a thread-safe global object, instantiated once.
We'll create a HibernateUtil
helper class which takes care
of startup and makes accessing a SessionFactory
convenient.
Let's have a look at the implementation:
package util; import org.hibernate.*; import org.hibernate.cfg.*; public class HibernateUtil { private static final SessionFactory sessionFactory; static { try { // Create the SessionFactory from hibernate.cfg.xml sessionFactory = new Configuration().configure().buildSessionFactory(); } catch (Throwable ex) { // Make sure you log the exception, as it might be swallowed System.err.println("Initial SessionFactory creation failed." + ex); throw new ExceptionInInitializerError(ex); } } public static SessionFactory getSessionFactory() { return sessionFactory; } }
This class does not only produce the global SessionFactory
in
its static initializer (called once by the JVM when the class is loaded), but also
hides the fact that it uses a static singleton. It might as well lookup the
SessionFactory
from JNDI in an application server.
If you give the SessionFactory
a name in your configuration
file, Hibernate will in fact try to bind it to JNDI after it has been built.
To avoid this code completely you could also use JMX deployment and let the
JMX-capable container instantiate and bind a HibernateService
to JNDI. These advanced options are discussed in the Hibernate reference
documentation.
Place HibernateUtil.java
in the development source directory, in
a package next to events
:
. +lib <Hibernate and third-party libraries> +src +events Event.java Event.hbm.xml +util HibernateUtil.java hibernate.cfg.xml +data build.xml
This should again compile without problems. We finally need to configure a logging
system - Hibernate uses commons logging and leaves you the choice between Log4j and
JDK 1.4 logging. Most developers prefer Log4j: copy log4j.properties
from the Hibernate distribution (it's in the etc/
directory) to
your src
directory, next to hibernate.cfg.xml
.
Have a look at the example configuration and change the settings if you like to have
more verbose output. By default, only Hibernate startup message are shown on stdout.
The tutorial infrastructure is complete - and we are ready to do some real work with Hibernate.
Finally, we can use Hibernate to load and store objects. We write an
EventManager
class with a main()
method:
package events; import org.hibernate.Session; import java.util.Date; import util.HibernateUtil; public class EventManager { public static void main(String[] args) { EventManager mgr = new EventManager(); if (args[0].equals("store")) { mgr.createAndStoreEvent("My Event", new Date()); } HibernateUtil.getSessionFactory().close(); } private void createAndStoreEvent(String title, Date theDate) { Session session = HibernateUtil.getSessionFactory().getCurrentSession(); session.beginTransaction(); Event theEvent = new Event(); theEvent.setTitle(title); theEvent.setDate(theDate); session.save(theEvent); session.getTransaction().commit(); } }
We create a new Event
object, and hand it over to Hibernate.
Hibernate now takes care of the SQL and executes INSERT
s
on the database. Let's have a look at the Session
and
Transaction
-handling code before we run this.
A Session
is a single unit of work. For now we'll keep things
simple and assume a one-to-one granularity between a Hibernate Session
and a database transaction. To shield our code from the actual underlying transaction
system (in this case plain JDBC, but it could also run with JTA) we use the
Transaction
API that is available on the Hibernate Session
.
What does sessionFactory.getCurrentSession()
do? First, you can call it
as many times and anywhere you like, once you get hold of your SessionFactory
(easy thanks to HibernateUtil
). The getCurrentSession()
method always returns the "current" unit of work. Remember that we switched the configuration
option for this mechanism to "thread" in hibernate.cfg.xml
? Hence,
the current unit of work is bound to the current Java thread that executes our application.
However, this is not the full picture, you also have to consider scope, when a unit of work
begins and when it ends.
A Session
begins when it is first needed, when the first call to
getCurrentSession()
is made. It is then bound by Hibernate to the current
thread. When the transaction ends, either through commit or rollback, Hibernate automatically
unbinds the Session
from the thread and closes it for you. If you call
getCurrentSession()
again, you get a new Session
and can
start a new unit of work. This thread-bound programming model is the most
popular way of using Hibernate, as it allows flexible layering of your code (transaction
demarcation code can be separated from data access code, we'll do this later in this tutorial).
Related to the unit of work scope, should the Hibernate Session
be used to
execute one or several database operations? The above example uses one Session
for one operation. This is pure coincidence, the example is just not complex enough to show any
other approach. The scope of a Hibernate Session
is flexible but you should
never design your application to use a new Hibernate Session
for
every database operation. So even if you see it a few more times in
the following (very trivial) examples, consider session-per-operation
an anti-pattern. A real (web) application is shown later in this tutorial.
Have a look at Chapter 11, Transactions And Concurrency for more information about transaction handling and demarcation. We also skipped any error handling and rollback in the previous example.
To run this first routine we have to add a callable target to the Ant build file:
<target name="run" depends="compile"> <java fork="true" classname="events.EventManager" classpathref="libraries"> <classpath path="${targetdir}"/> <arg value="${action}"/> </java> </target>
The value of the action
argument is set on the command line when
calling the target:
C:\hibernateTutorial\>ant run -Daction=store
You should see, after compilation, Hibernate starting up and, depending on your configuration, lots of log output. At the end you will find the following line:
[java] Hibernate: insert into EVENTS (EVENT_DATE, title, EVENT_ID) values (?, ?, ?)
This is the INSERT
executed by Hibernate, the question marks
represent JDBC bind parameters. To see the values bound as arguments, or to reduce
the verbosity of the log, check your log4j.properties
.
Now we'd like to list stored events as well, so we add an option to the main method:
if (args[0].equals("store")) { mgr.createAndStoreEvent("My Event", new Date()); } else if (args[0].equals("list")) { List events = mgr.listEvents(); for (int i = 0; i < events.size(); i++) { Event theEvent = (Event) events.get(i); System.out.println("Event: " + theEvent.getTitle() + " Time: " + theEvent.getDate()); } }
We also add a new listEvents() method
:
private List listEvents() { Session session = HibernateUtil.getSessionFactory().getCurrentSession(); session.beginTransaction(); List result = session.createQuery("from Event").list(); session.getTransaction().commit(); return result; }
What we do here is use an HQL (Hibernate Query Language) query to load all existing
Event
objects from the database. Hibernate will generate the
appropriate SQL, send it to the database and populate Event
objects
with the data. You can create more complex queries with HQL, of course.
Now, to execute and test all of this, follow these steps:
Run ant run -Daction=store
to store something into the database
and, of course, to generate the database schema before through hbm2ddl.
Now disable hbm2ddl by commenting out the property in your hibernate.cfg.xml
file. Usually you only leave it turned on in continuous unit testing, but another
run of hbm2ddl would drop everything you have stored - the
create
configuration setting actually translates into "drop all
tables from the schema, then re-create all tables, when the SessionFactory is build".
If you now call Ant with -Daction=list
, you should see the events
you have stored so far. You can of course also call the store
action a few
times more.
Note: Most new Hibernate users fail at this point and we see questions about Table not found error messages regularly. However, if you follow the steps outlined above you will not have this problem, as hbm2ddl creates the database schema on the first run, and subsequent application restarts will use this schema. If you change the mapping and/or database schema, you have to re-enable hbm2ddl once again.
We mapped a persistent entity class to a table. Let's build on this and add some class associations. First we'll add people to our application, and store a list of events they participate in.
The first cut of the Person
class is simple:
package events; public class Person { private Long id; private int age; private String firstname; private String lastname; public Person() {} // Accessor methods for all properties, private setter for 'id' }
Create a new mapping file called Person.hbm.xml
(don't forget the
DTD reference at the top):
<hibernate-mapping> <class name="events.Person" table="PERSON"> <id name="id" column="PERSON_ID"> <generator class="native"/> </id> <property name="age"/> <property name="firstname"/> <property name="lastname"/> </class> </hibernate-mapping>
Finally, add the new mapping to Hibernate's configuration:
<mapping resource="events/Event.hbm.xml"/> <mapping resource="events/Person.hbm.xml"/>
We'll now create an association between these two entities. Obviously, persons can participate in events, and events have participants. The design questions we have to deal with are: directionality, multiplicity, and collection behavior.
We'll add a collection of events to the Person
class. That way we can
easily navigate to the events for a particular person, without executing an explicit query -
by calling aPerson.getEvents()
. We use a Java collection, a Set
,
because the collection will not contain duplicate elements and the ordering is not relevant for us.
We need a unidirectional, many-valued associations, implemented with a Set
.
Let's write the code for this in the Java classes and then map it:
public class Person { private Set events = new HashSet(); public Set getEvents() { return events; } public void setEvents(Set events) { this.events = events; } }
Before we map this association, think about the other side. Clearly, we could just keep this
unidirectional. Or, we could create another collection on the Event
, if we
want to be able to navigate it bi-directional, i.e. anEvent.getParticipants()
.
This is not necessary, from a functional perspective. You could always execute an explicit query
to retrieve the participants for a particular event. This is a design choice left to you, but what
is clear from this discussion is the multiplicity of the association: "many" valued on both sides,
we call this a many-to-many association. Hence, we use Hibernate's
many-to-many mapping:
<class name="events.Person" table="PERSON"> <id name="id" column="PERSON_ID"> <generator class="native"/> </id> <property name="age"/> <property name="firstname"/> <property name="lastname"/> <set name="events" table="PERSON_EVENT"> <key column="PERSON_ID"/> <many-to-many column="EVENT_ID" class="events.Event"/> </set> </class>
Hibernate supports all kinds of collection mappings, a <set>
being most
common. For a many-to-many association (or n:m entity relationship), an
association table is needed. Each row in this table represents a link between a person and an event.
The table name is configured with the table
attribute of the set
element. The identifier column name in the association, for the person's side, is defined with the
<key>
element, the column name for the event's side with the
column
attribute of the <many-to-many>
. You also
have to tell Hibernate the class of the objects in your collection (correct: the class on the
other side of the collection of references).
The database schema for this mapping is therefore:
_____________ __________________ | | | | _____________ | EVENTS | | PERSON_EVENT | | | |_____________| |__________________| | PERSON | | | | | |_____________| | *EVENT_ID | <--> | *EVENT_ID | | | | EVENT_DATE | | *PERSON_ID | <--> | *PERSON_ID | | TITLE | |__________________| | AGE | |_____________| | FIRSTNAME | | LASTNAME | |_____________|
Let's bring some people and events together in a new method in EventManager
:
private void addPersonToEvent(Long personId, Long eventId) { Session session = HibernateUtil.getSessionFactory().getCurrentSession(); session.beginTransaction(); Person aPerson = (Person) session.load(Person.class, personId); Event anEvent = (Event) session.load(Event.class, eventId); aPerson.getEvents().add(anEvent); session.getTransaction().commit(); }
After loading a Person
and an Event
, simply
modify the collection using the normal collection methods. As you can see, there is no explicit call
to update()
or save()
, Hibernate automatically
detects that the collection has been modified and needs to be updated. This is called automatic
dirty checking, and you can also try it by modifying the name or the date property of
any of your objects. As long as they are in persistent state, that is, bound
to a particular Hibernate Session
(i.e. they have been just loaded or saved in
a unit of work), Hibernate monitors any changes and executes SQL in a write-behind fashion. The
process of synchronizing the memory state with the database, usually only at the end of a unit of
work, is called flushing. In our code, the unit of work ends with a commit
(or rollback) of the database transaction - as defined by the thread
configuration
option for the CurrentSessionContext
class.
You might of course load person and event in different units of work. Or you modify an object
outside of a Session
, when it is not in persistent state (if it was persistent
before, we call this state detached). You can even modify a collection when
it is detached:
private void addPersonToEvent(Long personId, Long eventId) { Session session = HibernateUtil.getSessionFactory().getCurrentSession(); session.beginTransaction(); Person aPerson = (Person) session .createQuery("select p from Person p left join fetch p.events where p.id = :pid") .setParameter("pid", personId) .uniqueResult(); // Eager fetch the collection so we can use it detached Event anEvent = (Event) session.load(Event.class, eventId); session.getTransaction().commit(); // End of first unit of work aPerson.getEvents().add(anEvent); // aPerson (and its collection) is detached // Begin second unit of work Session session2 = HibernateUtil.getSessionFactory().getCurrentSession(); session2.beginTransaction(); session2.update(aPerson); // Reattachment of aPerson session2.getTransaction().commit(); }
The call to update
makes a detached object persistent again, you could
say it binds it to a new unit of work, so any modifications you made to it while detached
can be saved to the database. This includes any modifications (additions/deletions) you
made to a collection of that entity object.
Well, this is not much use in our current situation, but it's an important concept you can
design into your own application. For now, complete this exercise by adding a new action
to the EventManager
's main method and call it from the command line. If
you need the identifiers of a person and an event - the save()
method
returns it (you might have to modify some of the previous methods to return that identifier):
else if (args[0].equals("addpersontoevent")) { Long eventId = mgr.createAndStoreEvent("My Event", new Date()); Long personId = mgr.createAndStorePerson("Foo", "Bar"); mgr.addPersonToEvent(personId, eventId); System.out.println("Added person " + personId + " to event " + eventId); }
This was an example of an association between two equally important classes, two entities.
As mentioned earlier, there are other classes and types in a typical model, usually "less
important". Some you have already seen, like an int
or a String
.
We call these classes value types, and their instances depend
on a particular entity. Instances of these types don't have their own identity, nor are they
shared between entities (two persons don't reference the same firstname
object, even if they have the same first name). Of course, value types can not only be found in
the JDK (in fact, in a Hibernate application all JDK classes are considered value types), but
you can also write dependent classes yourself, Address
or MonetaryAmount
,
for example.
You can also design a collection of value types. This is conceptually very different from a collection of references to other entities, but looks almost the same in Java.
We add a collection of value typed objects to the Person
entity. We want to
store email addresses, so the type we use is String
, and the collection is
again a Set
:
private Set emailAddresses = new HashSet(); public Set getEmailAddresses() { return emailAddresses; } public void setEmailAddresses(Set emailAddresses) { this.emailAddresses = emailAddresses; }
The mapping of this Set
:
<set name="emailAddresses" table="PERSON_EMAIL_ADDR"> <key column="PERSON_ID"/> <element type="string" column="EMAIL_ADDR"/> </set>
The difference compared with the earlier mapping is the element
part, which tells Hibernate that the collection
does not contain references to another entity, but a collection of elements of type
String
(the lowercase name tells you it's a Hibernate mapping type/converter).
Once again, the table
attribute of the set
element determines
the table name for the collection. The key
element defines the foreign-key column
name in the collection table. The column
attribute in the element
element defines the column name where the String
values will actually be stored.
Have a look at the updated schema:
_____________ __________________ | | | | _____________ | EVENTS | | PERSON_EVENT | | | ___________________ |_____________| |__________________| | PERSON | | | | | | | |_____________| | PERSON_EMAIL_ADDR | | *EVENT_ID | <--> | *EVENT_ID | | | |___________________| | EVENT_DATE | | *PERSON_ID | <--> | *PERSON_ID | <--> | *PERSON_ID | | TITLE | |__________________| | AGE | | *EMAIL_ADDR | |_____________| | FIRSTNAME | |___________________| | LASTNAME | |_____________|
You can see that the primary key of the collection table is in fact a composite key, using both columns. This also implies that there can't be duplicate email addresses per person, which is exactly the semantics we need for a set in Java.
You can now try and add elements to this collection, just like we did before by linking persons and events. It's the same code in Java:
private void addEmailToPerson(Long personId, String emailAddress) { Session session = HibernateUtil.getSessionFactory().getCurrentSession(); session.beginTransaction(); Person aPerson = (Person) session.load(Person.class, personId); // The getEmailAddresses() might trigger a lazy load of the collection aPerson.getEmailAddresses().add(emailAddress); session.getTransaction().commit(); }
This time we didn't use a fetch query to initialize the collection. Hence, the call to its getter method will trigger an additional select to initialize it, so we can add an element to it. Monitor the SQL log and try to optimize this with an eager fetch.
Next we are going to map a bi-directional association - making the association between person and event work from both sides in Java. Of course, the database schema doesn't change, we still have many-to-many multiplicity. A relational database is more flexible than a network programming language, so it doesn't need anything like a navigation direction - data can be viewed and retrieved in any possible way.
First, add a collection of participants to the Event
Event class:
private Set participants = new HashSet(); public Set getParticipants() { return participants; } public void setParticipants(Set participants) { this.participants = participants; }
Now map this side of the association too, in Event.hbm.xml
.
<set name="participants" table="PERSON_EVENT" inverse="true"> <key column="EVENT_ID"/> <many-to-many column="PERSON_ID" class="events.Person"/> </set>
As you see, these are normal set
mappings in both mapping documents.
Notice that the column names in key
and many-to-many
are
swapped in both mapping documents. The most important addition here is the
inverse="true"
attribute in the set
element of the
Event
's collection mapping.
What this means is that Hibernate should take the other side - the Person
class -
when it needs to find out information about the link between the two. This will be a lot easier to
understand once you see how the bi-directional link between our two entities is created .
First, keep in mind that Hibernate does not affect normal Java semantics. How did we create a
link between a Person
and an Event
in the unidirectional
example? We added an instance of Event
to the collection of event references,
of an instance of Person
. So, obviously, if we want to make this link working
bi-directional, we have to do the same on the other side - adding a Person
reference to the collection in an Event
. This "setting the link on both sides"
is absolutely necessary and you should never forget doing it.
Many developers program defensively and create link management methods to
correctly set both sides, e.g. in Person
:
protected Set getEvents() { return events; } protected void setEvents(Set events) { this.events = events; } public void addToEvent(Event event) { this.getEvents().add(event); event.getParticipants().add(this); } public void removeFromEvent(Event event) { this.getEvents().remove(event); event.getParticipants().remove(this); }
Notice that the get and set methods for the collection are now protected - this allows classes in the same package and subclasses to still access the methods, but prevents everybody else from messing with the collections directly (well, almost). You should probably do the same with the collection on the other side.
What about the inverse
mapping attribute? For you, and for Java, a bi-directional
link is simply a matter of setting the references on both sides correctly. Hibernate however doesn't
have enough information to correctly arrange SQL INSERT
and UPDATE
statements (to avoid constraint violations), and needs some help to handle bi-directional associations
properly. Making one side of the association inverse
tells Hibernate to basically
ignore it, to consider it a mirror of the other side. That's all that is necessary
for Hibernate to work out all of the issues when transformation a directional navigation model to
a SQL database schema. The rules you have to remember are straightforward: All bi-directional associations
need one side as inverse
. In a one-to-many association it has to be the many-side,
in many-to-many association you can pick either side, there is no difference.
Let's turn the following discussion into a small web application...
A Hibernate web application uses Session
and Transaction
almost like a standalone application. However, some common patterns are useful. We now write
an EventManagerServlet
. This servlet can list all events stored in the
database, and it provides an HTML form to enter new events.
Create a new class in your source directory, in the events
package:
package events; // Imports public class EventManagerServlet extends HttpServlet { // Servlet code }
The servlet handles HTTP GET
requests only, hence, the method
we implement is doGet()
:
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { SimpleDateFormat dateFormatter = new SimpleDateFormat("dd.MM.yyyy"); try { // Begin unit of work HibernateUtil.getSessionFactory() .getCurrentSession().beginTransaction(); // Process request and render page... // End unit of work HibernateUtil.getSessionFactory() .getCurrentSession().getTransaction().commit(); } catch (Exception ex) { HibernateUtil.getSessionFactory() .getCurrentSession().getTransaction().rollback(); throw new ServletException(ex); } }
The pattern we are applying here is called session-per-request.
When a request hits the servlet, a new Hibernate Session
is
opened through the first call to getCurrentSession()
on the
SessionFactory
. Then a database transaction is started-all
data access as to occur inside a transaction, no matter if data is read or written
(we don't use the auto-commit mode in applications).
Do not use a new Hibernate Session
for
every database operation. Use one Hibernate Session
that is
scoped to the whole request. Use getCurrentSession()
, so that
it is automatically bound to the current Java thread.
Next, the possible actions of the request are processed and the response HTML is rendered. We'll get to that part soon.
Finally, the unit of work ends when processing and rendering is complete. If any
problem occurred during processing or rendering, an exception will be thrown
and the database transaction rolled back. This completes the
session-per-request
pattern. Instead of the transaction
demarcation code in every servlet you could also write a servlet filter.
See the Hibernate website and Wiki for more information about this pattern,
called Open Session in View-you'll need it as soon
as you consider rendering your view in JSP, not in a servlet.
Let's implement the processing of the request and rendering of the page.
// Write HTML header PrintWriter out = response.getWriter(); out.println("<html><head><title>Event Manager</title></head><body>"); // Handle actions if ( "store".equals(request.getParameter("action")) ) { String eventTitle = request.getParameter("eventTitle"); String eventDate = request.getParameter("eventDate"); if ( "".equals(eventTitle) || "".equals(eventDate) ) { out.println("<b><i>Please enter event title and date.</i></b>"); } else { createAndStoreEvent(eventTitle, dateFormatter.parse(eventDate)); out.println("<b><i>Added event.</i></b>"); } } // Print page printEventForm(out); listEvents(out, dateFormatter); // Write HTML footer out.println("</body></html>"); out.flush(); out.close();
Granted, this coding style with a mix of Java and HTML would not scale in a more complex application-keep in mind that we are only illustrating basic Hibernate concepts in this tutorial. The code prints an HTML header and a footer. Inside this page, an HTML form for event entry and a list of all events in the database are printed. The first method is trivial and only outputs HTML:
private void printEventForm(PrintWriter out) { out.println("<h2>Add new event:</h2>"); out.println("<form>"); out.println("Title: <input name='eventTitle' length='50'/><br/>"); out.println("Date (e.g. 24.12.2009): <input name='eventDate' length='10'/><br/>"); out.println("<input type='submit' name='action' value='store'/>"); out.println("</form>"); }
The listEvents()
method uses the Hibernate
Session
bound to the current thread to execute
a query:
private void listEvents(PrintWriter out, SimpleDateFormat dateFormatter) { List result = HibernateUtil.getSessionFactory() .getCurrentSession().createCriteria(Event.class).list(); if (result.size() > 0) { out.println("<h2>Events in database:</h2>"); out.println("<table border='1'>"); out.println("<tr>"); out.println("<th>Event title</th>"); out.println("<th>Event date</th>"); out.println("</tr>"); for (Iterator it = result.iterator(); it.hasNext();) { Event event = (Event) it.next(); out.println("<tr>"); out.println("<td>" + event.getTitle() + "</td>"); out.println("<td>" + dateFormatter.format(event.getDate()) + "</td>"); out.println("</tr>"); } out.println("</table>"); } }
Finally, the store
action is dispatched to the
createAndStoreEvent()
method, which also uses
the Session
of the current thread:
protected void createAndStoreEvent(String title, Date theDate) { Event theEvent = new Event(); theEvent.setTitle(title); theEvent.setDate(theDate); HibernateUtil.getSessionFactory() .getCurrentSession().save(theEvent); }
That's it, the servlet is complete. A request to the servlet will be processed
in a single Session
and Transaction
. As
earlier in the standalone application, Hibernate can automatically bind these
objects to the current thread of execution. This gives you the freedom to layer
your code and access the SessionFactory
in any way you like.
Usually you'd use a more sophisticated design and move the data access code
into data access objects (the DAO pattern). See the Hibernate Wiki for more
examples.
To deploy this application you have to create a web archive, a WAR. Add the
following Ant target to your build.xml
:
<target name="war" depends="compile"> <war destfile="hibernate-tutorial.war" webxml="web.xml"> <lib dir="${librarydir}"> <exclude name="jsdk*.jar"/> </lib> <classes dir="${targetdir}"/> </war> </target>
This target creates a file called hibernate-tutorial.war
in your project directory. It packages all libraries and the web.xml
descriptor, which is expected in the base directory of your project:
<?xml version="1.0" encoding="UTF-8"?> <web-app version="2.4" xmlns="http://java.sun.com/xml/ns/j2ee" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://java.sun.com/xml/ns/j2ee http://java.sun.com/xml/ns/j2ee/web-app_2_4.xsd"> <servlet> <servlet-name>Event Manager</servlet-name> <servlet-class>events.EventManagerServlet</servlet-class> </servlet> <servlet-mapping> <servlet-name>Event Manager</servlet-name> <url-pattern>/eventmanager</url-pattern> </servlet-mapping> </web-app>
Before you compile and deploy the web application, note that an additional library
is required: jsdk.jar
. This is the Java servlet development kit,
if you don't have this library already, get it from the Sun website and copy it to
your library directory. However, it will be only used for compilation and excluded
from the WAR package.
To build and deploy call ant war
in your project directory
and copy the hibernate-tutorial.war
file into your Tomcat
webapp
directory. If you don't have Tomcat installed, download
it and follow the installation instructions. You don't have to change any Tomcat
configuration to deploy this application though.
Once deployed and Tomcat is running, access the application at
http://localhost:8080/hibernate-tutorial/eventmanager
. Make
sure you watch the Tomcat log to see Hibernate initialize when the first
request hits your servlet (the static initializer in HibernateUtil
is called) and to get the detailed output if any exceptions occurs.
This tutorial covered the basics of writing a simple standalone Hibernate application and a small web application.
If you already feel confident with Hibernate, continue browsing through the reference documentation table of contents for topics you find interesting - most asked are transactional processing (Chapter 11, Transactions And Concurrency), fetch performance (Chapter 19, Improving performance), or the usage of the API (Chapter 10, Working with objects) and the query features (Section 10.4, “Querying”).
Don't forget to check the Hibernate website for more (specialized) tutorials.
A (very) high-level view of the Hibernate architecture:
This diagram shows Hibernate using the database and configuration data to provide persistence services (and persistent objects) to the application.
We would like to show a more detailed view of the runtime architecture. Unfortunately, Hibernate is flexible and supports several approaches. We will show the two extremes. The "lite" architecture has the application provide its own JDBC connections and manage its own transactions. This approach uses a minimal subset of Hibernate's APIs:
The "full cream" architecture abstracts the application away from the underlying JDBC/JTA APIs and lets Hibernate take care of the details.
Heres some definitions of the objects in the diagrams:
org.hibernate.SessionFactory
)
A threadsafe (immutable) cache of compiled mappings for a single database.
A factory for Session
and a client of
ConnectionProvider
. Might hold an optional (second-level)
cache of data that is reusable between transactions, at a
process- or cluster-level.
org.hibernate.Session
)
A single-threaded, short-lived object representing a conversation between
the application and the persistent store. Wraps a JDBC connection. Factory
for Transaction
. Holds a mandatory (first-level) cache
of persistent objects, used when navigating the object graph or looking up
objects by identifier.
Short-lived, single threaded objects containing persistent state and business
function. These might be ordinary JavaBeans/POJOs, the only special thing about
them is that they are currently associated with (exactly one)
Session
. As soon as the Session
is closed,
they will be detached and free to use in any application layer (e.g. directly
as data transfer objects to and from presentation).
Instances of persistent classes that are not currently associated with a
Session
. They may have been instantiated by
the application and not (yet) persisted or they may have been instantiated by a
closed Session
.
org.hibernate.Transaction
)
(Optional) A single-threaded, short-lived object used by the application to
specify atomic units of work. Abstracts application from underlying JDBC,
JTA or CORBA transaction. A Session
might span several
Transaction
s in some cases. However, transaction demarcation,
either using the underlying API or Transaction
, is never
optional!
org.hibernate.connection.ConnectionProvider
)
(Optional) A factory for (and pool of) JDBC connections. Abstracts application from
underlying Datasource
or DriverManager
.
Not exposed to application, but can be extended/implemented by the developer.
org.hibernate.TransactionFactory
)
(Optional) A factory for Transaction
instances. Not exposed to the
application, but can be extended/implemented by the developer.
Hibernate offers many optional extension interfaces you can implement to customize the behavior of your persistence layer. See the API documentation for details.
Given a "lite" architecture, the application bypasses the
Transaction
/TransactionFactory
and/or
ConnectionProvider
APIs to talk to JTA or JDBC directly.
An instance of a persistent classes may be in one of three different states,
which are defined with respect to a persistence context.
The Hibernate Session
object is the persistence context:
The instance is not, and has never been associated with any persistence context. It has no persistent identity (primary key value).
The instance is currently associated with a persistence context. It has a persistent identity (primary key value) and, perhaps, a corresponding row in the database. For a particular persistence context, Hibernate guarantees that persistent identity is equivalent to Java identity (in-memory location of the object).
The instance was once associated with a persistence context, but that context was closed, or the instance was serialized to another process. It has a persistent identity and, perhaps, a corresponding row in the database. For detached instances, Hibernate makes no guarantees about the relationship between persistent identity and Java identity.
JMX is the J2EE standard for management of Java components. Hibernate may be managed via
a JMX standard service. We provide an MBean implementation in the distribution,
org.hibernate.jmx.HibernateService
.
For an example how to deploy Hibernate as a JMX service on the JBoss Application Server, please see the JBoss User Guide. On JBoss AS, you also get these benefits if you deploy using JMX:
Session Management: The Hibernate Session
's life cycle
can be automatically bound to the scope of a JTA transaction. This means you no
longer have to manually open and close the Session
, this
becomes the job of a JBoss EJB interceptor. You also don't have to worry about
transaction demarcation in your code anymore (unless you'd like to write a portable
persistence layer of course, use the optional Hibernate Transaction
API for this). You call the HibernateContext
to access a
Session
.
HAR deployment: Usually you deploy the Hibernate JMX service using a JBoss
service deployment descriptor (in an EAR and/or SAR file), it supports all the usual
configuration options of a Hibernate SessionFactory
. However, you still
have to name all your mapping files in the deployment descriptor. If you decide to use
the optional HAR deployment, JBoss will automatically detect all mapping files in your
HAR file.
Consult the JBoss AS user guide for more information about these options.
Another feature available as a JMX service are runtime Hibernate statistics. See Section 3.4.6, “Hibernate statistics”.
Hibernate may also be configured as a JCA connector. Please see the website for more details. Please note that Hibernate JCA support is still considered experimental.
Most applications using Hibernate need some form of "contextual" sessions, where a given
session is in effect throughout the scope of a given context. However, across applications
the definition of what constitutes a context is typically different; and different contexts
define different scopes to the notion of current. Applications using Hibernate prior
to version 3.0 tended to utilize either home-grown ThreadLocal
-based
contextual sessions, helper classes such as HibernateUtil
, or utilized
third-party frameworks (such as Spring or Pico) which provided proxy/interception-based contextual sessions.
Starting with version 3.0.1, Hibernate added the SessionFactory.getCurrentSession()
method. Initially, this assumed usage of JTA
transactions, where the
JTA
transaction defined both the scope and context of a current session.
The Hibernate team maintains that, given the maturity of the numerous stand-alone
JTA TransactionManager
implementations out there, most (if not all)
applications should be using JTA
transaction management whether or not
they are deployed into a J2EE
container. Based on that, the
JTA
-based contextual sessions is all you should ever need to use.
However, as of version 3.1, the processing behind
SessionFactory.getCurrentSession()
is now pluggable. To that
end, a new extension interface (org.hibernate.context.CurrentSessionContext
)
and a new configuration parameter (hibernate.current_session_context_class
)
have been added to allow pluggability of the scope and context of defining current sessions.
See the Javadocs for the org.hibernate.context.CurrentSessionContext
interface for a detailed discussion of its contract. It defines a single method,
currentSession()
, by which the implementation is responsible for
tracking the current contextual session. Out-of-the-box, Hibernate comes with three
implementations of this interface.
org.hibernate.context.JTASessionContext
- current sessions
are tracked and scoped by a JTA
transaction. The processing
here is exactly the same as in the older JTA-only approach. See the Javadocs
for details.
org.hibernate.context.ThreadLocalSessionContext
- current
sessions are tracked by thread of execution. Again, see the Javadocs for details.
org.hibernate.context.ManagedSessionContext
- current
sessions are tracked by thread of execution. However, you are responsible to
bind and unbind a Session
instance with static methods
on this class, it does never open, flush, or close a Session
.
The first two implementations provide a "one session - one database transaction" programming
model, also known and used as session-per-request. The beginning
and end of a Hibernate session is defined by the duration of a database transaction.
If you use programmatic transaction demarcation in plain JSE without JTA, you are advised to
use the Hibernate Transaction
API to hide the underlying transaction system
from your code. If you use JTA, use the JTA interfaces to demarcate transactions. If you
execute in an EJB container that supports CMT, transaction boundaries are defined declaratively
and you don't need any transaction or session demarcation operations in your code.
Refer to Chapter 11, Transactions And Concurrency for more information and code examples.
The hibernate.current_session_context_class
configuration parameter
defines which org.hibernate.context.CurrentSessionContext
implementation
should be used. Note that for backwards compatibility, if this config param is not set
but a org.hibernate.transaction.TransactionManagerLookup
is configured,
Hibernate will use the org.hibernate.context.JTASessionContext
.
Typically, the value of this parameter would just name the implementation class to
use; for the three out-of-the-box implementations, however, there are three corresponding
short names, "jta", "thread", and "managed".
Because Hibernate is designed to operate in many different environments, there
are a large number of configuration parameters. Fortunately, most have sensible
default values and Hibernate is distributed with an example
hibernate.properties
file in etc/
that shows
the various options. Just put the example file in your classpath and customize it.
An instance of org.hibernate.cfg.Configuration
represents an entire set of mappings
of an application's Java types to an SQL database. The org.hibernate.cfg.Configuration
is used to build an (immutable) org.hibernate.SessionFactory
. The mappings
are compiled from various XML mapping files.
You may obtain a org.hibernate.cfg.Configuration
instance by instantiating
it directly and specifying XML mapping documents. If the mapping files are in the classpath,
use addResource()
:
Configuration cfg = new Configuration() .addResource("Item.hbm.xml") .addResource("Bid.hbm.xml");
An alternative (sometimes better) way is to specify the mapped class, and let Hibernate find the mapping document for you:
Configuration cfg = new Configuration() .addClass(org.hibernate.auction.Item.class) .addClass(org.hibernate.auction.Bid.class);
Then Hibernate will look for mapping files named /org/hibernate/auction/Item.hbm.xml
and /org/hibernate/auction/Bid.hbm.xml
in the classpath. This approach eliminates any
hardcoded filenames.
A org.hibernate.cfg.Configuration
also allows you to specify configuration
properties:
Configuration cfg = new Configuration() .addClass(org.hibernate.auction.Item.class) .addClass(org.hibernate.auction.Bid.class) .setProperty("hibernate.dialect", "org.hibernate.dialect.MySQLInnoDBDialect") .setProperty("hibernate.connection.datasource", "java:comp/env/jdbc/test") .setProperty("hibernate.order_updates", "true");
This is not the only way to pass configuration properties to Hibernate. The various options include:
Pass an instance of java.util.Properties
to
Configuration.setProperties()
.
Place a file named hibernate.properties
in a root directory of the classpath.
Set System
properties using java -Dproperty=value
.
Include <property>
elements in
hibernate.cfg.xml
(discussed later).
hibernate.properties
is the easiest approach if you want to get started quickly.
The org.hibernate.cfg.Configuration
is intended as a startup-time object,
to be discarded once a SessionFactory
is created.
When all mappings have been parsed by the org.hibernate.cfg.Configuration
,
the application must obtain a factory for org.hibernate.Session
instances.
This factory is intended to be shared by all application threads:
SessionFactory sessions = cfg.buildSessionFactory();
Hibernate does allow your application to instantiate more than one
org.hibernate.SessionFactory
. This is useful if you are using more than
one database.
Usually, you want to have the org.hibernate.SessionFactory
create and pool
JDBC connections for you. If you take this approach, opening a org.hibernate.Session
is as simple as:
Session session = sessions.openSession(); // open a new Session
As soon as you do something that requires access to the database, a JDBC connection will be obtained from the pool.
For this to work, we need to pass some JDBC connection properties to Hibernate. All Hibernate property
names and semantics are defined on the class org.hibernate.cfg.Environment
. We will
now describe the most important settings for JDBC connection configuration.
Hibernate will obtain (and pool) connections using java.sql.DriverManager
if you set the following properties:
Table 3.1. Hibernate JDBC Properties
Property name | Purpose |
---|---|
hibernate.connection.driver_class | JDBC driver class |
hibernate.connection.url | JDBC URL |
hibernate.connection.username | database user |
hibernate.connection.password | database user password |
hibernate.connection.pool_size | maximum number of pooled connections |
Hibernate's own connection pooling algorithm is however quite rudimentary. It is intended to help you get started and is not intended for use in a production system or even for performance testing. You should use a third party pool for best performance and stability. Just replace the hibernate.connection.pool_size property with connection pool specific settings. This will turn off Hibernate's internal pool. For example, you might like to use C3P0.
C3P0 is an open source JDBC connection pool distributed along with Hibernate in the lib
directory. Hibernate will use its org.hibernate.connection.C3P0ConnectionProvider
for connection pooling if you set hibernate.c3p0.* properties. If you'd like to use Proxool
refer to the packaged hibernate.properties
and the Hibernate web site for more
information.
Here is an example hibernate.properties
file for C3P0:
hibernate.connection.driver_class = org.postgresql.Driver hibernate.connection.url = jdbc:postgresql://localhost/mydatabase hibernate.connection.username = myuser hibernate.connection.password = secret hibernate.c3p0.min_size=5 hibernate.c3p0.max_size=20 hibernate.c3p0.timeout=1800 hibernate.c3p0.max_statements=50 hibernate.dialect = org.hibernate.dialect.PostgreSQLDialect
For use inside an application server, you should almost always configure Hibernate to obtain connections
from an application server javax.sql.Datasource
registered in JNDI. You'll
need to set at least one of the following properties:
Table 3.2. Hibernate Datasource Properties
Property name | Purpose |
---|---|
hibernate.connection.datasource | datasource JNDI name |
hibernate.jndi.url | URL of the JNDI provider (optional) |
hibernate.jndi.class |
class of the JNDI InitialContextFactory (optional)
|
hibernate.connection.username | database user (optional) |
hibernate.connection.password | database user password (optional) |
Here's an example hibernate.properties
file for an application server provided JNDI
datasource:
hibernate.connection.datasource = java:/comp/env/jdbc/test hibernate.transaction.factory_class = \ org.hibernate.transaction.JTATransactionFactory hibernate.transaction.manager_lookup_class = \ org.hibernate.transaction.JBossTransactionManagerLookup hibernate.dialect = org.hibernate.dialect.PostgreSQLDialect
JDBC connections obtained from a JNDI datasource will automatically participate in the container-managed transactions of the application server.
Arbitrary connection properties may be given by prepending "hibernate.connection
" to the
connection property name. For example, you may specify a charSet
connection property using hibernate.connection.charSet.
You may define your own plugin strategy for obtaining JDBC connections by implementing the
interface org.hibernate.connection.ConnectionProvider
, and specifying your
custom implementation via the hibernate.connection.provider_class property.
There are a number of other properties that control the behaviour of Hibernate at runtime. All are optional and have reasonable default values.
Warning: some of these properties are "system-level" only. System-level properties can
be set only via java -Dproperty=value
or hibernate.properties
. They
may not be set by the other techniques described above.
Table 3.3. Hibernate Configuration Properties
Property name | Purpose |
---|---|
hibernate.dialect |
The classname of a Hibernate org.hibernate.dialect.Dialect which
allows Hibernate to generate SQL optimized for a particular relational database.
eg.
In most cases Hibernate will actually be able to chose the correct
|
hibernate.show_sql |
Write all SQL statements to console. This is an alternative
to setting the log category org.hibernate.SQL
to debug .
eg.
|
hibernate.format_sql |
Pretty print the SQL in the log and console.
eg.
|
hibernate.default_schema |
Qualify unqualified table names with the given schema/tablespace
in generated SQL.
eg.
|
hibernate.default_catalog |
Qualify unqualified table names with the given catalog
in generated SQL.
eg.
|
hibernate.session_factory_name |
The org.hibernate.SessionFactory will be automatically
bound to this name in JNDI after it has been created.
eg.
|
hibernate.max_fetch_depth |
Set a maximum "depth" for the outer join fetch tree
for single-ended associations (one-to-one, many-to-one).
A 0 disables default outer join fetching.
eg.
recommended values between |
hibernate.default_batch_fetch_size |
Set a default size for Hibernate batch fetching of associations.
eg.
recommended values |
hibernate.default_entity_mode |
Set a default mode for entity representation for all sessions
opened from this SessionFactory
|
hibernate.order_updates |
Force Hibernate to order SQL updates by the primary key value
of the items being updated. This will result in fewer transaction
deadlocks in highly concurrent systems.
eg.
|
hibernate.generate_statistics |
If enabled, Hibernate will collect statistics useful for
performance tuning.
eg.
|
hibernate.use_identifier_rollback |
If enabled, generated identifier properties will be
reset to default values when objects are deleted.
eg.
|
hibernate.use_sql_comments |
If turned on, Hibernate will generate comments inside the SQL, for
easier debugging, defaults to false .
eg.
|
Table 3.4. Hibernate JDBC and Connection Properties
Property name | Purpose |
---|---|
hibernate.jdbc.fetch_size |
A non-zero value determines the JDBC fetch size (calls
Statement.setFetchSize() ).
|
hibernate.jdbc.batch_size |
A non-zero value enables use of JDBC2 batch updates by Hibernate.
eg.
recommended values between |
hibernate.jdbc.batch_versioned_data |
Set this property to true if your JDBC driver returns
correct row counts from executeBatch() (it is usually
safe to turn this option on). Hibernate will then use batched DML for
automatically versioned data. Defaults to false .
eg.
|
hibernate.jdbc.factory_class |
Select a custom org.hibernate.jdbc.Batcher . Most applications
will not need this configuration property.
eg.
|
hibernate.jdbc.use_scrollable_resultset |
Enables use of JDBC2 scrollable resultsets by Hibernate.
This property is only necessary when using user supplied
JDBC connections, Hibernate uses connection metadata otherwise.
eg.
|
hibernate.jdbc.use_streams_for_binary |
Use streams when writing/reading binary or serializable
types to/from JDBC. *system-level property*
eg.
|
hibernate.jdbc.use_get_generated_keys |
Enable use of JDBC3 PreparedStatement.getGeneratedKeys()
to retrieve natively generated keys after insert. Requires JDBC3+ driver
and JRE1.4+, set to false if your driver has problems with the Hibernate
identifier generators. By default, tries to determine the driver capabilities
using connection metadata.
eg.
|
hibernate.connection.provider_class |
The classname of a custom org.hibernate.connection.ConnectionProvider
which provides JDBC connections to Hibernate.
eg.
|
hibernate.connection.isolation |
Set the JDBC transaction isolation level. Check java.sql.Connection
for meaningful values but note that most databases do not support all isolation levels and some
define additional, non-standard isolations.
eg.
|
hibernate.connection.autocommit |
Enables autocommit for JDBC pooled connections (not recommended).
eg.
|
hibernate.connection.release_mode |
Specify when Hibernate should release JDBC connections. By default,
a JDBC connection is held until the session is explicitly closed or
disconnected. For an application server JTA datasource, you should use
after_statement to aggressively release connections
after every JDBC call. For a non-JTA connection, it often makes sense to
release the connection at the end of each transaction, by using
after_transaction . auto will
choose after_statement for the JTA and CMT transaction
strategies and after_transaction for the JDBC
transaction strategy.
eg.
Note that this setting only affects |
hibernate.connection.<propertyName> |
Pass the JDBC property <propertyName>
to DriverManager.getConnection() .
|
hibernate.jndi.<propertyName> |
Pass the property <propertyName> to
the JNDI InitialContextFactory .
|
Table 3.5. Hibernate Cache Properties
Property name | Purpose |
---|---|
hibernate.cache.provider_class
|
The classname of a custom CacheProvider .
eg.
|
hibernate.cache.use_minimal_puts
|
Optimize second-level cache operation to minimize writes, at the
cost of more frequent reads. This setting is most useful for
clustered caches and, in Hibernate3, is enabled by default for
clustered cache implementations.
eg.
|
hibernate.cache.use_query_cache
|
Enable the query cache, individual queries still have to be set cachable.
eg.
|
hibernate.cache.use_second_level_cache
|
May be used to completely disable the second level cache, which is enabled
by default for classes which specify a <cache>
mapping.
eg.
|
hibernate.cache.query_cache_factory
|
The classname of a custom QueryCache interface,
defaults to the built-in StandardQueryCache .
eg.
|
hibernate.cache.region_prefix
|
A prefix to use for second-level cache region names.
eg.
|
hibernate.cache.use_structured_entries
|
Forces Hibernate to store data in the second-level cache
in a more human-friendly format.
eg.
|
Table 3.6. Hibernate Transaction Properties
Property name | Purpose |
---|---|
hibernate.transaction.factory_class
|
The classname of a TransactionFactory
to use with Hibernate Transaction API
(defaults to JDBCTransactionFactory ).
eg.
|
jta.UserTransaction
|
A JNDI name used by JTATransactionFactory to
obtain the JTA UserTransaction from the
application server.
eg.
|
hibernate.transaction.manager_lookup_class
|
The classname of a TransactionManagerLookup
- required when JVM-level caching is enabled or when using hilo
generator in a JTA environment.
eg.
|
hibernate.transaction.flush_before_completion
|
If enabled, the session will be automatically flushed during the
before completion phase of the transaction. Built-in and
automatic session context management is preferred, see
Section 2.5, “Contextual Sessions”.
eg.
|
hibernate.transaction.auto_close_session
|
If enabled, the session will be automatically closed during the
after completion phase of the transaction. Built-in and
utomatic session context management is preferred, see
Section 2.5, “Contextual Sessions”.
eg.
|
Table 3.7. Miscellaneous Properties
Property name | Purpose |
---|---|
hibernate.current_session_context_class
|
Supply a (custom) strategy for the scoping of the "current"
Session . See
Section 2.5, “Contextual Sessions” for more
information about the built-in strategies.
eg.
|
hibernate.query.factory_class
|
Chooses the HQL parser implementation.
eg.
|
hibernate.query.substitutions
|
Mapping from tokens in Hibernate queries to SQL tokens
(tokens might be function or literal names, for example).
eg.
|
hibernate.hbm2ddl.auto
|
Automatically validate or export schema DDL to the database
when the SessionFactory is created. With
create-drop , the database schema will be
dropped when the SessionFactory is closed
explicitly.
eg.
|
hibernate.cglib.use_reflection_optimizer
|
Enables use of CGLIB instead of runtime reflection (System-level
property). Reflection can sometimes be useful when troubleshooting,
note that Hibernate always requires CGLIB even if you turn off the
optimizer. You can not set this property in hibernate.cfg.xml .
eg.
|
You should always set the hibernate.dialect
property to the correct
org.hibernate.dialect.Dialect
subclass for your database. If you
specify a dialect, Hibernate will use sensible defaults for some of the
other properties listed above, saving you the effort of specifying them manually.
Table 3.8. Hibernate SQL Dialects (hibernate.dialect
)
RDBMS | Dialect |
---|---|
DB2 | org.hibernate.dialect.DB2Dialect |
DB2 AS/400 | org.hibernate.dialect.DB2400Dialect |
DB2 OS390 | org.hibernate.dialect.DB2390Dialect |
PostgreSQL | org.hibernate.dialect.PostgreSQLDialect |
MySQL | org.hibernate.dialect.MySQLDialect |
MySQL with InnoDB | org.hibernate.dialect.MySQLInnoDBDialect |
MySQL with MyISAM | org.hibernate.dialect.MySQLMyISAMDialect |
Oracle (any version) | org.hibernate.dialect.OracleDialect |
Oracle 9i/10g | org.hibernate.dialect.Oracle9Dialect |
Sybase | org.hibernate.dialect.SybaseDialect |
Sybase Anywhere | org.hibernate.dialect.SybaseAnywhereDialect |
Microsoft SQL Server | org.hibernate.dialect.SQLServerDialect |
SAP DB | org.hibernate.dialect.SAPDBDialect |
Informix | org.hibernate.dialect.InformixDialect |
HypersonicSQL | org.hibernate.dialect.HSQLDialect |
Ingres | org.hibernate.dialect.IngresDialect |
Progress | org.hibernate.dialect.ProgressDialect |
Mckoi SQL | org.hibernate.dialect.MckoiDialect |
Interbase | org.hibernate.dialect.InterbaseDialect |
Pointbase | org.hibernate.dialect.PointbaseDialect |
FrontBase | org.hibernate.dialect.FrontbaseDialect |
Firebird | org.hibernate.dialect.FirebirdDialect |
If your database supports ANSI, Oracle or Sybase style outer joins, outer join
fetching will often increase performance by limiting the number of round
trips to and from the database (at the cost of possibly more work performed by
the database itself). Outer join fetching allows a whole graph of objects connected
by many-to-one, one-to-many, many-to-many and one-to-one associations to be retrieved
in a single SQL SELECT
.
Outer join fetching may be disabled globally by setting
the property hibernate.max_fetch_depth
to 0
.
A setting of 1
or higher enables outer join fetching for
one-to-one and many-to-one associations which have been mapped with
fetch="join"
.
See Section 19.1, “Fetching strategies” for more information.
Oracle limits the size of byte
arrays that may
be passed to/from its JDBC driver. If you wish to use large instances of
binary
or serializable
type, you should
enable hibernate.jdbc.use_streams_for_binary
.
This is a system-level setting only.
The properties prefixed by hibernate.cache
allow you to use a process or cluster scoped second-level cache system
with Hibernate. See the Section 19.2, “The Second Level Cache” for
more details.
You may define new Hibernate query tokens using hibernate.query.substitutions
.
For example:
hibernate.query.substitutions true=1, false=0
would cause the tokens true
and false
to be translated to
integer literals in the generated SQL.
hibernate.query.substitutions toLowercase=LOWER
would allow you to rename the SQL LOWER
function.
If you enable hibernate.generate_statistics
, Hibernate will
expose a number of metrics that are useful when tuning a running system via
SessionFactory.getStatistics()
. Hibernate can even be configured
to expose these statistics via JMX. Read the Javadoc of the interfaces in
org.hibernate.stats
for more information.
Hibernate utilizes Simple Logging Facade for Java
(SLF4J) in order to log various system events. SLF4J can direct your logging output to
several logging frameworks (NOP, Simple, log4j version 1.2, JDK 1.4 logging, JCL or logback) depending on your
chosen binding. In order to setup logging properly you will need slf4j-api.jar
in
your classpath together with the jar file for your preferred binding - slf4j-log4j12.jar
in the case of Log4J. See the SLF4J documentation for more detail.
To use Log4j you will also need to place a log4j.properties
file in your classpath,
an example properties file is distributed with Hibernate in the src/
directory.
We strongly recommend that you familiarize yourself with Hibernate's log messages. A lot of work has been put into making the Hibernate log as detailed as possible, without making it unreadable. It is an essential troubleshooting device. The most interesting log categories are the following:
Table 3.9. Hibernate Log Categories
Category | Function |
---|---|
org.hibernate.SQL |
Log all SQL DML statements as they are executed |
org.hibernate.type |
Log all JDBC parameters |
org.hibernate.tool.hbm2ddl |
Log all SQL DDL statements as they are executed |
org.hibernate.pretty |
Log the state of all entities (max 20 entities) associated with the session at flush time |
org.hibernate.cache |
Log all second-level cache activity |
org.hibernate.transaction |
Log transaction related activity |
org.hibernate.jdbc |
Log all JDBC resource acquisition |
org.hibernate.hql.ast.AST |
Log HQL and SQL ASTs during query parsing |
org.hibernate.secure |
Log all JAAS authorization requests |
org.hibernate |
Log everything (a lot of information, but very useful for troubleshooting) |
When developing applications with Hibernate, you should almost always work with
debug
enabled for the category org.hibernate.SQL
,
or, alternatively, the property hibernate.show_sql
enabled.
The interface org.hibernate.cfg.NamingStrategy
allows you
to specify a "naming standard" for database objects and schema elements.
You may provide rules for automatically generating database identifiers from
Java identifiers or for processing "logical" column and table names given in
the mapping file into "physical" table and column names. This feature helps
reduce the verbosity of the mapping document, eliminating repetitive noise
(TBL_
prefixes, for example). The default strategy used by
Hibernate is quite minimal.
You may specify a different strategy by calling
Configuration.setNamingStrategy()
before adding mappings:
SessionFactory sf = new Configuration() .setNamingStrategy(ImprovedNamingStrategy.INSTANCE) .addFile("Item.hbm.xml") .addFile("Bid.hbm.xml") .buildSessionFactory();
org.hibernate.cfg.ImprovedNamingStrategy
is a built-in
strategy that might be a useful starting point for some applications.
An alternative approach to configuration is to specify a full configuration in
a file named hibernate.cfg.xml
. This file can be used as a
replacement for the hibernate.properties
file or, if both
are present, to override properties.
The XML configuration file is by default expected to be in the root o
your CLASSPATH
. Here is an example:
<?xml version='1.0' encoding='utf-8'?> <!DOCTYPE hibernate-configuration PUBLIC "-//Hibernate/Hibernate Configuration DTD//EN" "http://hibernate.sourceforge.net/hibernate-configuration-3.0.dtd"> <hibernate-configuration> <!-- a SessionFactory instance listed as /jndi/name --> <session-factory name="java:hibernate/SessionFactory"> <!-- properties --> <property name="connection.datasource">java:/comp/env/jdbc/MyDB</property> <property name="dialect">org.hibernate.dialect.MySQLDialect</property> <property name="show_sql">false</property> <property name="transaction.factory_class"> org.hibernate.transaction.JTATransactionFactory </property> <property name="jta.UserTransaction">java:comp/UserTransaction</property> <!-- mapping files --> <mapping resource="org/hibernate/auction/Item.hbm.xml"/> <mapping resource="org/hibernate/auction/Bid.hbm.xml"/> <!-- cache settings --> <class-cache class="org.hibernate.auction.Item" usage="read-write"/> <class-cache class="org.hibernate.auction.Bid" usage="read-only"/> <collection-cache collection="org.hibernate.auction.Item.bids" usage="read-write"/> </session-factory> </hibernate-configuration>
As you can see, the advantage of this approach is the externalization of the
mapping file names to configuration. The hibernate.cfg.xml
is also more convenient once you have to tune the Hibernate cache. Note that is
your choice to use either hibernate.properties
or
hibernate.cfg.xml
, both are equivalent, except for the above
mentioned benefits of using the XML syntax.
With the XML configuration, starting Hibernate is then as simple as
SessionFactory sf = new Configuration().configure().buildSessionFactory();
You can pick a different XML configuration file using
SessionFactory sf = new Configuration() .configure("catdb.cfg.xml") .buildSessionFactory();
Hibernate has the following integration points for J2EE infrastructure:
Container-managed datasources: Hibernate can use
JDBC connections managed by the container and provided through JNDI. Usually,
a JTA compatible TransactionManager
and a
ResourceManager
take care of transaction management (CMT),
esp. distributed transaction handling across several datasources. You may
of course also demarcate transaction boundaries programmatically (BMT) or
you might want to use the optional Hibernate Transaction
API for this to keep your code portable.
Automatic JNDI binding: Hibernate can bind its
SessionFactory
to JNDI after startup.
JTA Session binding: The Hibernate Session
may be automatically bound to the scope of JTA transactions. Simply
lookup the SessionFactory
from JNDI and get the current
Session
. Let Hibernate take care of flushing and closing the
Session
when your JTA transaction completes. Transaction
demarcation is either declarative (CMT) or programmatic (BMT/UserTransaction).
JMX deployment: If you have a JMX capable application server
(e.g. JBoss AS), you can chose to deploy Hibernate as a managed MBean. This saves
you the one line startup code to build your SessionFactory
from
a Configuration
. The container will startup your
HibernateService
, and ideally also take care of service
dependencies (Datasource has to be available before Hibernate starts, etc).
Depending on your environment, you might have to set the configuration option
hibernate.connection.aggressive_release
to true if your
application server shows "connection containment" exceptions.
The Hibernate Session
API is independent of any transaction
demarcation system in your architecture. If you let Hibernate use JDBC directly,
through a connection pool, you may begin and end your transactions by calling
the JDBC API. If you run in a J2EE application server, you might want to use bean-managed
transactions and call the JTA API and UserTransaction
when needed.
To keep your code portable between these two (and other) environments we recommend the optional
Hibernate Transaction
API, which wraps and hides the underlying system.
You have to specify a factory class for Transaction
instances by setting the
Hibernate configuration property hibernate.transaction.factory_class
.
There are three standard (built-in) choices:
org.hibernate.transaction.JDBCTransactionFactory
delegates to database (JDBC) transactions (default)
org.hibernate.transaction.JTATransactionFactory
delegates to container-managed transaction if an existing transaction is underway in this context (e.g. EJB session bean method), otherwise a new transaction is started and bean-managed transaction are used.
org.hibernate.transaction.CMTTransactionFactory
delegates to container-managed JTA transactions
You may also define your own transaction strategies (for a CORBA transaction service, for example).
Some features in Hibernate (i.e. the second level cache, Contextual Sessions with JTA, etc.)
require access to the JTA TransactionManager
in a managed environment.
In an application server you have to specify how Hibernate should obtain a reference to the
TransactionManager
, since J2EE does not standardize a single mechanism:
Table 3.10. JTA TransactionManagers
Transaction Factory | Application Server |
---|---|
org.hibernate.transaction.JBossTransactionManagerLookup |
JBoss |
org.hibernate.transaction.WeblogicTransactionManagerLookup |
Weblogic |
org.hibernate.transaction.WebSphereTransactionManagerLookup |
WebSphere |
org.hibernate.transaction.WebSphereExtendedJTATransactionLookup |
WebSphere 6 |
org.hibernate.transaction.OrionTransactionManagerLookup |
Orion |
org.hibernate.transaction.ResinTransactionManagerLookup |
Resin |
org.hibernate.transaction.JOTMTransactionManagerLookup |
JOTM |
org.hibernate.transaction.JOnASTransactionManagerLookup |
JOnAS |
org.hibernate.transaction.JRun4TransactionManagerLookup |
JRun4 |
org.hibernate.transaction.BESTransactionManagerLookup |
Borland ES |
A JNDI bound Hibernate SessionFactory
can simplify the lookup
of the factory and the creation of new Session
s. Note that this
is not related to a JNDI bound Datasource
, both simply use the
same registry!
If you wish to have the SessionFactory
bound to a JNDI namespace, specify
a name (eg. java:hibernate/SessionFactory
) using the property
hibernate.session_factory_name
. If this property is omitted, the
SessionFactory
will not be bound to JNDI. (This is especially useful in
environments with a read-only JNDI default implementation, e.g. Tomcat.)
When binding the SessionFactory
to JNDI, Hibernate will use the values of
hibernate.jndi.url
, hibernate.jndi.class
to instantiate
an initial context. If they are not specified, the default InitialContext
will be used.
Hibernate will automatically place the SessionFactory
in JNDI after
you call cfg.buildSessionFactory()
. This means you will at least have
this call in some startup code (or utility class) in your application, unless you use
JMX deployment with the HibernateService
(discussed later).
If you use a JNDI SessionFactory
, an EJB or any other class may
obtain the SessionFactory
using a JNDI lookup.
We recommend that you bind the SessionFactory
to JNDI in
a managed environment and use a static
singleton otherwise.
To shield your application code from these details, we also recommend to hide the
actual lookup code for a SessionFactory
in a helper class,
such as HibernateUtil.getSessionFactory()
. Note that such a
class is also a convenient way to startup Hibernatesee chapter 1.
The easiest way to handle Session
s and transactions is
Hibernates automatic "current" Session
management.
See the discussion of current sessions.
Using the "jta"
session context, if there is no Hibernate
Session
associated with the current JTA transaction, one will
be started and associated with that JTA transaction the first time you call
sessionFactory.getCurrentSession()
. The Session
s
retrieved via getCurrentSession()
in "jta"
context
will be set to automatically flush before the transaction completes, close
after the transaction completes, and aggressively release JDBC connections
after each statement. This allows the Session
s to
be managed by the life cycle of the JTA transaction to which it is associated,
keeping user code clean of such management concerns. Your code can either use
JTA programmatically through UserTransaction
, or (recommended
for portable code) use the Hibernate Transaction
API to set
transaction boundaries. If you run in an EJB container, declarative transaction
demarcation with CMT is preferred.
The line cfg.buildSessionFactory()
still has to be executed
somewhere to get a SessionFactory
into JNDI. You can do this
either in a static
initializer block (like the one in
HibernateUtil
) or you deploy Hibernate as a managed
service.
Hibernate is distributed with org.hibernate.jmx.HibernateService
for deployment on an application server with JMX capabilities, such as JBoss AS.
The actual deployment and configuration is vendor specific. Here is an example
jboss-service.xml
for JBoss 4.0.x:
<?xml version="1.0"?> <server> <mbean code="org.hibernate.jmx.HibernateService" name="jboss.jca:service=HibernateFactory,name=HibernateFactory"> <!-- Required services --> <depends>jboss.jca:service=RARDeployer</depends> <depends>jboss.jca:service=LocalTxCM,name=HsqlDS</depends> <!-- Bind the Hibernate service to JNDI --> <attribute name="JndiName">java:/hibernate/SessionFactory</attribute> <!-- Datasource settings --> <attribute name="Datasource">java:HsqlDS</attribute> <attribute name="Dialect">org.hibernate.dialect.HSQLDialect</attribute> <!-- Transaction integration --> <attribute name="TransactionStrategy"> org.hibernate.transaction.JTATransactionFactory</attribute> <attribute name="TransactionManagerLookupStrategy"> org.hibernate.transaction.JBossTransactionManagerLookup</attribute> <attribute name="FlushBeforeCompletionEnabled">true</attribute> <attribute name="AutoCloseSessionEnabled">true</attribute> <!-- Fetching options --> <attribute name="MaximumFetchDepth">5</attribute> <!-- Second-level caching --> <attribute name="SecondLevelCacheEnabled">true</attribute> <attribute name="CacheProviderClass">org.hibernate.cache.EhCacheProvider</attribute> <attribute name="QueryCacheEnabled">true</attribute> <!-- Logging --> <attribute name="ShowSqlEnabled">true</attribute> <!-- Mapping files --> <attribute name="MapResources">auction/Item.hbm.xml,auction/Category.hbm.xml</attribute> </mbean> </server>
This file is deployed in a directory called META-INF
and packaged
in a JAR file with the extension .sar
(service archive). You also need
to package Hibernate, its required third-party libraries, your compiled persistent classes,
as well as your mapping files in the same archive. Your enterprise beans (usually session
beans) may be kept in their own JAR file, but you may include this EJB JAR file in the
main service archive to get a single (hot-)deployable unit. Consult the JBoss AS
documentation for more information about JMX service and EJB deployment.
Persistent classes are classes in an application that implement the entities of the business problem (e.g. Customer and Order in an E-commerce application). Not all instances of a persistent class are considered to be in the persistent state - an instance may instead be transient or detached.
Hibernate works best if these classes follow some simple rules, also known
as the Plain Old Java Object (POJO) programming model. However, none of these
rules are hard requirements. Indeed, Hibernate3 assumes very little about
the nature of your persistent objects. You may express a domain model in other
ways: using trees of Map
instances, for example.
Most Java applications require a persistent class representing felines.
package eg; import java.util.Set; import java.util.Date; public class Cat { private Long id; // identifier private Date birthdate; private Color color; private char sex; private float weight; private int litterId; private Cat mother; private Set kittens = new HashSet(); private void setId(Long id) { this.id=id; } public Long getId() { return id; } void setBirthdate(Date date) { birthdate = date; } public Date getBirthdate() { return birthdate; } void setWeight(float weight) { this.weight = weight; } public float getWeight() { return weight; } public Color getColor() { return color; } void setColor(Color color) { this.color = color; } void setSex(char sex) { this.sex=sex; } public char getSex() { return sex; } void setLitterId(int id) { this.litterId = id; } public int getLitterId() { return litterId; } void setMother(Cat mother) { this.mother = mother; } public Cat getMother() { return mother; } void setKittens(Set kittens) { this.kittens = kittens; } public Set getKittens() { return kittens; } // addKitten not needed by Hibernate public void addKitten(Cat kitten) { kitten.setMother(this); kitten.setLitterId( kittens.size() ); kittens.add(kitten); } }
There are four main rules to follow here:
Cat
has a no-argument constructor. All persistent classes must
have a default constructor (which may be non-public) so that Hibernate can instantiate
them using Constructor.newInstance()
. We strongly recommend having a
default constructor with at least package visibility for runtime proxy
generation in Hibernate.
Cat
has a property called id
. This property
maps to the primary key column of a database table. The property might have been called
anything, and its type might have been any primitive type, any primitive "wrapper"
type, java.lang.String
or java.util.Date
. (If
your legacy database table has composite keys, you can even use a user-defined class
with properties of these types - see the section on composite identifiers later.)
The identifier property is strictly optional. You can leave them off and let Hibernate keep track of object identifiers internally. We do not recommend this, however.
In fact, some functionality is available only to classes which declare an identifier property:
Transitive reattachment for detached objects (cascade update or cascade merge) - see Section 10.11, “Transitive persistence”
Session.saveOrUpdate()
Session.merge()
We recommend you declare consistently-named identifier properties on persistent classes. We further recommend that you use a nullable (ie. non-primitive) type.
A central feature of Hibernate, proxies, depends upon the persistent class being either non-final, or the implementation of an interface that declares all public methods.
You can persist final
classes that do not implement an interface
with Hibernate, but you won't be able to use proxies for lazy association fetching -
which will limit your options for performance tuning.
You should also avoid declaring public final
methods on the
non-final classes. If you want to use a class with a public final
method, you must explicitly disable proxying by setting lazy="false"
.
Cat
declares accessor methods for all its persistent fields.
Many other ORM tools directly persist instance variables. We believe it is
better to provide an indirection between the relational schema and internal
data structures of the class. By default, Hibernate persists JavaBeans style
properties, and recognizes method names of the form getFoo
,
isFoo
and setFoo
. You may switch to direct
field access for particular properties, if needed.
Properties need not be declared public - Hibernate can
persist a property with a default, protected
or
private
get / set pair.
A subclass must also observe the first and second rules. It inherits its
identifier property from the superclass, Cat
.
package eg; public class DomesticCat extends Cat { private String name; public String getName() { return name; } protected void setName(String name) { this.name=name; } }
You have to override the equals()
and hashCode()
methods if you
intend to put instances of persistent classes in a Set
(the recommended way to represent many-valued associations)
and
intend to use reattachment of detached instances
Hibernate guarantees equivalence of persistent identity (database row) and Java identity
only inside a particular session scope. So as soon as we mix instances retrieved in
different sessions, we must implement equals()
and
hashCode()
if we wish to have meaningful semantics for
Set
s.
The most obvious way is to implement equals()
/hashCode()
by comparing the identifier value of both objects. If the value is the same, both must
be the same database row, they are therefore equal (if both are added to a Set
,
we will only have one element in the Set
). Unfortunately, we can't use that
approach with generated identifiers! Hibernate will only assign identifier values to objects
that are persistent, a newly created instance will not have any identifier value! Furthermore,
if an instance is unsaved and currently in a Set
, saving it will assign
an identifier value to the object. If equals()
and hashCode()
are based on the identifier value, the hash code would change, breaking the contract of the
Set
. See the Hibernate website for a full discussion of this problem. Note
that this is not a Hibernate issue, but normal Java semantics of object identity and equality.
We recommend implementing equals()
and hashCode()
using Business key equality. Business key equality means that the
equals()
method compares only the properties that form the business
key, a key that would identify our instance in the real world (a
natural candidate key):
public class Cat { ... public boolean equals(Object other) { if (this == other) return true; if ( !(other instanceof Cat) ) return false; final Cat cat = (Cat) other; if ( !cat.getLitterId().equals( getLitterId() ) ) return false; if ( !cat.getMother().equals( getMother() ) ) return false; return true; } public int hashCode() { int result; result = getMother().hashCode(); result = 29 * result + getLitterId(); return result; } }
Note that a business key does not have to be as solid as a database primary key candidate (see Section 11.1.3, “Considering object identity”). Immutable or unique properties are usually good candidates for a business key.
Note that the following features are currently considered experimental and may change in the near future.
Persistent entities don't necessarily have to be represented as POJO classes
or as JavaBean objects at runtime. Hibernate also supports dynamic models
(using Map
s of Map
s at runtime) and the
representation of entities as DOM4J trees. With this approach, you don't
write persistent classes, only mapping files.
By default, Hibernate works in normal POJO mode. You may set a default entity
representation mode for a particular SessionFactory
using the
default_entity_mode
configuration option (see
Table 3.3, “Hibernate Configuration Properties”.
The following examples demonstrates the representation using Map
s.
First, in the mapping file, an entity-name
has to be declared
instead of (or in addition to) a class name:
<hibernate-mapping> <class entity-name="Customer"> <id name="id" type="long" column="ID"> <generator class="sequence"/> </id> <property name="name" column="NAME" type="string"/> <property name="address" column="ADDRESS" type="string"/> <many-to-one name="organization" column="ORGANIZATION_ID" class="Organization"/> <bag name="orders" inverse="true" lazy="false" cascade="all"> <key column="CUSTOMER_ID"/> <one-to-many class="Order"/> </bag> </class> </hibernate-mapping>
Note that even though associations are declared using target class names, the target type of an associations may also be a dynamic entity instead of a POJO.
After setting the default entity mode to dynamic-map
for the SessionFactory
, we can at runtime work with
Map
s of Map
s:
Session s = openSession(); Transaction tx = s.beginTransaction(); Session s = openSession(); // Create a customer Map david = new HashMap(); david.put("name", "David"); // Create an organization Map foobar = new HashMap(); foobar.put("name", "Foobar Inc."); // Link both david.put("organization", foobar); // Save both s.save("Customer", david); s.save("Organization", foobar); tx.commit(); s.close();
The advantages of a dynamic mapping are quick turnaround time for prototyping without the need for entity class implementation. However, you lose compile-time type checking and will very likely deal with many exceptions at runtime. Thanks to the Hibernate mapping, the database schema can easily be normalized and sound, allowing to add a proper domain model implementation on top later on.
Entity representation modes can also be set on a per Session
basis:
Session dynamicSession = pojoSession.getSession(EntityMode.MAP); // Create a customer Map david = new HashMap(); david.put("name", "David"); dynamicSession.save("Customer", david); ... dynamicSession.flush(); dynamicSession.close() ... // Continue on pojoSession
Please note that the call to getSession()
using an
EntityMode
is on the Session
API, not the
SessionFactory
. That way, the new Session
shares the underlying JDBC connection, transaction, and other context
information. This means you don't have tocall flush()
and close()
on the secondary Session
, and
also leave the transaction and connection handling to the primary unit of work.
More information about the XML representation capabilities can be found in Chapter 18, XML Mapping.
org.hibernate.tuple.Tuplizer
, and its sub-interfaces, are responsible
for managing a particular representation of a piece of data, given that representation's
org.hibernate.EntityMode
. If a given piece of data is thought of as
a data structure, then a tuplizer is the thing which knows how to create such a data structure
and how to extract values from and inject values into such a data structure. For example,
for the POJO entity mode, the correpsonding tuplizer knows how create the POJO through its
constructor and how to access the POJO properties using the defined property accessors.
There are two high-level types of Tuplizers, represented by the
org.hibernate.tuple.entity.EntityTuplizer
and org.hibernate.tuple.component.ComponentTuplizer
interfaces. EntityTuplizer
s are responsible for managing the above mentioned
contracts in regards to entities, while ComponentTuplizer
s do the same for
components.
Users may also plug in their own tuplizers. Perhaps you require that a java.util.Map
implementation other than java.util.HashMap
be used while in the
dynamic-map entity-mode; or perhaps you need to define a different proxy generation strategy
than the one used by default. Both would be achieved by defining a custom tuplizer
implementation. Tuplizers definitions are attached to the entity or component mapping they
are meant to manage. Going back to the example of our customer entity:
<hibernate-mapping> <class entity-name="Customer"> <!-- Override the dynamic-map entity-mode tuplizer for the customer entity --> <tuplizer entity-mode="dynamic-map" class="CustomMapTuplizerImpl"/> <id name="id" type="long" column="ID"> <generator class="sequence"/> </id> <!-- other properties --> ... </class> </hibernate-mapping> public class CustomMapTuplizerImpl extends org.hibernate.tuple.entity.DynamicMapEntityTuplizer { // override the buildInstantiator() method to plug in our custom map... protected final Instantiator buildInstantiator( org.hibernate.mapping.PersistentClass mappingInfo) { return new CustomMapInstantiator( mappingInfo ); } private static final class CustomMapInstantiator extends org.hibernate.tuple.DynamicMapInstantitor { // override the generateMap() method to return our custom map... protected final Map generateMap() { return new CustomMap(); } } }
Object/relational mappings are usually defined in an XML document. The mapping document is designed to be readable and hand-editable. The mapping language is Java-centric, meaning that mappings are constructed around persistent class declarations, not table declarations.
Note that, even though many Hibernate users choose to write the XML by hand, a number of tools exist to generate the mapping document, including XDoclet, Middlegen and AndroMDA.
Lets kick off with an example mapping:
<?xml version="1.0"?> <!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN" "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd"> <hibernate-mapping package="eg"> <class name="Cat" table="cats" discriminator-value="C"> <id name="id"> <generator class="native"/> </id> <discriminator column="subclass" type="character"/> <property name="weight"/> <property name="birthdate" type="date" not-null="true" update="false"/> <property name="color" type="eg.types.ColorUserType" not-null="true" update="false"/> <property name="sex" not-null="true" update="false"/> <property name="litterId" column="litterId" update="false"/> <many-to-one name="mother" column="mother_id" update="false"/> <set name="kittens" inverse="true" order-by="litter_id"> <key column="mother_id"/> <one-to-many class="Cat"/> </set> <subclass name="DomesticCat" discriminator-value="D"> <property name="name" type="string"/> </subclass> </class> <class name="Dog"> <!-- mapping for Dog could go here --> </class> </hibernate-mapping>
We will now discuss the content of the mapping document. We will only describe the
document elements and attributes that are used by Hibernate at runtime. The mapping
document also contains some extra optional attributes and elements that affect the
database schemas exported by the schema export tool. (For example the
not-null
attribute.)
All XML mappings should declare the doctype shown. The actual DTD may be found
at the URL above, in the directory hibernate-x.x.x/src/org/hibernate
or in hibernate3.jar
. Hibernate will always look for
the DTD in its classpath first. If you experience lookups of the DTD using an
Internet connection, check your DTD declaration against the contents of your
claspath.
As mentioned previously, Hibernate will first attempt to resolve DTDs in its classpath. The
manner in which it does this is by registering a custom org.xml.sax.EntityResolver
implementation with the SAXReader it uses to read in the xml files. This custom
EntityResolver
recognizes two different systemId namespaces.
a hibernate namespace
is recognized whenever the
resolver encounteres a systemId starting with
http://hibernate.sourceforge.net/
; the resolver
attempts to resolve these entities via the classlaoder which loaded
the Hibernate classes.
a user namespace
is recognized whenever the
resolver encounteres a systemId using a classpath://
URL protocol; the resolver will attempt to resolve these entities
via (1) the current thread context classloader and (2) the
classloader which loaded the Hibernate classes.
An example of utilizing user namespacing:
<?xml version="1.0"?> <!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN" "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd" [ <!ENTITY types SYSTEM "classpath://your/domain/types.xml"> ]> <hibernate-mapping package="your.domain"> <class name="MyEntity"> <id name="id" type="my-custom-id-type"> ... </id> <class> &types; </hibernate-mapping>
Where types.xml
is a resource in the your.domain
package and contains a custom typedef.
This element has several optional attributes. The schema
and
catalog
attributes specify that tables referred to in this mapping
belong to the named schema and/or catalog. If specified, tablenames will be qualified
by the given schema and catalog names. If missing, tablenames will be unqualified.
The default-cascade
attribute specifies what cascade style
should be assumed for properties and collections which do not specify a
cascade
attribute. The auto-import
attribute lets us
use unqualified class names in the query language, by default.
<hibernate-mapping schema="schemaName" (1) catalog="catalogName" (2) default-cascade="cascade_style" (3) default-access="field|property|ClassName" (4) default-lazy="true|false" (5) auto-import="true|false" (6) package="package.name" (7) />
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If you have two persistent classes with the same (unqualified) name, you should set
auto-import="false"
. Hibernate will throw an exception if you attempt
to assign two classes to the same "imported" name.
Note that the hibernate-mapping
element allows you to nest
several persistent <class>
mappings, as shown above.
It is however good practice (and expected by some tools) to map only a single
persistent class (or a single class hierarchy) in one mapping file and name
it after the persistent superclass, e.g. Cat.hbm.xml
,
Dog.hbm.xml
, or if using inheritance,
Animal.hbm.xml
.
You may declare a persistent class using the class
element:
<class name="ClassName" (1) table="tableName" (2) discriminator-value="discriminator_value" (3) mutable="true|false" (4) schema="owner" (5) catalog="catalog" (6) proxy="ProxyInterface" (7) dynamic-update="true|false" (8) dynamic-insert="true|false" (9) select-before-update="true|false" (10) polymorphism="implicit|explicit" (11) where="arbitrary sql where condition" (12) persister="PersisterClass" (13) batch-size="N" (14) optimistic-lock="none|version|dirty|all" (15) lazy="true|false" (16) entity-name="EntityName" (17) check="arbitrary sql check condition" (18) rowid="rowid" (19) subselect="SQL expression" (20) abstract="true|false" (21) node="element-name" />
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It is perfectly acceptable for the named persistent class to be an interface. You would then
declare implementing classes of that interface using the <subclass>
element. You may persist any static inner class. You should specify the
class name using the standard form ie. eg.Foo$Bar
.
Immutable classes, mutable="false"
, may not be updated or deleted by the
application. This allows Hibernate to make some minor performance optimizations.
The optional proxy
attribute enables lazy initialization of persistent
instances of the class. Hibernate will initially return CGLIB proxies which implement
the named interface. The actual persistent object will be loaded when a method of the
proxy is invoked. See "Initializing collections and proxies" below.
Implicit polymorphism means that instances of the class will be returned
by a query that names any superclass or implemented interface or the class and that instances
of any subclass of the class will be returned by a query that names the class itself.
Explicit polymorphism means that class instances will be returned only
by queries that explicitly name that class and that queries that name the class will return
only instances of subclasses mapped inside this <class>
declaration
as a <subclass>
or <joined-subclass>
. For
most purposes the default, polymorphism="implicit"
, is appropriate.
Explicit polymorphism is useful when two different classes are mapped to the same table
(this allows a "lightweight" class that contains a subset of the table columns).
The persister
attribute lets you customize the persistence strategy used for
the class. You may, for example, specify your own subclass of
org.hibernate.persister.EntityPersister
or you might even provide a
completely new implementation of the interface
org.hibernate.persister.ClassPersister
that implements persistence via,
for example, stored procedure calls, serialization to flat files or LDAP. See
org.hibernate.test.CustomPersister
for a simple example (of "persistence"
to a Hashtable
).
Note that the dynamic-update
and dynamic-insert
settings are not inherited by subclasses and so may also be specified on the
<subclass>
or <joined-subclass>
elements.
These settings may increase performance in some cases, but might actually decrease
performance in others. Use judiciously.
Use of select-before-update
will usually decrease performance. It is very
useful to prevent a database update trigger being called unnecessarily if you reattach a
graph of detached instances to a Session
.
If you enable dynamic-update
, you will have a choice of optimistic
locking strategies:
version
check the version/timestamp columns
all
check all columns
dirty
check the changed columns, allowing some concurrent updates
none
do not use optimistic locking
We very strongly recommend that you use version/timestamp
columns for optimistic locking with Hibernate. This is the optimal strategy with
respect to performance and is the only strategy that correctly handles modifications
made to detached instances (ie. when Session.merge()
is used).
There is no difference between a view and a base table for a Hibernate mapping, as expected this is transparent at the database level (note that some DBMS don't support views properly, especially with updates). Sometimes you want to use a view, but can't create one in the database (ie. with a legacy schema). In this case, you can map an immutable and read-only entity to a given SQL subselect expression:
<class name="Summary"> <subselect> select item.name, max(bid.amount), count(*) from item join bid on bid.item_id = item.id group by item.name </subselect> <synchronize table="item"/> <synchronize table="bid"/> <id name="name"/> ... </class>
Declare the tables to synchronize this entity with, ensuring that auto-flush happens
correctly, and that queries against the derived entity do not return stale data.
The <subselect>
is available as both as an attribute and
a nested mapping element.
Mapped classes must declare the primary key column of the database
table. Most classes will also have a JavaBeans-style property holding the unique identifier
of an instance. The <id>
element defines the mapping from that
property to the primary key column.
<id name="propertyName" (1) type="typename" (2) column="column_name" (3) unsaved-value="null|any|none|undefined|id_value" (4) access="field|property|ClassName"> (5) node="element-name|@attribute-name|element/@attribute|." <generator class="generatorClass"/> </id>
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If the name
attribute is missing, it is assumed that the class has no
identifier property.
The unsaved-value
attribute is almost never needed in Hibernate3.
There is an alternative <composite-id>
declaration to allow access to
legacy data with composite keys. We strongly discourage its use for anything else.
The optional <generator>
child element names a Java class used
to generate unique identifiers for instances of the persistent class. If any parameters
are required to configure or initialize the generator instance, they are passed using the
<param>
element.
<id name="id" type="long" column="cat_id"> <generator class="org.hibernate.id.TableHiLoGenerator"> <param name="table">uid_table</param> <param name="column">next_hi_value_column</param> </generator> </id>
All generators implement the interface org.hibernate.id.IdentifierGenerator
.
This is a very simple interface; some applications may choose to provide their own specialized
implementations. However, Hibernate provides a range of built-in implementations. There are shortcut
names for the built-in generators:
increment
generates identifiers of type long
, short
or
int
that are unique only when no other process is inserting data
into the same table.
Do not use in a cluster.
identity
supports identity columns in DB2, MySQL, MS SQL Server, Sybase and
HypersonicSQL. The returned identifier is of type long
,
short
or int
.
sequence
uses a sequence in DB2, PostgreSQL, Oracle, SAP DB, McKoi or a generator
in Interbase. The returned identifier is of type long
,
short
or int
hilo
uses a hi/lo algorithm to efficiently generate identifiers of
type long
, short
or int
,
given a table and column (by default hibernate_unique_key
and
next_hi
respectively) as a source of hi values. The hi/lo
algorithm generates identifiers that are unique only for a particular database.
seqhilo
uses a hi/lo algorithm to efficiently generate identifiers of type
long
, short
or int
,
given a named database sequence.
uuid
uses a 128-bit UUID algorithm to generate identifiers of type string, unique within a network (the IP address is used). The UUID is encoded as a string of hexadecimal digits of length 32.
guid
uses a database-generated GUID string on MS SQL Server and MySQL.
native
picks identity
, sequence
or
hilo
depending upon the capabilities of the
underlying database.
assigned
lets the application to assign an identifier to the object before
save()
is called. This is the default strategy
if no <generator>
element is specified.
select
retrieves a primary key assigned by a database trigger by selecting the row by some unique key and retrieving the primary key value.
foreign
uses the identifier of another associated object. Usually used in conjunction
with a <one-to-one>
primary key association.
sequence-identity
a specialized sequence generation strategy which utilizes a database sequence for the actual value generation, but combines this with JDBC3 getGeneratedKeys to actually return the generated identifier value as part of the insert statement execution. This strategy is only known to be supported on Oracle 10g drivers targetted for JDK 1.4. Note comments on these insert statements are disabled due to a bug in the Oracle drivers.
The hilo
and seqhilo
generators provide two alternate
implementations of the hi/lo algorithm, a favorite approach to identifier generation. The
first implementation requires a "special" database table to hold the next available "hi" value.
The second uses an Oracle-style sequence (where supported).
<id name="id" type="long" column="cat_id"> <generator class="hilo"> <param name="table">hi_value</param> <param name="column">next_value</param> <param name="max_lo">100</param> </generator> </id>
<id name="id" type="long" column="cat_id"> <generator class="seqhilo"> <param name="sequence">hi_value</param> <param name="max_lo">100</param> </generator> </id>
Unfortunately, you can't use hilo
when supplying your own
Connection
to Hibernate. When Hibernate is using an application
server datasource to obtain connections enlisted with JTA, you must properly configure
the hibernate.transaction.manager_lookup_class
.
The UUID contains: IP address, startup time of the JVM (accurate to a quarter second), system time and a counter value (unique within the JVM). It's not possible to obtain a MAC address or memory address from Java code, so this is the best we can do without using JNI.
For databases which support identity columns (DB2, MySQL, Sybase, MS SQL), you
may use identity
key generation. For databases that support
sequences (DB2, Oracle, PostgreSQL, Interbase, McKoi, SAP DB) you may use
sequence
style key generation. Both these strategies require
two SQL queries to insert a new object.
<id name="id" type="long" column="person_id"> <generator class="sequence"> <param name="sequence">person_id_sequence</param> </generator> </id>
<id name="id" type="long" column="person_id" unsaved-value="0"> <generator class="identity"/> </id>
For cross-platform development, the native
strategy will
choose from the identity
, sequence
and
hilo
strategies, dependant upon the capabilities of the
underlying database.
If you want the application to assign identifiers (as opposed to having
Hibernate generate them), you may use the assigned
generator.
This special generator will use the identifier value already assigned to the
object's identifier property. This generator is used when the primary key
is a natural key instead of a surrogate key. This is the default behavior
if you do no specify a <generator>
element.
Choosing the assigned
generator makes Hibernate use
unsaved-value="undefined"
, forcing Hibernate to go to
the database to determine if an instance is transient or detached, unless
there is a version or timestamp property, or you define
Interceptor.isUnsaved()
.
For legacy schemas only (Hibernate does not generate DDL with triggers).
<id name="id" type="long" column="person_id"> <generator class="select"> <param name="key">socialSecurityNumber</param> </generator> </id>
In the above example, there is a unique valued property named
socialSecurityNumber
defined by the class, as a
natural key, and a surrogate key named person_id
whose value is generated by a trigger.
Starting with release 3.2.3, there are 2 new generators which represent a re-thinking of 2 different aspects of identifier generation. The first aspect is database portability; the second is optimization (not having to query the database for every request for a new identifier value). These two new generators are intended to take the place of some of the named generators described above (starting in 3.3.x); however, they are included in the current releases and can be referenced by FQN.
The first of these new generators is org.hibernate.id.enhanced.SequenceStyleGenerator
which is intended firstly as a replacement for the sequence
generator and secondly as
a better portability generator than native
(because native
(generally) chooses between identity
and sequence
which have
largely different semantics which can cause subtle isssues in applications eyeing portability).
org.hibernate.id.enhanced.SequenceStyleGenerator
however achieves portability in
a different manner. It chooses between using a table or a sequence in the database to store its
incrementing values depending on the capabilities of the dialect being used. The difference between this
and native
is that table-based and sequence-based storage have the same exact
semantic (in fact sequences are exactly what Hibernate tries to emmulate with its table-based
generators). This generator has a number of configuration parameters:
sequence_name
(optional, defaults to hibernate_sequence
):
The name of the sequence (or table) to be used.
initial_value
(optional, defaults to 1
): The initial
value to be retrieved from the sequence/table. In sequence creation terms, this is analogous
to the clause typical named "STARTS WITH".
increment_size
(optional, defaults to 1
): The value by
which subsequent calls to the sequence/table should differ. In sequence creation terms, this
is analogous to the clause typical named "INCREMENT BY".
force_table_use
(optional, defaults to false
): Should
we force the use of a table as the backing structure even though the dialect might support
sequence?
value_column
(optional, defaults to next_val
): Only
relevant for table structures! The name of the column on the table which is used to
hold the value.
optimizer
(optional, defaults to none
):
See Section 5.1.6, “Identifier generator optimization”
The second of these new generators is org.hibernate.id.enhanced.TableGenerator
which
is intended firstly as a replacement for the table
generator (although it actually
functions much more like org.hibernate.id.MultipleHiLoPerTableGenerator
) and secondly
as a re-implementation of org.hibernate.id.MultipleHiLoPerTableGenerator
utilizing the
notion of pluggable optimiziers. Essentially this generator defines a table capable of holding
a number of different increment values simultaneously by using multiple distinctly keyed rows. This
generator has a number of configuration parameters:
table_name
(optional, defaults to hibernate_sequences
):
The name of the table to be used.
value_column_name
(optional, defaults to next_val
):
The name of the column on the table which is used to hold the value.
segment_column_name
(optional, defaults to sequence_name
):
The name of the column on the table which is used to hold the "segement key". This is the
value which distinctly identifies which increment value to use.
segment_value
(optional, defaults to default
):
The "segment key" value for the segment from which we want to pull increment values for
this generator.
segment_value_length
(optional, defaults to 255
):
Used for schema generation; the column size to create this segment key column.
initial_value
(optional, defaults to 1
):
The initial value to be retrieved from the table.
increment_size
(optional, defaults to 1
):
The value by which subsequent calls to the table should differ.
optimizer
(optional, defaults to ):
See Section 5.1.6, “Identifier generator optimization”
For identifier generators which store values in the database, it is inefficient for them to hit the database on each and every call to generate a new identifier value. Instead, you'd ideally want to group a bunch of them in memory and only hit the database when you have exhausted your in-memory value group. This is the role of the pluggable optimizers. Currently only the two enhanced generators (Section 5.1.5, “Enhanced identifier generators” support this notion.
none
(generally this is the default if no optimizer was specified): This
says to not perform any optimizations, and hit the database each and every request.
hilo
: applies a hi/lo algorithm around the database retrieved values. The
values from the database for this optimizer are expected to be sequential. The values
retrieved from the database structure for this optimizer indicates the "group number"; the
increment_size
is multiplied by that value in memory to define a group
"hi value".
pooled
: like was discussed for hilo
, this optimizers
attempts to minimize the number of hits to the database. Here, however, we simply store
the starting value for the "next group" into the database structure rather than a sequential
value in combination with an in-memory grouping algorithm. increment_size
here refers to the values coming from the database.
<composite-id name="propertyName" class="ClassName" mapped="true|false" access="field|property|ClassName"> node="element-name|." <key-property name="propertyName" type="typename" column="column_name"/> <key-many-to-one name="propertyName class="ClassName" column="column_name"/> ...... </composite-id>
For a table with a composite key, you may map multiple properties of the class
as identifier properties. The <composite-id>
element
accepts <key-property>
property mappings and
<key-many-to-one>
mappings as child elements.
<composite-id> <key-property name="medicareNumber"/> <key-property name="dependent"/> </composite-id>
Your persistent class must override equals()
and hashCode()
to implement composite identifier equality. It must
also implements Serializable
.
Unfortunately, this approach to composite identifiers means that a persistent object
is its own identifier. There is no convenient "handle" other than the object itself.
You must instantiate an instance of the persistent class itself and populate its
identifier properties before you can load()
the persistent state
associated with a composite key. We call this approach an embedded
composite identifier, and discourage it for serious applications.
A second approach is what we call a mapped composite identifier,
where the identifier properties named inside the <composite-id>
element are duplicated on both the persistent class and a separate identifier class.
<composite-id class="MedicareId" mapped="true"> <key-property name="medicareNumber"/> <key-property name="dependent"/> </composite-id>
In this example, both the composite identifier class, MedicareId
,
and the entity class itself have properties named medicareNumber
and dependent
. The identifier class must override
equals()
and hashCode()
and implement.
Serializable
. The disadvantage of this approach is quite
obviouscode duplication.
The following attributes are used to specify a mapped composite identifier:
mapped
(optional, defaults to false
):
indicates that a mapped composite identifier is used, and that the contained
property mappings refer to both the entity class and the composite identifier
class.
class
(optional, but required for a mapped composite identifier):
The class used as a composite identifier.
We will describe a third, even more convenient approach where the composite identifier is implemented as a component class in Section 8.4, “Components as composite identifiers”. The attributes described below apply only to this alternative approach:
name
(optional, required for this approach): A property of
component type that holds the composite identifier (see chapter 9).
access
(optional - defaults to property
):
The strategy Hibernate should use for accessing the property value.
class
(optional - defaults to the property type determined by
reflection): The component class used as a composite identifier (see next section).
This third approach, an identifier component is the one we recommend for almost all applications.
The <discriminator>
element is required for polymorphic persistence
using the table-per-class-hierarchy mapping strategy and declares a discriminator column of the
table. The discriminator column contains marker values that tell the persistence layer what
subclass to instantiate for a particular row. A restricted set of types may be used:
string
, character
, integer
,
byte
, short
, boolean
,
yes_no
, true_false
.
<discriminator column="discriminator_column" (1) type="discriminator_type" (2) force="true|false" (3) insert="true|false" (4) formula="arbitrary sql expression" (5) />
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Actual values of the discriminator column are specified by the
discriminator-value
attribute of the <class>
and
<subclass>
elements.
The force
attribute is (only) useful if the table contains rows with
"extra" discriminator values that are not mapped to a persistent class. This will not
usually be the case.
Using the formula
attribute you can declare an arbitrary SQL expression
that will be used to evaluate the type of a row:
<discriminator formula="case when CLASS_TYPE in ('a', 'b', 'c') then 0 else 1 end" type="integer"/>
The <version>
element is optional and indicates that
the table contains versioned data. This is particularly useful if you plan to
use long transactions (see below).
<version column="version_column" (1) name="propertyName" (2) type="typename" (3) access="field|property|ClassName" (4) unsaved-value="null|negative|undefined" (5) generated="never|always" (6) insert="true|false" (7) node="element-name|@attribute-name|element/@attribute|." />
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Version numbers may be of Hibernate type long
, integer
,
short
, timestamp
or calendar
.
A version or timestamp property should never be null for a detached instance, so
Hibernate will detect any instance with a null version or timestamp as transient,
no matter what other unsaved-value
strategies are specified.
Declaring a nullable version or timestamp property is an easy way to avoid
any problems with transitive reattachment in Hibernate, especially useful for people
using assigned identifiers or composite keys!
The optional <timestamp>
element indicates that the table contains
timestamped data. This is intended as an alternative to versioning. Timestamps are by nature
a less safe implementation of optimistic locking. However, sometimes the application might
use the timestamps in other ways.
<timestamp column="timestamp_column" (1) name="propertyName" (2) access="field|property|ClassName" (3) unsaved-value="null|undefined" (4) source="vm|db" (5) generated="never|always" (6) node="element-name|@attribute-name|element/@attribute|." />
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Note that <timestamp>
is equivalent to
<version type="timestamp">
. And
<timestamp source="db">
is equivalent to
<version type="dbtimestamp">
The <property>
element declares a persistent, JavaBean style
property of the class.
<property name="propertyName" (1) column="column_name" (2) type="typename" (3) update="true|false" (4) insert="true|false" (4) formula="arbitrary SQL expression" (5) access="field|property|ClassName" (6) lazy="true|false" (7) unique="true|false" (8) not-null="true|false" (9) optimistic-lock="true|false" (10) generated="never|insert|always" (11) node="element-name|@attribute-name|element/@attribute|." index="index_name" unique_key="unique_key_id" length="L" precision="P" scale="S" />
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typename could be:
The name of a Hibernate basic type (eg. integer, string, character,
date, timestamp, float, binary, serializable, object, blob
).
The name of a Java class with a default basic type (eg. int, float,
char, java.lang.String, java.util.Date, java.lang.Integer, java.sql.Clob
).
The name of a serializable Java class.
The class name of a custom type (eg. com.illflow.type.MyCustomType
).
If you do not specify a type, Hibernate will use reflection upon the named
property to take a guess at the correct Hibernate type. Hibernate will try to
interpret the name of the return class of the property getter using rules 2, 3,
4 in that order. However, this is not always enough.
In certain cases you will still need the type
attribute. (For example, to distinguish between Hibernate.DATE
and
Hibernate.TIMESTAMP
, or to specify a custom type.)
The access
attribute lets you control how Hibernate will access
the property at runtime. By default, Hibernate will call the property get/set pair.
If you specify access="field"
, Hibernate will bypass the get/set
pair and access the field directly, using reflection. You may specify your own
strategy for property access by naming a class that implements the interface
org.hibernate.property.PropertyAccessor
.
An especially powerful feature are derived properties. These properties are by
definition read-only, the property value is computed at load time. You declare
the computation as a SQL expression, this translates to a SELECT
clause subquery in the SQL query that loads an instance:
<property name="totalPrice" formula="( SELECT SUM (li.quantity*p.price) FROM LineItem li, Product p WHERE li.productId = p.productId AND li.customerId = customerId AND li.orderNumber = orderNumber )"/>
Note that you can reference the entities own table by not declaring an alias on
a particular column (customerId
in the given example). Also note
that you can use the nested <formula>
mapping element
if you don't like to use the attribute.
An ordinary association to another persistent class is declared using a
many-to-one
element. The relational model is a
many-to-one association: a foreign key in one table is referencing
the primary key column(s) of the target table.
<many-to-one name="propertyName" (1) column="column_name" (2) class="ClassName" (3) cascade="cascade_style" (4) fetch="join|select" (5) update="true|false" (6) insert="true|false" (6) property-ref="propertyNameFromAssociatedClass" (7) access="field|property|ClassName" (8) unique="true|false" (9) not-null="true|false" (10) optimistic-lock="true|false" (11) lazy="proxy|no-proxy|false" (12) not-found="ignore|exception" (13) entity-name="EntityName" (14) formula="arbitrary SQL expression" (15) node="element-name|@attribute-name|element/@attribute|." embed-xml="true|false" index="index_name" unique_key="unique_key_id" foreign-key="foreign_key_name" />
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Setting a value of the cascade
attribute to any meaningful
value other than none
will propagate certain operations to the
associated object. The meaningful values are the names of Hibernate's basic
operations, persist, merge, delete, save-update, evict, replicate, lock,
refresh
, as well as the special values delete-orphan
and all
and comma-separated combinations of operation
names, for example, cascade="persist,merge,evict"
or
cascade="all,delete-orphan"
. See Section 10.11, “Transitive persistence”
for a full explanation. Note that single valued associations (many-to-one and
one-to-one associations) do not support orphan delete.
A typical many-to-one
declaration looks as simple as this:
<many-to-one name="product" class="Product" column="PRODUCT_ID"/>
The property-ref
attribute should only be used for mapping legacy
data where a foreign key refers to a unique key of the associated table other than
the primary key. This is an ugly relational model. For example, suppose the
Product
class had a unique serial number, that is not the primary
key. (The unique
attribute controls Hibernate's DDL generation with
the SchemaExport tool.)
<property name="serialNumber" unique="true" type="string" column="SERIAL_NUMBER"/>
Then the mapping for OrderItem
might use:
<many-to-one name="product" property-ref="serialNumber" column="PRODUCT_SERIAL_NUMBER"/>
This is certainly not encouraged, however.
If the referenced unique key comprises multiple properties of the associated entity, you should
map the referenced properties inside a named <properties>
element.
If the referenced unique key is the property of a component, you may specify a property path:
<many-to-one name="owner" property-ref="identity.ssn" column="OWNER_SSN"/>
A one-to-one association to another persistent class is declared using a
one-to-one
element.
<one-to-one name="propertyName" (1) class="ClassName" (2) cascade="cascade_style" (3) constrained="true|false" (4) fetch="join|select" (5) property-ref="propertyNameFromAssociatedClass" (6) access="field|property|ClassName" (7) formula="any SQL expression" (8) lazy="proxy|no-proxy|false" (9) entity-name="EntityName" (10) node="element-name|@attribute-name|element/@attribute|." embed-xml="true|false" foreign-key="foreign_key_name" />
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There are two varieties of one-to-one association:
primary key associations
unique foreign key associations
Primary key associations don't need an extra table column; if two rows are related by the association then the two table rows share the same primary key value. So if you want two objects to be related by a primary key association, you must make sure that they are assigned the same identifier value!
For a primary key association, add the following mappings to Employee
and
Person
, respectively.
<one-to-one name="person" class="Person"/>
<one-to-one name="employee" class="Employee" constrained="true"/>
Now we must ensure that the primary keys of related rows in the PERSON and
EMPLOYEE tables are equal. We use a special Hibernate identifier generation strategy
called foreign
:
<class name="person" table="PERSON"> <id name="id" column="PERSON_ID"> <generator class="foreign"> <param name="property">employee</param> </generator> </id> ... <one-to-one name="employee" class="Employee" constrained="true"/> </class>
A newly saved instance of Person
is then assigned the same primary
key value as the Employee
instance refered with the employee
property of that Person
.
Alternatively, a foreign key with a unique constraint, from Employee
to
Person
, may be expressed as:
<many-to-one name="person" class="Person" column="PERSON_ID" unique="true"/>
And this association may be made bidirectional by adding the following to the
Person
mapping:
<one-to-one name="employee" class="Employee" property-ref="person"/>
<natural-id mutable="true|false"/> <property ... /> <many-to-one ... /> ...... </natural-id>
Even though we recommend the use of surrogate keys as primary keys, you should still try
to identify natural keys for all entities. A natural key is a property or combination of
properties that is unique and non-null. If it is also immutable, even better. Map the
properties of the natural key inside the <natural-id>
element.
Hibernate will generate the necessary unique key and nullability constraints, and your
mapping will be more self-documenting.
We strongly recommend that you implement equals()
and
hashCode()
to compare the natural key properties of the entity.
This mapping is not intended for use with entities with natural primary keys.
mutable
(optional, defaults to false
):
By default, natural identifier properties as assumed to be immutable (constant).
The <component>
element maps properties of a
child object to columns of the table of a parent class. Components may, in
turn, declare their own properties, components or collections. See
"Components" below.
<component name="propertyName" (1) class="className" (2) insert="true|false" (3) update="true|false" (4) access="field|property|ClassName" (5) lazy="true|false" (6) optimistic-lock="true|false" (7) unique="true|false" (8) node="element-name|." > <property ...../> <many-to-one .... /> ........ </component>
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The child <property>
tags map properties of the
child class to table columns.
The <component>
element allows a <parent>
subelement that maps a property of the component class as a reference back to the
containing entity.
The <dynamic-component>
element allows a Map
to be mapped as a component, where the property names refer to keys of the map, see
Section 8.5, “Dynamic components”.
The <properties>
element allows the definition of a named,
logical grouping of properties of a class. The most important use of the construct
is that it allows a combination of properties to be the target of a
property-ref
. It is also a convenient way to define a multi-column
unique constraint.
<properties name="logicalName" (1) insert="true|false" (2) update="true|false" (3) optimistic-lock="true|false" (4) unique="true|false" (5) > <property ...../> <many-to-one .... /> ........ </properties>
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For example, if we have the following <properties>
mapping:
<class name="Person"> <id name="personNumber"/> ... <properties name="name" unique="true" update="false"> <property name="firstName"/> <property name="initial"/> <property name="lastName"/> </properties> </class>
Then we might have some legacy data association which refers to this unique key of
the Person
table, instead of to the primary key:
<many-to-one name="person" class="Person" property-ref="name"> <column name="firstName"/> <column name="initial"/> <column name="lastName"/> </many-to-one>
We don't recommend the use of this kind of thing outside the context of mapping legacy data.
Finally, polymorphic persistence requires the declaration of each subclass of
the root persistent class. For the table-per-class-hierarchy
mapping strategy, the <subclass>
declaration is used.
<subclass name="ClassName" (1) discriminator-value="discriminator_value" (2) proxy="ProxyInterface" (3) lazy="true|false" (4) dynamic-update="true|false" dynamic-insert="true|false" entity-name="EntityName" node="element-name" extends="SuperclassName"> <property .... /> ..... </subclass>
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Each subclass should declare its own persistent properties and subclasses.
<version>
and <id>
properties
are assumed to be inherited from the root class. Each subclass in a heirarchy must
define a unique discriminator-value
. If none is specified, the
fully qualified Java class name is used.
For information about inheritance mappings, see Chapter 9, Inheritance Mapping.
Alternatively, each subclass may be mapped to its own table (table-per-subclass
mapping strategy). Inherited state is retrieved by joining with the table of the
superclass. We use the <joined-subclass>
element.
<joined-subclass name="ClassName" (1) table="tablename" (2) proxy="ProxyInterface" (3) lazy="true|false" (4) dynamic-update="true|false" dynamic-insert="true|false" schema="schema" catalog="catalog" extends="SuperclassName" persister="ClassName" subselect="SQL expression" entity-name="EntityName" node="element-name"> <key .... > <property .... /> ..... </joined-subclass>
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No discriminator column is required for this mapping strategy. Each subclass must,
however, declare a table column holding the object identifier using the
<key>
element. The mapping at the start of the chapter
would be re-written as:
<?xml version="1.0"?> <!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD//EN" "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd"> <hibernate-mapping package="eg"> <class name="Cat" table="CATS"> <id name="id" column="uid" type="long"> <generator class="hilo"/> </id> <property name="birthdate" type="date"/> <property name="color" not-null="true"/> <property name="sex" not-null="true"/> <property name="weight"/> <many-to-one name="mate"/> <set name="kittens"> <key column="MOTHER"/> <one-to-many class="Cat"/> </set> <joined-subclass name="DomesticCat" table="DOMESTIC_CATS"> <key column="CAT"/> <property name="name" type="string"/> </joined-subclass> </class> <class name="eg.Dog"> <!-- mapping for Dog could go here --> </class> </hibernate-mapping>
For information about inheritance mappings, see Chapter 9, Inheritance Mapping.
A third option is to map only the concrete classes of an inheritance hierarchy
to tables, (the table-per-concrete-class strategy) where each table defines all
persistent state of the class, including inherited state. In Hibernate, it is
not absolutely necessary to explicitly map such inheritance hierarchies. You
can simply map each class with a separate <class>
declaration. However, if you wish use polymorphic associations (e.g. an association
to the superclass of your hierarchy), you need to
use the <union-subclass>
mapping.
<union-subclass name="ClassName" (1) table="tablename" (2) proxy="ProxyInterface" (3) lazy="true|false" (4) dynamic-update="true|false" dynamic-insert="true|false" schema="schema" catalog="catalog" extends="SuperclassName" abstract="true|false" persister="ClassName" subselect="SQL expression" entity-name="EntityName" node="element-name"> <property .... /> ..... </union-subclass>
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No discriminator column or key column is required for this mapping strategy.
For information about inheritance mappings, see Chapter 9, Inheritance Mapping.
Using the <join>
element, it is possible to map
properties of one class to several tables, when there's a 1-to-1 relationship between the tables.
<join table="tablename" (1) schema="owner" (2) catalog="catalog" (3) fetch="join|select" (4) inverse="true|false" (5) optional="true|false"> (6) <key ... /> <property ... /> ... </join>
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For example, the address information for a person can be mapped to a separate table (while preserving value type semantics for all properties):
<class name="Person" table="PERSON"> <id name="id" column="PERSON_ID">...</id> <join table="ADDRESS"> <key column="ADDRESS_ID"/> <property name="address"/> <property name="zip"/> <property name="country"/> </join> ...
This feature is often only useful for legacy data models, we recommend fewer tables than classes and a fine-grained domain model. However, it is useful for switching between inheritance mapping strategies in a single hierarchy, as explained later.
We've seen the <key>
element crop up a few times
now. It appears anywhere the parent mapping element defines a join to
a new table, and defines the foreign key in the joined table, that references
the primary key of the original table.
<key column="columnname" (1) on-delete="noaction|cascade" (2) property-ref="propertyName" (3) not-null="true|false" (4) update="true|false" (5) unique="true|false" (6) />
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We recommend that for systems where delete performance is important, all keys should be
defined on-delete="cascade"
, and Hibernate will use a database-level
ON CASCADE DELETE
constraint, instead of many individual
DELETE
statements. Be aware that this feature bypasses Hibernate's
usual optimistic locking strategy for versioned data.
The not-null
and update
attributes are useful when
mapping a unidirectional one to many association. If you map a unidirectional one to many
to a non-nullable foreign key, you must declare the key column using
<key not-null="true">
.
Any mapping element which accepts a column
attribute will alternatively
accept a <column>
subelement. Likewise, <formula>
is an alternative to the formula
attribute.
<column name="column_name" length="N" precision="N" scale="N" not-null="true|false" unique="true|false" unique-key="multicolumn_unique_key_name" index="index_name" sql-type="sql_type_name" check="SQL expression" default="SQL expression"/>
<formula>SQL expression</formula>
column
and formula
attributes may even be combined
within the same property or association mapping to express, for example, exotic join
conditions.
<many-to-one name="homeAddress" class="Address" insert="false" update="false"> <column name="person_id" not-null="true" length="10"/> <formula>'MAILING'</formula> </many-to-one>
Suppose your application has two persistent classes with the same name, and you don't want to
specify the fully qualified (package) name in Hibernate queries. Classes may be "imported"
explicitly, rather than relying upon auto-import="true"
. You may even import
classes and interfaces that are not explicitly mapped.
<import class="java.lang.Object" rename="Universe"/>
<import class="ClassName" (1) rename="ShortName" (2) />
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There is one further type of property mapping. The <any>
mapping element
defines a polymorphic association to classes from multiple tables. This type of mapping always
requires more than one column. The first column holds the type of the associated entity.
The remaining columns hold the identifier. It is impossible to specify a foreign key constraint
for this kind of association, so this is most certainly not meant as the usual way of mapping
(polymorphic) associations. You should use this only in very special cases (eg. audit logs,
user session data, etc).
The meta-type
attribute lets the application specify a custom type that
maps database column values to persistent classes which have identifier properties of the
type specified by id-type
. You must specify the mapping from values of
the meta-type to class names.
<any name="being" id-type="long" meta-type="string"> <meta-value value="TBL_ANIMAL" class="Animal"/> <meta-value value="TBL_HUMAN" class="Human"/> <meta-value value="TBL_ALIEN" class="Alien"/> <column name="table_name"/> <column name="id"/> </any>
<any name="propertyName" (1) id-type="idtypename" (2) meta-type="metatypename" (3) cascade="cascade_style" (4) access="field|property|ClassName" (5) optimistic-lock="true|false" (6) > <meta-value ... /> <meta-value ... /> ..... <column .... /> <column .... /> ..... </any>
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To understand the behaviour of various Java language-level objects with respect to the persistence service, we need to classify them into two groups:
An entity exists independently of any other objects holding references to the entity. Contrast this with the usual Java model where an unreferenced object is garbage collected. Entities must be explicitly saved and deleted (except that saves and deletions may be cascaded from a parent entity to its children). This is different from the ODMG model of object persistence by reachablity - and corresponds more closely to how application objects are usually used in large systems. Entities support circular and shared references. They may also be versioned.
An entity's persistent state consists of references to other entities and instances of value types. Values are primitives, collections (not what's inside a collection), components and certain immutable objects. Unlike entities, values (in particular collections and components) are persisted and deleted by reachability. Since value objects (and primitives) are persisted and deleted along with their containing entity they may not be independently versioned. Values have no independent identity, so they cannot be shared by two entities or collections.
Up until now, we've been using the term "persistent class" to refer to
entities. We will continue to do that. Strictly speaking, however, not all
user-defined classes with persistent state are entities. A
component is a user defined class with value semantics.
A Java property of type java.lang.String
also has value
semantics. Given this definition, we can say that all types (classes) provided
by the JDK have value type semantics in Java, while user-defined types may
be mapped with entity or value type semantics. This decision is up to the
application developer. A good hint for an entity class in a domain model are
shared references to a single instance of that class, while composition or
aggregation usually translates to a value type.
We'll revisit both concepts throughout the documentation.
The challenge is to map the Java type system (and the developers' definition of
entities and value types) to the SQL/database type system. The bridge between
both systems is provided by Hibernate: for entities we use
<class>
, <subclass>
and so on.
For value types we use <property>
,
<component>
, etc, usually with a type
attribute. The value of this attribute is the name of a Hibernate
mapping type. Hibernate provides many mappings (for standard
JDK value types) out of the box. You can write your own mapping types and implement your
custom conversion strategies as well, as you'll see later.
All built-in Hibernate types except collections support null semantics.
The built-in basic mapping types may be roughly categorized into
integer, long, short, float, double, character, byte,
boolean, yes_no, true_false
Type mappings from Java primitives or wrapper classes to appropriate
(vendor-specific) SQL column types. boolean, yes_no
and true_false
are all alternative encodings for
a Java boolean
or java.lang.Boolean
.
string
A type mapping from java.lang.String
to
VARCHAR
(or Oracle VARCHAR2
).
date, time, timestamp
Type mappings from java.util.Date
and its subclasses
to SQL types DATE
, TIME
and
TIMESTAMP
(or equivalent).
calendar, calendar_date
Type mappings from java.util.Calendar
to
SQL types TIMESTAMP
and DATE
(or equivalent).
big_decimal, big_integer
Type mappings from java.math.BigDecimal
and
java.math.BigInteger
to NUMERIC
(or Oracle NUMBER
).
locale, timezone, currency
Type mappings from java.util.Locale
,
java.util.TimeZone
and
java.util.Currency
to VARCHAR
(or Oracle VARCHAR2
).
Instances of Locale
and Currency
are
mapped to their ISO codes. Instances of TimeZone
are
mapped to their ID
.
class
A type mapping from java.lang.Class
to
VARCHAR
(or Oracle VARCHAR2
).
A Class
is mapped to its fully qualified name.
binary
Maps byte arrays to an appropriate SQL binary type.
text
Maps long Java strings to a SQL CLOB
or
TEXT
type.
serializable
Maps serializable Java types to an appropriate SQL binary type. You
may also indicate the Hibernate type serializable
with
the name of a serializable Java class or interface that does not default
to a basic type.
clob, blob
Type mappings for the JDBC classes java.sql.Clob
and
java.sql.Blob
. These types may be inconvenient for some
applications, since the blob or clob object may not be reused outside of
a transaction. (Furthermore, driver support is patchy and inconsistent.)
imm_date, imm_time, imm_timestamp, imm_calendar, imm_calendar_date,
imm_serializable, imm_binary
Type mappings for what are usually considered mutable Java types, where
Hibernate makes certain optimizations appropriate only for immutable
Java types, and the application treats the object as immutable. For
example, you should not call Date.setTime()
for an
instance mapped as imm_timestamp
. To change the
value of the property, and have that change made persistent, the
application must assign a new (nonidentical) object to the property.
Unique identifiers of entities and collections may be of any basic type except
binary
, blob
and clob
.
(Composite identifiers are also allowed, see below.)
The basic value types have corresponding Type
constants defined on
org.hibernate.Hibernate
. For example, Hibernate.STRING
represents the string
type.
It is relatively easy for developers to create their own value types. For example,
you might want to persist properties of type java.lang.BigInteger
to VARCHAR
columns. Hibernate does not provide a built-in type
for this. But custom types are not limited to mapping a property (or collection element)
to a single table column. So, for example, you might have a Java property
getName()
/setName()
of type
java.lang.String
that is persisted to the columns
FIRST_NAME
, INITIAL
, SURNAME
.
To implement a custom type, implement either org.hibernate.UserType
or org.hibernate.CompositeUserType
and declare properties using the
fully qualified classname of the type. Check out
org.hibernate.test.DoubleStringType
to see the kind of things that
are possible.
<property name="twoStrings" type="org.hibernate.test.DoubleStringType"> <column name="first_string"/> <column name="second_string"/> </property>
Notice the use of <column>
tags to map a property to multiple
columns.
The CompositeUserType
, EnhancedUserType
,
UserCollectionType
, and UserVersionType
interfaces provide support for more specialized uses.
You may even supply parameters to a UserType
in the mapping file. To
do this, your UserType
must implement the
org.hibernate.usertype.ParameterizedType
interface. To supply parameters
to your custom type, you can use the <type>
element in your mapping
files.
<property name="priority"> <type name="com.mycompany.usertypes.DefaultValueIntegerType"> <param name="default">0</param> </type> </property>
The UserType
can now retrieve the value for the parameter named
default
from the Properties
object passed to it.
If you use a certain UserType
very often, it may be useful to define a
shorter name for it. You can do this using the <typedef>
element.
Typedefs assign a name to a custom type, and may also contain a list of default
parameter values if the type is parameterized.
<typedef class="com.mycompany.usertypes.DefaultValueIntegerType" name="default_zero"> <param name="default">0</param> </typedef>
<property name="priority" type="default_zero"/>
It is also possible to override the parameters supplied in a typedef on a case-by-case basis by using type parameters on the property mapping.
Even though Hibernate's rich range of built-in types and support for components means you
will very rarely need to use a custom type, it is nevertheless
considered good form to use custom types for (non-entity) classes that occur frequently
in your application. For example, a MonetaryAmount
class is a good
candidate for a CompositeUserType
, even though it could easily be mapped
as a component. One motivation for this is abstraction. With a custom type, your mapping
documents would be future-proofed against possible changes in your way of representing
monetary values.
It is possible to provide more than one mapping for a particular persistent class. In this case you must specify an entity name do disambiguate between instances of the two mapped entities. (By default, the entity name is the same as the class name.) Hibernate lets you specify the entity name when working with persistent objects, when writing queries, or when mapping associations to the named entity.
<class name="Contract" table="Contracts" entity-name="CurrentContract"> ... <set name="history" inverse="true" order-by="effectiveEndDate desc"> <key column="currentContractId"/> <one-to-many entity-name="HistoricalContract"/> </set> </class> <class name="Contract" table="ContractHistory" entity-name="HistoricalContract"> ... <many-to-one name="currentContract" column="currentContractId" entity-name="CurrentContract"/> </class>
Notice how associations are now specified using entity-name
instead of
class
.
You may force Hibernate to quote an identifier in the generated SQL by enclosing the table or
column name in backticks in the mapping document. Hibernate will use the correct quotation
style for the SQL Dialect
(usually double quotes, but brackets for SQL
Server and backticks for MySQL).
<class name="LineItem" table="`Line Item`"> <id name="id" column="`Item Id`"/><generator class="assigned"/></id> <property name="itemNumber" column="`Item #`"/> ... </class>
XML isn't for everyone, and so there are some alternative ways to define O/R mapping metadata in Hibernate.
Many Hibernate users prefer to embed mapping information directly in sourcecode using
XDoclet @hibernate.tags
. We will not cover this approach in this
document, since strictly it is considered part of XDoclet. However, we include the
following example of the Cat
class with XDoclet mappings.
package eg; import java.util.Set; import java.util.Date; /** * @hibernate.class * table="CATS" */ public class Cat { private Long id; // identifier private Date birthdate; private Cat mother; private Set kittens private Color color; private char sex; private float weight; /* * @hibernate.id * generator-class="native" * column="CAT_ID" */ public Long getId() { return id; } private void setId(Long id) { this.id=id; } /** * @hibernate.many-to-one * column="PARENT_ID" */ public Cat getMother() { return mother; } void setMother(Cat mother) { this.mother = mother; } /** * @hibernate.property * column="BIRTH_DATE" */ public Date getBirthdate() { return birthdate; } void setBirthdate(Date date) { birthdate = date; } /** * @hibernate.property * column="WEIGHT" */ public float getWeight() { return weight; } void setWeight(float weight) { this.weight = weight; } /** * @hibernate.property * column="COLOR" * not-null="true" */ public Color getColor() { return color; } void setColor(Color color) { this.color = color; } /** * @hibernate.set * inverse="true" * order-by="BIRTH_DATE" * @hibernate.collection-key * column="PARENT_ID" * @hibernate.collection-one-to-many */ public Set getKittens() { return kittens; } void setKittens(Set kittens) { this.kittens = kittens; } // addKitten not needed by Hibernate public void addKitten(Cat kitten) { kittens.add(kitten); } /** * @hibernate.property * column="SEX" * not-null="true" * update="false" */ public char getSex() { return sex; } void setSex(char sex) { this.sex=sex; } }
See the Hibernate web site for more examples of XDoclet and Hibernate.
JDK 5.0 introduced XDoclet-style annotations at the language level, type-safe and
checked at compile time. This mechnism is more powerful than XDoclet annotations and
better supported by tools and IDEs. IntelliJ IDEA, for example, supports auto-completion
and syntax highlighting of JDK 5.0 annotations. The new revision of the EJB specification
(JSR-220) uses JDK 5.0 annotations as the primary metadata mechanism for entity beans.
Hibernate3 implements the EntityManager
of JSR-220 (the persistence API),
support for mapping metadata is available via the Hibernate Annotations
package, as a separate download. Both EJB3 (JSR-220) and Hibernate3 metadata is supported.
This is an example of a POJO class annotated as an EJB entity bean:
@Entity(access = AccessType.FIELD) public class Customer implements Serializable { @Id; Long id; String firstName; String lastName; Date birthday; @Transient Integer age; @Embedded private Address homeAddress; @OneToMany(cascade=CascadeType.ALL) @JoinColumn(name="CUSTOMER_ID") Set<Order> orders; // Getter/setter and business methods }
Note that support for JDK 5.0 Annotations (and JSR-220) is still work in progress and not completed. Please refer to the Hibernate Annotations module for more details.
Generated properties are properties which have their values generated by the
database. Typically, Hibernate applications needed to refresh
objects which contain any properties for which the database was generating values.
Marking properties as generated, however, lets the application delegate this
responsibility to Hibernate. Essentially, whenever Hibernate issues an SQL INSERT
or UPDATE for an entity which has defined generated properties, it immediately
issues a select afterwards to retrieve the generated values.
Properties marked as generated must additionally be non-insertable and non-updateable. Only versions, timestamps, and simple properties can be marked as generated.
never
(the default) - means that the given property value
is not generated within the database.
insert
- states that the given property value is generated on
insert, but is not regenerated on subsequent updates. Things like created-date would
fall into this category. Note that even thought
version and
timestamp properties can
be marked as generated, this option is not available there...
always
- states that the property value is generated both
on insert and on update.
Allows CREATE and DROP of arbitrary database objects, in conjunction with
Hibernate's schema evolution tools, to provide the ability to fully define
a user schema within the Hibernate mapping files. Although designed specifically
for creating and dropping things like triggers or stored procedures, really any
SQL command that can be run via a java.sql.Statement.execute()
method is valid here (ALTERs, INSERTS, etc). There are essentially two modes for
defining auxiliary database objects...
The first mode is to explicitly list the CREATE and DROP commands out in the mapping file:
<hibernate-mapping> ... <database-object> <create>CREATE TRIGGER my_trigger ...</create> <drop>DROP TRIGGER my_trigger</drop> </database-object> </hibernate-mapping>
The second mode is to supply a custom class which knows how to construct the
CREATE and DROP commands. This custom class must implement the
org.hibernate.mapping.AuxiliaryDatabaseObject
interface.
<hibernate-mapping> ... <database-object> <definition class="MyTriggerDefinition"/> </database-object> </hibernate-mapping>
Additionally, these database objects can be optionally scoped such that they only apply when certain dialects are used.
<hibernate-mapping> ... <database-object> <definition class="MyTriggerDefinition"/> <dialect-scope name="org.hibernate.dialect.Oracle9Dialect"/> <dialect-scope name="org.hibernate.dialect.OracleDialect"/> </database-object> </hibernate-mapping>
Hibernate requires that persistent collection-valued fields be declared as an interface type, for example:
public class Product { private String serialNumber; private Set parts = new HashSet(); public Set getParts() { return parts; } void setParts(Set parts) { this.parts = parts; } public String getSerialNumber() { return serialNumber; } void setSerialNumber(String sn) { serialNumber = sn; } }
The actual interface might be java.util.Set
,
java.util.Collection
, java.util.List
,
java.util.Map
, java.util.SortedSet
,
java.util.SortedMap
or ... anything you like! (Where
"anything you like" means you will have to write an implementation of
org.hibernate.usertype.UserCollectionType
.)
Notice how we initialized the instance variable with an instance of
HashSet
. This is the best way to initialize collection
valued properties of newly instantiated (non-persistent) instances. When
you make the instance persistent - by calling persist()
,
for example - Hibernate will actually replace the HashSet
with an instance of Hibernate's own implementation of Set
.
Watch out for errors like this:
Cat cat = new DomesticCat(); Cat kitten = new DomesticCat(); .... Set kittens = new HashSet(); kittens.add(kitten); cat.setKittens(kittens); session.persist(cat); kittens = cat.getKittens(); // Okay, kittens collection is a Set (HashSet) cat.getKittens(); // Error!
The persistent collections injected by Hibernate behave like
HashMap
, HashSet
,
TreeMap
, TreeSet
or
ArrayList
, depending upon the interface type.
Collections instances have the usual behavior of value types. They are automatically persisted when referenced by a persistent object and automatically deleted when unreferenced. If a collection is passed from one persistent object to another, its elements might be moved from one table to another. Two entities may not share a reference to the same collection instance. Due to the underlying relational model, collection-valued properties do not support null value semantics; Hibernate does not distinguish between a null collection reference and an empty collection.
You shouldn't have to worry much about any of this. Use persistent collections the same way you use ordinary Java collections. Just make sure you understand the semantics of bidirectional associations (discussed later).
There are quite a range of mappings that can be generated for collections, covering many common relational models. We suggest you experiment with the schema generation tool to get a feeling for how various mapping declarations translate to database tables.
The Hibernate mapping element used for mapping a collection depends upon
the type of the interface. For example, a <set>
element is used for mapping properties of type Set
.
<class name="Product"> <id name="serialNumber" column="productSerialNumber"/> <set name="parts"> <key column="productSerialNumber" not-null="true"/> <one-to-many class="Part"/> </set> </class>
Apart from <set>
, there is also
<list>
, <map>
,
<bag>
, <array>
and
<primitive-array>
mapping elements. The
<map>
element is representative:
<map name="propertyName" (1) table="table_name" (2) schema="schema_name" (3) lazy="true|extra|false" (4) inverse="true|false" (5) cascade="all|none|save-update|delete|all-delete-orphan|delet(6)e-orphan" sort="unsorted|natural|comparatorClass" (7) order-by="column_name asc|desc" (8) where="arbitrary sql where condition" (9) fetch="join|select|subselect" (10) batch-size="N" (11) access="field|property|ClassName" (12) optimistic-lock="true|false" (13) mutable="true|false" (14) node="element-name|." embed-xml="true|false" > <key .... /> <map-key .... /> <element .... /> </map>
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Collection instances are distinguished in the database by the foreign key of
the entity that owns the collection. This foreign key is referred to as the
collection key column (or columns) of the collection
table. The collection key column is mapped by the <key>
element.
There may be a nullability constraint on the foreign key column. For most
collections, this is implied. For unidirectional one to many associations,
the foreign key column is nullable by default, so you might need to specify
not-null="true"
.
<key column="productSerialNumber" not-null="true"/>
The foreign key constraint may use ON DELETE CASCADE
.
<key column="productSerialNumber" on-delete="cascade"/>
See the previous chapter for a full definition of the <key>
element.
Collections may contain almost any other Hibernate type, including all basic types, custom types, components, and of course, references to other entities. This is an important distinction: an object in a collection might be handled with "value" semantics (its life cycle fully depends on the collection owner) or it might be a reference to another entity, with its own life cycle. In the latter case, only the "link" between the two objects is considered to be state held by the collection.
The contained type is referred to as the collection element type.
Collection elements are mapped by <element>
or
<composite-element>
, or in the case of entity references,
with <one-to-many>
or <many-to-many>
.
The first two map elements with value semantics, the next two are used to map entity
associations.
All collection mappings, except those with set and bag semantics, need an
index column in the collection table - a column that maps to an
array index, or List
index, or Map
key. The
index of a Map
may be of any basic type, mapped with
<map-key>
, it may be an entity reference mapped with
<map-key-many-to-many>
, or it may be a composite type,
mapped with <composite-map-key>
. The index of an array or
list is always of type integer
and is mapped using the
<list-index>
element. The mapped column contains
sequential integers (numbered from zero, by default).
<list-index
column="column_name" (1)
base="0|1|..."/>
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<map-key-many-to-many column="column_name" (1) formula="any SQL expression" (2)(3) class="ClassName" />
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If your table doesn't have an index column, and you still wish to use List
as the property type, you should map the property as a Hibernate <bag>.
A bag does not retain its order when it is retrieved from the database, but it may be
optionally sorted or ordered.
Any collection of values or many-to-many association requires a dedicated collection table with a foreign key column or columns, collection element column or columns and possibly an index column or columns.
For a collection of values, we use the <element>
tag.
<element column="column_name" (1) formula="any SQL expression" (2) type="typename" (3) length="L" precision="P" scale="S" not-null="true|false" unique="true|false" node="element-name" />
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A many-to-many association is specified using the
<many-to-many>
element.
<many-to-many column="column_name" (1) formula="any SQL expression" (2) class="ClassName" (3) fetch="select|join" (4) unique="true|false" (5) not-found="ignore|exception" (6) entity-name="EntityName" (7) property-ref="propertyNameFromAssociatedClass" (8) node="element-name" embed-xml="true|false" />
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Some examples, first, a set of strings:
<set name="names" table="person_names"> <key column="person_id"/> <element column="person_name" type="string"/> </set>
A bag containing integers (with an iteration order determined by the
order-by
attribute):
<bag name="sizes" table="item_sizes" order-by="size asc"> <key column="item_id"/> <element column="size" type="integer"/> </bag>
An array of entities - in this case, a many to many association:
<array name="addresses" table="PersonAddress" cascade="persist"> <key column="personId"/> <list-index column="sortOrder"/> <many-to-many column="addressId" class="Address"/> </array>
A map from string indices to dates:
<map name="holidays" table="holidays" schema="dbo" order-by="hol_name asc"> <key column="id"/> <map-key column="hol_name" type="string"/> <element column="hol_date" type="date"/> </map>
A list of components (discussed in the next chapter):
<list name="carComponents" table="CarComponents"> <key column="carId"/> <list-index column="sortOrder"/> <composite-element class="CarComponent"> <property name="price"/> <property name="type"/> <property name="serialNumber" column="serialNum"/> </composite-element> </list>
A one to many association links the tables of two classes via a foreign key, with no intervening collection table. This mapping loses certain semantics of normal Java collections:
An instance of the contained entity class may not belong to more than one instance of the collection
An instance of the contained entity class may not appear at more than one value of the collection index
An association from Product
to Part
requires
existence of a foreign key column and possibly an index column to the Part
table. A <one-to-many>
tag indicates that this is a one to many
association.
<one-to-many class="ClassName" (1) not-found="ignore|exception" (2) entity-name="EntityName" (3) node="element-name" embed-xml="true|false" />
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Notice that the <one-to-many>
element does not need to
declare any columns. Nor is it necessary to specify the table
name anywhere.
Very important note: If the foreign key column of a
<one-to-many>
association is declared NOT NULL
,
you must declare the <key>
mapping
not-null="true"
or use a bidirectional association
with the collection mapping marked inverse="true"
. See the discussion
of bidirectional associations later in this chapter.
This example shows a map of Part
entities by name (where
partName
is a persistent property of Part
).
Notice the use of a formula-based index.
<map name="parts" cascade="all"> <key column="productId" not-null="true"/> <map-key formula="partName"/> <one-to-many class="Part"/> </map>
Hibernate supports collections implementing java.util.SortedMap
and
java.util.SortedSet
. You must specify a comparator in the mapping file:
<set name="aliases" table="person_aliases" sort="natural"> <key column="person"/> <element column="name" type="string"/> </set> <map name="holidays" sort="my.custom.HolidayComparator"> <key column="year_id"/> <map-key column="hol_name" type="string"/> <element column="hol_date" type="date"/> </map>
Allowed values of the sort
attribute are unsorted
,
natural
and the name of a class implementing
java.util.Comparator
.
Sorted collections actually behave like java.util.TreeSet
or
java.util.TreeMap
.
If you want the database itself to order the collection elements use the
order-by
attribute of set
, bag
or map
mappings. This solution is only available under
JDK 1.4 or higher (it is implemented using LinkedHashSet
or
LinkedHashMap
). This performs the ordering in the SQL query,
not in memory.
<set name="aliases" table="person_aliases" order-by="lower(name) asc"> <key column="person"/> <element column="name" type="string"/> </set> <map name="holidays" order-by="hol_date, hol_name"> <key column="year_id"/> <map-key column="hol_name" type="string"/> <element column="hol_date type="date"/> </map>
Note that the value of the order-by
attribute is an SQL ordering, not
a HQL ordering!
Associations may even be sorted by some arbitrary criteria at runtime using a collection
filter()
.
sortedUsers = s.createFilter( group.getUsers(), "order by this.name" ).list();
A bidirectional association allows navigation from both "ends" of the association. Two kinds of bidirectional association are supported:
set or bag valued at one end, single-valued at the other
set or bag valued at both ends
You may specify a bidirectional many-to-many association simply by mapping two many-to-many associations to the same database table and declaring one end as inverse (which one is your choice, but it can not be an indexed collection).
Here's an example of a bidirectional many-to-many association; each category can have many items and each item can be in many categories:
<class name="Category"> <id name="id" column="CATEGORY_ID"/> ... <bag name="items" table="CATEGORY_ITEM"> <key column="CATEGORY_ID"/> <many-to-many class="Item" column="ITEM_ID"/> </bag> </class> <class name="Item"> <id name="id" column="ITEM_ID"/> ... <!-- inverse end --> <bag name="categories" table="CATEGORY_ITEM" inverse="true"> <key column="ITEM_ID"/> <many-to-many class="Category" column="CATEGORY_ID"/> </bag> </class>
Changes made only to the inverse end of the association are not persisted. This means that Hibernate has two representations in memory for every bidirectional association, one link from A to B and another link from B to A. This is easier to understand if you think about the Java object model and how we create a many-to-many relationship in Java:
category.getItems().add(item); // The category now "knows" about the relationship item.getCategories().add(category); // The item now "knows" about the relationship session.persist(item); // The relationship won't be saved! session.persist(category); // The relationship will be saved
The non-inverse side is used to save the in-memory representation to the database.
You may define a bidirectional one-to-many association by mapping a one-to-many association
to the same table column(s) as a many-to-one association and declaring the many-valued
end inverse="true"
.
<class name="Parent"> <id name="id" column="parent_id"/> .... <set name="children" inverse="true"> <key column="parent_id"/> <one-to-many class="Child"/> </set> </class> <class name="Child"> <id name="id" column="child_id"/> .... <many-to-one name="parent" class="Parent" column="parent_id" not-null="true"/> </class>
Mapping one end of an association with inverse="true"
doesn't
affect the operation of cascades, these are orthogonal concepts!
A bidirectional association where one end is represented as a <list>
or <map>
requires special consideration. If there is a property of
the child class which maps to the index column, no problem, we can continue using
inverse="true"
on the collection mapping:
<class name="Parent"> <id name="id" column="parent_id"/> .... <map name="children" inverse="true"> <key column="parent_id"/> <map-key column="name" type="string"/> <one-to-many class="Child"/> </map> </class> <class name="Child"> <id name="id" column="child_id"/> .... <property name="name" not-null="true"/> <many-to-one name="parent" class="Parent" column="parent_id" not-null="true"/> </class>
But, if there is no such property on the child class, we can't think of the association as
truly bidirectional (there is information available at one end of the association that is
not available at the other end). In this case, we can't map the collection
inverse="true"
. Instead, we could use the following mapping:
<class name="Parent"> <id name="id" column="parent_id"/> .... <map name="children"> <key column="parent_id" not-null="true"/> <map-key column="name" type="string"/> <one-to-many class="Child"/> </map> </class> <class name="Child"> <id name="id" column="child_id"/> .... <many-to-one name="parent" class="Parent" column="parent_id" insert="false" update="false" not-null="true"/> </class>
Note that in this mapping, the collection-valued end of the association is responsible for updates to the foreign key. TODO: Does this really result in some unnecessary update statements?
There are three possible approaches to mapping a ternary association. One is to use a
Map
with an association as its index:
<map name="contracts"> <key column="employer_id" not-null="true"/> <map-key-many-to-many column="employee_id" class="Employee"/> <one-to-many class="Contract"/> </map>
<map name="connections"> <key column="incoming_node_id"/> <map-key-many-to-many column="outgoing_node_id" class="Node"/> <many-to-many column="connection_id" class="Connection"/> </map>
A second approach is to simply remodel the association as an entity class. This is the approach we use most commonly.
A final alternative is to use composite elements, which we will discuss later.
If you've fully embraced our view that composite keys are a bad thing and that entities should have synthetic identifiers (surrogate keys), then you might find it a bit odd that the many to many associations and collections of values that we've shown so far all map to tables with composite keys! Now, this point is quite arguable; a pure association table doesn't seem to benefit much from a surrogate key (though a collection of composite values might). Nevertheless, Hibernate provides a feature that allows you to map many to many associations and collections of values to a table with a surrogate key.
The <idbag>
element lets you map a List
(or Collection
) with bag semantics.
<idbag name="lovers" table="LOVERS"> <collection-id column="ID" type="long"> <generator class="sequence"/> </collection-id> <key column="PERSON1"/> <many-to-many column="PERSON2" class="Person" fetch="join"/> </idbag>
As you can see, an <idbag>
has a synthetic id generator,
just like an entity class! A different surrogate key is assigned to each collection
row. Hibernate does not provide any mechanism to discover the surrogate key value
of a particular row, however.
Note that the update performance of an <idbag>
is
much better than a regular <bag>
!
Hibernate can locate individual rows efficiently and update or delete them
individually, just like a list, map or set.
In the current implementation, the native
identifier generation
strategy is not supported for <idbag>
collection identifiers.
The previous sections are pretty confusing. So lets look at an example. This class:
package eg; import java.util.Set; public class Parent { private long id; private Set children; public long getId() { return id; } private void setId(long id) { this.id=id; } private Set getChildren() { return children; } private void setChildren(Set children) { this.children=children; } .... .... }
has a collection of Child
instances. If each
child has at most one parent, the most natural mapping is a
one-to-many association:
<hibernate-mapping> <class name="Parent"> <id name="id"> <generator class="sequence"/> </id> <set name="children"> <key column="parent_id"/> <one-to-many class="Child"/> </set> </class> <class name="Child"> <id name="id"> <generator class="sequence"/> </id> <property name="name"/> </class> </hibernate-mapping>
This maps to the following table definitions:
create table parent ( id bigint not null primary key ) create table child ( id bigint not null primary key, name varchar(255), parent_id bigint ) alter table child add constraint childfk0 (parent_id) references parent
If the parent is required, use a bidirectional one-to-many association:
<hibernate-mapping> <class name="Parent"> <id name="id"> <generator class="sequence"/> </id> <set name="children" inverse="true"> <key column="parent_id"/> <one-to-many class="Child"/> </set> </class> <class name="Child"> <id name="id"> <generator class="sequence"/> </id> <property name="name"/> <many-to-one name="parent" class="Parent" column="parent_id" not-null="true"/> </class> </hibernate-mapping>
Notice the NOT NULL
constraint:
create table parent ( id bigint not null primary key ) create table child ( id bigint not null primary key, name varchar(255), parent_id bigint not null ) alter table child add constraint childfk0 (parent_id) references parent
Alternatively, if you absolutely insist that this association should be unidirectional,
you can declare the NOT NULL
constraint on the <key>
mapping:
<hibernate-mapping> <class name="Parent"> <id name="id"> <generator class="sequence"/> </id> <set name="children"> <key column="parent_id" not-null="true"/> <one-to-many class="Child"/> </set> </class> <class name="Child"> <id name="id"> <generator class="sequence"/> </id> <property name="name"/> </class> </hibernate-mapping>
On the other hand, if a child might have multiple parents, a many-to-many association is appropriate:
<hibernate-mapping> <class name="Parent"> <id name="id"> <generator class="sequence"/> </id> <set name="children" table="childset"> <key column="parent_id"/> <many-to-many class="Child" column="child_id"/> </set> </class> <class name="Child"> <id name="id"> <generator class="sequence"/> </id> <property name="name"/> </class> </hibernate-mapping>
Table definitions:
create table parent ( id bigint not null primary key ) create table child ( id bigint not null primary key, name varchar(255) ) create table childset ( parent_id bigint not null, child_id bigint not null, primary key ( parent_id, child_id ) ) alter table childset add constraint childsetfk0 (parent_id) references parent alter table childset add constraint childsetfk1 (child_id) references child
For more examples and a complete walk-through a parent/child relationship mapping, see Chapter 21, Example: Parent/Child.
Even more exotic association mappings are possible, we will catalog all possibilities in the next chapter.
Association mappings are the often most difficult thing to get right. In
this section we'll go through the canonical cases one by one, starting
with unidirectional mappings, and then considering the bidirectional cases.
We'll use Person
and Address
in all
the examples.
We'll classify associations by whether or not they map to an intervening join table, and by multiplicity.
Nullable foreign keys are not considered good practice in traditional data modelling, so all our examples use not null foreign keys. This is not a requirement of Hibernate, and the mappings will all work if you drop the nullability constraints.
A unidirectional many-to-one association is the most common kind of unidirectional association.
<class name="Person"> <id name="id" column="personId"> <generator class="native"/> </id> <many-to-one name="address" column="addressId" not-null="true"/> </class> <class name="Address"> <id name="id" column="addressId"> <generator class="native"/> </id> </class>
create table Person ( personId bigint not null primary key, addressId bigint not null ) create table Address ( addressId bigint not null primary key )
A unidirectional one-to-one association on a foreign key is almost identical. The only difference is the column unique constraint.
<class name="Person"> <id name="id" column="personId"> <generator class="native"/> </id> <many-to-one name="address" column="addressId" unique="true" not-null="true"/> </class> <class name="Address"> <id name="id" column="addressId"> <generator class="native"/> </id> </class>
create table Person ( personId bigint not null primary key, addressId bigint not null unique ) create table Address ( addressId bigint not null primary key )
A unidirectional one-to-one association on a primary key usually uses a special id generator. (Notice that we've reversed the direction of the association in this example.)
<class name="Person"> <id name="id" column="personId"> <generator class="native"/> </id> </class> <class name="Address"> <id name="id" column="personId"> <generator class="foreign"> <param name="property">person</param> </generator> </id> <one-to-one name="person" constrained="true"/> </class>
create table Person ( personId bigint not null primary key ) create table Address ( personId bigint not null primary key )
A unidirectional one-to-many association on a foreign key is a very unusual case, and is not really recommended.
<class name="Person"> <id name="id" column="personId"> <generator class="native"/> </id> <set name="addresses"> <key column="personId" not-null="true"/> <one-to-many class="Address"/> </set> </class> <class name="Address"> <id name="id" column="addressId"> <generator class="native"/> </id> </class>
create table Person ( personId bigint not null primary key ) create table Address ( addressId bigint not null primary key, personId bigint not null )
We think it's better to use a join table for this kind of association.
A unidirectional one-to-many association on a join table
is much preferred. Notice that by specifying unique="true"
,
we have changed the multiplicity from many-to-many to one-to-many.
<class name="Person"> <id name="id" column="personId"> <generator class="native"/> </id> <set name="addresses" table="PersonAddress"> <key column="personId"/> <many-to-many column="addressId" unique="true" class="Address"/> </set> </class> <class name="Address"> <id name="id" column="addressId"> <generator class="native"/> </id> </class>
create table Person ( personId bigint not null primary key ) create table PersonAddress ( personId not null, addressId bigint not null primary key ) create table Address ( addressId bigint not null primary key )
A unidirectional many-to-one association on a join table is quite common when the association is optional.
<class name="Person"> <id name="id" column="personId"> <generator class="native"/> </id> <join table="PersonAddress" optional="true"> <key column="personId" unique="true"/> <many-to-one name="address" column="addressId" not-null="true"/> </join> </class> <class name="Address"> <id name="id" column="addressId"> <generator class="native"/> </id> </class>
create table Person ( personId bigint not null primary key ) create table PersonAddress ( personId bigint not null primary key, addressId bigint not null ) create table Address ( addressId bigint not null primary key )
A unidirectional one-to-one association on a join table is extremely unusual, but possible.
<class name="Person"> <id name="id" column="personId"> <generator class="native"/> </id> <join table="PersonAddress" optional="true"> <key column="personId" unique="true"/> <many-to-one name="address" column="addressId" not-null="true" unique="true"/> </join> </class> <class name="Address"> <id name="id" column="addressId"> <generator class="native"/> </id> </class>
create table Person ( personId bigint not null primary key ) create table PersonAddress ( personId bigint not null primary key, addressId bigint not null unique ) create table Address ( addressId bigint not null primary key )
Finally, we have a unidirectional many-to-many association.
<class name="Person"> <id name="id" column="personId"> <generator class="native"/> </id> <set name="addresses" table="PersonAddress"> <key column="personId"/> <many-to-many column="addressId" class="Address"/> </set> </class> <class name="Address"> <id name="id" column="addressId"> <generator class="native"/> </id> </class>
create table Person ( personId bigint not null primary key ) create table PersonAddress ( personId bigint not null, addressId bigint not null, primary key (personId, addressId) ) create table Address ( addressId bigint not null primary key )
A bidirectional many-to-one association is the most common kind of association. (This is the standard parent/child relationship.)
<class name="Person"> <id name="id" column="personId"> <generator class="native"/> </id> <many-to-one name="address" column="addressId" not-null="true"/> </class> <class name="Address"> <id name="id" column="addressId"> <generator class="native"/> </id> <set name="people" inverse="true"> <key column="addressId"/> <one-to-many class="Person"/> </set> </class>
create table Person ( personId bigint not null primary key, addressId bigint not null ) create table Address ( addressId bigint not null primary key )
If you use a List
(or other indexed collection) you need
to set the key
column of the foreign key to not null
,
and let Hibernate manage the association from the collections side to maintain the index
of each element (making the other side virtually inverse by setting
update="false"
and insert="false"
):
<class name="Person"> <id name="id"/> ... <many-to-one name="address" column="addressId" not-null="true" insert="false" update="false"/> </class> <class name="Address"> <id name="id"/> ... <list name="people"> <key column="addressId" not-null="true"/> <list-index column="peopleIdx"/> <one-to-many class="Person"/> </list> </class>
It is important that you define not-null="true"
on the
<key>
element of the collection mapping if the
underlying foreign key column is NOT NULL
. Don't only
declare not-null="true"
on a possible nested
<column>
element, but on the <key>
element.
A bidirectional one-to-one association on a foreign key is quite common.
<class name="Person"> <id name="id" column="personId"> <generator class="native"/> </id> <many-to-one name="address" column="addressId" unique="true" not-null="true"/> </class> <class name="Address"> <id name="id" column="addressId"> <generator class="native"/> </id> <one-to-one name="person" property-ref="address"/> </class>
create table Person ( personId bigint not null primary key, addressId bigint not null unique ) create table Address ( addressId bigint not null primary key )
A bidirectional one-to-one association on a primary key uses the special id generator.
<class name="Person"> <id name="id" column="personId"> <generator class="native"/> </id> <one-to-one name="address"/> </class> <class name="Address"> <id name="id" column="personId"> <generator class="foreign"> <param name="property">person</param> </generator> </id> <one-to-one name="person" constrained="true"/> </class>
create table Person ( personId bigint not null primary key ) create table Address ( personId bigint not null primary key )
A bidirectional one-to-many association on a join table.
Note that the inverse="true"
can go on either end of the
association, on the collection, or on the join.
<class name="Person"> <id name="id" column="personId"> <generator class="native"/> </id> <set name="addresses" table="PersonAddress"> <key column="personId"/> <many-to-many column="addressId" unique="true" class="Address"/> </set> </class> <class name="Address"> <id name="id" column="addressId"> <generator class="native"/> </id> <join table="PersonAddress" inverse="true" optional="true"> <key column="addressId"/> <many-to-one name="person" column="personId" not-null="true"/> </join> </class>
create table Person ( personId bigint not null primary key ) create table PersonAddress ( personId bigint not null, addressId bigint not null primary key ) create table Address ( addressId bigint not null primary key )
A bidirectional one-to-one association on a join table is extremely unusual, but possible.
<class name="Person"> <id name="id" column="personId"> <generator class="native"/> </id> <join table="PersonAddress" optional="true"> <key column="personId" unique="true"/> <many-to-one name="address" column="addressId" not-null="true" unique="true"/> </join> </class> <class name="Address"> <id name="id" column="addressId"> <generator class="native"/> </id> <join table="PersonAddress" optional="true" inverse="true"> <key column="addressId" unique="true"/> <many-to-one name="person" column="personId" not-null="true" unique="true"/> </join> </class>
create table Person ( personId bigint not null primary key ) create table PersonAddress ( personId bigint not null primary key, addressId bigint not null unique ) create table Address ( addressId bigint not null primary key )
Finally, we have a bidirectional many-to-many association.
<class name="Person"> <id name="id" column="personId"> <generator class="native"/> </id> <set name="addresses" table="PersonAddress"> <key column="personId"/> <many-to-many column="addressId" class="Address"/> </set> </class> <class name="Address"> <id name="id" column="addressId"> <generator class="native"/> </id> <set name="people" inverse="true" table="PersonAddress"> <key column="addressId"/> <many-to-many column="personId" class="Person"/> </set> </class>
create table Person ( personId bigint not null primary key ) create table PersonAddress ( personId bigint not null, addressId bigint not null, primary key (personId, addressId) ) create table Address ( addressId bigint not null primary key )
More complex association joins are extremely rare.
Hibernate makes it possible to handle more complex situations using
SQL fragments embedded in the mapping document. For example, if a table
with historical account information data defines
accountNumber
, effectiveEndDate
and effectiveStartDate
columns, mapped as follows:
<properties name="currentAccountKey"> <property name="accountNumber" type="string" not-null="true"/> <property name="currentAccount" type="boolean"> <formula>case when effectiveEndDate is null then 1 else 0 end</formula> </property> </properties> <property name="effectiveEndDate" type="date"/> <property name="effectiveStateDate" type="date" not-null="true"/>
Then we can map an association to the current instance
(the one with null effectiveEndDate
) using:
<many-to-one name="currentAccountInfo" property-ref="currentAccountKey" class="AccountInfo"> <column name="accountNumber"/> <formula>'1'</formula> </many-to-one>
In a more complex example, imagine that the association between
Employee
and Organization
is maintained
in an Employment
table full of historical employment data.
Then an association to the employee's most recent employer
(the one with the most recent startDate
) might be mapped this way:
<join> <key column="employeeId"/> <subselect> select employeeId, orgId from Employments group by orgId having startDate = max(startDate) </subselect> <many-to-one name="mostRecentEmployer" class="Organization" column="orgId"/> </join>
You can get quite creative with this functionality, but it is usually more practical to handle these kinds of cases using HQL or a criteria query.
The notion of a component is re-used in several different contexts, for different purposes, throughout Hibernate.
A component is a contained object that is persisted as a value type, not an entity reference. The term "component" refers to the object-oriented notion of composition (not to architecture-level components). For example, you might model a person like this:
public class Person { private java.util.Date birthday; private Name name; private String key; public String getKey() { return key; } private void setKey(String key) { this.key=key; } public java.util.Date getBirthday() { return birthday; } public void setBirthday(java.util.Date birthday) { this.birthday = birthday; } public Name getName() { return name; } public void setName(Name name) { this.name = name; } ...... ...... }
public class Name { char initial; String first; String last; public String getFirst() { return first; } void setFirst(String first) { this.first = first; } public String getLast() { return last; } void setLast(String last) { this.last = last; } public char getInitial() { return initial; } void setInitial(char initial) { this.initial = initial; } }
Now Name
may be persisted as a component of
Person
. Notice that Name
defines getter
and setter methods for its persistent properties, but doesn't need to declare
any interfaces or identifier properties.
Our Hibernate mapping would look like:
<class name="eg.Person" table="person"> <id name="Key" column="pid" type="string"> <generator class="uuid"/> </id> <property name="birthday" type="date"/> <component name="Name" class="eg.Name"> <!-- class attribute optional --> <property name="initial"/> <property name="first"/> <property name="last"/> </component> </class>
The person table would have the columns pid
,
birthday
,
initial
,
first
and
last
.
Like all value types, components do not support shared references. In other words, two persons could have the same name, but the two person objects would contain two independent name ojects, only "the same" by value. The null value semantics of a component are ad hoc. When reloading the containing object, Hibernate will assume that if all component columns are null, then the entire component is null. This should be okay for most purposes.
The properties of a component may be of any Hibernate type (collections, many-to-one associations, other components, etc). Nested components should not be considered an exotic usage. Hibernate is intended to support a very fine-grained object model.
The <component>
element allows a <parent>
subelement that maps a property of the component class as a reference back to the
containing entity.
<class name="eg.Person" table="person"> <id name="Key" column="pid" type="string"> <generator class="uuid"/> </id> <property name="birthday" type="date"/> <component name="Name" class="eg.Name" unique="true"> <parent name="namedPerson"/> <!-- reference back to the Person --> <property name="initial"/> <property name="first"/> <property name="last"/> </component> </class>
Collections of components are supported (eg. an array of type
Name
). Declare your component collection by
replacing the <element>
tag with a
<composite-element>
tag.
<set name="someNames" table="some_names" lazy="true"> <key column="id"/> <composite-element class="eg.Name"> <!-- class attribute required --> <property name="initial"/> <property name="first"/> <property name="last"/> </composite-element> </set>
Note: if you define a Set
of composite elements, it is
very important to implement equals()
and
hashCode()
correctly.
Composite elements may contain components but not collections. If your
composite element itself contains
components, use the <nested-composite-element>
tag. This is a pretty exotic case - a collection of components which
themselves have components. By this stage you should be asking yourself
if a one-to-many association is more appropriate. Try remodelling the
composite element as an entity - but note that even though the Java model
is the same, the relational model and persistence semantics are still
slightly different.
Please note that a composite element mapping doesn't support null-able properties
if you're using a <set>
. Hibernate
has to use each columns value to identify a record when deleting objects
(there is no separate primary key column in the composite element table),
which is not possible with null values. You have to either use only
not-null properties in a composite-element or choose a
<list>
, <map>
,
<bag>
or <idbag>
.
A special case of a composite element is a composite element with a nested
<many-to-one>
element. A mapping like this allows
you to map extra columns of a many-to-many association table to the
composite element class. The following is a many-to-many association
from Order
to Item
where
purchaseDate
, price
and
quantity
are properties of the association:
<class name="eg.Order" .... > .... <set name="purchasedItems" table="purchase_items" lazy="true"> <key column="order_id"> <composite-element class="eg.Purchase"> <property name="purchaseDate"/> <property name="price"/> <property name="quantity"/> <many-to-one name="item" class="eg.Item"/> <!-- class attribute is optional --> </composite-element> </set> </class>
Of course, there can't be a reference to the purchae on the other side, for
bidirectional association navigation. Remember that components are value types and
don't allow shared references. A single Purchase
can be in the
set of an Order
, but it can't be referenced by the Item
at the same time.
Even ternary (or quaternary, etc) associations are possible:
<class name="eg.Order" .... > .... <set name="purchasedItems" table="purchase_items" lazy="true"> <key column="order_id"> <composite-element class="eg.OrderLine"> <many-to-one name="purchaseDetails class="eg.Purchase"/> <many-to-one name="item" class="eg.Item"/> </composite-element> </set> </class>
Composite elements may appear in queries using the same syntax as associations to other entities.
The <composite-map-key>
element lets you map a
component class as the key of a Map
. Make sure you override
hashCode()
and equals()
correctly on
the component class.
You may use a component as an identifier of an entity class. Your component class must satisfy certain requirements:
It must implement java.io.Serializable
.
It must re-implement equals()
and
hashCode()
, consistently with the database's
notion of composite key equality.
Note: in Hibernate3, the second requirement is not an absolutely hard requirement of Hibernate. But do it anyway.
You can't use an IdentifierGenerator
to generate composite keys.
Instead the application must assign its own identifiers.
Use the <composite-id>
tag (with nested
<key-property>
elements) in place of the usual
<id>
declaration. For example, the
OrderLine
class has a primary key that depends upon
the (composite) primary key of Order
.
<class name="OrderLine"> <composite-id name="id" class="OrderLineId"> <key-property name="lineId"/> <key-property name="orderId"/> <key-property name="customerId"/> </composite-id> <property name="name"/> <many-to-one name="order" class="Order" insert="false" update="false"> <column name="orderId"/> <column name="customerId"/> </many-to-one> .... </class>
Now, any foreign keys referencing the OrderLine
table are also
composite. You must declare this in your mappings for other classes. An association
to OrderLine
would be mapped like this:
<many-to-one name="orderLine" class="OrderLine"> <!-- the "class" attribute is optional, as usual --> <column name="lineId"/> <column name="orderId"/> <column name="customerId"/> </many-to-one>
(Note that the <column>
tag is an alternative to the
column
attribute everywhere.)
A many-to-many
association to OrderLine
also
uses the composite foreign key:
<set name="undeliveredOrderLines"> <key column name="warehouseId"/> <many-to-many class="OrderLine"> <column name="lineId"/> <column name="orderId"/> <column name="customerId"/> </many-to-many> </set>
The collection of OrderLine
s in Order
would
use:
<set name="orderLines" inverse="true"> <key> <column name="orderId"/> <column name="customerId"/> </key> <one-to-many class="OrderLine"/> </set>
(The <one-to-many>
element, as usual, declares no columns.)
If OrderLine
itself owns a collection, it also has a composite
foreign key.
<class name="OrderLine"> .... .... <list name="deliveryAttempts"> <key> <!-- a collection inherits the composite key type --> <column name="lineId"/> <column name="orderId"/> <column name="customerId"/> </key> <list-index column="attemptId" base="1"/> <composite-element class="DeliveryAttempt"> ... </composite-element> </set> </class>
You may even map a property of type Map
:
<dynamic-component name="userAttributes"> <property name="foo" column="FOO" type="string"/> <property name="bar" column="BAR" type="integer"/> <many-to-one name="baz" class="Baz" column="BAZ_ID"/> </dynamic-component>
The semantics of a <dynamic-component>
mapping are identical
to <component>
. The advantage of this kind of mapping is
the ability to determine the actual properties of the bean at deployment time, just
by editing the mapping document. Runtime manipulation of the mapping document is
also possible, using a DOM parser. Even better, you can access (and change) Hibernate's
configuration-time metamodel via the Configuration
object.
Hibernate supports the three basic inheritance mapping strategies:
table per class hierarchy
table per subclass
table per concrete class
In addition, Hibernate supports a fourth, slightly different kind of polymorphism:
implicit polymorphism
It is possible to use different mapping strategies for different
branches of the same inheritance hierarchy, and then make use of implicit
polymorphism to achieve polymorphism across the whole hierarchy. However,
Hibernate does not support mixing <subclass>
,
and <joined-subclass>
and
<union-subclass>
mappings under the same root
<class>
element. It is possible to mix together
the table per hierarchy and table per subclass strategies, under the
the same <class>
element, by combining the
<subclass>
and <join>
elements (see below).
It is possible to define subclass
, union-subclass
,
and joined-subclass
mappings in separate mapping documents, directly beneath
hibernate-mapping
. This allows you to extend a class hierachy just by adding
a new mapping file. You must specify an extends
attribute in the subclass mapping,
naming a previously mapped superclass. Note: Previously this feature made the ordering of the mapping
documents important. Since Hibernate3, the ordering of mapping files does not matter when using the
extends keyword. The ordering inside a single mapping file still needs to be defined as superclasses
before subclasses.
<hibernate-mapping> <subclass name="DomesticCat" extends="Cat" discriminator-value="D"> <property name="name" type="string"/> </subclass> </hibernate-mapping>
Suppose we have an interface Payment
, with implementors
CreditCardPayment
, CashPayment
,
ChequePayment
. The table per hierarchy mapping would
look like:
<class name="Payment" table="PAYMENT"> <id name="id" type="long" column="PAYMENT_ID"> <generator class="native"/> </id> <discriminator column="PAYMENT_TYPE" type="string"/> <property name="amount" column="AMOUNT"/> ... <subclass name="CreditCardPayment" discriminator-value="CREDIT"> <property name="creditCardType" column="CCTYPE"/> ... </subclass> <subclass name="CashPayment" discriminator-value="CASH"> ... </subclass> <subclass name="ChequePayment" discriminator-value="CHEQUE"> ... </subclass> </class>
Exactly one table is required. There is one big limitation of this mapping
strategy: columns declared by the subclasses, such as CCTYPE
,
may not have NOT NULL
constraints.
A table per subclass mapping would look like:
<class name="Payment" table="PAYMENT"> <id name="id" type="long" column="PAYMENT_ID"> <generator class="native"/> </id> <property name="amount" column="AMOUNT"/> ... <joined-subclass name="CreditCardPayment" table="CREDIT_PAYMENT"> <key column="PAYMENT_ID"/> <property name="creditCardType" column="CCTYPE"/> ... </joined-subclass> <joined-subclass name="CashPayment" table="CASH_PAYMENT"> <key column="PAYMENT_ID"/> ... </joined-subclass> <joined-subclass name="ChequePayment" table="CHEQUE_PAYMENT"> <key column="PAYMENT_ID"/> ... </joined-subclass> </class>
Four tables are required. The three subclass tables have primary key associations to the superclass table (so the relational model is actually a one-to-one association).
Note that Hibernate's implementation of table per subclass requires
no discriminator column. Other object/relational mappers use a
different implementation of table per subclass which requires a type
discriminator column in the superclass table. The approach taken by
Hibernate is much more difficult to implement but arguably more
correct from a relational point of view. If you would like to use
a discriminator column with the table per subclass strategy, you
may combine the use of <subclass>
and
<join>
, as follow:
<class name="Payment" table="PAYMENT"> <id name="id" type="long" column="PAYMENT_ID"> <generator class="native"/> </id> <discriminator column="PAYMENT_TYPE" type="string"/> <property name="amount" column="AMOUNT"/> ... <subclass name="CreditCardPayment" discriminator-value="CREDIT"> <join table="CREDIT_PAYMENT"> <key column="PAYMENT_ID"/> <property name="creditCardType" column="CCTYPE"/> ... </join> </subclass> <subclass name="CashPayment" discriminator-value="CASH"> <join table="CASH_PAYMENT"> <key column="PAYMENT_ID"/> ... </join> </subclass> <subclass name="ChequePayment" discriminator-value="CHEQUE"> <join table="CHEQUE_PAYMENT" fetch="select"> <key column="PAYMENT_ID"/> ... </join> </subclass> </class>
The optional fetch="select"
declaration tells Hibernate
not to fetch the ChequePayment
subclass data using an
outer join when querying the superclass.
You may even mix the table per hierarchy and table per subclass strategies using this approach:
<class name="Payment" table="PAYMENT"> <id name="id" type="long" column="PAYMENT_ID"> <generator class="native"/> </id> <discriminator column="PAYMENT_TYPE" type="string"/> <property name="amount" column="AMOUNT"/> ... <subclass name="CreditCardPayment" discriminator-value="CREDIT"> <join table="CREDIT_PAYMENT"> <property name="creditCardType" column="CCTYPE"/> ... </join> </subclass> <subclass name="CashPayment" discriminator-value="CASH"> ... </subclass> <subclass name="ChequePayment" discriminator-value="CHEQUE"> ... </subclass> </class>
For any of these mapping strategies, a polymorphic association to the root
Payment
class is mapped using
<many-to-one>
.
<many-to-one name="payment" column="PAYMENT_ID" class="Payment"/>
There are two ways we could go about mapping the table per concrete class
strategy. The first is to use <union-subclass>
.
<class name="Payment"> <id name="id" type="long" column="PAYMENT_ID"> <generator class="sequence"/> </id> <property name="amount" column="AMOUNT"/> ... <union-subclass name="CreditCardPayment" table="CREDIT_PAYMENT"> <property name="creditCardType" column="CCTYPE"/> ... </union-subclass> <union-subclass name="CashPayment" table="CASH_PAYMENT"> ... </union-subclass> <union-subclass name="ChequePayment" table="CHEQUE_PAYMENT"> ... </union-subclass> </class>
Three tables are involved for the subclasses. Each table defines columns for all properties of the class, including inherited properties.
The limitation of this approach is that if a property is mapped on the superclass, the column name must be the same on all subclass tables. (We might relax this in a future release of Hibernate.) The identity generator strategy is not allowed in union subclass inheritance, indeed the primary key seed has to be shared accross all unioned subclasses of a hierarchy.
If your superclass is abstract, map it with abstract="true"
.
Of course, if it is not abstract, an additional table (defaults to
PAYMENT
in the example above) is needed to hold instances
of the superclass.
An alternative approach is to make use of implicit polymorphism:
<class name="CreditCardPayment" table="CREDIT_PAYMENT"> <id name="id" type="long" column="CREDIT_PAYMENT_ID"> <generator class="native"/> </id> <property name="amount" column="CREDIT_AMOUNT"/> ... </class> <class name="CashPayment" table="CASH_PAYMENT"> <id name="id" type="long" column="CASH_PAYMENT_ID"> <generator class="native"/> </id> <property name="amount" column="CASH_AMOUNT"/> ... </class> <class name="ChequePayment" table="CHEQUE_PAYMENT"> <id name="id" type="long" column="CHEQUE_PAYMENT_ID"> <generator class="native"/> </id> <property name="amount" column="CHEQUE_AMOUNT"/> ... </class>
Notice that nowhere do we mention the Payment
interface
explicitly. Also notice that properties of Payment
are
mapped in each of the subclasses. If you want to avoid duplication, consider
using XML entities
(e.g. [ <!ENTITY allproperties SYSTEM "allproperties.xml"> ]
in the DOCTYPE
declartion and
&allproperties;
in the mapping).
The disadvantage of this approach is that Hibernate does not generate SQL
UNION
s when performing polymorphic queries.
For this mapping strategy, a polymorphic association to Payment
is usually mapped using <any>
.
<any name="payment" meta-type="string" id-type="long"> <meta-value value="CREDIT" class="CreditCardPayment"/> <meta-value value="CASH" class="CashPayment"/> <meta-value value="CHEQUE" class="ChequePayment"/> <column name="PAYMENT_CLASS"/> <column name="PAYMENT_ID"/> </any>
There is one further thing to notice about this mapping. Since the subclasses
are each mapped in their own <class>
element (and since
Payment
is just an interface), each of the subclasses could
easily be part of another inheritance hierarchy! (And you can still use polymorphic
queries against the Payment
interface.)
<class name="CreditCardPayment" table="CREDIT_PAYMENT"> <id name="id" type="long" column="CREDIT_PAYMENT_ID"> <generator class="native"/> </id> <discriminator column="CREDIT_CARD" type="string"/> <property name="amount" column="CREDIT_AMOUNT"/> ... <subclass name="MasterCardPayment" discriminator-value="MDC"/> <subclass name="VisaPayment" discriminator-value="VISA"/> </class> <class name="NonelectronicTransaction" table="NONELECTRONIC_TXN"> <id name="id" type="long" column="TXN_ID"> <generator class="native"/> </id> ... <joined-subclass name="CashPayment" table="CASH_PAYMENT"> <key column="PAYMENT_ID"/> <property name="amount" column="CASH_AMOUNT"/> ... </joined-subclass> <joined-subclass name="ChequePayment" table="CHEQUE_PAYMENT"> <key column="PAYMENT_ID"/> <property name="amount" column="CHEQUE_AMOUNT"/> ... </joined-subclass> </class>
Once again, we don't mention Payment
explicitly. If we
execute a query against the Payment
interface - for
example, from Payment
- Hibernate
automatically returns instances of CreditCardPayment
(and its subclasses, since they also implement Payment
),
CashPayment
and ChequePayment
but
not instances of NonelectronicTransaction
.
There are certain limitations to the "implicit polymorphism" approach to
the table per concrete-class mapping strategy. There are somewhat less
restrictive limitations to <union-subclass>
mappings.
The following table shows the limitations of table per concrete-class mappings, and of implicit polymorphism, in Hibernate.
Table 9.1. Features of inheritance mappings
Inheritance strategy | Polymorphic many-to-one | Polymorphic one-to-one | Polymorphic one-to-many | Polymorphic many-to-many | Polymorphic load()/get() |
Polymorphic queries | Polymorphic joins | Outer join fetching |
---|---|---|---|---|---|---|---|---|
table per class-hierarchy | <many-to-one> |
<one-to-one> |
<one-to-many> |
<many-to-many> |
s.get(Payment.class, id) |
from Payment p |
from Order o join o.payment p |
supported |
table per subclass | <many-to-one> |
<one-to-one> |
<one-to-many> |
<many-to-many> |
s.get(Payment.class, id) |
from Payment p |
from Order o join o.payment p |
supported |
table per concrete-class (union-subclass) | <many-to-one> |
<one-to-one> |
<one-to-many> (for inverse="true" only)
|
<many-to-many> |
s.get(Payment.class, id) |
from Payment p |
from Order o join o.payment p |
supported |
table per concrete class (implicit polymorphism) | <any> |
not supported | not supported | <many-to-any> |
s.createCriteria(Payment.class).add( Restrictions.idEq(id) ).uniqueResult() |
from Payment p |
not supported | not supported |
Hibernate is a full object/relational mapping solution that not only shields
the developer from the details of the underlying database management
system, but also offers state management of objects. This is,
contrary to the management of SQL statements
in common JDBC/SQL
persistence layers, a very natural object-oriented view of persistence in Java
applications.
In other words, Hibernate application developers should always think about the state of their objects, and not necessarily about the execution of SQL statements. This part is taken care of by Hibernate and is only relevant for the application developer when tuning the performance of the system.
Hibernate defines and supports the following object states:
Transient - an object is transient if it has just
been instantiated using the new
operator, and it
is not associated with a Hibernate Session
. It has no
persistent representation in the database and no identifier value has been
assigned. Transient instances will be destroyed by the garbage collector if
the application doesn't hold a reference anymore. Use the Hibernate
Session
to make an object persistent (and let Hibernate
take care of the SQL statements that need to be executed for this transition).
Persistent - a persistent instance has a representation
in the database and an identifier value. It might just have been saved or loaded,
however, it is by definition in the scope of a Session
.
Hibernate will detect any changes made to an object in persistent state and
synchronize the state with the database when the unit of work completes.
Developers don't execute manual UPDATE
statements, or
DELETE
statements when an object should be made transient.
Detached - a detached instance is an object that has been
persistent, but its Session
has been closed. The reference
to the object is still valid, of course, and the detached instance might even
be modified in this state. A detached instance can be reattached to a new
Session
at a later point in time, making it (and all the
modifications) persistent again. This feature enables a programming model for
long running units of work that require user think-time. We call them
application transactions, i.e. a unit of work from the
point of view of the user.
We'll now discuss the states and state transitions (and the Hibernate methods that trigger a transition) in more detail.
Newly instantiated instances of a a persistent class are considered transient by Hibernate. We can make a transient instance persistent by associating it with a session:
DomesticCat fritz = new DomesticCat(); fritz.setColor(Color.GINGER); fritz.setSex('M'); fritz.setName("Fritz"); Long generatedId = (Long) sess.save(fritz);
If Cat
has a generated identifier, the identifier is
generated and assigned to the cat
when save()
is called. If Cat
has an assigned
identifier, or a composite key, the identifier should be assigned to
the cat
instance before calling save()
.
You may also use persist()
instead of save()
,
with the semantics defined in the EJB3 early draft.
persist()
makes a transient instance persistent.
However, it doesn't guarantee that the identifier value will be assigned to
the persistent instance immediately, the assignment might happen at flush time.
persist()
also guarantees that it will not execute an
INSERT
statement if it is called outside of transaction
boundaries. This is useful in long-running conversations with an extended
Session/persistence context.
save()
does guarantee to return an identifier. If an INSERT
has to be executed to get the identifier ( e.g. "identity" generator, not
"sequence"), this INSERT happens immediately, no matter if you are inside or
outside of a transaction. This is problematic in a long-running conversation
with an extended Session/persistence context.
Alternatively, you may assign the identifier using an overloaded version
of save()
.
DomesticCat pk = new DomesticCat(); pk.setColor(Color.TABBY); pk.setSex('F'); pk.setName("PK"); pk.setKittens( new HashSet() ); pk.addKitten(fritz); sess.save( pk, new Long(1234) );
If the object you make persistent has associated objects (e.g. the
kittens
collection in the previous example),
these objects may be made persistent in any order you like unless you
have a NOT NULL
constraint upon a foreign key column.
There is never a risk of violating foreign key constraints. However, you
might violate a NOT NULL
constraint if you
save()
the objects in the wrong order.
Usually you don't bother with this detail, as you'll very likely use Hibernate's
transitive persistence feature to save the associated
objects automatically. Then, even NOT NULL
constraint violations don't occur - Hibernate will take care of everything.
Transitive persistence is discussed later in this chapter.
The load()
methods of Session
gives you
a way to retrieve a persistent instance if you already know its identifier.
load()
takes a class object and will load the state into
a newly instantiated instance of that class, in persistent state.
Cat fritz = (Cat) sess.load(Cat.class, generatedId);
// you need to wrap primitive identifiers long id = 1234; DomesticCat pk = (DomesticCat) sess.load( DomesticCat.class, new Long(id) );
Alternatively, you can load state into a given instance:
Cat cat = new DomesticCat(); // load pk's state into cat sess.load( cat, new Long(pkId) ); Set kittens = cat.getKittens();
Note that load()
will throw an unrecoverable exception if
there is no matching database row. If the class is mapped with a proxy,
load()
just returns an uninitialized proxy and does not
actually hit the database until you invoke a method of the proxy. This
behaviour is very useful if you wish to create an association to an object
without actually loading it from the database. It also allows multiple
instances to be loaded as a batch if batch-size
is
defined for the class mapping.
If you are not certain that a matching row exists, you should use the
get()
method, which hits the database immediately and
returns null if there is no matching row.
Cat cat = (Cat) sess.get(Cat.class, id); if (cat==null) { cat = new Cat(); sess.save(cat, id); } return cat;
You may even load an object using an SQL SELECT ... FOR UPDATE
,
using a LockMode
. See the API documentation for more information.
Cat cat = (Cat) sess.get(Cat.class, id, LockMode.UPGRADE);
Note that any associated instances or contained collections are
not selected FOR UPDATE
, unless you decide
to specify lock
or all
as a
cascade style for the association.
It is possible to re-load an object and all its collections at any time, using the
refresh()
method. This is useful when database triggers are used to
initialize some of the properties of the object.
sess.save(cat); sess.flush(); //force the SQL INSERT sess.refresh(cat); //re-read the state (after the trigger executes)
An important question usually appears at this point: How much does Hibernate load
from the database and how many SQL SELECT
s will it use? This
depends on the fetching strategy and is explained in
Section 19.1, “Fetching strategies”.
If you don't know the identifiers of the objects you are looking for, you need a query. Hibernate supports an easy-to-use but powerful object oriented query language (HQL). For programmatic query creation, Hibernate supports a sophisticated Criteria and Example query feature (QBC and QBE). You may also express your query in the native SQL of your database, with optional support from Hibernate for result set conversion into objects.
HQL and native SQL queries are represented with an instance of org.hibernate.Query
.
This interface offers methods for parameter binding, result set handling, and for the execution
of the actual query. You always obtain a Query
using the current
Session
:
List cats = session.createQuery( "from Cat as cat where cat.birthdate < ?") .setDate(0, date) .list(); List mothers = session.createQuery( "select mother from Cat as cat join cat.mother as mother where cat.name = ?") .setString(0, name) .list(); List kittens = session.createQuery( "from Cat as cat where cat.mother = ?") .setEntity(0, pk) .list(); Cat mother = (Cat) session.createQuery( "select cat.mother from Cat as cat where cat = ?") .setEntity(0, izi) .uniqueResult();]] Query mothersWithKittens = (Cat) session.createQuery( "select mother from Cat as mother left join fetch mother.kittens"); Set uniqueMothers = new HashSet(mothersWithKittens.list());
A query is usually executed by invoking list()
, the
result of the query will be loaded completely into a collection in memory.
Entity instances retrieved by a query are in persistent state. The
uniqueResult()
method offers a shortcut if you
know your query will only return a single object. Note that queries that
make use of eager fetching of collections usually return duplicates of
the root objects (but with their collections initialized). You can filter
these duplicates simply through a Set
.
Occasionally, you might be able to achieve better performance by
executing the query using the iterate()
method.
This will only usually be the case if you expect that the actual
entity instances returned by the query will already be in the session
or second-level cache. If they are not already cached,
iterate()
will be slower than list()
and might require many database hits for a simple query, usually
1 for the initial select which only returns identifiers,
and n additional selects to initialize the actual instances.
// fetch ids Iterator iter = sess.createQuery("from eg.Qux q order by q.likeliness").iterate(); while ( iter.hasNext() ) { Qux qux = (Qux) iter.next(); // fetch the object // something we couldnt express in the query if ( qux.calculateComplicatedAlgorithm() ) { // delete the current instance iter.remove(); // dont need to process the rest break; } }
Hibernate queries sometimes return tuples of objects, in which case each tuple is returned as an array:
Iterator kittensAndMothers = sess.createQuery( "select kitten, mother from Cat kitten join kitten.mother mother") .list() .iterator(); while ( kittensAndMothers.hasNext() ) { Object[] tuple = (Object[]) kittensAndMothers.next(); Cat kitten = (Cat) tuple[0]; Cat mother = (Cat) tuple[1]; .... }
Queries may specify a property of a class in the select
clause.
They may even call SQL aggregate functions. Properties or aggregates are considered
"scalar" results (and not entities in persistent state).
Iterator results = sess.createQuery( "select cat.color, min(cat.birthdate), count(cat) from Cat cat " + "group by cat.color") .list() .iterator(); while ( results.hasNext() ) { Object[] row = (Object[]) results.next(); Color type = (Color) row[0]; Date oldest = (Date) row[1]; Integer count = (Integer) row[2]; ..... }
Methods on Query
are provided for binding values to
named parameters or JDBC-style ?
parameters.
Contrary to JDBC, Hibernate numbers parameters from zero.
Named parameters are identifiers of the form :name
in
the query string. The advantages of named parameters are:
named parameters are insensitive to the order they occur in the query string
they may occur multiple times in the same query
they are self-documenting
//named parameter (preferred) Query q = sess.createQuery("from DomesticCat cat where cat.name = :name"); q.setString("name", "Fritz"); Iterator cats = q.iterate();
//positional parameter Query q = sess.createQuery("from DomesticCat cat where cat.name = ?"); q.setString(0, "Izi"); Iterator cats = q.iterate();
//named parameter list List names = new ArrayList(); names.add("Izi"); names.add("Fritz"); Query q = sess.createQuery("from DomesticCat cat where cat.name in (:namesList)"); q.setParameterList("namesList", names); List cats = q.list();
If you need to specify bounds upon your result set (the maximum number of rows
you want to retrieve and / or the first row you want to retrieve) you should
use methods of the Query
interface:
Query q = sess.createQuery("from DomesticCat cat"); q.setFirstResult(20); q.setMaxResults(10); List cats = q.list();
Hibernate knows how to translate this limit query into the native SQL of your DBMS.
If your JDBC driver supports scrollable ResultSet
s, the
Query
interface may be used to obtain a
ScrollableResults
object, which allows flexible
navigation of the query results.
Query q = sess.createQuery("select cat.name, cat from DomesticCat cat " + "order by cat.name"); ScrollableResults cats = q.scroll(); if ( cats.first() ) { // find the first name on each page of an alphabetical list of cats by name firstNamesOfPages = new ArrayList(); do { String name = cats.getString(0); firstNamesOfPages.add(name); } while ( cats.scroll(PAGE_SIZE) ); // Now get the first page of cats pageOfCats = new ArrayList(); cats.beforeFirst(); int i=0; while( ( PAGE_SIZE > i++ ) && cats.next() ) pageOfCats.add( cats.get(1) ); } cats.close()
Note that an open database connection (and cursor) is required for this
functionality, use setMaxResult()
/setFirstResult()
if you need offline pagination functionality.
You may also define named queries in the mapping document. (Remember to use a
CDATA
section if your query contains characters that could
be interpreted as markup.)
<query name="ByNameAndMaximumWeight"><![CDATA[ from eg.DomesticCat as cat where cat.name = ? and cat.weight > ? ] ]></query>
Parameter binding and executing is done programatically:
Query q = sess.getNamedQuery("ByNameAndMaximumWeight"); q.setString(0, name); q.setInt(1, minWeight); List cats = q.list();
Note that the actual program code is independent of the query language that is used, you may also define native SQL queries in metadata, or migrate existing queries to Hibernate by placing them in mapping files.
Also note that a query declaration inside a <hibernate-mapping>
element requires a global unique name for the query, while a query declaration inside a
<class>
element is made unique automatically by prepending the
fully qualified name of the class, for example
eg.Cat.ByNameAndMaximumWeight
.
A collection filter is a special type of query that may be applied to
a persistent collection or array. The query string may refer to this
,
meaning the current collection element.
Collection blackKittens = session.createFilter( pk.getKittens(), "where this.color = ?") .setParameter( Color.BLACK, Hibernate.custom(ColorUserType.class) ) .list() );
The returned collection is considered a bag, and it's a copy of the given collection. The original collection is not modified (this is contrary to the implication of the name "filter", but consistent with expected behavior).
Observe that filters do not require a from
clause (though they may have
one if required). Filters are not limited to returning the collection elements themselves.
Collection blackKittenMates = session.createFilter( pk.getKittens(), "select this.mate where this.color = eg.Color.BLACK.intValue") .list();
Even an empty filter query is useful, e.g. to load a subset of elements in a huge collection:
Collection tenKittens = session.createFilter( mother.getKittens(), "") .setFirstResult(0).setMaxResults(10) .list();
HQL is extremely powerful but some developers prefer to build queries dynamically,
using an object-oriented API, rather than building query strings. Hibernate provides
an intuitive Criteria
query API for these cases:
Criteria crit = session.createCriteria(Cat.class); crit.add( Restrictions.eq( "color", eg.Color.BLACK ) ); crit.setMaxResults(10); List cats = crit.list();
The Criteria
and the associated Example
API are discussed in more detail in Chapter 15, Criteria Queries.
You may express a query in SQL, using createSQLQuery()
and
let Hibernate take care of the mapping from result sets to objects. Note
that you may at any time call session.connection()
and
use the JDBC Connection
directly. If you chose to use the
Hibernate API, you must enclose SQL aliases in braces:
List cats = session.createSQLQuery("SELECT {cat.*} FROM CAT {cat} WHERE ROWNUM<10") .addEntity("cat", Cat.class) .list();
List cats = session.createSQLQuery( "SELECT {cat}.ID AS {cat.id}, {cat}.SEX AS {cat.sex}, " + "{cat}.MATE AS {cat.mate}, {cat}.SUBCLASS AS {cat.class}, ... " + "FROM CAT {cat} WHERE ROWNUM<10") .addEntity("cat", Cat.class) .list()
SQL queries may contain named and positional parameters, just like Hibernate queries. More information about native SQL queries in Hibernate can be found in Chapter 16, Native SQL.
Transactional persistent instances (ie. objects loaded, saved, created or
queried by the Session
) may be manipulated by the application
and any changes to persistent state will be persisted when the Session
is flushed (discussed later in this chapter). There is no need
to call a particular method (like update()
, which has a different
purpose) to make your modifications persistent. So the most straightforward way to update
the state of an object is to load()
it,
and then manipulate it directly, while the Session
is open:
DomesticCat cat = (DomesticCat) sess.load( Cat.class, new Long(69) ); cat.setName("PK"); sess.flush(); // changes to cat are automatically detected and persisted
Sometimes this programming model is inefficient since it would require both an SQL
SELECT
(to load an object) and an SQL UPDATE
(to persist its updated state) in the same session. Therefore Hibernate offers an
alternate approach, using detached instances.
Note that Hibernate does not offer its own API for direct execution of
UPDATE
or DELETE
statements. Hibernate is a
state management service, you don't have to think in
statements to use it. JDBC is a perfect API for executing
SQL statements, you can get a JDBC Connection
at any time
by calling session.connection()
. Furthermore, the notion
of mass operations conflicts with object/relational mapping for online
transaction processing-oriented applications. Future versions of Hibernate
may however provide special mass operation functions. See Chapter 13, Batch processing
for some possible batch operation tricks.
Many applications need to retrieve an object in one transaction, send it to the UI layer for manipulation, then save the changes in a new transaction. Applications that use this kind of approach in a high-concurrency environment usually use versioned data to ensure isolation for the "long" unit of work.
Hibernate supports this model by providing for reattachment of detached instances
using the Session.update()
or Session.merge()
methods:
// in the first session Cat cat = (Cat) firstSession.load(Cat.class, catId); Cat potentialMate = new Cat(); firstSession.save(potentialMate); // in a higher layer of the application cat.setMate(potentialMate); // later, in a new session secondSession.update(cat); // update cat secondSession.update(mate); // update mate
If the Cat
with identifier catId
had already
been loaded by secondSession
when the application tried to
reattach it, an exception would have been thrown.
Use update()
if you are sure that the session does
not contain an already persistent instance with the same identifier, and
merge()
if you want to merge your modifications at any time
without consideration of the state of the session. In other words, update()
is usually the first method you would call in a fresh session, ensuring that
reattachment of your detached instances is the first operation that is executed.
The application should individually update()
detached instances
reachable from the given detached instance if and only if it wants
their state also updated. This can be automated of course, using transitive
persistence, see Section 10.11, “Transitive persistence”.
The lock()
method also allows an application to reassociate
an object with a new session. However, the detached instance has to be unmodified!
//just reassociate: sess.lock(fritz, LockMode.NONE); //do a version check, then reassociate: sess.lock(izi, LockMode.READ); //do a version check, using SELECT ... FOR UPDATE, then reassociate: sess.lock(pk, LockMode.UPGRADE);
Note that lock()
can be used with various
LockMode
s, see the API documentation and the
chapter on transaction handling for more information. Reattachment is not
the only usecase for lock()
.
Other models for long units of work are discussed in Section 11.3, “Optimistic concurrency control”.
Hibernate users have requested a general purpose method that either saves a
transient instance by generating a new identifier or updates/reattaches
the detached instances associated with its current identifier.
The saveOrUpdate()
method implements this functionality.
// in the first session Cat cat = (Cat) firstSession.load(Cat.class, catID); // in a higher tier of the application Cat mate = new Cat(); cat.setMate(mate); // later, in a new session secondSession.saveOrUpdate(cat); // update existing state (cat has a non-null id) secondSession.saveOrUpdate(mate); // save the new instance (mate has a null id)
The usage and semantics of saveOrUpdate()
seems to be confusing
for new users. Firstly, so long as you are not trying to use instances from one session
in another new session, you should not need to use update()
,
saveOrUpdate()
, or merge()
. Some whole
applications will never use either of these methods.
Usually update()
or saveOrUpdate()
are used in
the following scenario:
the application loads an object in the first session
the object is passed up to the UI tier
some modifications are made to the object
the object is passed back down to the business logic tier
the application persists these modifications by calling
update()
in a second session
saveOrUpdate()
does the following:
if the object is already persistent in this session, do nothing
if another object associated with the session has the same identifier, throw an exception
if the object has no identifier property, save()
it
if the object's identifier has the value assigned to a newly instantiated
object, save()
it
if the object is versioned (by a <version>
or
<timestamp>
), and the version property value
is the same value assigned to a newly instantiated object,
save()
it
otherwise update()
the object
and merge()
is very different:
if there is a persistent instance with the same identifier currently associated with the session, copy the state of the given object onto the persistent instance
if there is no persistent instance currently associated with the session, try to load it from the database, or create a new persistent instance
the persistent instance is returned
the given instance does not become associated with the session, it remains detached
Session.delete()
will remove an object's state from the database.
Of course, your application might still hold a reference to a deleted object.
It's best to think of delete()
as making a persistent instance
transient.
sess.delete(cat);
You may delete objects in any order you like, without risk of foreign key
constraint violations. It is still possible to violate a NOT
NULL
constraint on a foreign key column by deleting objects in
the wrong order, e.g. if you delete the parent, but forget to delete the
children.
It is occasionally useful to be able to take a graph of persistent instances and make them persistent in a different datastore, without regenerating identifier values.
//retrieve a cat from one database Session session1 = factory1.openSession(); Transaction tx1 = session1.beginTransaction(); Cat cat = session1.get(Cat.class, catId); tx1.commit(); session1.close(); //reconcile with a second database Session session2 = factory2.openSession(); Transaction tx2 = session2.beginTransaction(); session2.replicate(cat, ReplicationMode.LATEST_VERSION); tx2.commit(); session2.close();
The ReplicationMode
determines how replicate()
will deal with conflicts with existing rows in the database.
ReplicationMode.IGNORE
- ignore the object when there is
an existing database row with the same identifier
ReplicationMode.OVERWRITE
- overwrite any existing database
row with the same identifier
ReplicationMode.EXCEPTION
- throw an exception if there is
an existing database row with the same identifier
ReplicationMode.LATEST_VERSION
- overwrite the row if its
version number is earlier than the version number of the object, or ignore
the object otherwise
Usecases for this feature include reconciling data entered into different database instances, upgrading system configuration information during product upgrades, rolling back changes made during non-ACID transactions and more.
From time to time the Session
will execute the SQL statements
needed to synchronize the JDBC connection's state with the state of objects held in
memory. This process, flush, occurs by default at the following
points
before some query executions
from org.hibernate.Transaction.commit()
from Session.flush()
The SQL statements are issued in the following order
all entity insertions, in the same order the corresponding objects
were saved using Session.save()
all entity updates
all collection deletions
all collection element deletions, updates and insertions
all collection insertions
all entity deletions, in the same order the corresponding objects
were deleted using Session.delete()
(An exception is that objects using native
ID generation are
inserted when they are saved.)
Except when you explicity flush()
, there are absolutely no
guarantees about when the Session
executes
the JDBC calls, only the order in which they are executed.
However, Hibernate does guarantee that the Query.list(..)
will never return stale data; nor will they return the wrong data.
It is possible to change the default behavior so that flush occurs less frequently.
The FlushMode
class defines three different modes: only flush
at commit time (and only when the Hibernate Transaction
API
is used), flush automatically using the explained routine, or never flush unless
flush()
is called explicitly. The last mode is useful for long running
units of work, where a Session
is kept open and disconnected for
a long time (see Section 11.3.2, “Extended session and automatic versioning”).
sess = sf.openSession(); Transaction tx = sess.beginTransaction(); sess.setFlushMode(FlushMode.COMMIT); // allow queries to return stale state Cat izi = (Cat) sess.load(Cat.class, id); izi.setName(iznizi); // might return stale data sess.find("from Cat as cat left outer join cat.kittens kitten"); // change to izi is not flushed! ... tx.commit(); // flush occurs sess.close();
During flush, an exception might occur (e.g. if a DML operation violates a constraint). Since handling exceptions involves some understanding of Hibernate's transactional behavior, we discuss it in Chapter 11, Transactions And Concurrency.
It is quite cumbersome to save, delete, or reattach individual objects, especially if you deal with a graph of associated objects. A common case is a parent/child relationship. Consider the following example:
If the children in a parent/child relationship would be value typed (e.g. a collection of addresses or strings), their life cycle would depend on the parent and no further action would be required for convenient "cascading" of state changes. When the parent is saved, the value-typed child objects are saved as well, when the parent is deleted, the children will be deleted, etc. This even works for operations such as the removal of a child from the collection; Hibernate will detect this and, since value-typed objects can't have shared references, delete the child from the database.
Now consider the same scenario with parent and child objects being entities, not value-types (e.g. categories and items, or parent and child cats). Entities have their own life cycle, support shared references (so removing an entity from the collection does not mean it can be deleted), and there is by default no cascading of state from one entity to any other associated entities. Hibernate does not implement persistence by reachability by default.
For each basic operation of the Hibernate session - including persist(), merge(),
saveOrUpdate(), delete(), lock(), refresh(), evict(), replicate()
- there is a
corresponding cascade style. Respectively, the cascade styles are named create,
merge, save-update, delete, lock, refresh, evict, replicate
. If you want an
operation to be cascaded along an association, you must indicate that in the mapping
document. For example:
<one-to-one name="person" cascade="persist"/>
Cascade styles my be combined:
<one-to-one name="person" cascade="persist,delete,lock"/>
You may even use cascade="all"
to specify that all
operations should be cascaded along the association. The default cascade="none"
specifies that no operations are to be cascaded.
A special cascade style, delete-orphan
, applies only to one-to-many
associations, and indicates that the delete()
operation should
be applied to any child object that is removed from the association.
Recommendations:
It doesn't usually make sense to enable cascade on a <many-to-one>
or <many-to-many>
association. Cascade is often useful for
<one-to-one>
and <one-to-many>
associations.
If the child object's lifespan is bounded by the lifespan of the parent
object, make it a life cycle object by specifying
cascade="all,delete-orphan"
.
Otherwise, you might not need cascade at all. But if you think that you will often be
working with the parent and children together in the same transaction, and you want to save
yourself some typing, consider using cascade="persist,merge,save-update"
.
Mapping an association (either a single valued association, or a collection) with
cascade="all"
marks the association as a
parent/child style relationship where save/update/delete of the
parent results in save/update/delete of the child or children.
Futhermore, a mere reference to a child from a persistent parent will result in
save/update of the child. This metaphor is incomplete, however. A child which becomes
unreferenced by its parent is not automatically deleted, except
in the case of a <one-to-many>
association mapped with
cascade="delete-orphan"
. The precise semantics of cascading
operations for a parent/child relationship are as follows:
If a parent is passed to persist()
, all children are passed to
persist()
If a parent is passed to merge()
, all children are passed to
merge()
If a parent is passed to save()
, update()
or
saveOrUpdate()
, all children are passed to saveOrUpdate()
If a transient or detached child becomes referenced by a persistent parent,
it is passed to saveOrUpdate()
If a parent is deleted, all children are passed to delete()
If a child is dereferenced by a persistent parent, nothing
special happens - the application should explicitly delete
the child if necessary - unless cascade="delete-orphan"
,
in which case the "orphaned" child is deleted.
Finally, note that cascading of operations can be applied to an object graph at
call time or at flush time. All operations,
if enabled, are cascaded to associated entities reachable when the operation is
executed. However, save-upate
and delete-orphan
are transitive for all associated entities reachable during flush of the
Session
.
Hibernate requires a very rich meta-level model of all entity and value types. From time to time, this model is very useful to the application itself. For example, the application might use Hibernate's metadata to implement a "smart" deep-copy algorithm that understands which objects should be copied (eg. mutable value types) and which should not (eg. immutable value types and, possibly, associated entities).
Hibernate exposes metadata via the ClassMetadata
and
CollectionMetadata
interfaces and the Type
hierarchy. Instances of the metadata interfaces may be obtained from the
SessionFactory
.
Cat fritz = ......; ClassMetadata catMeta = sessionfactory.getClassMetadata(Cat.class); Object[] propertyValues = catMeta.getPropertyValues(fritz); String[] propertyNames = catMeta.getPropertyNames(); Type[] propertyTypes = catMeta.getPropertyTypes(); // get a Map of all properties which are not collections or associations Map namedValues = new HashMap(); for ( int i=0; i<propertyNames.length; i++ ) { if ( !propertyTypes[i].isEntityType() && !propertyTypes[i].isCollectionType() ) { namedValues.put( propertyNames[i], propertyValues[i] ); } }
The most important point about Hibernate and concurrency control is that it is very easy to understand. Hibernate directly uses JDBC connections and JTA resources without adding any additional locking behavior. We highly recommend you spend some time with the JDBC, ANSI, and transaction isolation specification of your database management system.
Hibernate does not lock objects in memory. Your application can expect the behavior as
defined by the isolation level of your database transactions. Note that thanks to the
Session
, which is also a transaction-scoped cache, Hibernate
provides repeatable reads for lookup by identifier and entity queries (not
reporting queries that return scalar values).
In addition to versioning for automatic optimistic concurrency control, Hibernate also
offers a (minor) API for pessimistic locking of rows, using the
SELECT FOR UPDATE
syntax. Optimistic concurrency control and
this API are discussed later in this chapter.
We start the discussion of concurrency control in Hibernate with the granularity of
Configuration
, SessionFactory
, and
Session
, as well as database transactions and long conversations.
A SessionFactory
is an expensive-to-create, threadsafe object
intended to be shared by all application threads. It is created once, usually on
application startup, from a Configuration
instance.
A Session
is an inexpensive, non-threadsafe object that should be
used once, for a single request, a conversation, single unit of work, and then discarded.
A Session
will not obtain a JDBC Connection
(or a Datasource
) unless it is needed, hence consume no
resources until used.
To complete this picture you also have to think about database transactions. A database transaction has to be as short as possible, to reduce lock contention in the database. Long database transactions will prevent your application from scaling to highly concurrent load. Hence, it is almost never good design to hold a database transaction open during user think time, until the unit of work is complete.
What is the scope of a unit of work? Can a single Hibernate Session
span several database transactions or is this a one-to-one relationship of scopes? When
should you open and close a Session
and how do you demarcate the
database transaction boundaries?
First, don't use the session-per-operation antipattern, that is,
don't open and close a Session
for every simple database call in
a single thread! Of course, the same is true for database transactions. Database calls
in an application are made using a planned sequence, they are grouped into atomic
units of work. (Note that this also means that auto-commit after every single
SQL statement is useless in an application, this mode is intended for ad-hoc SQL
console work. Hibernate disables, or expects the application server to do so,
auto-commit mode immediately.) Database transactions are never optional, all
communication with a database has to occur inside a transaction, no matter if
you read or write data. As explained, auto-commit behavior for reading data
should be avoided, as many small transactions are unlikely to perform better than
one clearly defined unit of work. The latter is also much more maintainable
and extensible.
The most common pattern in a multi-user client/server application is
session-per-request. In this model, a request from the client
is sent to the server (where the Hibernate persistence layer runs), a new Hibernate
Session
is opened, and all database operations are executed in this unit
of work. Once the work has been completed (and the response for the client has been prepared),
the session is flushed and closed. You would also use a single database transaction to
serve the clients request, starting and committing it when you open and close the
Session
. The relationship between the two is one-to-one and this
model is a perfect fit for many applications.
The challenge lies in the implementation. Hibernate provides built-in management of
the "current session" to simplify this pattern. All you have to do is start a
transaction when a server request has to be processed, and end the transaction
before the response is sent to the client. You can do this in any way you
like, common solutions are ServletFilter
, AOP interceptor with a
pointcut on the service methods, or a proxy/interception container. An EJB container
is a standardized way to implement cross-cutting aspects such as transaction
demarcation on EJB session beans, declaratively with CMT. If you decide to
use programmatic transaction demarcation, prefer the Hibernate Transaction
API shown later in this chapter, for ease of use and code portability.
Your application code can access a "current session" to process the request
by simply calling sessionFactory.getCurrentSession()
anywhere
and as often as needed. You will always get a Session
scoped
to the current database transaction. This has to be configured for either
resource-local or JTA environments, see Section 2.5, “Contextual Sessions”.
Sometimes it is convenient to extend the scope of a Session
and
database transaction until the "view has been rendered". This is especially useful
in servlet applications that utilize a separate rendering phase after the request
has been processed. Extending the database transaction until view rendering is
complete is easy to do if you implement your own interceptor. However, it is not
easily doable if you rely on EJBs with container-managed transactions, as a
transaction will be completed when an EJB method returns, before rendering of any
view can start. See the Hibernate website and forum for tips and examples around
this Open Session in View pattern.
The session-per-request pattern is not the only useful concept you can use to design units of work. Many business processes require a whole series of interactions with the user interleaved with database accesses. In web and enterprise applications it is not acceptable for a database transaction to span a user interaction. Consider the following example:
The first screen of a dialog opens, the data seen by the user has been loaded in
a particular Session
and database transaction. The user is free to
modify the objects.
The user clicks "Save" after 5 minutes and expects his modifications to be made persistent; he also expects that he was the only person editing this information and that no conflicting modification can occur.
We call this unit of work, from the point of view of the user, a long running conversation (or application transaction). There are many ways how you can implement this in your application.
A first naive implementation might keep the Session
and database
transaction open during user think time, with locks held in the database to prevent
concurrent modification, and to guarantee isolation and atomicity. This is of course
an anti-pattern, since lock contention would not allow the application to scale with
the number of concurrent users.
Clearly, we have to use several database transactions to implement the conversation. In this case, maintaining isolation of business processes becomes the partial responsibility of the application tier. A single conversation usually spans several database transactions. It will be atomic if only one of these database transactions (the last one) stores the updated data, all others simply read data (e.g. in a wizard-style dialog spanning several request/response cycles). This is easier to implement than it might sound, especially if you use Hibernate's features:
Automatic Versioning - Hibernate can do automatic optimistic concurrency control for you, it can automatically detect if a concurrent modification occurred during user think time. Usually we only check at the end of the conversation.
Detached Objects - If you decide to use the already discussed session-per-request pattern, all loaded instances will be in detached state during user think time. Hibernate allows you to reattach the objects and persist the modifications, the pattern is called session-per-request-with-detached-objects. Automatic versioning is used to isolate concurrent modifications.
Extended (or Long) Session - The Hibernate
Session
may be disconnected from the underlying JDBC
connection after the database transaction has been committed, and reconnected
when a new client request occurs. This pattern is known as
session-per-conversation and makes
even reattachment unnecessary. Automatic versioning is used to isolate
concurrent modifications and the Session
is usually
not allowed to be flushed automatically, but explicitly.
Both session-per-request-with-detached-objects and session-per-conversation have advantages and disadvantages, we discuss them later in this chapter in the context of optimistic concurrency control.
An application may concurrently access the same persistent state in two
different Session
s. However, an instance of a persistent class
is never shared between two Session
instances. Hence there are
two different notions of identity:
foo.getId().equals( bar.getId() )
foo==bar
Then for objects attached to a particular Session
(i.e. in the scope of a Session
) the two notions are equivalent, and
JVM identity for database identity is guaranteed by Hibernate. However, while the application
might concurrently access the "same" (persistent identity) business object in two different
sessions, the two instances will actually be "different" (JVM identity). Conflicts are
resolved using (automatic versioning) at flush/commit time, using an optimistic approach.
This approach leaves Hibernate and the database to worry about concurrency; it also provides
the best scalability, since guaranteeing identity in single-threaded units of work only doesn't
need expensive locking or other means of synchronization. The application never needs to
synchronize on any business object, as long as it sticks to a single thread per
Session
. Within a Session
the application may safely use
==
to compare objects.
However, an application that uses ==
outside of a Session
,
might see unexpected results. This might occur even in some unexpected places, for example,
if you put two detached instances into the same Set
. Both might have the same
database identity (i.e. they represent the same row), but JVM identity is by definition not
guaranteed for instances in detached state. The developer has to override the equals()
and hashCode()
methods in persistent classes and implement
his own notion of object equality. There is one caveat: Never use the database
identifier to implement equality, use a business key, a combination of unique, usually
immutable, attributes. The database identifier will change if a transient object is made
persistent. If the transient instance (usually together with detached instances) is held in a
Set
, changing the hashcode breaks the contract of the Set
.
Attributes for business keys don't have to be as stable as database primary keys, you only
have to guarantee stability as long as the objects are in the same Set
. See
the Hibernate website for a more thorough discussion of this issue. Also note that this is not
a Hibernate issue, but simply how Java object identity and equality has to be implemented.
Never use the anti-patterns session-per-user-session or session-per-application (of course, there are rare exceptions to this rule). Note that some of the following issues might also appear with the recommended patterns, make sure you understand the implications before making a design decision:
A Session
is not thread-safe. Things which are supposed to work
concurrently, like HTTP requests, session beans, or Swing workers, will cause race
conditions if a Session
instance would be shared. If you keep your
Hibernate Session
in your HttpSession
(discussed
later), you should consider synchronizing access to your Http session. Otherwise,
a user that clicks reload fast enough may use the same Session
in
two concurrently running threads.
An exception thrown by Hibernate means you have to rollback your database transaction
and close the Session
immediately (discussed later in more detail).
If your Session
is bound to the application, you have to stop
the application. Rolling back the database transaction doesn't put your business
objects back into the state they were at the start of the transaction. This means the
database state and the business objects do get out of sync. Usually this is not a
problem, because exceptions are not recoverable and you have to start over after
rollback anyway.
The Session
caches every object that is in persistent state (watched
and checked for dirty state by Hibernate). This means it grows endlessly until you
get an OutOfMemoryException, if you keep it open for a long time or simply load too
much data. One solution for this is to call clear()
and evict()
to manage the Session
cache, but you most likely should consider a
Stored Procedure if you need mass data operations. Some solutions are shown in
Chapter 13, Batch processing. Keeping a Session
open for the duration
of a user session also means a high probability of stale data.
Database (or system) transaction boundaries are always necessary. No communication with the database can occur outside of a database transaction (this seems to confuse many developers who are used to the auto-commit mode). Always use clear transaction boundaries, even for read-only operations. Depending on your isolation level and database capabilities this might not be required but there is no downside if you always demarcate transactions explicitly. Certainly, a single database transaction is going to perform better than many small transactions, even for reading data.
A Hibernate application can run in non-managed (i.e. standalone, simple Web- or Swing applications) and managed J2EE environments. In a non-managed environment, Hibernate is usually responsible for its own database connection pool. The application developer has to manually set transaction boundaries, in other words, begin, commit, or rollback database transactions himself. A managed environment usually provides container-managed transactions (CMT), with the transaction assembly defined declaratively in deployment descriptors of EJB session beans, for example. Programmatic transaction demarcation is then no longer necessary.
However, it is often desirable to keep your persistence layer portable between non-managed
resource-local environments, and systems that can rely on JTA but use BMT instead of CMT.
In both cases you'd use programmatic transaction demarcation. Hibernate offers a wrapper
API called Transaction
that translates into the native transaction system of
your deployment environment. This API is actually optional, but we strongly encourage its use
unless you are in a CMT session bean.
Usually, ending a Session
involves four distinct phases:
flush the session
commit the transaction
close the session
handle exceptions
Flushing the session has been discussed earlier, we'll now have a closer look at transaction demarcation and exception handling in both managed- and non-managed environments.
If a Hibernate persistence layer runs in a non-managed environment, database connections are usually handled by simple (i.e. non-DataSource) connection pools from which Hibernate obtains connections as needed. The session/transaction handling idiom looks like this:
// Non-managed environment idiom Session sess = factory.openSession(); Transaction tx = null; try { tx = sess.beginTransaction(); // do some work ... tx.commit(); } catch (RuntimeException e) { if (tx != null) tx.rollback(); throw e; // or display error message } finally { sess.close(); }
You don't have to flush()
the Session
explicitly -
the call to commit()
automatically triggers the synchronization (depending
upon the FlushMode for the session.
A call to close()
marks the end of a session. The main implication
of close()
is that the JDBC connection will be relinquished by the
session. This Java code is portable and runs in both non-managed and JTA environments.
A much more flexible solution is Hibernate's built-in "current session" context management, as described earlier:
// Non-managed environment idiom with getCurrentSession() try { factory.getCurrentSession().beginTransaction(); // do some work ... factory.getCurrentSession().getTransaction().commit(); } catch (RuntimeException e) { factory.getCurrentSession().getTransaction().rollback(); throw e; // or display error message }
You will very likely never see these code snippets in a regular application;
fatal (system) exceptions should always be caught at the "top". In other words, the
code that executes Hibernate calls (in the persistence layer) and the code that handles
RuntimeException
(and usually can only clean up and exit) are in
different layers. The current context management by Hibernate can significantly
simplify this design, as all you need is access to a SessionFactory
.
Exception handling is discussed later in this chapter.
Note that you should select org.hibernate.transaction.JDBCTransactionFactory
(which is the default), and for the second example "thread"
as your
hibernate.current_session_context_class
.
If your persistence layer runs in an application server (e.g. behind EJB session beans), every datasource connection obtained by Hibernate will automatically be part of the global JTA transaction. You can also install a standalone JTA implementation and use it without EJB. Hibernate offers two strategies for JTA integration.
If you use bean-managed transactions (BMT) Hibernate will tell the application server to start
and end a BMT transaction if you use the Transaction
API. So, the
transaction management code is identical to the non-managed environment.
// BMT idiom Session sess = factory.openSession(); Transaction tx = null; try { tx = sess.beginTransaction(); // do some work ... tx.commit(); } catch (RuntimeException e) { if (tx != null) tx.rollback(); throw e; // or display error message } finally { sess.close(); }
If you want to use a transaction-bound Session
, that is, the
getCurrentSession()
functionality for easy context propagation,
you will have to use the JTA UserTransaction
API directly:
// BMT idiom with getCurrentSession() try { UserTransaction tx = (UserTransaction)new InitialContext() .lookup("java:comp/UserTransaction"); tx.begin(); // Do some work on Session bound to transaction factory.getCurrentSession().load(...); factory.getCurrentSession().persist(...); tx.commit(); } catch (RuntimeException e) { tx.rollback(); throw e; // or display error message }
With CMT, transaction demarcation is done in session bean deployment descriptors, not programmatically, hence, the code is reduced to:
// CMT idiom Session sess = factory.getCurrentSession(); // do some work ...
In a CMT/EJB even rollback happens automatically, since an unhandled RuntimeException
thrown by a session bean method tells the container to set the global transaction to rollback.
This means you do not need to use the Hibernate Transaction
API at
all with BMT or CMT, and you get automatic propagation of the "current" Session bound to the
transaction.
Note that you should choose org.hibernate.transaction.JTATransactionFactory
if you use JTA directly (BMT), and org.hibernate.transaction.CMTTransactionFactory
in a CMT session bean, when you configure Hibernate's transaction factory. Remember to also set
hibernate.transaction.manager_lookup_class
. Furthermore, make sure
that your hibernate.current_session_context_class
is either unset (backwards
compatibility), or set to "jta"
.
The getCurrentSession()
operation has one downside in a JTA environment.
There is one caveat to the use of after_statement
connection release
mode, which is then used by default. Due to a silly limitation of the JTA spec, it is not
possible for Hibernate to automatically clean up any unclosed ScrollableResults
or
Iterator
instances returned by scroll()
or
iterate()
. You must release the underlying database
cursor by calling ScrollableResults.close()
or
Hibernate.close(Iterator)
explicitly from a finally
block. (Of course, most applications can easily avoid using scroll()
or
iterate()
at all from the JTA or CMT code.)
If the Session
throws an exception (including any
SQLException
), you should immediately rollback the database
transaction, call Session.close()
and discard the
Session
instance. Certain methods of Session
will not leave the session in a consistent state. No
exception thrown by Hibernate can be treated as recoverable. Ensure that the
Session
will be closed by calling close()
in a finally
block.
The HibernateException
, which wraps most of the errors that
can occur in a Hibernate persistence layer, is an unchecked exception (it wasn't
in older versions of Hibernate). In our opinion, we shouldn't force the application
developer to catch an unrecoverable exception at a low layer. In most systems, unchecked
and fatal exceptions are handled in one of the first frames of the method call
stack (i.e. in higher layers) and an error message is presented to the application
user (or some other appropriate action is taken). Note that Hibernate might also throw
other unchecked exceptions which are not a HibernateException
. These
are, again, not recoverable and appropriate action should be taken.
Hibernate wraps SQLException
s thrown while interacting with the database
in a JDBCException
. In fact, Hibernate will attempt to convert the exception
into a more meaningful subclass of JDBCException
. The underlying
SQLException
is always available via JDBCException.getCause()
.
Hibernate converts the SQLException
into an appropriate
JDBCException
subclass using the SQLExceptionConverter
attached to the SessionFactory
. By default, the
SQLExceptionConverter
is defined by the configured dialect; however, it is
also possible to plug in a custom implementation (see the javadocs for the
SQLExceptionConverterFactory
class for details). The standard
JDBCException
subtypes are:
JDBCConnectionException
- indicates an error
with the underlying JDBC communication.
SQLGrammarException
- indicates a grammar
or syntax problem with the issued SQL.
ConstraintViolationException
- indicates some
form of integrity constraint violation.
LockAcquisitionException
- indicates an error
acquiring a lock level necessary to perform the requested operation.
GenericJDBCException
- a generic exception
which did not fall into any of the other categories.
One extremely important feature provided by a managed environment like EJB
that is never provided for non-managed code is transaction timeout. Transaction
timeouts ensure that no misbehaving transaction can indefinitely tie up
resources while returning no response to the user. Outside a managed (JTA)
environment, Hibernate cannot fully provide this functionality. However,
Hibernate can at least control data access operations, ensuring that database
level deadlocks and queries with huge result sets are limited by a defined
timeout. In a managed environment, Hibernate can delegate transaction timeout
to JTA. This functionality is abstracted by the Hibernate
Transaction
object.
Session sess = factory.openSession(); try { //set transaction timeout to 3 seconds sess.getTransaction().setTimeout(3); sess.getTransaction().begin(); // do some work ... sess.getTransaction().commit() } catch (RuntimeException e) { sess.getTransaction().rollback(); throw e; // or display error message } finally { sess.close(); }
Note that setTimeout()
may not be called in a CMT bean,
where transaction timeouts must be defined declaratively.
The only approach that is consistent with high concurrency and high scalability is optimistic concurrency control with versioning. Version checking uses version numbers, or timestamps, to detect conflicting updates (and to prevent lost updates). Hibernate provides for three possible approaches to writing application code that uses optimistic concurrency. The use cases we show are in the context of long conversations, but version checking also has the benefit of preventing lost updates in single database transactions.
In an implementation without much help from Hibernate, each interaction with the
database occurs in a new Session
and the developer is responsible
for reloading all persistent instances from the database before manipulating them.
This approach forces the application to carry out its own version checking to ensure
conversation transaction isolation. This approach is the least efficient in terms of
database access. It is the approach most similar to entity EJBs.
// foo is an instance loaded by a previous Session session = factory.openSession(); Transaction t = session.beginTransaction(); int oldVersion = foo.getVersion(); session.load( foo, foo.getKey() ); // load the current state if ( oldVersion != foo.getVersion() ) throw new StaleObjectStateException(); foo.setProperty("bar"); t.commit(); session.close();
The version
property is mapped using <version>
,
and Hibernate will automatically increment it during flush if the entity is
dirty.
Of course, if you are operating in a low-data-concurrency environment and don't require version checking, you may use this approach and just skip the version check. In that case, last commit wins will be the default strategy for your long conversations. Keep in mind that this might confuse the users of the application, as they might experience lost updates without error messages or a chance to merge conflicting changes.
Clearly, manual version checking is only feasible in very trivial circumstances
and not practical for most applications. Often not only single instances, but
complete graphs of modified objects have to be checked. Hibernate offers automatic
version checking with either an extended Session
or detached instances
as the design paradigm.
A single Session
instance and its persistent instances are
used for the whole conversation, known as session-per-conversation.
Hibernate checks instance versions at flush time, throwing an exception if concurrent
modification is detected. It's up to the developer to catch and handle this exception
(common options are the opportunity for the user to merge changes or to restart the
business conversation with non-stale data).
The Session
is disconnected from any underlying JDBC connection
when waiting for user interaction. This approach is the most efficient in terms
of database access. The application need not concern itself with version checking or
with reattaching detached instances, nor does it have to reload instances in every
database transaction.
// foo is an instance loaded earlier by the old session Transaction t = session.beginTransaction(); // Obtain a new JDBC connection, start transaction foo.setProperty("bar"); session.flush(); // Only for last transaction in conversation t.commit(); // Also return JDBC connection session.close(); // Only for last transaction in conversation
The foo
object still knows which Session
it was
loaded in. Beginning a new database transaction on an old session obtains a new connection
and resumes the session. Committing a database transaction disconnects a session
from the JDBC connection and returns the connection to the pool. After reconnection, to
force a version check on data you aren't updating, you may call Session.lock()
with LockMode.READ
on any objects that might have been updated by another
transaction. You don't need to lock any data that you are updating.
Usually you would set FlushMode.MANUAL
on an extended Session
,
so that only the last database transaction cycle is allowed to actually persist all
modifications made in this conversation. Hence, only this last database transaction
would include the flush()
operation, and then also
close()
the session to end the conversation.
This pattern is problematic if the Session
is too big to
be stored during user think time, e.g. an HttpSession
should
be kept as small as possible. As the Session
is also the
(mandatory) first-level cache and contains all loaded objects, we can probably
use this strategy only for a few request/response cycles. You should use a
Session
only for a single conversation, as it will soon also
have stale data.
(Note that earlier Hibernate versions required explicit disconnection and reconnection
of a Session
. These methods are deprecated, as beginning and
ending a transaction has the same effect.)
Also note that you should keep the disconnected Session
close
to the persistence layer. In other words, use an EJB stateful session bean to
hold the Session
in a three-tier environment, and don't transfer
it to the web layer (or even serialize it to a separate tier) to store it in the
HttpSession
.
The extended session pattern, or session-per-conversation, is
more difficult to implement with automatic current session context management.
You need to supply your own implementation of the CurrentSessionContext
for this, see the Hibernate Wiki for examples.
Each interaction with the persistent store occurs in a new Session
.
However, the same persistent instances are reused for each interaction with the database.
The application manipulates the state of detached instances originally loaded in another
Session
and then reattaches them using Session.update()
,
Session.saveOrUpdate()
, or Session.merge()
.
// foo is an instance loaded by a previous Session foo.setProperty("bar"); session = factory.openSession(); Transaction t = session.beginTransaction(); session.saveOrUpdate(foo); // Use merge() if "foo" might have been loaded already t.commit(); session.close();
Again, Hibernate will check instance versions during flush, throwing an exception if conflicting updates occurred.
You may also call lock()
instead of update()
and use LockMode.READ
(performing a version check, bypassing all
caches) if you are sure that the object has not been modified.
You may disable Hibernate's automatic version increment for particular properties and
collections by setting the optimistic-lock
mapping attribute to
false
. Hibernate will then no longer increment versions if the
property is dirty.
Legacy database schemas are often static and can't be modified. Or, other applications
might also access the same database and don't know how to handle version numbers or
even timestamps. In both cases, versioning can't rely on a particular column in a table.
To force a version check without a version or timestamp property mapping, with a
comparison of the state of all fields in a row, turn on optimistic-lock="all"
in the <class>
mapping. Note that this conceptually only works
if Hibernate can compare the old and new state, i.e. if you use a single long
Session
and not session-per-request-with-detached-objects.
Sometimes concurrent modification can be permitted as long as the changes that have been
made don't overlap. If you set optimistic-lock="dirty"
when mapping the
<class>
, Hibernate will only compare dirty fields during flush.
In both cases, with dedicated version/timestamp columns or with full/dirty field
comparison, Hibernate uses a single UPDATE
statement (with an
appropriate WHERE
clause) per entity to execute the version check
and update the information. If you use transitive persistence to cascade reattachment
to associated entities, Hibernate might execute unnecessary updates. This is usually
not a problem, but on update triggers in the database might be
executed even when no changes have been made to detached instances. You can customize
this behavior by setting select-before-update="true"
in the
<class>
mapping, forcing Hibernate to SELECT
the instance to ensure that changes did actually occur, before updating the row.
It is not intended that users spend much time worrying about locking strategies. It's usually enough to specify an isolation level for the JDBC connections and then simply let the database do all the work. However, advanced users may sometimes wish to obtain exclusive pessimistic locks, or re-obtain locks at the start of a new transaction.
Hibernate will always use the locking mechanism of the database, never lock objects in memory!
The LockMode
class defines the different lock levels that may be acquired
by Hibernate. A lock is obtained by the following mechanisms:
LockMode.WRITE
is acquired automatically when Hibernate updates or inserts
a row.
LockMode.UPGRADE
may be acquired upon explicit user request using
SELECT ... FOR UPDATE
on databases which support that syntax.
LockMode.UPGRADE_NOWAIT
may be acquired upon explicit user request using a
SELECT ... FOR UPDATE NOWAIT
under Oracle.
LockMode.READ
is acquired automatically when Hibernate reads data
under Repeatable Read or Serializable isolation level. May be re-acquired by explicit user
request.
LockMode.NONE
represents the absence of a lock. All objects switch to this
lock mode at the end of a Transaction
. Objects associated with the session
via a call to update()
or saveOrUpdate()
also start out
in this lock mode.
The "explicit user request" is expressed in one of the following ways:
A call to Session.load()
, specifying a LockMode
.
A call to Session.lock()
.
A call to Query.setLockMode()
.
If Session.load()
is called with UPGRADE
or
UPGRADE_NOWAIT
, and the requested object was not yet loaded by
the session, the object is loaded using SELECT ... FOR UPDATE
.
If load()
is called for an object that is already loaded with
a less restrictive lock than the one requested, Hibernate calls
lock()
for that object.
Session.lock()
performs a version number check if the specified lock
mode is READ
, UPGRADE
or
UPGRADE_NOWAIT
. (In the case of UPGRADE
or
UPGRADE_NOWAIT
, SELECT ... FOR UPDATE
is used.)
If the database does not support the requested lock mode, Hibernate will use an appropriate alternate mode (instead of throwing an exception). This ensures that applications will be portable.
The legacy (2.x) behavior of Hibernate in regards to JDBC connection management
was that a Session
would obtain a connection when it was first
needed and then hold unto that connection until the session was closed.
Hibernate 3.x introduced the notion of connection release modes to tell a session
how to handle its JDBC connections. Note that the following discussion is pertinent
only to connections provided through a configured ConnectionProvider
;
user-supplied connections are outside the breadth of this discussion. The different
release modes are identified by the enumerated values of
org.hibernate.ConnectionReleaseMode
:
ON_CLOSE
- is essentially the legacy behavior described above. The
Hibernate session obtains a connection when it first needs to perform some JDBC access
and holds unto that connection until the session is closed.
AFTER_TRANSACTION
- says to release connections after a
org.hibernate.Transaction
has completed.
AFTER_STATEMENT
(also referred to as aggressive release) - says to
release connections after each and every statement execution. This aggressive releasing
is skipped if that statement leaves open resources associated with the given session;
currently the only situation where this occurs is through the use of
org.hibernate.ScrollableResults
.
The configuration parameter hibernate.connection.release_mode
is used
to specify which release mode to use. The possible values:
auto
(the default) - this choice delegates to the release mode
returned by the org.hibernate.transaction.TransactionFactory.getDefaultReleaseMode()
method. For JTATransactionFactory, this returns ConnectionReleaseMode.AFTER_STATEMENT; for
JDBCTransactionFactory, this returns ConnectionReleaseMode.AFTER_TRANSACTION. It is rarely
a good idea to change this default behavior as failures due to the value of this setting
tend to indicate bugs and/or invalid assumptions in user code.
on_close
- says to use ConnectionReleaseMode.ON_CLOSE. This setting
is left for backwards compatibility, but its use is highly discouraged.
after_transaction
- says to use ConnectionReleaseMode.AFTER_TRANSACTION.
This setting should not be used in JTA environments. Also note that with
ConnectionReleaseMode.AFTER_TRANSACTION, if a session is considered to be in auto-commit
mode connections will be released as if the release mode were AFTER_STATEMENT.
after_statement
- says to use ConnectionReleaseMode.AFTER_STATEMENT. Additionally,
the configured ConnectionProvider
is consulted to see if it supports this
setting (supportsAggressiveRelease()
). If not, the release mode is reset
to ConnectionReleaseMode.AFTER_TRANSACTION. This setting is only safe in environments where
we can either re-acquire the same underlying JDBC connection each time we make a call into
ConnectionProvider.getConnection()
or in auto-commit environments where
it does not matter whether we get back the same connection.
It is often useful for the application to react to certain events that occur inside Hibernate. This allows implementation of certain kinds of generic functionality, and extension of Hibernate functionality.
The Interceptor
interface provides callbacks from the session to the
application allowing the application to inspect and/or manipulate properties of a
persistent object before it is saved, updated, deleted or loaded. One
possible use for this is to track auditing information. For example, the following
Interceptor
automatically sets the createTimestamp
when an Auditable
is created and updates the
lastUpdateTimestamp
property when an Auditable
is
updated.
You may either implement Interceptor
directly or (better) extend
EmptyInterceptor
.
package org.hibernate.test; import java.io.Serializable; import java.util.Date; import java.util.Iterator; import org.hibernate.EmptyInterceptor; import org.hibernate.Transaction; import org.hibernate.type.Type; public class AuditInterceptor extends EmptyInterceptor { private int updates; private int creates; private int loads; public void onDelete(Object entity, Serializable id, Object[] state, String[] propertyNames, Type[] types) { // do nothing } public boolean onFlushDirty(Object entity, Serializable id, Object[] currentState, Object[] previousState, String[] propertyNames, Type[] types) { if ( entity instanceof Auditable ) { updates++; for ( int i=0; i < propertyNames.length; i++ ) { if ( "lastUpdateTimestamp".equals( propertyNames[i] ) ) { currentState[i] = new Date(); return true; } } } return false; } public boolean onLoad(Object entity, Serializable id, Object[] state, String[] propertyNames, Type[] types) { if ( entity instanceof Auditable ) { loads++; } return false; } public boolean onSave(Object entity, Serializable id, Object[] state, String[] propertyNames, Type[] types) { if ( entity instanceof Auditable ) { creates++; for ( int i=0; i<propertyNames.length; i++ ) { if ( "createTimestamp".equals( propertyNames[i] ) ) { state[i] = new Date(); return true; } } } return false; } public void afterTransactionCompletion(Transaction tx) { if ( tx.wasCommitted() ) { System.out.println("Creations: " + creates + ", Updates: " + updates, "Loads: " + loads); } updates=0; creates=0; loads=0; } }
Interceptors come in two flavors: Session
-scoped and
SessionFactory
-scoped.
A Session
-scoped interceptor is specified
when a session is opened using one of the overloaded SessionFactory.openSession()
methods accepting an Interceptor
.
Session session = sf.openSession( new AuditInterceptor() );
A SessionFactory
-scoped interceptor is registered with the Configuration
object prior to building the SessionFactory
. In this case, the supplied interceptor
will be applied to all sessions opened from that SessionFactory
; this is true unless
a session is opened explicitly specifying the interceptor to use. SessionFactory
-scoped
interceptors must be thread safe, taking care to not store session-specific state since multiple
sessions will use this interceptor (potentially) concurrently.
new Configuration().setInterceptor( new AuditInterceptor() );
If you have to react to particular events in your persistence layer, you may also use the Hibernate3 event architecture. The event system can be used in addition or as a replacement for interceptors.
Essentially all of the methods of the Session
interface correlate
to an event. You have a LoadEvent
, a FlushEvent
, etc
(consult the XML configuration-file DTD or the org.hibernate.event
package for the full list of defined event types). When a request is made of one of
these methods, the Hibernate Session
generates an appropriate
event and passes it to the configured event listeners for that type. Out-of-the-box,
these listeners implement the same processing in which those methods always resulted.
However, you are free to implement a customization of one of the listener interfaces
(i.e., the LoadEvent
is processed by the registered implemenation
of the LoadEventListener
interface), in which case their
implementation would be responsible for processing any load()
requests
made of the Session
.
The listeners should be considered effectively singletons; meaning, they are shared between requests, and thus should not save any state as instance variables.
A custom listener should implement the appropriate interface for the event it wants to
process and/or extend one of the convenience base classes (or even the default event
listeners used by Hibernate out-of-the-box as these are declared non-final for this
purpose). Custom listeners can either be registered programmatically through the
Configuration
object, or specified in the Hibernate configuration
XML (declarative configuration through the properties file is not supported). Here's an
example of a custom load event listener:
public class MyLoadListener implements LoadEventListener { // this is the single method defined by the LoadEventListener interface public void onLoad(LoadEvent event, LoadEventListener.LoadType loadType) throws HibernateException { if ( !MySecurity.isAuthorized( event.getEntityClassName(), event.getEntityId() ) ) { throw MySecurityException("Unauthorized access"); } } }
You also need a configuration entry telling Hibernate to use the listener in addition to the default listener:
<hibernate-configuration> <session-factory> ... <event type="load"> <listener class="com.eg.MyLoadListener"/> <listener class="org.hibernate.event.def.DefaultLoadEventListener"/> </event> </session-factory> </hibernate-configuration>
Instead, you may register it programmatically:
Configuration cfg = new Configuration(); LoadEventListener[] stack = { new MyLoadListener(), new DefaultLoadEventListener() }; cfg.EventListeners().setLoadEventListeners(stack);
Listeners registered declaratively cannot share instances. If the same class name is
used in multiple <listener/>
elements, each reference will
result in a separate instance of that class. If you need the capability to share
listener instances between listener types you must use the programmatic registration
approach.
Why implement an interface and define the specific type during configuration? Well, a listener implementation could implement multiple event listener interfaces. Having the type additionally defined during registration makes it easier to turn custom listeners on or off during configuration.
Usually, declarative security in Hibernate applications is managed in a session facade layer. Now, Hibernate3 allows certain actions to be permissioned via JACC, and authorized via JAAS. This is optional functionality built on top of the event architecture.
First, you must configure the appropriate event listeners, to enable the use of JAAS authorization.
<listener type="pre-delete" class="org.hibernate.secure.JACCPreDeleteEventListener"/> <listener type="pre-update" class="org.hibernate.secure.JACCPreUpdateEventListener"/> <listener type="pre-insert" class="org.hibernate.secure.JACCPreInsertEventListener"/> <listener type="pre-load" class="org.hibernate.secure.JACCPreLoadEventListener"/>
Note that <listener type="..." class="..."/>
is just a shorthand
for <event type="..."><listener class="..."/></event>
when there is exactly one listener for a particular event type.
Next, still in hibernate.cfg.xml
, bind the permissions to roles:
<grant role="admin" entity-name="User" actions="insert,update,read"/> <grant role="su" entity-name="User" actions="*"/>
The role names are the roles understood by your JACC provider.
A naive approach to inserting 100 000 rows in the database using Hibernate might look like this:
Session session = sessionFactory.openSession(); Transaction tx = session.beginTransaction(); for ( int i=0; i<100000; i++ ) { Customer customer = new Customer(.....); session.save(customer); } tx.commit(); session.close();
This would fall over with an OutOfMemoryException
somewhere
around the 50 000th row. That's because Hibernate caches all the newly inserted
Customer
instances in the session-level cache.
In this chapter we'll show you how to avoid this problem. First, however, if you are doing batch processing, it is absolutely critical that you enable the use of JDBC batching, if you intend to achieve reasonable performance. Set the JDBC batch size to a reasonable number (say, 10-50):
hibernate.jdbc.batch_size 20
Note that Hibernate disables insert batching at the JDBC level transparently if you
use an identiy
identifier generator.
You also might like to do this kind of work in a process where interaction with the second-level cache is completely disabled:
hibernate.cache.use_second_level_cache false
However, this is not absolutely necessary, since we can explicitly set the
CacheMode
to disable interaction with the second-level cache.
When making new objects persistent, you must flush()
and
then clear()
the session regularly, to control the size of
the first-level cache.
Session session = sessionFactory.openSession(); Transaction tx = session.beginTransaction(); for ( int i=0; i<100000; i++ ) { Customer customer = new Customer(.....); session.save(customer); if ( i % 20 == 0 ) { //20, same as the JDBC batch size //flush a batch of inserts and release memory: session.flush(); session.clear(); } } tx.commit(); session.close();
For retrieving and updating data the same ideas apply. In addition, you need to
use scroll()
to take advantage of server-side cursors for
queries that return many rows of data.
Session session = sessionFactory.openSession(); Transaction tx = session.beginTransaction(); ScrollableResults customers = session.getNamedQuery("GetCustomers") .setCacheMode(CacheMode.IGNORE) .scroll(ScrollMode.FORWARD_ONLY); int count=0; while ( customers.next() ) { Customer customer = (Customer) customers.get(0); customer.updateStuff(...); if ( ++count % 20 == 0 ) { //flush a batch of updates and release memory: session.flush(); session.clear(); } } tx.commit(); session.close();
Alternatively, Hibernate provides a command-oriented API that may be used for
streaming data to and from the database in the form of detached objects. A
StatelessSession
has no persistence context associated
with it and does not provide many of the higher-level life cycle semantics.
In particular, a stateless session does not implement a first-level cache nor
interact with any second-level or query cache. It does not implement
transactional write-behind or automatic dirty checking. Operations performed
using a stateless session do not ever cascade to associated instances. Collections
are ignored by a stateless session. Operations performed via a stateless session
bypass Hibernate's event model and interceptors. Stateless sessions are vulnerable
to data aliasing effects, due to the lack of a first-level cache. A stateless
session is a lower-level abstraction, much closer to the underlying JDBC.
StatelessSession session = sessionFactory.openStatelessSession(); Transaction tx = session.beginTransaction(); ScrollableResults customers = session.getNamedQuery("GetCustomers") .scroll(ScrollMode.FORWARD_ONLY); while ( customers.next() ) { Customer customer = (Customer) customers.get(0); customer.updateStuff(...); session.update(customer); } tx.commit(); session.close();
Note that in this code example, the Customer
instances returned
by the query are immediately detached. They are never associated with any persistence
context.
The insert(), update()
and delete()
operations
defined by the StatelessSession
interface are considered to be
direct database row-level operations, which result in immediate execution of a SQL
INSERT, UPDATE
or DELETE
respectively. Thus,
they have very different semantics to the save(), saveOrUpdate()
and delete()
operations defined by the Session
interface.
As already discussed, automatic and transparent object/relational mapping is concerned
with the management of object state. This implies that the object state is available
in memory, hence manipulating (using the SQL Data Manipulation Language
(DML) statements: INSERT
, UPDATE
, DELETE
)
data directly in the database will not affect in-memory state. However, Hibernate provides methods
for bulk SQL-style DML statement execution which are performed through the
Hibernate Query Language (HQL).
The pseudo-syntax for UPDATE
and DELETE
statements
is: ( UPDATE | DELETE ) FROM? EntityName (WHERE where_conditions)?
. Some
points to note:
In the from-clause, the FROM keyword is optional
There can only be a single entity named in the from-clause; it can optionally be aliased. If the entity name is aliased, then any property references must be qualified using that alias; if the entity name is not aliased, then it is illegal for any property references to be qualified.
No joins (either implicit or explicit) can be specified in a bulk HQL query. Sub-queries may be used in the where-clause; the subqueries, themselves, may contain joins.
The where-clause is also optional.
As an example, to execute an HQL UPDATE
, use the
Query.executeUpdate()
method (the method is named for
those familiar with JDBC's PreparedStatement.executeUpdate()
):
Session session = sessionFactory.openSession(); Transaction tx = session.beginTransaction(); String hqlUpdate = "update Customer c set c.name = :newName where c.name = :oldName"; // or String hqlUpdate = "update Customer set name = :newName where name = :oldName"; int updatedEntities = s.createQuery( hqlUpdate ) .setString( "newName", newName ) .setString( "oldName", oldName ) .executeUpdate(); tx.commit(); session.close();
HQL UPDATE
statements, by default do not effect the
version
or the timestamp property values
for the affected entities; this is in keeping with the EJB3 specification. However,
you can force Hibernate to properly reset the version
or
timestamp
property values through the use of a versioned update
.
This is achieved by adding the VERSIONED
keyword after the UPDATE
keyword.
Session session = sessionFactory.openSession(); Transaction tx = session.beginTransaction(); String hqlVersionedUpdate = "update versioned Customer set name = :newName where name = :oldName"; int updatedEntities = s.createQuery( hqlUpdate ) .setString( "newName", newName ) .setString( "oldName", oldName ) .executeUpdate(); tx.commit(); session.close();
Note that custom version types (org.hibernate.usertype.UserVersionType
)
are not allowed in conjunction with a update versioned
statement.
To execute an HQL DELETE
, use the same Query.executeUpdate()
method:
Session session = sessionFactory.openSession(); Transaction tx = session.beginTransaction(); String hqlDelete = "delete Customer c where c.name = :oldName"; // or String hqlDelete = "delete Customer where name = :oldName"; int deletedEntities = s.createQuery( hqlDelete ) .setString( "oldName", oldName ) .executeUpdate(); tx.commit(); session.close();
The int
value returned by the Query.executeUpdate()
method indicate the number of entities effected by the operation. Consider this may or may not
correlate to the number of rows effected in the database. An HQL bulk operation might result in
multiple actual SQL statements being executed, for joined-subclass, for example. The returned
number indicates the number of actual entities affected by the statement. Going back to the
example of joined-subclass, a delete against one of the subclasses may actually result
in deletes against not just the table to which that subclass is mapped, but also the "root"
table and potentially joined-subclass tables further down the inheritence hierarchy.
The pseudo-syntax for INSERT
statements is:
INSERT INTO EntityName properties_list select_statement
. Some
points to note:
Only the INSERT INTO ... SELECT ... form is supported; not the INSERT INTO ... VALUES ... form.
The properties_list is analogous to the column speficiation
in the SQL INSERT
statement. For entities involved in mapped
inheritence, only properties directly defined on that given class-level can be
used in the properties_list. Superclass properties are not allowed; and subclass
properties do not make sense. In other words, INSERT
statements are inherently non-polymorphic.
select_statement can be any valid HQL select query, with the caveat that the return types
must match the types expected by the insert. Currently, this is checked during query
compilation rather than allowing the check to relegate to the database. Note however
that this might cause problems between Hibernate Type
s which are
equivalent as opposed to equal. This might cause
issues with mismatches between a property defined as a org.hibernate.type.DateType
and a property defined as a org.hibernate.type.TimestampType
, even though the
database might not make a distinction or might be able to handle the conversion.
For the id property, the insert statement gives you two options. You can either
explicitly specify the id property in the properties_list (in which case its value
is taken from the corresponding select expression) or omit it from the properties_list
(in which case a generated value is used). This later option is only available when
using id generators that operate in the database; attempting to use this option with
any "in memory" type generators will cause an exception during parsing. Note that
for the purposes of this discussion, in-database generators are considered to be
org.hibernate.id.SequenceGenerator
(and its subclasses) and
any implementors of org.hibernate.id.PostInsertIdentifierGenerator
.
The most notable exception here is org.hibernate.id.TableHiLoGenerator
,
which cannot be used because it does not expose a selectable way to get its values.
For properties mapped as either version
or timestamp
,
the insert statement gives you two options. You can either specify the property in the
properties_list (in which case its value is taken from the corresponding select expressions)
or omit it from the properties_list (in which case the seed value
defined
by the org.hibernate.type.VersionType
is used).
An example HQL INSERT
statement execution:
Session session = sessionFactory.openSession(); Transaction tx = session.beginTransaction(); String hqlInsert = "insert into DelinquentAccount (id, name) select c.id, c.name from Customer c where ..."; int createdEntities = s.createQuery( hqlInsert ) .executeUpdate(); tx.commit(); session.close();
Hibernate is equipped with an extremely powerful query language that (quite intentionally) looks very much like SQL. But don't be fooled by the syntax; HQL is fully object-oriented, understanding notions like inheritence, polymorphism and association.
Queries are case-insensitive, except for names of Java classes and properties.
So SeLeCT
is the same as
sELEct
is the same as
SELECT
but
org.hibernate.eg.FOO
is not
org.hibernate.eg.Foo
and
foo.barSet
is not
foo.BARSET
.
This manual uses lowercase HQL keywords. Some users find queries with uppercase keywords more readable, but we find this convention ugly when embedded in Java code.
The simplest possible Hibernate query is of the form:
from eg.Cat
which simply returns all instances of the class eg.Cat
.
We don't usually need to qualify the class name, since auto-import
is the default. So we almost always just write:
from Cat
Most of the time, you will need to assign an alias, since
you will want to refer to the Cat
in other parts of the
query.
from Cat as cat
This query assigns the alias cat
to Cat
instances, so we could use that alias later in the query. The as
keyword is optional; we could also write:
from Cat cat
Multiple classes may appear, resulting in a cartesian product or "cross" join.
from Formula, Parameter
from Formula as form, Parameter as param
It is considered good practice to name query aliases using an initial lowercase,
consistent with Java naming standards for local variables
(eg. domesticCat
).
We may also assign aliases to associated entities, or even to elements of a
collection of values, using a join
.
from Cat as cat inner join cat.mate as mate left outer join cat.kittens as kitten
from Cat as cat left join cat.mate.kittens as kittens
from Formula form full join form.parameter param
The supported join types are borrowed from ANSI SQL
inner join
left outer join
right outer join
full join
(not usually useful)
The inner join
, left outer join
and
right outer join
constructs may be abbreviated.
from Cat as cat join cat.mate as mate left join cat.kittens as kitten
You may supply extra join conditions using the HQL with
keyword.
from Cat as cat left join cat.kittens as kitten with kitten.bodyWeight > 10.0
In addition, a "fetch" join allows associations or collections of values to be initialized along with their parent objects, using a single select. This is particularly useful in the case of a collection. It effectively overrides the outer join and lazy declarations of the mapping file for associations and collections. See Section 19.1, “Fetching strategies” for more information.
from Cat as cat inner join fetch cat.mate left join fetch cat.kittens
A fetch join does not usually need to assign an alias, because the associated objects
should not be used in the where
clause (or any other clause). Also,
the associated objects are not returned directly in the query results. Instead, they may
be accessed via the parent object. The only reason we might need an alias is if we are
recursively join fetching a further collection:
from Cat as cat inner join fetch cat.mate left join fetch cat.kittens child left join fetch child.kittens
Note that the fetch
construct may not be used in queries called using
iterate()
(though scroll()
can be used). Nor should
fetch
be used together with setMaxResults()
or
setFirstResult()
as these operations are based on the result rows, which
usually contain duplicates for eager collection fetching, hence, the number of rows is not what
you'd expect.
Nor may fetch
be used together with an ad hoc with
condition.
It is possible to create a cartesian product by join fetching more than one collection in a
query, so take care in this case. Join fetching multiple collection roles also sometimes gives
unexpected results for bag mappings, so be careful about how you formulate your queries in this
case. Finally, note that full join fetch
and right join fetch
are not meaningful.
If you are using property-level lazy fetching (with bytecode instrumentation), it is
possible to force Hibernate to fetch the lazy properties immediately (in the first
query) using fetch all properties
.
from Document fetch all properties order by name
from Document doc fetch all properties where lower(doc.name) like '%cats%'
HQL supports two forms of association joining: implicit
and explicit
.
The queries shown in the previous section all use the explicit
form where
the join keyword is explicitly used in the from clause. This is the recommended form.
The implicit
form does not use the join keyword. Instead, the
associations are "dereferenced" using dot-notation. implicit
joins
can appear in any of the HQL clauses. implicit
join result
in inner joins in the resulting SQL statement.
from Cat as cat where cat.mate.name like '%s%'
There are, generally speaking, 2 ways to refer to an entity's identifier property:
The special property (lowercase) id
may be used to reference the identifier
property of an entity provided that entity does not define a non-identifier property
named id.
If the entity defines a named identifier property, you may use that property name.
References to composite identifier properties follow the same naming rules. If the
entity has a non-identifier property named id, the composite identifier property can only
be referenced by its defined named; otherwise, the special id
property
can be used to rerference the identifier property.
Note: this has changed significantly starting in version 3.2.2. In previous versions,
id
always referred to the identifier property no
matter what its actual name. A ramification of that decision was that non-identifier
properties named id
could never be referenced in Hibernate queries.
The select
clause picks which objects and properties to return in
the query result set. Consider:
select mate from Cat as cat inner join cat.mate as mate
The query will select mate
s of other Cat
s.
Actually, you may express this query more compactly as:
select cat.mate from Cat cat
Queries may return properties of any value type including properties of component type:
select cat.name from DomesticCat cat where cat.name like 'fri%'
select cust.name.firstName from Customer as cust
Queries may return multiple objects and/or properties as an array of type
Object[]
,
select mother, offspr, mate.name from DomesticCat as mother inner join mother.mate as mate left outer join mother.kittens as offspr
or as a List
,
select new list(mother, offspr, mate.name) from DomesticCat as mother inner join mother.mate as mate left outer join mother.kittens as offspr
or as an actual typesafe Java object,
select new Family(mother, mate, offspr) from DomesticCat as mother join mother.mate as mate left join mother.kittens as offspr
assuming that the class Family
has an appropriate constructor.
You may assign aliases to selected expressions using as
:
select max(bodyWeight) as max, min(bodyWeight) as min, count(*) as n from Cat cat
This is most useful when used together with select new map
:
select new map( max(bodyWeight) as max, min(bodyWeight) as min, count(*) as n ) from Cat cat
This query returns a Map
from aliases to selected values.
HQL queries may even return the results of aggregate functions on properties:
select avg(cat.weight), sum(cat.weight), max(cat.weight), count(cat) from Cat cat
The supported aggregate functions are
avg(...), sum(...), min(...), max(...)
count(*)
count(...), count(distinct ...), count(all...)
You may use arithmetic operators, concatenation, and recognized SQL functions in the select clause:
select cat.weight + sum(kitten.weight) from Cat cat join cat.kittens kitten group by cat.id, cat.weight
select firstName||' '||initial||' '||upper(lastName) from Person
The distinct
and all
keywords may be used and
have the same semantics as in SQL.
select distinct cat.name from Cat cat select count(distinct cat.name), count(cat) from Cat cat
A query like:
from Cat as cat
returns instances not only of Cat
, but also of subclasses like
DomesticCat
. Hibernate queries may name any Java
class or interface in the from
clause. The query will return instances
of all persistent classes that extend that class or implement the interface. The following
query would return all persistent objects:
from java.lang.Object o
The interface Named
might be implemented by various persistent
classes:
from Named n, Named m where n.name = m.name
Note that these last two queries will require more than one SQL SELECT
. This
means that the order by
clause does not correctly order the whole result set.
(It also means you can't call these queries using Query.scroll()
.)
The where
clause allows you to narrow the list of instances returned.
If no alias exists, you may refer to properties by name:
from Cat where name='Fritz'
If there is an alias, use a qualified property name:
from Cat as cat where cat.name='Fritz'
returns instances of Cat
named 'Fritz'.
select foo from Foo foo, Bar bar where foo.startDate = bar.date
will return all instances of Foo
for which
there exists an instance of bar
with a
date
property equal to the
startDate
property of the
Foo
. Compound path expressions make the
where
clause extremely powerful. Consider:
from Cat cat where cat.mate.name is not null
This query translates to an SQL query with a table (inner) join. If you were to write something like
from Foo foo where foo.bar.baz.customer.address.city is not null
you would end up with a query that would require four table joins in SQL.
The =
operator may be used to compare not only properties, but also
instances:
from Cat cat, Cat rival where cat.mate = rival.mate
select cat, mate from Cat cat, Cat mate where cat.mate = mate
The special property (lowercase) id
may be used to reference the
unique identifier of an object. See Section 14.5, “Refering to identifier property”
for more information.
from Cat as cat where cat.id = 123 from Cat as cat where cat.mate.id = 69
The second query is efficient. No table join is required!
Properties of composite identifiers may also be used. Suppose Person
has a composite identifier consisting of country
and
medicareNumber
. Again, see Section 14.5, “Refering to identifier property”
for more information regarding referencing identifier properties.
from bank.Person person where person.id.country = 'AU' and person.id.medicareNumber = 123456
from bank.Account account where account.owner.id.country = 'AU' and account.owner.id.medicareNumber = 123456
Once again, the second query requires no table join.
Likewise, the special property class
accesses the discriminator value
of an instance in the case of polymorphic persistence. A Java class name embedded in the
where clause will be translated to its discriminator value.
from Cat cat where cat.class = DomesticCat
You may also use components or composite user types, or properties of said component types. See Section 14.17, “Components” for more details.
An "any" type has the special properties id
and class
,
allowing us to express a join in the following way (where AuditLog.item
is a property mapped with <any>
).
from AuditLog log, Payment payment where log.item.class = 'Payment' and log.item.id = payment.id
Notice that log.item.class
and payment.class
would refer to the values of completely different database columns in the above query.
Expressions allowed in the where
clause include
most of the kind of things you could write in SQL:
mathematical operators +, -, *, /
binary comparison operators =, >=, <=, <>, !=, like
logical operations and, or, not
Parentheses ( )
, indicating grouping
in
,
not in
,
between
,
is null
,
is not null
,
is empty
,
is not empty
,
member of
and
not member of
"Simple" case, case ... when ... then ... else ... end
, and
"searched" case, case when ... then ... else ... end
string concatenation ...||...
or concat(...,...)
current_date()
, current_time()
,
current_timestamp()
second(...)
, minute(...)
,
hour(...)
, day(...)
,
month(...)
, year(...)
,
Any function or operator defined by EJB-QL 3.0: substring(), trim(),
lower(), upper(), length(), locate(), abs(), sqrt(), bit_length(), mod()
coalesce()
and nullif()
str()
for converting numeric or temporal values to a
readable string
cast(... as ...)
, where the second argument is the name of
a Hibernate type, and extract(... from ...)
if ANSI
cast()
and extract()
is supported by
the underlying database
the HQL index()
function, that applies to aliases of
a joined indexed collection
HQL functions that take collection-valued path expressions: size(),
minelement(), maxelement(), minindex(), maxindex()
, along with the
special elements()
and indices
functions
which may be quantified using some, all, exists, any, in
.
Any database-supported SQL scalar function like sign()
,
trunc()
, rtrim()
, sin()
JDBC-style positional parameters ?
named parameters :name
, :start_date
, :x1
SQL literals 'foo'
, 69
, 6.66E+2
,
'1970-01-01 10:00:01.0'
Java public static final
constants eg.Color.TABBY
in
and between
may be used as follows:
from DomesticCat cat where cat.name between 'A' and 'B'
from DomesticCat cat where cat.name in ( 'Foo', 'Bar', 'Baz' )
and the negated forms may be written
from DomesticCat cat where cat.name not between 'A' and 'B'
from DomesticCat cat where cat.name not in ( 'Foo', 'Bar', 'Baz' )
Likewise, is null
and is not null
may be used to test
for null values.
Booleans may be easily used in expressions by declaring HQL query substitutions in Hibernate configuration:
<property name="hibernate.query.substitutions">true 1, false 0</property>
This will replace the keywords true
and false
with the
literals 1
and 0
in the translated SQL from this HQL:
from Cat cat where cat.alive = true
You may test the size of a collection with the special property size
, or
the special size()
function.
from Cat cat where cat.kittens.size > 0
from Cat cat where size(cat.kittens) > 0
For indexed collections, you may refer to the minimum and maximum indices using
minindex
and maxindex
functions. Similarly,
you may refer to the minimum and maximum elements of a collection of basic type
using the minelement
and maxelement
functions.
from Calendar cal where maxelement(cal.holidays) > current_date
from Order order where maxindex(order.items) > 100
from Order order where minelement(order.items) > 10000
The SQL functions any, some, all, exists, in
are supported when passed the element
or index set of a collection (elements
and indices
functions)
or the result of a subquery (see below).
select mother from Cat as mother, Cat as kit where kit in elements(foo.kittens)
select p from NameList list, Person p where p.name = some elements(list.names)
from Cat cat where exists elements(cat.kittens)
from Player p where 3 > all elements(p.scores)
from Show show where 'fizard' in indices(show.acts)
Note that these constructs - size
, elements
,
indices
, minindex
, maxindex
,
minelement
, maxelement
- may only be used in
the where clause in Hibernate3.
Elements of indexed collections (arrays, lists, maps) may be referred to by index (in a where clause only):
from Order order where order.items[0].id = 1234
select person from Person person, Calendar calendar where calendar.holidays['national day'] = person.birthDay and person.nationality.calendar = calendar
select item from Item item, Order order where order.items[ order.deliveredItemIndices[0] ] = item and order.id = 11
select item from Item item, Order order where order.items[ maxindex(order.items) ] = item and order.id = 11
The expression inside []
may even be an arithmetic expression.
select item from Item item, Order order where order.items[ size(order.items) - 1 ] = item
HQL also provides the built-in index()
function, for elements
of a one-to-many association or collection of values.
select item, index(item) from Order order join order.items item where index(item) < 5
Scalar SQL functions supported by the underlying database may be used
from DomesticCat cat where upper(cat.name) like 'FRI%'
If you are not yet convinced by all this, think how much longer and less readable the following query would be in SQL:
select cust from Product prod, Store store inner join store.customers cust where prod.name = 'widget' and store.location.name in ( 'Melbourne', 'Sydney' ) and prod = all elements(cust.currentOrder.lineItems)
Hint: something like
SELECT cust.name, cust.address, cust.phone, cust.id, cust.current_order FROM customers cust, stores store, locations loc, store_customers sc, product prod WHERE prod.name = 'widget' AND store.loc_id = loc.id AND loc.name IN ( 'Melbourne', 'Sydney' ) AND sc.store_id = store.id AND sc.cust_id = cust.id AND prod.id = ALL( SELECT item.prod_id FROM line_items item, orders o WHERE item.order_id = o.id AND cust.current_order = o.id )
The list returned by a query may be ordered by any property of a returned class or components:
from DomesticCat cat order by cat.name asc, cat.weight desc, cat.birthdate
The optional asc
or desc
indicate ascending or descending order
respectively.
A query that returns aggregate values may be grouped by any property of a returned class or components:
select cat.color, sum(cat.weight), count(cat) from Cat cat group by cat.color
select foo.id, avg(name), max(name) from Foo foo join foo.names name group by foo.id
A having
clause is also allowed.
select cat.color, sum(cat.weight), count(cat) from Cat cat group by cat.color having cat.color in (eg.Color.TABBY, eg.Color.BLACK)
SQL functions and aggregate functions are allowed in the having
and order by
clauses, if supported by the underlying database
(eg. not in MySQL).
select cat from Cat cat join cat.kittens kitten group by cat.id, cat.name, cat.other, cat.properties having avg(kitten.weight) > 100 order by count(kitten) asc, sum(kitten.weight) desc
Note that neither the group by
clause nor the
order by
clause may contain arithmetic expressions.
Also note that Hibernate currently does not expand a grouped entity,
so you can't write group by cat
if all properties
of cat
are non-aggregated. You have to list all
non-aggregated properties explicitly.
For databases that support subselects, Hibernate supports subqueries within queries. A subquery must be surrounded by parentheses (often by an SQL aggregate function call). Even correlated subqueries (subqueries that refer to an alias in the outer query) are allowed.
from Cat as fatcat where fatcat.weight > ( select avg(cat.weight) from DomesticCat cat )
from DomesticCat as cat where cat.name = some ( select name.nickName from Name as name )
from Cat as cat where not exists ( from Cat as mate where mate.mate = cat )
from DomesticCat as cat where cat.name not in ( select name.nickName from Name as name )
select cat.id, (select max(kit.weight) from cat.kitten kit) from Cat as cat
Note that HQL subqueries may occur only in the select or where clauses.
Note that subqueries can also utilize row value constructor
syntax. See
Section 14.18, “Row value constructor syntax” for more details.
Hibernate queries can be quite powerful and complex. In fact, the power of the query language is one of Hibernate's main selling points. Here are some example queries very similar to queries that I used on a recent project. Note that most queries you will write are much simpler than these!
The following query returns the order id, number of items and total value of the order for all
unpaid orders for a particular customer and given minimum total value, ordering the results by
total value. In determining the prices, it uses the current catalog. The resulting SQL query,
against the ORDER
, ORDER_LINE
, PRODUCT
,
CATALOG
and PRICE
tables has four inner joins and an
(uncorrelated) subselect.
select order.id, sum(price.amount), count(item) from Order as order join order.lineItems as item join item.product as product, Catalog as catalog join catalog.prices as price where order.paid = false and order.customer = :customer and price.product = product and catalog.effectiveDate < sysdate and catalog.effectiveDate >= all ( select cat.effectiveDate from Catalog as cat where cat.effectiveDate < sysdate ) group by order having sum(price.amount) > :minAmount order by sum(price.amount) desc
What a monster! Actually, in real life, I'm not very keen on subqueries, so my query was really more like this:
select order.id, sum(price.amount), count(item) from Order as order join order.lineItems as item join item.product as product, Catalog as catalog join catalog.prices as price where order.paid = false and order.customer = :customer and price.product = product and catalog = :currentCatalog group by order having sum(price.amount) > :minAmount order by sum(price.amount) desc
The next query counts the number of payments in each status, excluding all payments in the
AWAITING_APPROVAL
status where the most recent status change was made by the
current user. It translates to an SQL query with two inner joins and a correlated subselect
against the PAYMENT
, PAYMENT_STATUS
and
PAYMENT_STATUS_CHANGE
tables.
select count(payment), status.name from Payment as payment join payment.currentStatus as status join payment.statusChanges as statusChange where payment.status.name <> PaymentStatus.AWAITING_APPROVAL or ( statusChange.timeStamp = ( select max(change.timeStamp) from PaymentStatusChange change where change.payment = payment ) and statusChange.user <> :currentUser ) group by status.name, status.sortOrder order by status.sortOrder
If I would have mapped the statusChanges
collection as a list, instead of a set,
the query would have been much simpler to write.
select count(payment), status.name from Payment as payment join payment.currentStatus as status where payment.status.name <> PaymentStatus.AWAITING_APPROVAL or payment.statusChanges[ maxIndex(payment.statusChanges) ].user <> :currentUser group by status.name, status.sortOrder order by status.sortOrder
The next query uses the MS SQL Server isNull()
function to return all
the accounts and unpaid payments for the organization to which the current user belongs.
It translates to an SQL query with three inner joins, an outer join and a subselect against
the ACCOUNT
, PAYMENT
, PAYMENT_STATUS
,
ACCOUNT_TYPE
, ORGANIZATION
and
ORG_USER
tables.
select account, payment from Account as account left outer join account.payments as payment where :currentUser in elements(account.holder.users) and PaymentStatus.UNPAID = isNull(payment.currentStatus.name, PaymentStatus.UNPAID) order by account.type.sortOrder, account.accountNumber, payment.dueDate
For some databases, we would need to do away with the (correlated) subselect.
select account, payment from Account as account join account.holder.users as user left outer join account.payments as payment where :currentUser = user and PaymentStatus.UNPAID = isNull(payment.currentStatus.name, PaymentStatus.UNPAID) order by account.type.sortOrder, account.accountNumber, payment.dueDate
HQL now supports update
, delete
and
insert ... select ...
statements.
See Section 13.4, “DML-style operations” for details.
You can count the number of query results without actually returning them:
( (Integer) session.createQuery("select count(*) from ....").iterate().next() ).intValue()
To order a result by the size of a collection, use the following query:
select usr.id, usr.name from User as usr left join usr.messages as msg group by usr.id, usr.name order by count(msg)
If your database supports subselects, you can place a condition upon selection size in the where clause of your query:
from User usr where size(usr.messages) >= 1
If your database doesn't support subselects, use the following query:
select usr.id, usr.name from User usr.name join usr.messages msg group by usr.id, usr.name having count(msg) >= 1
As this solution can't return a User
with zero messages
because of the inner join, the following form is also useful:
select usr.id, usr.name from User as usr left join usr.messages as msg group by usr.id, usr.name having count(msg) = 0
Properties of a JavaBean can be bound to named query parameters:
Query q = s.createQuery("from foo Foo as foo where foo.name=:name and foo.size=:size"); q.setProperties(fooBean); // fooBean has getName() and getSize() List foos = q.list();
Collections are pageable by using the Query
interface with a filter:
Query q = s.createFilter( collection, "" ); // the trivial filter q.setMaxResults(PAGE_SIZE); q.setFirstResult(PAGE_SIZE * pageNumber); List page = q.list();
Collection elements may be ordered or grouped using a query filter:
Collection orderedCollection = s.filter( collection, "order by this.amount" ); Collection counts = s.filter( collection, "select this.type, count(this) group by this.type" );
You can find the size of a collection without initializing it:
( (Integer) session.createQuery("select count(*) from ....").iterate().next() ).intValue();
Components might be used in just about every way that simple value types can be used in HQL
queries. They can appear in the select
clause:
select p.name from Person p
select p.name.first from Person p
where the Person's name property is a component. Components can also be used
in the where
clause:
from Person p where p.name = :name
from Person p where p.name.first = :firstName
Components can also be used in the order by
clause:
from Person p order by p.name
from Person p order by p.name.first
Another common use of components is in row value constructors.
HQL supports the use of ANSI SQL row value constructor
syntax (sometimes
called tuple
syntax), even though the underlying database may not support
that notion. Here we are generally referring to multi-valued comparisons, typically associated
with components. Consider an entity Person which defines a name component:
from Person p where p.name.first='John' and p.name.last='Jingleheimer-Schmidt'
That's valid syntax, although a little verbose. It be nice to make this a bit more concise and use
row value constructor
syntax:
from Person p where p.name=('John', 'Jingleheimer-Schmidt')
It can also be useful to specify this in the select
clause:
select p.name from Person p
Another time using row value constructor
syntax can be beneficial
is when using subqueries needing to compare against multiple values:
from Cat as cat where not ( cat.name, cat.color ) in ( select cat.name, cat.color from DomesticCat cat )
One thing to consider when deciding if you want to use this syntax is that the query will be dependent upon the ordering of the component sub-properties in the metadata.
Hibernate features an intuitive, extensible criteria query API.
The interface org.hibernate.Criteria
represents a query against
a particular persistent class. The Session
is a factory for
Criteria
instances.
Criteria crit = sess.createCriteria(Cat.class); crit.setMaxResults(50); List cats = crit.list();
An individual query criterion is an instance of the interface
org.hibernate.criterion.Criterion
. The class
org.hibernate.criterion.Restrictions
defines
factory methods for obtaining certain built-in
Criterion
types.
List cats = sess.createCriteria(Cat.class) .add( Restrictions.like("name", "Fritz%") ) .add( Restrictions.between("weight", minWeight, maxWeight) ) .list();
Restrictions may be grouped logically.
List cats = sess.createCriteria(Cat.class) .add( Restrictions.like("name", "Fritz%") ) .add( Restrictions.or( Restrictions.eq( "age", new Integer(0) ), Restrictions.isNull("age") ) ) .list();
List cats = sess.createCriteria(Cat.class) .add( Restrictions.in( "name", new String[] { "Fritz", "Izi", "Pk" } ) ) .add( Restrictions.disjunction() .add( Restrictions.isNull("age") ) .add( Restrictions.eq("age", new Integer(0) ) ) .add( Restrictions.eq("age", new Integer(1) ) ) .add( Restrictions.eq("age", new Integer(2) ) ) ) ) .list();
There are quite a range of built-in criterion types (Restrictions
subclasses), but one that is especially useful lets you specify SQL directly.
List cats = sess.createCriteria(Cat.class) .add( Restrictions.sqlRestriction("lower({alias}.name) like lower(?)", "Fritz%", Hibernate.STRING) ) .list();
The {alias}
placeholder with be replaced by the row alias
of the queried entity.
An alternative approach to obtaining a criterion is to get it from a
Property
instance. You can create a Property
by calling Property.forName()
.
Property age = Property.forName("age"); List cats = sess.createCriteria(Cat.class) .add( Restrictions.disjunction() .add( age.isNull() ) .add( age.eq( new Integer(0) ) ) .add( age.eq( new Integer(1) ) ) .add( age.eq( new Integer(2) ) ) ) ) .add( Property.forName("name").in( new String[] { "Fritz", "Izi", "Pk" } ) ) .list();
You may order the results using org.hibernate.criterion.Order
.
List cats = sess.createCriteria(Cat.class) .add( Restrictions.like("name", "F%") .addOrder( Order.asc("name") ) .addOrder( Order.desc("age") ) .setMaxResults(50) .list();
List cats = sess.createCriteria(Cat.class) .add( Property.forName("name").like("F%") ) .addOrder( Property.forName("name").asc() ) .addOrder( Property.forName("age").desc() ) .setMaxResults(50) .list();
You may easily specify constraints upon related entities by navigating
associations using createCriteria()
.
List cats = sess.createCriteria(Cat.class) .add( Restrictions.like("name", "F%") ) .createCriteria("kittens") .add( Restrictions.like("name", "F%") ) .list();
note that the second createCriteria()
returns a new
instance of Criteria
, which refers to the elements of
the kittens
collection.
The following, alternate form is useful in certain circumstances.
List cats = sess.createCriteria(Cat.class) .createAlias("kittens", "kt") .createAlias("mate", "mt") .add( Restrictions.eqProperty("kt.name", "mt.name") ) .list();
(createAlias()
does not create a new instance of
Criteria
.)
Note that the kittens collections held by the Cat
instances
returned by the previous two queries are not pre-filtered
by the criteria! If you wish to retrieve just the kittens that match the
criteria, you must use a ResultTransformer
.
List cats = sess.createCriteria(Cat.class) .createCriteria("kittens", "kt") .add( Restrictions.eq("name", "F%") ) .setResultTransformer(Criteria.ALIAS_TO_ENTITY_MAP) .list(); Iterator iter = cats.iterator(); while ( iter.hasNext() ) { Map map = (Map) iter.next(); Cat cat = (Cat) map.get(Criteria.ROOT_ALIAS); Cat kitten = (Cat) map.get("kt"); }
You may specify association fetching semantics at runtime using
setFetchMode()
.
List cats = sess.createCriteria(Cat.class) .add( Restrictions.like("name", "Fritz%") ) .setFetchMode("mate", FetchMode.EAGER) .setFetchMode("kittens", FetchMode.EAGER) .list();
This query will fetch both mate
and kittens
by outer join. See Section 19.1, “Fetching strategies” for more information.
The class org.hibernate.criterion.Example
allows
you to construct a query criterion from a given instance.
Cat cat = new Cat(); cat.setSex('F'); cat.setColor(Color.BLACK); List results = session.createCriteria(Cat.class) .add( Example.create(cat) ) .list();
Version properties, identifiers and associations are ignored. By default, null valued properties are excluded.
You can adjust how the Example
is applied.
Example example = Example.create(cat) .excludeZeroes() //exclude zero valued properties .excludeProperty("color") //exclude the property named "color" .ignoreCase() //perform case insensitive string comparisons .enableLike(); //use like for string comparisons List results = session.createCriteria(Cat.class) .add(example) .list();
You can even use examples to place criteria upon associated objects.
List results = session.createCriteria(Cat.class) .add( Example.create(cat) ) .createCriteria("mate") .add( Example.create( cat.getMate() ) ) .list();
The class org.hibernate.criterion.Projections
is a
factory for Projection
instances. We apply a
projection to a query by calling setProjection()
.
List results = session.createCriteria(Cat.class) .setProjection( Projections.rowCount() ) .add( Restrictions.eq("color", Color.BLACK) ) .list();
List results = session.createCriteria(Cat.class) .setProjection( Projections.projectionList() .add( Projections.rowCount() ) .add( Projections.avg("weight") ) .add( Projections.max("weight") ) .add( Projections.groupProperty("color") ) ) .list();
There is no explicit "group by" necessary in a criteria query. Certain
projection types are defined to be grouping projections,
which also appear in the SQL group by
clause.
An alias may optionally be assigned to a projection, so that the projected value may be referred to in restrictions or orderings. Here are two different ways to do this:
List results = session.createCriteria(Cat.class) .setProjection( Projections.alias( Projections.groupProperty("color"), "colr" ) ) .addOrder( Order.asc("colr") ) .list();
List results = session.createCriteria(Cat.class) .setProjection( Projections.groupProperty("color").as("colr") ) .addOrder( Order.asc("colr") ) .list();
The alias()
and as()
methods simply wrap a
projection instance in another, aliased, instance of Projection
.
As a shortcut, you can assign an alias when you add the projection to a
projection list:
List results = session.createCriteria(Cat.class) .setProjection( Projections.projectionList() .add( Projections.rowCount(), "catCountByColor" ) .add( Projections.avg("weight"), "avgWeight" ) .add( Projections.max("weight"), "maxWeight" ) .add( Projections.groupProperty("color"), "color" ) ) .addOrder( Order.desc("catCountByColor") ) .addOrder( Order.desc("avgWeight") ) .list();
List results = session.createCriteria(Domestic.class, "cat") .createAlias("kittens", "kit") .setProjection( Projections.projectionList() .add( Projections.property("cat.name"), "catName" ) .add( Projections.property("kit.name"), "kitName" ) ) .addOrder( Order.asc("catName") ) .addOrder( Order.asc("kitName") ) .list();
You can also use Property.forName()
to express projections:
List results = session.createCriteria(Cat.class) .setProjection( Property.forName("name") ) .add( Property.forName("color").eq(Color.BLACK) ) .list();
List results = session.createCriteria(Cat.class) .setProjection( Projections.projectionList() .add( Projections.rowCount().as("catCountByColor") ) .add( Property.forName("weight").avg().as("avgWeight") ) .add( Property.forName("weight").max().as("maxWeight") ) .add( Property.forName("color").group().as("color" ) ) .addOrder( Order.desc("catCountByColor") ) .addOrder( Order.desc("avgWeight") ) .list();
The DetachedCriteria
class lets you create a query outside the scope
of a session, and then later execute it using some arbitrary Session
.
DetachedCriteria query = DetachedCriteria.forClass(Cat.class) .add( Property.forName("sex").eq('F') ); Session session = ....; Transaction txn = session.beginTransaction(); List results = query.getExecutableCriteria(session).setMaxResults(100).list(); txn.commit(); session.close();
A DetachedCriteria
may also be used to express a subquery. Criterion
instances involving subqueries may be obtained via Subqueries
or
Property
.
DetachedCriteria avgWeight = DetachedCriteria.forClass(Cat.class) .setProjection( Property.forName("weight").avg() ); session.createCriteria(Cat.class) .add( Property.forName("weight").gt(avgWeight) ) .list();
DetachedCriteria weights = DetachedCriteria.forClass(Cat.class) .setProjection( Property.forName("weight") ); session.createCriteria(Cat.class) .add( Subqueries.geAll("weight", weights) ) .list();
Even correlated subqueries are possible:
DetachedCriteria avgWeightForSex = DetachedCriteria.forClass(Cat.class, "cat2") .setProjection( Property.forName("weight").avg() ) .add( Property.forName("cat2.sex").eqProperty("cat.sex") ); session.createCriteria(Cat.class, "cat") .add( Property.forName("weight").gt(avgWeightForSex) ) .list();
For most queries, including criteria queries, the query cache is not very efficient, because query cache invalidation occurs too frequently. However, there is one special kind of query where we can optimize the cache invalidation algorithm: lookups by a constant natural key. In some applications, this kind of query occurs frequently. The criteria API provides special provision for this use case.
First, you should map the natural key of your entity using
<natural-id>
, and enable use of the second-level cache.
<class name="User"> <cache usage="read-write"/> <id name="id"> <generator class="increment"/> </id> <natural-id> <property name="name"/> <property name="org"/> </natural-id> <property name="password"/> </class>
Note that this functionality is not intended for use with entities with mutable natural keys.
Next, enable the Hibernate query cache.
Now, Restrictions.naturalId()
allows us to make use of
the more efficient cache algorithm.
session.createCriteria(User.class) .add( Restrictions.naturalId() .set("name", "gavin") .set("org", "hb") ).setCacheable(true) .uniqueResult();
You may also express queries in the native SQL dialect of your
database. This is useful if you want to utilize database specific features
such as query hints or the CONNECT
keyword in Oracle. It
also provides a clean migration path from a direct SQL/JDBC based
application to Hibernate.
Hibernate3 allows you to specify handwritten SQL (including stored procedures) for all create, update, delete, and load operations.
Execution of native SQL queries is controlled via the
SQLQuery
interface, which is obtained by calling
Session.createSQLQuery()
. The following describes how
to use this API for querying.
The most basic SQL query is to get a list of scalars (values).
sess.createSQLQuery("SELECT * FROM CATS").list(); sess.createSQLQuery("SELECT ID, NAME, BIRTHDATE FROM CATS").list();
These will both return a List of Object arrays (Object[]) with scalar values for each column in the CATS table. Hibernate will use ResultSetMetadata to deduce the actual order and types of the returned scalar values.
To avoid the overhead of using
ResultSetMetadata
or simply to be more explicit in
what is returned one can use addScalar()
.
sess.createSQLQuery("SELECT * FROM CATS") .addScalar("ID", Hibernate.LONG) .addScalar("NAME", Hibernate.STRING) .addScalar("BIRTHDATE", Hibernate.DATE)
This query specified:
the SQL query string
the columns and types to return
This will still return Object arrays, but now it will not use
ResultSetMetadata
but will instead explicitly get the
ID, NAME and BIRTHDATE column as respectively a Long, String and a Short
from the underlying resultset. This also means that only these three
columns will be returned, even though the query is using
*
and could return more than the three listed
columns.
It is possible to leave out the type information for all or some of the scalars.
sess.createSQLQuery("SELECT * FROM CATS") .addScalar("ID", Hibernate.LONG) .addScalar("NAME") .addScalar("BIRTHDATE")
This is essentially the same query as before, but now
ResultSetMetaData
is used to decide the type of NAME
and BIRTHDATE where as the type of ID is explicitly specified.
How the java.sql.Types returned from ResultSetMetaData is mapped
to Hibernate types is controlled by the Dialect. If a specific type is
not mapped or does not result in the expected type it is possible to
customize it via calls to registerHibernateType
in
the Dialect.
The above queries were all about returning scalar values,
basically returning the "raw" values from the resultset. The following
shows how to get entity objects from a native sql query via
addEntity()
.
sess.createSQLQuery("SELECT * FROM CATS").addEntity(Cat.class); sess.createSQLQuery("SELECT ID, NAME, BIRTHDATE FROM CATS").addEntity(Cat.class);
This query specified:
the SQL query string
the entity returned by the query
Assuming that Cat is mapped as a class with the columns ID, NAME and BIRTHDATE the above queries will both return a List where each element is a Cat entity.
If the entity is mapped with a many-to-one
to
another entity it is required to also return this when performing the
native query, otherwise a database specific "column not found" error
will occur. The additional columns will automatically be returned when
using the * notation, but we prefer to be explicit as in the following
example for a many-to-one
to a
Dog
:
sess.createSQLQuery("SELECT ID, NAME, BIRTHDATE, DOG_ID FROM CATS").addEntity(Cat.class);
This will allow cat.getDog() to function properly.
It is possible to eagerly join in the Dog
to
avoid the possible extra roundtrip for initializing the proxy. This is
done via the addJoin()
method, which allows you to
join in an association or collection.
sess.createSQLQuery("SELECT c.ID, NAME, BIRTHDATE, DOG_ID, D_ID, D_NAME FROM CATS c, DOGS d WHERE c.DOG_ID = d.D_ID") .addEntity("cat", Cat.class) .addJoin("cat.dog");
In this example the returned Cat
's will have
their dog
property fully initialized without any
extra roundtrip to the database. Notice that we added a alias name
("cat") to be able to specify the target property path of the join. It
is possible to do the same eager joining for collections, e.g. if the
Cat
had a one-to-many to Dog
instead.
sess.createSQLQuery("SELECT ID, NAME, BIRTHDATE, D_ID, D_NAME, CAT_ID FROM CATS c, DOGS d WHERE c.ID = d.CAT_ID") .addEntity("cat", Cat.class) .addJoin("cat.dogs");
At this stage we are reaching the limits of what is possible with native queries without starting to enhance the sql queries to make them usable in Hibernate; the problems starts to arise when returning multiple entities of the same type or when the default alias/column names are not enough.
Until now the result set column names are assumed to be the same as the column names specified in the mapping document. This can be problematic for SQL queries which join multiple tables, since the same column names may appear in more than one table.
Column alias injection is needed in the following query (which most likely will fail):
sess.createSQLQuery("SELECT c.*, m.* FROM CATS c, CATS m WHERE c.MOTHER_ID = c.ID") .addEntity("cat", Cat.class) .addEntity("mother", Cat.class)
The intention for this query is to return two Cat instances per row, a cat and its mother. This will fail since there is a conflict of names since they are mapped to the same column names and on some databases the returned column aliases will most likely be on the form "c.ID", "c.NAME", etc. which are not equal to the columns specified in the mappings ("ID" and "NAME").
The following form is not vulnerable to column name duplication:
sess.createSQLQuery("SELECT {cat.*}, {mother.*} FROM CATS c, CATS m WHERE c.MOTHER_ID = c.ID") .addEntity("cat", Cat.class) .addEntity("mother", Cat.class)
This query specified:
the SQL query string, with placeholders for Hibernate to inject column aliases
the entities returned by the query
The {cat.*} and {mother.*} notation used above is a shorthand for "all properties". Alternatively, you may list the columns explicitly, but even in this case we let Hibernate inject the SQL column aliases for each property. The placeholder for a column alias is just the property name qualified by the table alias. In the following example, we retrieve Cats and their mothers from a different table (cat_log) to the one declared in the mapping metadata. Notice that we may even use the property aliases in the where clause if we like.
String sql = "SELECT ID as {c.id}, NAME as {c.name}, " + "BIRTHDATE as {c.birthDate}, MOTHER_ID as {c.mother}, {mother.*} " + "FROM CAT_LOG c, CAT_LOG m WHERE {c.mother} = c.ID"; List loggedCats = sess.createSQLQuery(sql) .addEntity("cat", Cat.class) .addEntity("mother", Cat.class).list()
For most cases the above alias injection is needed, but for queries relating to more complex mappings like composite properties, inheritance discriminators, collections etc. there are some specific aliases to use to allow Hibernate to inject the proper aliases.
The following table shows the different possibilities of using the alias injection. Note: the alias names in the result are examples, each alias will have a unique and probably different name when used.
Table 16.1. Alias injection names
Description | Syntax | Example |
---|---|---|
A simple property | {[aliasname].[propertyname] |
A_NAME as {item.name} |
A composite property | {[aliasname].[componentname].[propertyname]} |
CURRENCY as {item.amount.currency}, VALUE as
{item.amount.value} |
Discriminator of an entity | {[aliasname].class} |
DISC as {item.class} |
All properties of an entity | {[aliasname].*} |
{item.*} |
A collection key | {[aliasname].key} |
ORGID as {coll.key} |
The id of an collection | {[aliasname].id} |
EMPID as {coll.id} |
The element of an collection | {[aliasname].element} |
XID as {coll.element} |
roperty of the element in the collection | {[aliasname].element.[propertyname]} |
NAME as {coll.element.name} |
All properties of the element in the collection | {[aliasname].element.*} |
{coll.element.*} |
All properties of the the collection | {[aliasname].*} |
{coll.*} |
It is possible to apply a ResultTransformer to native sql queries. Allowing it to e.g. return non-managed entities.
sess.createSQLQuery("SELECT NAME, BIRTHDATE FROM CATS") .setResultTransformer(Transformers.aliasToBean(CatDTO.class))
This query specified:
the SQL query string
a result transformer
The above query will return a list of CatDTO
which has been instantiated and injected the values of NAME and BIRTHNAME into its corresponding
properties or fields.
Native sql queries which query for entities that is mapped as part of an inheritance must include all properties for the baseclass and all it subclasses.
Native sql queries support positional as well as named parameters:
Query query = sess.createSQLQuery("SELECT * FROM CATS WHERE NAME like ?").addEntity(Cat.class); List pusList = query.setString(0, "Pus%").list(); query = sess.createSQLQuery("SELECT * FROM CATS WHERE NAME like :name").addEntity(Cat.class); List pusList = query.setString("name", "Pus%").list();
Named SQL queries may be defined in the mapping document and called
in exactly the same way as a named HQL query. In this case, we do
not need to call
addEntity()
.
<sql-query name="persons"> <return alias="person" class="eg.Person"/> SELECT person.NAME AS {person.name}, person.AGE AS {person.age}, person.SEX AS {person.sex} FROM PERSON person WHERE person.NAME LIKE :namePattern </sql-query>
List people = sess.getNamedQuery("persons") .setString("namePattern", namePattern) .setMaxResults(50) .list();
The <return-join>
and
<load-collection>
elements are used to join
associations and define queries which initialize collections,
respectively.
<sql-query name="personsWith"> <return alias="person" class="eg.Person"/> <return-join alias="address" property="person.mailingAddress"/> SELECT person.NAME AS {person.name}, person.AGE AS {person.age}, person.SEX AS {person.sex}, address.STREET AS {address.street}, address.CITY AS {address.city}, address.STATE AS {address.state}, address.ZIP AS {address.zip} FROM PERSON person JOIN ADDRESS address ON person.ID = address.PERSON_ID AND address.TYPE='MAILING' WHERE person.NAME LIKE :namePattern </sql-query>
A named SQL query may return a scalar value. You must declare the
column alias and Hibernate type using the
<return-scalar>
element:
<sql-query name="mySqlQuery"> <return-scalar column="name" type="string"/> <return-scalar column="age" type="long"/> SELECT p.NAME AS name, p.AGE AS age, FROM PERSON p WHERE p.NAME LIKE 'Hiber%' </sql-query>
You can externalize the resultset mapping informations in a
<resultset>
element to either reuse them across
several named queries or through the
setResultSetMapping()
API.
<resultset name="personAddress"> <return alias="person" class="eg.Person"/> <return-join alias="address" property="person.mailingAddress"/> </resultset> <sql-query name="personsWith" resultset-ref="personAddress"> SELECT person.NAME AS {person.name}, person.AGE AS {person.age}, person.SEX AS {person.sex}, address.STREET AS {address.street}, address.CITY AS {address.city}, address.STATE AS {address.state}, address.ZIP AS {address.zip} FROM PERSON person JOIN ADDRESS address ON person.ID = address.PERSON_ID AND address.TYPE='MAILING' WHERE person.NAME LIKE :namePattern </sql-query>
You can alternatively use the resultset mapping information in your hbm files directly in java code.
List cats = sess.createSQLQuery( "select {cat.*}, {kitten.*} from cats cat, cats kitten where kitten.mother = cat.id" ) .setResultSetMapping("catAndKitten") .list();
With <return-property>
you can explicitly
tell Hibernate what column aliases to use, instead of using the
{}
-syntax to let Hibernate inject its own
aliases.
<sql-query name="mySqlQuery"> <return alias="person" class="eg.Person"> <return-property name="name" column="myName"/> <return-property name="age" column="myAge"/> <return-property name="sex" column="mySex"/> </return> SELECT person.NAME AS myName, person.AGE AS myAge, person.SEX AS mySex, FROM PERSON person WHERE person.NAME LIKE :name </sql-query>
<return-property>
also works with
multiple columns. This solves a limitation with the
{}
-syntax which can not allow fine grained control of
multi-column properties.
<sql-query name="organizationCurrentEmployments"> <return alias="emp" class="Employment"> <return-property name="salary"> <return-column name="VALUE"/> <return-column name="CURRENCY"/> </return-property> <return-property name="endDate" column="myEndDate"/> </return> SELECT EMPLOYEE AS {emp.employee}, EMPLOYER AS {emp.employer}, STARTDATE AS {emp.startDate}, ENDDATE AS {emp.endDate}, REGIONCODE as {emp.regionCode}, EID AS {emp.id}, VALUE, CURRENCY FROM EMPLOYMENT WHERE EMPLOYER = :id AND ENDDATE IS NULL ORDER BY STARTDATE ASC </sql-query>
Notice that in this example we used
<return-property>
in combination with the
{}
-syntax for injection. Allowing users to choose how
they want to refer column and properties.
If your mapping has a discriminator you must use
<return-discriminator>
to specify the
discriminator column.
Hibernate 3 introduces support for queries via stored procedures and functions. Most of the following documentation is equivalent for both. The stored procedure/function must return a resultset as the first out-parameter to be able to work with Hibernate. An example of such a stored function in Oracle 9 and higher is as follows:
CREATE OR REPLACE FUNCTION selectAllEmployments RETURN SYS_REFCURSOR AS st_cursor SYS_REFCURSOR; BEGIN OPEN st_cursor FOR SELECT EMPLOYEE, EMPLOYER, STARTDATE, ENDDATE, REGIONCODE, EID, VALUE, CURRENCY FROM EMPLOYMENT; RETURN st_cursor; END;
To use this query in Hibernate you need to map it via a named query.
<sql-query name="selectAllEmployees_SP" callable="true"> <return alias="emp" class="Employment"> <return-property name="employee" column="EMPLOYEE"/> <return-property name="employer" column="EMPLOYER"/> <return-property name="startDate" column="STARTDATE"/> <return-property name="endDate" column="ENDDATE"/> <return-property name="regionCode" column="REGIONCODE"/> <return-property name="id" column="EID"/> <return-property name="salary"> <return-column name="VALUE"/> <return-column name="CURRENCY"/> </return-property> </return> { ? = call selectAllEmployments() } </sql-query>
Notice stored procedures currently only return scalars and
entities. <return-join>
and
<load-collection>
are not supported.
To use stored procedures with Hibernate the procedures/functions
have to follow some rules. If they do not follow those rules they are
not usable with Hibernate. If you still want to use these procedures
you have to execute them via session.connection()
.
The rules are different for each database, since database vendors have
different stored procedure semantics/syntax.
Stored procedure queries can't be paged with
setFirstResult()/setMaxResults()
.
Recommended call form is standard SQL92: { ? = call
functionName(<parameters>) }
or { ? = call
procedureName(<parameters>}
. Native call syntax is not
supported.
For Oracle the following rules apply:
A function must return a result set. The first parameter of
a procedure must be an OUT
that returns a
result set. This is done by using a
SYS_REFCURSOR
type in Oracle 9 or 10. In Oracle
you need to define a REF CURSOR
type, see
Oracle literature.
For Sybase or MS SQL server the following rules apply:
The procedure must return a result set. Note that since these servers can/will return multiple result sets and update counts, Hibernate will iterate the results and take the first result that is a result set as its return value. Everything else will be discarded.
If you can enable SET NOCOUNT ON
in your
procedure it will probably be more efficient, but this is not a
requirement.
Hibernate3 can use custom SQL statements for create, update, and
delete operations. The class and collection persisters in Hibernate
already contain a set of configuration time generated strings (insertsql,
deletesql, updatesql etc.). The mapping tags
<sql-insert>
,
<sql-delete>
, and
<sql-update>
override these strings:
<class name="Person"> <id name="id"> <generator class="increment"/> </id> <property name="name" not-null="true"/> <sql-insert>INSERT INTO PERSON (NAME, ID) VALUES ( UPPER(?), ? )</sql-insert> <sql-update>UPDATE PERSON SET NAME=UPPER(?) WHERE ID=?</sql-update> <sql-delete>DELETE FROM PERSON WHERE ID=?</sql-delete> </class>
The SQL is directly executed in your database, so you are free to use any dialect you like. This will of course reduce the portability of your mapping if you use database specific SQL.
Stored procedures are supported if the callable
attribute is set:
<class name="Person"> <id name="id"> <generator class="increment"/> </id> <property name="name" not-null="true"/> <sql-insert callable="true">{call createPerson (?, ?)}</sql-insert> <sql-delete callable="true">{? = call deletePerson (?)}</sql-delete> <sql-update callable="true">{? = call updatePerson (?, ?)}</sql-update> </class>
The order of the positional parameters are currently vital, as they must be in the same sequence as Hibernate expects them.
You can see the expected order by enabling debug logging for the
org.hibernate.persister.entity
level. With this level
enabled Hibernate will print out the static SQL that is used to create,
update, delete etc. entities. (To see the expected sequence, remember to
not include your custom SQL in the mapping files as that will override the
Hibernate generated static sql.)
The stored procedures are in most cases (read: better do it than not) required to return the number of rows inserted/updated/deleted, as Hibernate has some runtime checks for the success of the statement. Hibernate always registers the first statement parameter as a numeric output parameter for the CUD operations:
CREATE OR REPLACE FUNCTION updatePerson (uid IN NUMBER, uname IN VARCHAR2) RETURN NUMBER IS BEGIN update PERSON set NAME = uname, where ID = uid; return SQL%ROWCOUNT; END updatePerson;
You may also declare your own SQL (or HQL) queries for entity loading:
<sql-query name="person"> <return alias="pers" class="Person" lock-mode="upgrade"/> SELECT NAME AS {pers.name}, ID AS {pers.id} FROM PERSON WHERE ID=? FOR UPDATE </sql-query>
This is just a named query declaration, as discussed earlier. You may reference this named query in a class mapping:
<class name="Person"> <id name="id"> <generator class="increment"/> </id> <property name="name" not-null="true"/> <loader query-ref="person"/> </class>
This even works with stored procedures.
You may even define a query for collection loading:
<set name="employments" inverse="true"> <key/> <one-to-many class="Employment"/> <loader query-ref="employments"/> </set>
<sql-query name="employments"> <load-collection alias="emp" role="Person.employments"/> SELECT {emp.*} FROM EMPLOYMENT emp WHERE EMPLOYER = :id ORDER BY STARTDATE ASC, EMPLOYEE ASC </sql-query>
You could even define an entity loader that loads a collection by join fetching:
<sql-query name="person"> <return alias="pers" class="Person"/> <return-join alias="emp" property="pers.employments"/> SELECT NAME AS {pers.*}, {emp.*} FROM PERSON pers LEFT OUTER JOIN EMPLOYMENT emp ON pers.ID = emp.PERSON_ID WHERE ID=? </sql-query>
Hibernate3 provides an innovative new approach to handling data with "visibility" rules. A Hibernate filter is a global, named, parameterized filter that may be enabled or disabled for a particular Hibernate session.
Hibernate3 adds the ability to pre-define filter criteria and attach those filters at both a class and a collection level. A filter criteria is the ability to define a restriction clause very similiar to the existing "where" attribute available on the class and various collection elements. Except these filter conditions can be parameterized. The application can then make the decision at runtime whether given filters should be enabled and what their parameter values should be. Filters can be used like database views, but parameterized inside the application.
In order to use filters, they must first be defined and then attached to the appropriate
mapping elements. To define a filter, use the <filter-def/>
element
within a <hibernate-mapping/>
element:
<filter-def name="myFilter"> <filter-param name="myFilterParam" type="string"/> </filter-def>
Then, this filter can be attached to a class:
<class name="myClass" ...> ... <filter name="myFilter" condition=":myFilterParam = MY_FILTERED_COLUMN"/> </class>
or, to a collection:
<set ...> <filter name="myFilter" condition=":myFilterParam = MY_FILTERED_COLUMN"/> </set>
or, even to both (or multiples of each) at the same time.
The methods on Session
are: enableFilter(String filterName)
,
getEnabledFilter(String filterName)
, and disableFilter(String filterName)
.
By default, filters are not enabled for a given session; they must be explcitly
enabled through use of the Session.enableFilter()
method, which returns an
instance of the Filter
interface. Using the simple filter defined above, this
would look like:
session.enableFilter("myFilter").setParameter("myFilterParam", "some-value");
Note that methods on the org.hibernate.Filter interface do allow the method-chaining common to much of Hibernate.
A full example, using temporal data with an effective record date pattern:
<filter-def name="effectiveDate"> <filter-param name="asOfDate" type="date"/> </filter-def> <class name="Employee" ...> ... <many-to-one name="department" column="dept_id" class="Department"/> <property name="effectiveStartDate" type="date" column="eff_start_dt"/