Hibernate.orgCommunity Documentation
3.5.6-Final
Copyright © 2004 Red Hat, Inc.
September 15, 2010
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. It can also significantly reduce development time otherwise spent with manual data handling in SQL and JDBC.
Hibernate's 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, Tutorial 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.
View 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 [JPwH] 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 from [JPwH].
FAQs are answered on the Hibernate website.
Links to third party demos, examples, and tutorials are maintained 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 tracking 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.
Intended for new users, this chapter provides an step-by-step introduction
to Hibernate, starting with a simple application using an in-memory database. The
tutorial is based on an earlier tutorial 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 have a limited knowledge of JAVA or SQL, it is advised that you start with a good introduction to that technology prior to attempting to learn Hibernate.
The distribution contains another example application under
the tutorial/eg
project source
directory.
For this example, we will set up a small database application that can store events we want to attend and information about the host(s) of these events.
Although you can use whatever database you feel comfortable using, we will use HSQLDB (an in-memory, Java database) to avoid describing installation/setup of any particular database servers.
The first thing we need to do is to set up the development environment. We
will be using the "standard layout" advocated by alot of build tools such
as Maven. Maven, in particular, has a
good resource describing this layout.
As this tutorial is to be a web application, we will be creating and making
use of src/main/java
, src/main/resources
and src/main/webapp
directories.
We will be using Maven in this tutorial, taking advantage of its transitive dependency management capabilities as well as the ability of many IDEs to automatically set up a project for us based on the maven descriptor.
<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">
<modelVersion>4.0.0</modelVersion>
<groupId>org.hibernate.tutorials</groupId>
<artifactId>hibernate-tutorial</artifactId>
<version>1.0.0-SNAPSHOT</version>
<name>First Hibernate Tutorial</name>
<build>
<!-- we dont want the version to be part of the generated war file name -->
<finalName>${artifactId}</finalName>
</build>
<dependencies>
<dependency>
<groupId>org.hibernate</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>
<!-- Hibernate uses slf4j for logging, for our purposes here use the simple backend -->
<dependency>
<groupId>org.slf4j</groupId>
<artifactId>slf4j-simple</artifactId>
</dependency>
<!-- Hibernate gives you a choice of bytecode providers between cglib and javassist -->
<dependency>
<groupId>javassist</groupId>
<artifactId>javassist</artifactId>
</dependency>
</dependencies>
</project>
It is not a requirement to use Maven. If you wish to use something else to
build this tutorial (such as Ant), the layout will remain the same. The only
change is that you will need to manually account for all the needed
dependencies. If you use something like Ivy
providing transitive dependency management you would still use the dependencies
mentioned below. Otherwise, you'd need to grab all
dependencies, both explicit and transitive, and add them to the project's
classpath. If working from the Hibernate distribution bundle, this would mean
hibernate3.jar
, all artifacts in the
lib/required
directory and all files from either the
lib/bytecode/cglib
or lib/bytecode/javassist
directory; additionally you will need both the servlet-api jar and one of the slf4j
logging backends.
Save this file as pom.xml
in the project root directory.
Next, we create a class that represents the event we want to store in the database; it 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;
}
}
This class uses standard JavaBean naming conventions for property getter and setter methods, as well as private visibility for the fields. Although this is the recommended design, it is not required. Hibernate can also access fields directly, the benefit of accessor methods is robustness for refactoring.
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, especially web applications, need to distinguish
objects by identifier, so you should consider this a feature rather
than a limitation. However, we usually do not manipulate the identity
of an object, hence the setter method should be private. Only Hibernate
will assign identifiers when an object is saved. Hibernate can access
public, private, and protected accessor methods, as well as public,
private and 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 or public visibility is required for runtime proxy generation and efficient data retrieval without bytecode instrumentation.
Save this file to the src/main/java/org/hibernate/tutorial/domain
directory.
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 package="org.hibernate.tutorial.domain">
[...]
</hibernate-mapping>
Hibernate DTD is sophisticated. You can use it for auto-completion
of XML mapping elements and attributes in your editor or IDE.
Opening up the DTD file in your text editor is the easiest way to
get an overview of all elements and attributes, and to view the
defaults, as well as some comments. 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
hibernate-core.jar
(it is also included in the
hibernate3.jar
, if using the distribution bundle).
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
a mapping to a table in the SQL database:
<hibernate-mapping package="org.hibernate.tutorial.domain">
<class name="Event" table="EVENTS">
</class>
</hibernate-mapping>
So far we have told Hibernate how to persist and load object of
class Event
to the table
EVENTS
. Each instance is now represented by a
row in that table. Now we can continue by mapping the unique
identifier property to the tables primary key. As we do not want
to care about handling this identifier, we configure Hibernate's
identifier generation strategy for a surrogate primary key column:
<hibernate-mapping package="org.hibernate.tutorial.domain">
<class name="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. The name="id"
mapping
attribute declares the name of the JavaBean property and tells
Hibernate to use the getId()
and
setId()
methods to access the property. The
column attribute tells Hibernate which column of the
EVENTS
table holds the primary key value.
The nested generator
element specifies the
identifier generation strategy (aka how are identifier values
generated?). In this case we choose native
,
which offers a level of portability depending on the configured
database dialect. Hibernate supports database generated, globally
unique, as well as application assigned, identifiers. Identifier
value generation is also one of Hibernate's many extension points
and you can plugin in your own strategy.
native
is no longer consider the best strategy in terms of portability. for further
discussion, see Section 26.4, “Identifier generation”
Lastly, we need to tell Hibernate about the remaining entity class properties. By default, no properties of the class are considered persistent:
<hibernate-mapping package="org.hibernate.tutorial.domain">
<class name="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>
Similar to the id
element, the
name
attribute of the
property
element tells Hibernate which getter
and setter methods to use. In this case, Hibernate will search
for getDate()
, setDate()
,
getTitle()
and setTitle()
methods.
Why does the date
property mapping include the
column
attribute, but the title
does not? Without the column
attribute, Hibernate
by default uses the property name as the column name. This works for
title
, however, date
is a reserved
keyword in most databases so you will need to map it to a different name.
The title
mapping also lacks a type
attribute. The
types declared and used in the mapping files are not Java data types; they are not SQL
database types either. These types are 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 cannot
know if the property, which is of java.util.Date
, should map to a
SQL date
, timestamp
, or time
column.
Full date and time information is preserved by mapping the property with a
timestamp
converter.
Hibernate makes this mapping type determination using reflection when the mapping files are processed. This can take time and resources, so if startup performance is important you should consider explicitly defining the type to use.
Save this mapping file as
src/main/resources/org/hibernate/tutorial/domain/Event.hbm.xml
.
At this point, you should have the persistent class and its mapping file in place. It is now time to configure Hibernate. First let's set up HSQLDB to run in "server mode"
We do this do that the data remains between runs.
We will utilize the Maven exec plugin to launch the HSQLDB server
by running:
mvn exec:java -Dexec.mainClass="org.hsqldb.Server" -Dexec.args="-database.0 file:target/data/tutorial"
You will 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 HSQLDB, delete
all files in the target/data
directory,
and start HSQLDB again.
Hibernate will be connecting to the database on behalf of your application, so it needs to know
how to obtain connections. For this tutorial we will be using a standalone connection
pool (as opposed to a javax.sql.DataSource
). Hibernate comes with
support for two third-party open source JDBC connection pools:
c3p0 and
proxool. However, we will be using the
Hibernate built-in connection pool for this tutorial.
The built-in Hibernate connection pool is in no way intended for production use. It lacks several features found on any decent connection pool.
For Hibernate's configuration, we can use a simple hibernate.properties
file, a
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">update</property>
<mapping resource="org/hibernate/tutorial/domain/Event.hbm.xml"/>
</session-factory>
</hibernate-configuration>
Notice that this configuration file specifies a different DTD
You configure Hibernate's SessionFactory
. SessionFactory is a global
factory responsible for a particular database. If you have several databases, for easier
startup you should use several <session-factory>
configurations in
several configuration files.
The first four property
elements contain the necessary
configuration for the JDBC connection. The dialect property
element specifies the particular SQL variant Hibernate generates.
In most cases, Hibernate is able to properly determine which dialect to use. See Section 26.3, “Dialect resolution” for more information.
Hibernate's automatic session management for persistence contexts is particularly useful
in this context. The hbm2ddl.auto
option turns on automatic generation of
database schemas directly into the database. This can also be turned
off by removing the configuration option, or redirected to a file with the help of
the SchemaExport
Ant task. Finally, add the mapping file(s)
for persistent classes to the configuration.
Save this file as hibernate.cfg.xml
into the
src/main/resources
directory.
We will now build the tutorial with Maven. You will need to
have Maven installed; it is available from the
Maven download page.
Maven will read the /pom.xml
file we created
earlier and know how to perform some basic project tasks. First,
lets run the compile
goal to make sure we can compile
everything so far:
[hibernateTutorial]$ mvn compile [INFO] Scanning for projects... [INFO] ------------------------------------------------------------------------ [INFO] Building First Hibernate Tutorial [INFO] task-segment: [compile] [INFO] ------------------------------------------------------------------------ [INFO] [resources:resources] [INFO] Using default encoding to copy filtered resources. [INFO] [compiler:compile] [INFO] Compiling 1 source file to /home/steve/projects/sandbox/hibernateTutorial/target/classes [INFO] ------------------------------------------------------------------------ [INFO] BUILD SUCCESSFUL [INFO] ------------------------------------------------------------------------ [INFO] Total time: 2 seconds [INFO] Finished at: Tue Jun 09 12:25:25 CDT 2009 [INFO] Final Memory: 5M/547M [INFO] ------------------------------------------------------------------------
It is time to load and store some Event
objects, but first you have to complete the setup with some
infrastructure code. You have to startup Hibernate by building
a global org.hibernate.SessionFactory
object and storing it somewhere for easy access in application code. A
org.hibernate.SessionFactory
is used to
obtain org.hibernate.Session
instances.
A org.hibernate.Session
represents a
single-threaded unit of work. The
org.hibernate.SessionFactory
is a
thread-safe global object that is instantiated once.
We will create a HibernateUtil
helper class that
takes care of startup and makes accessing the
org.hibernate.SessionFactory
more convenient.
package org.hibernate.tutorial.util;
import org.hibernate.SessionFactory;
import org.hibernate.cfg.Configuration;
public class HibernateUtil {
private static final SessionFactory sessionFactory = buildSessionFactory();
private static SessionFactory buildSessionFactory() {
try {
// Create the SessionFactory from hibernate.cfg.xml
return 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;
}
}
Save this code as
src/main/java/org/hibernate/tutorial/util/HibernateUtil.java
This class not only produces the global
org.hibernate.SessionFactory
reference in
its static initializer; it also hides the fact that it uses a
static singleton. We might just as well have looked up the
org.hibernate.SessionFactory
reference from
JNDI in an application server or any other location for that matter.
If you give the org.hibernate.SessionFactory
a name in your configuration, Hibernate will try to bind it to
JNDI under that name after it has been built. Another, better option is to
use a JMX deployment and let the JMX-capable container instantiate and bind
a HibernateService
to JNDI. Such advanced options are
discussed later.
You now need to configure a logging
system. Hibernate uses commons logging and provides two choices: Log4j and
JDK 1.4 logging. Most developers prefer Log4j: copy log4j.properties
from the Hibernate distribution in the etc/
directory to
your src
directory, next to hibernate.cfg.xml
.
If you prefer to have
more verbose output than that provided in the example configuration, you can change the settings. By default, only the Hibernate startup message is shown on stdout.
The tutorial infrastructure is complete and you are now ready to do some real work with Hibernate.
We are now ready to start doing some real work with Hibernate.
Let's start by writing an EventManager
class
with a main()
method:
package org.hibernate.tutorial;
import org.hibernate.Session;
import java.util.*;
import org.hibernate.tutorial.domain.Event;
import org.hibernate.tutorial.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();
}
}
In createAndStoreEvent()
we created a new
Event
object and handed it over to Hibernate.
At that point, Hibernate takes care of the SQL and executes an
INSERT
on the database.
A org.hibernate.Session is designed to
represent a single unit of work (a single atomic piece of work
to be performed). For now we will keep things simple and assume
a one-to-one granularity between a Hibernate
org.hibernate.Session and a database
transaction. To shield our code from the actual underlying
transaction system we use the Hibernate
org.hibernate.Transaction
API.
In this particular case we are using JDBC-based transactional
semantics, but it could also run with JTA.
What does sessionFactory.getCurrentSession()
do?
First, you can call it as many times and anywhere you like
once you get hold of your
org.hibernate.SessionFactory
.
The getCurrentSession()
method always returns
the "current" unit of work. Remember that we switched
the configuration option for this mechanism to "thread" in our
src/main/resources/hibernate.cfg.xml
?
Due to that setting, the context of a current unit of work is bound
to the current Java thread that executes the application.
Hibernate offers three methods of current session tracking. The "thread" based method is not intended for production use; it is merely useful for prototyping and tutorials such as this one. Current session tracking is discussed in more detail later on.
A org.hibernate.Session begins when the
first call to getCurrentSession()
is made for
the current thread. It is then bound by Hibernate to the current
thread. When the transaction ends, either through commit or
rollback, Hibernate automatically unbinds the
org.hibernate.Session from the thread
and closes it for you. If you call
getCurrentSession()
again, you get a new
org.hibernate.Session and can start a
new unit of work.
Related to the unit of work scope, should the Hibernate org.hibernate.Session be used to execute one or several database operations? The above example uses one org.hibernate.Session for one operation. However this is pure coincidence; the example is just not complex enough to show any other approach. The scope of a Hibernate org.hibernate.Session is flexible but you should never design your application to use a new Hibernate org.hibernate.Session for every database operation. Even though it is used in the following examples, consider session-per-operation an anti-pattern. A real web application is shown later in the tutorial which will help illustrate this.
See Chapter 12, Transactions and Concurrency for more information about transaction handling and demarcation. The previous example also skipped any error handling and rollback.
To run this, we will make use of the Maven exec plugin to call our class
with the necessary classpath setup:
mvn exec:java -Dexec.mainClass="org.hibernate.tutorial.EventManager" -Dexec.args="store"
You may need to perform mvn compile
first.
You should see Hibernate starting up and, depending on your configuration, lots of log output. Towards the end, the following line will be displayed:
[java] Hibernate: insert into EVENTS (EVENT_DATE, title, EVENT_ID) values (?, ?, ?)
This is the INSERT
executed by Hibernate.
To list stored events an option is added 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()
);
}
}
A new listEvents() method is also added
:
private List listEvents() {
Session session = HibernateUtil.getSessionFactory().getCurrentSession();
session.beginTransaction();
List result = session.createQuery("from Event").list();
session.getTransaction().commit();
return result;
}
Here, we are using a Hibernate Query Language (HQL) 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. See Chapter 15, HQL: The Hibernate Query Language
for more information.
Now we can call our new functionality, again using the Maven exec plugin:
mvn exec:java -Dexec.mainClass="org.hibernate.tutorial.EventManager" -Dexec.args="list"
So far we have mapped a single persistent entity class to a table in isolation. Let's expand on that a bit and add some class associations. We will add people to the application and store a list of events in which they participate.
The first cut of the Person
class looks like this:
package org.hibernate.tutorial.domain;
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'
}
Save this to a file named
src/main/java/org/hibernate/tutorial/domain/Person.java
Next, create the new mapping file as
src/main/resources/org/hibernate/tutorial/domain/Person.hbm.xml
<hibernate-mapping package="org.hibernate.tutorial.domain">
<class name="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"/>
Create an association between these two entities. Persons can participate in events, and events have participants. The design questions you have to deal with are: directionality, multiplicity, and collection behavior.
By adding a collection of events to the Person
class, you can easily navigate to the events for a particular person,
without executing an explicit query - by calling
Person#getEvents
. Multi-valued associations
are represented in Hibernate by one of the Java Collection Framework
contracts; here we choose a java.util.Set
because the collection will not contain duplicate elements and the ordering
is not relevant to our examples:
public class Person {
private Set events = new HashSet();
public Set getEvents() {
return events;
}
public void setEvents(Set events) {
this.events = events;
}
}
Before mapping this association, let's consider the other side.
We could just keep this unidirectional or create another
collection on the Event
, if we wanted to be
able to navigate it from both directions. This is not necessary,
from a functional perspective. You can 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 is called a many-to-many
association. Hence, we use Hibernate's many-to-many mapping:
<class name="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="Event"/>
</set>
</class>
Hibernate supports a broad range of collection mappings, a
set
being most common. For a many-to-many
association, or n:m entity relationship, an
association table is required. Each row in this table represents
a link between a person and an event. The table name is
decalred using the table
attribute of the
set
element. The identifier column name in
the association, for the person 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 (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 | |_____________|
Now we will 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. 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. 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
org.hibernate.Session
, 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.
You can load person and event in different units of work. Or
you can modify an object outside of a
org.hibernate.Session
, when it
is not in persistent state (if it was persistent before, this
state is called 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 by binding 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.
This is not much use in our example, but it is an important concept you can
incorporate into your own application. Complete this exercise by adding a new action
to the main method of the EventManager
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 is 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
java.lang.String
. We call these classes
value types, and their instances
depend on a particular entity. Instances of
these types do not have their own identity, nor are they shared
between entities. Two persons do not reference the same
firstname
object, even if they have the same
first name. Value types cannot only be found in the JDK , but
you can also write dependent classes yourself
such as an Address
or
MonetaryAmount
class. In fact, in a Hibernate
application all JDK classes are considered value types.
You can also design a collection of value types. This is conceptually different from a collection of references to other entities, but looks almost the same in Java.
Let's add a collection of email addresses to the
Person
entity. This will be represented as a
java.util.Set
of
java.lang.String
instances:
private Set emailAddresses = new HashSet();
public Set getEmailAddresses() {
return emailAddresses;
}
public void setEmailAddresses(Set emailAddresses) {
this.emailAddresses = emailAddresses;
}
The mapping of this Set
is as follows:
<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 use of
the element
part which tells Hibernate that the
collection does not contain references to another entity, but is
rather a collection whose elements are values types, here specifically
of type string
. The lowercase name tells you
it is a Hibernate mapping type/converter. 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 email address values will actually
be stored.
Here is 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 that uses both columns. This also implies that there cannot be duplicate email addresses per person, which is exactly the semantics we need for a set in Java.
You can now try to add elements to this collection, just like we did before by linking persons and events. It is 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);
// adding to the emailAddress collection might trigger a lazy load of the collection
aPerson.getEmailAddresses().add(emailAddress);
session.getTransaction().commit();
}
This time we did not use a fetch query to initialize the collection. Monitor the SQL log and try to optimize this with an eager fetch.
Next you will map a bi-directional association. You will make the association between person and event work from both sides in Java. The database schema does not change, so you will still have many-to-many multiplicity.
A relational database is more flexible than a network programming language, in that it does not need a navigation direction; data can be viewed and retrieved in any possible way.
First, add a collection of participants to the
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 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>
These are normal set
mappings in both mapping documents.
Notice that the column names in key
and many-to-many
swap 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? You add an instance of Event
to the collection of event references,
of an instance of Person
. If you want to make this link
bi-directional, you have to do the same on the other side by adding a Person
reference to the collection in an Event
. This process of "setting the link on both sides"
is absolutely necessary with bi-directional links.
Many developers program defensively and create link management methods to
correctly set both sides (for example, 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);
}
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 altering the collections directly. Repeat the steps for 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, does not
have enough information to correctly arrange SQL INSERT
and UPDATE
statements (to avoid constraint violations). Making one side of the association inverse
tells Hibernate to consider it a mirror of the other side. That is all that is necessary
for Hibernate to resolve any issues that arise when transforming a directional navigation model to
a SQL database schema. The rules are straightforward: all bi-directional associations
need one side as inverse
. In a one-to-many association it has to be the many-side,
and in many-to-many association you can select either side.
A Hibernate web application uses Session
and Transaction
almost like a standalone application. However, some common patterns are useful. You can now write
an EventManagerServlet
. This servlet can list all events stored in the
database, and it provides an HTML form to enter new events.
First we need create our basic processing servlet. Since our
servlet only handles HTTP GET
requests, we
will only implement the doGet()
method:
package org.hibernate.tutorial.web;
// Imports
public class EventManagerServlet extends HttpServlet {
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();
if ( ServletException.class.isInstance( ex ) ) {
throw ( ServletException ) ex;
}
else {
throw new ServletException( ex );
}
}
}
}
Save this servlet as
src/main/java/org/hibernate/tutorial/web/EventManagerServlet.java
The pattern applied 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
. A database transaction is then started. All
data access occurs inside a transaction irrespective of whether the data is read or written.
Do not 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 will get to that part soon.
Finally, the unit of work ends when processing and rendering are complete. If any
problems 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 will need it as soon
as you consider rendering your view in JSP, not in a servlet.
Now you can implement the processing of the request and the 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();
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>");
Iterator it = result.iterator();
while (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);
}
The servlet is now 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 would 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 for testing we must create a
Web ARchive (WAR). First we must define the WAR descriptor
as src/main/webapp/WEB-INF/web.xml
<?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>org.hibernate.tutorial.web.EventManagerServlet</servlet-class>
</servlet>
<servlet-mapping>
<servlet-name>Event Manager</servlet-name>
<url-pattern>/eventmanager</url-pattern>
</servlet-mapping>
</web-app>
To build and deploy call mvn package
in your
project directory and copy the hibernate-tutorial.war
file into your Tomcat webapps
directory.
If you do not have Tomcat installed, download it from http://tomcat.apache.org/ and follow the installation instructions. Our application requires no changes to the standard Tomcat configuration.
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. More tutorials are available from the Hibernate website.
The diagram below provides a high-level view of the Hibernate architecture:
We do not have the scope in this document to provide a more detailed view of all the runtime architectures available; Hibernate is flexible and supports several different approaches. We will, however, show the two extremes: "minimal" architecture and "comprehensive" architecture.
This next diagram illustrates how Hibernate utilizes database and configuration data to provide persistence services, and persistent objects, to the application.
The "minimal" 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 "comprehensive" architecture abstracts the application away from the underlying JDBC/JTA APIs and allows Hibernate to manage the details.
Here are some definitions of the objects depicted 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
, SessionFactory
can 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. It wraps a JDBC connection and is a factory
for Transaction
. Session
holds a mandatory first-level cache
of persistent objects that are used when navigating the object graph or looking up
objects by identifier.
Short-lived, single threaded objects containing persistent state and business
function. These can be ordinary JavaBeans/POJOs. They are associated with exactly one
Session
. Once the Session
is closed,
they will be detached and free to use in any application layer (for example, 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. It abstracts the application from the 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. It abstracts the application from
underlying Datasource
or DriverManager
.
It is not exposed to application, but it can be extended and/or implemented by the developer.
org.hibernate.TransactionFactory
)
(Optional) A factory for Transaction
instances. It is not exposed to the
application, but it can be extended and/or implemented by the developer.
Hibernate offers a range of optional extension interfaces you can implement to customize the behavior of your persistence layer. See the API documentation for details.
Given a "minimal" architecture, the application bypasses the
Transaction
/TransactionFactory
and/or
ConnectionProvider
APIs to communicate with JTA or JDBC directly.
An instance of a persistent class can be in one of three different states. These states are
defined in relation to a persistence context.
The Hibernate Session
object is the persistence context. The three different states are as follows:
The instance is not associated with any persistence context. It has no persistent identity or primary key value.
The instance is currently associated with a persistence context. It has a persistent identity (primary key value) and can have a corresponding row in the database. For a particular persistence context, Hibernate guarantees that persistent identity is equivalent to Java identity in relation to the 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 can have a corresponding row in the database. For detached instances, Hibernate does not guarantee the relationship between persistent identity and Java identity.
JMX is the J2EE standard for the management of Java components. Hibernate can be managed via
a JMX standard service. AN MBean implementation is provided in the distribution:
org.hibernate.jmx.HibernateService
.
For an example of how to deploy Hibernate as a JMX service on the JBoss Application Server, please see the JBoss User Guide. JBoss AS also provides 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 that you no
longer have to manually open and close the Session
; this
becomes the job of a JBoss EJB interceptor. You also do not have to worry about
transaction demarcation in your code (if you would like to write a portable
persistence layer use the optional Hibernate Transaction
API for this). You call the HibernateContext
to access a
Session
.
HAR deployment: the Hibernate JMX service is deployed using a JBoss
service deployment descriptor in an EAR and/or SAR file, as it supports all the usual
configuration options of a Hibernate SessionFactory
. However, you still
need to name all your mapping files in the deployment descriptor. If you 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 is runtime Hibernate statistics. See Section 3.4.6, “Hibernate statistics” for more information.
Hibernate can also be configured as a JCA connector. Please see the website for more information. Please note, however, that at this stage Hibernate JCA support is under development.
Most applications using Hibernate need some form of "contextual" session, 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; 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.
Given the maturity of the numerous stand-alone
JTA TransactionManager
implementations, 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 are all you 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. 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 not open, flush, or close a Session
.
The first two implementations provide a "one session - one database transaction" programming
model. This is 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, you can utilize the JTA interfaces to demarcate transactions. If you
execute in an EJB container that supports CMT, transaction boundaries are defined declaratively
and you do not need any transaction or session demarcation operations in your code.
Refer to Chapter 12, 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. For backwards compatibility, if this configuration parameter 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".
Hibernate is designed to operate in many different environments and, as such, there
is a broad range of configuration parameters. Fortunately, most have sensible
default values and Hibernate is distributed with an example
hibernate.properties
file in etc/
that displays
the various options. Simply put the example file in your classpath and customize it to suit your needs.
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 can 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()
. For example:
Configuration cfg = new Configuration()
.addResource("Item.hbm.xml")
.addResource("Bid.hbm.xml");
An alternative way is to specify the mapped class and allow Hibernate to find the mapping document for you:
Configuration cfg = new Configuration()
.addClass(org.hibernate.auction.Item.class)
.addClass(org.hibernate.auction.Bid.class);
Hibernate will then search 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. For example:
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. Some alternative 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
(this is discussed later).
If you want to get started quicklyhibernate.properties
is the easiest approach.
The org.hibernate.cfg.Configuration
is intended as a startup-time object that will
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.
It is advisable 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
Once you start a task that requires access to the database, a JDBC connection will be obtained from the pool.
Before you can do this, you first need to pass some JDBC connection properties to Hibernate. All Hibernate property
names and semantics are defined on the class org.hibernate.cfg.Environment
.
The most important settings for JDBC connection configuration are outlined below.
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 would like to use Proxool,
refer to the packaged hibernate.properties
and the Hibernate web site for more
information.
The following 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 will
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 is 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 can be given by prepending "hibernate.connection
" to the
connection property name. For example, you can specify a charSet
connection property using hibernate.connection.charSet.
You can 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 behavior of Hibernate at runtime. All are optional and have reasonable default values.
Some of these properties are "system-level" only. System-level properties can
be set only via java -Dproperty=value
or hibernate.properties
. They
cannot 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.
e.g.
In most cases Hibernate will actually be able to choose 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 .
e.g.
|
hibernate.format_sql |
Pretty print the SQL in the log and console.
e.g.
|
hibernate.default_schema |
Qualify unqualified table names with the given schema/tablespace
in generated SQL.
e.g.
|
hibernate.default_catalog |
Qualifies unqualified table names with the given catalog
in generated SQL.
e.g.
|
hibernate.session_factory_name |
The org.hibernate.SessionFactory will be automatically
bound to this name in JNDI after it has been created.
e.g.
|
hibernate.max_fetch_depth |
Sets 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.
e.g.
recommended values between |
hibernate.default_batch_fetch_size |
Sets a default size for Hibernate batch fetching of associations.
e.g.
recommended values |
hibernate.default_entity_mode |
Sets a default mode for entity representation for all sessions
opened from this SessionFactory
|
hibernate.order_updates |
Forces 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.
e.g.
|
hibernate.generate_statistics |
If enabled, Hibernate will collect statistics useful for
performance tuning.
e.g.
|
hibernate.use_identifier_rollback |
If enabled, generated identifier properties will be
reset to default values when objects are deleted.
e.g.
|
hibernate.use_sql_comments |
If turned on, Hibernate will generate comments inside the SQL, for
easier debugging, defaults to false .
e.g.
|
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.
e.g.
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 .
e.g.
|
hibernate.jdbc.factory_class |
Select a custom org.hibernate.jdbc.Batcher . Most applications
will not need this configuration property.
e.g.
|
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.
e.g.
|
hibernate.jdbc.use_streams_for_binary |
Use streams when writing/reading binary or serializable
types to/from JDBC. *system-level property*
e.g.
|
hibernate.jdbc.use_get_generated_keys |
Enables 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, it tries to determine the driver capabilities
using connection metadata.
e.g.
|
hibernate.connection.provider_class |
The classname of a custom org.hibernate.connection.ConnectionProvider
which provides JDBC connections to Hibernate.
e.g.
|
hibernate.connection.isolation |
Sets 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.
e.g.
|
hibernate.connection.autocommit |
Enables autocommit for JDBC pooled connections (it is not recommended).
e.g.
|
hibernate.connection.release_mode |
Specifies 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, 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.
e.g.
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 .
e.g.
|
hibernate.cache.use_minimal_puts
|
Optimizes 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.
e.g.
|
hibernate.cache.use_query_cache
|
Enables the query cache. Individual queries still have to be set cachable.
e.g.
|
hibernate.cache.use_second_level_cache
|
Can be used to completely disable the second level cache, which is enabled
by default for classes which specify a <cache>
mapping.
e.g.
|
hibernate.cache.query_cache_factory
|
The classname of a custom QueryCache interface,
defaults to the built-in StandardQueryCache .
e.g.
|
hibernate.cache.region_prefix
|
A prefix to use for second-level cache region names.
e.g.
|
hibernate.cache.use_structured_entries
|
Forces Hibernate to store data in the second-level cache
in a more human-friendly format.
e.g.
|
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 ).
e.g.
|
jta.UserTransaction
|
A JNDI name used by JTATransactionFactory to
obtain the JTA UserTransaction from the
application server.
e.g.
|
hibernate.transaction.manager_lookup_class
|
The classname of a TransactionManagerLookup . It is
required when JVM-level caching is enabled or when using hilo
generator in a JTA environment.
e.g.
|
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”.
e.g.
|
hibernate.transaction.auto_close_session
|
If enabled, the session will be automatically closed during the
after completion phase of the transaction. Built-in and
automatic session context management is preferred, see
Section 2.5, “Contextual sessions”.
e.g.
|
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.
e.g.
|
hibernate.query.factory_class
|
Chooses the HQL parser implementation.
e.g.
|
hibernate.query.substitutions
|
Is used to map from tokens in Hibernate queries to SQL tokens
(tokens might be function or literal names, for example).
e.g.
|
hibernate.hbm2ddl.auto
|
Automatically validates or exports 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.
e.g.
|
hibernate.bytecode.use_reflection_optimizer
|
Enables the use of bytecode manipulation instead of
runtime reflection. This is a System-level property and cannot be set
in e.g. |
hibernate.bytecode.provider
| Both javassist or cglib can be used as byte manipulation
engines; the default is e.g. |
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. This means that you will not have to specify 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 | org.hibernate.dialect.Oracle9iDialect |
Oracle 10g | org.hibernate.dialect.Oracle10gDialect |
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. This is, however, 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 can 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 that have been mapped with
fetch="join"
.
See Section 20.1, “Fetching strategies” for more information.
Oracle limits the size of byte
arrays that can
be passed to and/or 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 20.2, “The Second Level Cache” for
more information.
You can define new Hibernate query tokens using hibernate.query.substitutions
.
For example:
hibernate.query.substitutions true=1, false=0
This would cause the tokens true
and false
to be translated to
integer literals in the generated SQL.
hibernate.query.substitutions toLowercase=LOWER
This would allow you to rename the SQL LOWER
function.
If you enable hibernate.generate_statistics
, Hibernate
exposes 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 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.
It is recommended 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. This is a lot of information but it is 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 can 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 can 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 of
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>
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. It 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 select 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),
especially distributed transaction handling across several datasources. You can
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
can be automatically bound to the scope of JTA transactions. Simply
lookup the SessionFactory
from JNDI and get the current
Session
. Let Hibernate manage 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 choose 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 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 can 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, or built-in, choices:
org.hibernate.transaction.JDBCTransactionFactory
delegates to database (JDBC) transactions (default)
org.hibernate.transaction.JTATransactionFactory
delegates to container-managed transactions if an existing transaction is underway in this context (for example, EJB session bean method). Otherwise, a new transaction is started and bean-managed transactions are used.
org.hibernate.transaction.CMTTransactionFactory
delegates to container-managed JTA transactions
You can 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, since J2EE does not standardize a single mechanism, you have to specify how Hibernate should obtain a reference to the
TransactionManager
:
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
function of the factory and create new Session
s. This
is not, however, 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 (e.g. 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 (in Tomcat, for example).
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 have
this call in some startup code, or utility class in your application, unless you use
JMX deployment with the HibernateService
(this is discussed later in greater detail).
If you use a JNDI SessionFactory
, an EJB or any other class, you can
obtain the SessionFactory
using a JNDI lookup.
It is recommended 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 Hibernate—see chapter 1.
The easiest way to handle Sessions
and transactions is
Hibernate's automatic "current" Session
management.
For a discussion of contextual sessions see Section 2.5, “Contextual 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 the "jta"
context
are 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 can 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) can be kept in their own JAR file, but you can 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. For example, an instance can 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 can express a domain model in other
ways (using trees of Map
instances, for example).
Most Java applications require a persistent class representing felines. For example:
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);
}
}
The four main rules of persistent classes are explored in more detail in the following sections.
Cat
has a no-argument constructor. All persistent classes must
have a default constructor (which can be non-public) so that Hibernate can instantiate
them using Constructor.newInstance()
. It is recommended that you have 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 use a user-defined class
with properties of these types (see the section on composite identifiers later in the chapter.)
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 that 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 that you declare consistently-named identifier properties on persistent classes and that you use a nullable (i.e., 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. You will not, however, be able to use proxies for lazy association fetching which
will ultimately 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. 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
. If required, you can switch to direct
field access for particular properties.
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
. For example:
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. When you mix instances retrieved in
different sessions, you must implement equals()
and
hashCode()
if you 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, because they are equal. If both are added to a Set
,
you will only have one element in the Set
). Unfortunately, you cannot 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. This is not
a Hibernate issue, but normal Java semantics of object identity and equality.
It is recommended that you implement equals()
and hashCode()
using Business key equality. Business key equality means that the
equals()
method compares only the properties that form the business
key. It is 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;
}
}
A business key does not have to be as solid as a database primary key candidate (see Section 12.1.3, “Considering object identity”). Immutable or unique properties are usually good candidates for a business key.
The following features are currently considered experimental and may change in the near future.
Persistent entities do not 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 do not
write persistent classes, only mapping files.
By default, Hibernate works in normal POJO mode. You can 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 demonstrate 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>
Even though associations are declared using target class names, the target type of associations can also be a dynamic entity instead of a POJO.
After setting the default entity mode to dynamic-map
for the SessionFactory
, you can, at runtime, work with
Map
s of Map
s:
Session s = openSession();
Transaction tx = s.beginTransaction();
// 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();
One of the main advantages of dynamic mapping is quick turnaround time for prototyping, without the need for entity class implementation. However, you lose compile-time type checking and will likely deal with many exceptions at runtime. As a result of 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 do not have to call 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 19, 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 that 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 corresponding tuplizer knows how create the POJO through its
constructor. It also knows 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 can 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. Tuplizer 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();
}
}
}
The org.hibernate.EntityNameResolver
interface is a contract for resolving the
entity name of a given entity instance. The interface defines a single method resolveEntityName
which is passed the entity instance and is expected to return the appropriate entity name (null is allowed and
would indicate that the resolver does not know how to resolve the entity name of the given entity instance).
Generally speaking, an org.hibernate.EntityNameResolver
is going to be most
useful in the case of dynamic models. One example might be using proxied interfaces as your domain model. The
hibernate test suite has an example of this exact style of usage under the
org.hibernate.test.dynamicentity.tuplizer2. Here is some of the code from that package
for illustration.
/**
* A very trivial JDK Proxy InvocationHandler implementation where we proxy an interface as
* the domain model and simply store persistent state in an internal Map. This is an extremely
* trivial example meant only for illustration.
*/
public final class DataProxyHandler implements InvocationHandler {
private String entityName;
private HashMap data = new HashMap();
public DataProxyHandler(String entityName, Serializable id) {
this.entityName = entityName;
data.put( "Id", id );
}
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
String methodName = method.getName();
if ( methodName.startsWith( "set" ) ) {
String propertyName = methodName.substring( 3 );
data.put( propertyName, args[0] );
}
else if ( methodName.startsWith( "get" ) ) {
String propertyName = methodName.substring( 3 );
return data.get( propertyName );
}
else if ( "toString".equals( methodName ) ) {
return entityName + "#" + data.get( "Id" );
}
else if ( "hashCode".equals( methodName ) ) {
return new Integer( this.hashCode() );
}
return null;
}
public String getEntityName() {
return entityName;
}
public HashMap getData() {
return data;
}
}
/**
*
*/
public class ProxyHelper {
public static String extractEntityName(Object object) {
// Our custom java.lang.reflect.Proxy instances actually bundle
// their appropriate entity name, so we simply extract it from there
// if this represents one of our proxies; otherwise, we return null
if ( Proxy.isProxyClass( object.getClass() ) ) {
InvocationHandler handler = Proxy.getInvocationHandler( object );
if ( DataProxyHandler.class.isAssignableFrom( handler.getClass() ) ) {
DataProxyHandler myHandler = ( DataProxyHandler ) handler;
return myHandler.getEntityName();
}
}
return null;
}
// various other utility methods ....
}
/**
* The EntityNameResolver implementation.
* IMPL NOTE : An EntityNameResolver really defines a strategy for how entity names should be
* resolved. Since this particular impl can handle resolution for all of our entities we want to
* take advantage of the fact that SessionFactoryImpl keeps these in a Set so that we only ever
* have one instance registered. Why? Well, when it comes time to resolve an entity name,
* Hibernate must iterate over all the registered resolvers. So keeping that number down
* helps that process be as speedy as possible. Hence the equals and hashCode impls
*/
public class MyEntityNameResolver implements EntityNameResolver {
public static final MyEntityNameResolver INSTANCE = new MyEntityNameResolver();
public String resolveEntityName(Object entity) {
return ProxyHelper.extractEntityName( entity );
}
public boolean equals(Object obj) {
return getClass().equals( obj.getClass() );
}
public int hashCode() {
return getClass().hashCode();
}
}
public class MyEntityTuplizer extends PojoEntityTuplizer {
public MyEntityTuplizer(EntityMetamodel entityMetamodel, PersistentClass mappedEntity) {
super( entityMetamodel, mappedEntity );
}
public EntityNameResolver[] getEntityNameResolvers() {
return new EntityNameResolver[] { MyEntityNameResolver.INSTANCE };
}
public String determineConcreteSubclassEntityName(Object entityInstance, SessionFactoryImplementor factory) {
String entityName = ProxyHelper.extractEntityName( entityInstance );
if ( entityName == null ) {
entityName = super.determineConcreteSubclassEntityName( entityInstance, factory );
}
return entityName;
}
...
}
In order to register an org.hibernate.EntityNameResolver
users must either:
Implement a custom Tuplizer, implementing
the getEntityNameResolvers
method.
Register it with the org.hibernate.impl.SessionFactoryImpl
(which is the
implementation class for org.hibernate.SessionFactory
) using the
registerEntityNameResolver
method.
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 and not table declarations.
Please note that even though many Hibernate users choose to write the XML by hand, a number of tools exist to generate the mapping document. These include XDoclet, Middlegen and AndroMDA.
Here is 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, however, 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 can 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 the DTD declaration against the contents of your
classpath.
Hibernate will first attempt to resolve DTDs in its classpath.
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 encounters a systemId starting with
http://hibernate.sourceforge.net/
. The resolver
attempts to resolve these entities via the classloader which loaded
the Hibernate classes.
a user namespace
is recognized whenever the
resolver encounters 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.
The following is 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 version "3.5.6-Final">
<!ENTITY today "September 15, 2010">
<!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 they are specified, tablenames will be qualified
by the given schema and catalog names. If they are missing, tablenames will be unqualified.
The default-cascade
attribute specifies what cascade style
should be assumed for properties and collections that do not specify a
cascade
attribute. By default, the auto-import
attribute allows you
to use unqualified class names in the query language.
<hibernate-mapping schema="schemaName" catal
og="catalogName" defau
lt-cascade="cascade_style" defau
lt-access="field|property|ClassName" defau
lt-lazy="true|false" auto-
import="true|false" packa
ge="package.name" />
| |
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| |
|
If you have two persistent classes with the same unqualified name, you should set
auto-import="false"
. An exception will result if you attempt
to assign two classes to the same "imported" name.
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. For example, Cat.hbm.xml
,
Dog.hbm.xml
, or if using inheritance,
Animal.hbm.xml
.
You can declare a persistent class using the class
element. For example:
<class name="ClassName" table=
"tableName" discri
minator-value="discriminator_value" mutabl
e="true|false" schema
="owner" catalo
g="catalog" proxy=
"ProxyInterface" dynami
c-update="true|false" dynami
c-insert="true|false" select
-before-update="true|false" polymo
rphism="implicit|explicit" where=
"arbitrary sql where condition" persis
ter="PersisterClass" batch-
size="N" optimi
stic-lock="none|version|dirty|all" lazy="(16)true|false" entity(17)-name="EntityName" check=(18)"arbitrary sql check condition" rowid=(19)"rowid" subsel(20)ect="SQL expression" abstra(21)ct="true|false" node="element-name" />
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| |
(16) |
|
(17) |
|
(18) |
|
(19) |
|
(20) |
|
(21) |
|
It is acceptable for the named persistent class to be an interface. You can
declare implementing classes of that interface using the <subclass>
element. You can persist any static inner class. Specify the
class name using the standard form i.e. e.g.Foo$Bar
.
Immutable classes, mutable="false"
, cannot 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 that implement
the named interface. The persistent object will load 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 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. 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 can, for example, specify your own subclass of
org.hibernate.persister.EntityPersister
, or you can even provide a
completely new implementation of the interface
org.hibernate.persister.ClassPersister
that implements, for example, persistence via
stored procedure calls, serialization to flat files or LDAP. See
org.hibernate.test.CustomPersister
for a simple example of "persistence"
to a Hashtable
.
The dynamic-update
and dynamic-insert
settings are not inherited by subclasses, so they can also be specified on the
<subclass>
or <joined-subclass>
elements.
Although these settings can increase performance in some cases, they can actually decrease
performance in others.
Use of select-before-update
will usually decrease performance. It is
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
It is strongly recommended that you use version/timestamp
columns for optimistic locking with Hibernate.
This strategy optimizes performance and correctly handles modifications
made to detached instances (i.e. when Session.merge()
is used).
There is no difference between a view and a base table for a Hibernate mapping. This is transparent at the database level, although some DBMS do not support views properly, especially with updates. Sometimes you want to use a view, but you cannot create one in the database (i.e. 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 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" type="
typename" column
="column_name" unsave
d-value="null|any|none|undefined|id_value" access
="field|property|ClassName"> node="element-name|@attribute-name|element/@attribute|." <generator class="generatorClass"/> </id>
| |
| |
| |
| |
|
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 that allows access to
legacy data with composite keys. Its use is strongly discouraged 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 can choose to provide their own specialized
implementations, however, Hibernate provides a range of built-in implementations. The shortcut
names for the built-in generators are as follows:
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 that are unique within a network (the IP address is used). The UUID is encoded as a string of 32 hexadecimal digits in length.
guid
uses a database-generated GUID string on MS SQL Server and MySQL.
native
selects identity
, sequence
or
hilo
depending upon the capabilities of the
underlying database.
assigned
lets the application 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. It is usually used in conjunction
with a <one-to-one>
primary key association.
sequence-identity
a specialized sequence generation strategy that utilizes a database sequence for the actual value generation, but combines this with JDBC3 getGeneratedKeys to return the generated identifier value as part of the insert statement execution. This strategy is only supported on Oracle 10g drivers targeted for JDK 1.4. 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. The
first implementation requires a "special" database table to hold the next available "hi" value.
Where supported, the second uses an Oracle-style sequence.
<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 cannot use hilo
when supplying your own
Connection
to Hibernate. When Hibernate uses an application
server datasource to obtain connections enlisted with JTA, you must configure
the hibernate.transaction.manager_lookup_class
.
The UUID contains: IP address, startup time of the JVM that is accurate to a quarter second, system time and a counter value that is unique within the JVM. It is not possible to obtain a MAC address or memory address from Java code, so this is the best option without using JNI.
For databases that support identity columns (DB2, MySQL, Sybase, MS SQL), you
can use identity
key generation. For databases that support
sequences (DB2, Oracle, PostgreSQL, Interbase, McKoi, SAP DB) you can use
sequence
style key generation. Both of these strategies require
two SQL queries to insert a new object. For example:
<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, depending on the capabilities of the underlying database,
choose from the identity
, sequence
and
hilo
strategies.
If you want the application to assign identifiers, as opposed to having
Hibernate generate them, you can use the assigned
generator.
This special generator uses the identifier value already assigned to the
object's identifier property. The generator is used when the primary key
is a natural key instead of a surrogate key. This is the default behavior
if you do not specify a <generator>
element.
The assigned
generator makes Hibernate use
unsaved-value="undefined"
. This forces 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()
.
Hibernate does not generate DDL with triggers. It is for legacy schemas only.
<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
. It is 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 Optimization means that you do not have 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
. This is because native
generally chooses between identity
and sequence
which have
largely different semantics that can cause subtle issues in applications eyeing portability.
org.hibernate.id.enhanced.SequenceStyleGenerator
, however, achieves portability in
a different manner. It chooses between 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 emulate 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 typically 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 typically 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, it is 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, even though it actually
functions much more like org.hibernate.id.MultipleHiLoPerTableGenerator
, and secondly,
as a re-implementation of org.hibernate.id.MultipleHiLoPerTableGenerator
that utilizes the
notion of pluggable optimizers. 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 that is used to hold the value.
segment_column_name
(optional - defaults to sequence_name
):
the name of the column on the table that is used to hold the "segment key". This is the
value which 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 that 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 can 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 operation.
none
(generally this is the default if no optimizer was specified): this
will not perform any optimizations and hit the database for 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
: as with the case of hilo
, this optimizer
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. Here, increment_size
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>
A table with a composite key can be mapped with 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>
The persistent class must override equals()
and hashCode()
to implement composite identifier equality. It must
also implement Serializable
.
Unfortunately, this approach 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 main disadvantage of this approach is
code 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. Please see chapter 9 for more information.
access
(optional - defaults to property
):
the strategy Hibernate uses for accessing the property value.
class
(optional - defaults to the property type determined by
reflection): the component class used as a composite identifier. Please see the next section for more information.
The third approach, an identifier component, is recommended for almost all applications.
The <discriminator>
element is required for polymorphic persistence
using the table-per-class-hierarchy mapping strategy. It 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 can be used:
string
, character
, integer
,
byte
, short
, boolean
,
yes_no
, true_false
.
<discriminator column="discriminator_column" type="
discriminator_type" force=
"true|false" insert
="true|false" formul
a="arbitrary sql expression" />
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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.
The formula
attribute allows you to declare an arbitrary SQL expression
that will be used to evaluate the type of a row. For example:
<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 for more information:
<version column="version_column" name="
propertyName" type="
typename" access
="field|property|ClassName" unsave
d-value="null|negative|undefined" genera
ted="never|always" insert
="true|false" node="element-name|@attribute-name|element/@attribute|." />
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Version numbers can be of Hibernate type long
, integer
,
short
, timestamp
or calendar
.
A version or timestamp property should never be null for a detached instance.
Hibernate will detect any instance with a null version or timestamp as transient,
irrespective of what other unsaved-value
strategies are specified.
Declaring a nullable version or timestamp property is an easy way to avoid
problems with transitive reattachment in Hibernate. It is especially useful for people
using assigned identifiers or composite keys.
The optional <timestamp>
element indicates that the table contains
timestamped data. This provides an alternative to versioning. Timestamps are
a less safe implementation of optimistic locking. However, sometimes the application might
use the timestamps in other ways.
<timestamp column="timestamp_column" name="
propertyName" access
="field|property|ClassName" unsave
d-value="null|undefined" source
="vm|db" genera
ted="never|always" node="element-name|@attribute-name|element/@attribute|." />
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<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" column
="column_name" type="
typename" update
="true|false" insert
="true|false" formul
a="arbitrary SQL expression" access
="field|property|ClassName" lazy="
true|false" unique
="true|false" not-nu
ll="true|false" optimi
stic-lock="true|false" genera
ted="never|insert|always" 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: integer, string, character,
date, timestamp, float, binary, serializable, object, blob
etc.
The name of a Java class with a default basic type: int, float,
char, java.lang.String, java.util.Date, java.lang.Integer, java.sql.Clob
etc.
The name of a serializable Java class.
The class name of a custom type: com.illflow.type.MyCustomType
etc.
If you do not specify a type, Hibernate will use reflection upon the named
property and guess the correct Hibernate type. Hibernate will attempt to
interpret the name of the return class of the property getter using, in order, rules 2, 3,
and 4.
In certain cases you will need the type
attribute. For example, to distinguish between Hibernate.DATE
and
Hibernate.TIMESTAMP
, or to specify a custom type.
The access
attribute allows you to control how Hibernate accesses
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 can specify your own
strategy for property access by naming a class that implements the interface
org.hibernate.property.PropertyAccessor
.
A powerful feature is derived properties. These properties are by
definition read-only. The property value is computed at load time. You declare
the computation as an SQL expression. This then 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 )"/>
You can reference the entity table by not declaring an alias on
a particular column. This would be customerId
in the given example. You can also use
the nested <formula>
mapping element
if you do not want 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" column
="column_name" class=
"ClassName" cascad
e="cascade_style" fetch=
"join|select" update
="true|false" insert
="true|false" proper
ty-ref="propertyNameFromAssociatedClass" access
="field|property|ClassName" unique
="true|false" not-nu
ll="true|false" optimi
stic-lock="true|false" lazy="
proxy|no-proxy|false" not-fo
und="ignore|exception" entity
-name="EntityName" formul
a="arbitrary SQL expression" 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 divided into three categories. First, basic
operations, which include: persist, merge, delete, save-update, evict, replicate, lock and
refresh
; second, special values: delete-orphan
;
and third, all
comma-separated combinations of operation
names: cascade="persist,merge,evict"
or
cascade="all,delete-orphan"
. See Section 10.11, “Transitive persistence”
for a full explanation. Note that single valued, many-to-one and
one-to-one, associations do not support orphan delete.
Here is an example of a typical many-to-one
declaration:
<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 a complicated and confusing relational model. For example, if 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 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 can 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" class=
"ClassName" cascad
e="cascade_style" constr
ained="true|false" fetch=
"join|select" proper
ty-ref="propertyNameFromAssociatedClass" access
="field|property|ClassName" formul
a="any SQL expression" lazy="
proxy|no-proxy|false" entity
-name="EntityName" 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 associations:
primary key associations
unique foreign key associations
Primary key associations do not need an extra table column. If two rows are related by the association, then the two table rows share the same primary key value. To relate two objects by a primary key association, ensure 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"/>
Ensure that the primary keys of the related rows in the PERSON and
EMPLOYEE tables are equal. You 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 assigned the same primary
key value as the Employee
instance referred with the employee
property of that Person
.
Alternatively, a foreign key with a unique constraint, from Employee
to
Person
, can be expressed as:
<many-to-one name="person" class="Person" column="PERSON_ID" unique="true"/>
This association can 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>
Although we recommend the use of surrogate keys as primary keys, you should try
to identify natural keys for all entities. A natural key is a property or combination of
properties that is unique and non-null. It is also immutable. Map the
properties of the natural key inside the <natural-id>
element.
Hibernate will generate the necessary unique key and nullability constraints and, as a result, your
mapping will be more self-documenting.
It is recommended that you implement equals()
and
hashCode()
to compare the natural key properties of the entity.
This mapping is not intended for use with entities that have natural primary keys.
mutable
(optional - defaults to false
):
by default, natural identifier properties are assumed to be immutable (constant).
The <component>
element maps properties of a
child object to columns of the table of a parent class. Components can, in
turn, declare their own properties, components or collections. See
the "Component" examples below:
<component name="propertyName" class=
"className" insert
="true|false" update
="true|false" access
="field|property|ClassName" lazy="
true|false" optimi
stic-lock="true|false" unique
="true|false" 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” for more information.
The <properties>
element allows the definition of a named,
logical grouping of the 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. For example:
<properties name="logicalName" insert
="true|false" update
="true|false" optimi
stic-lock="true|false" unique
="true|false" > <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>
You might have some legacy data association that 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>
The use of this outside the context of mapping legacy data is not recommended.
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. For example:
<subclass name="ClassName" discri
minator-value="discriminator_value" proxy=
"ProxyInterface" lazy="
true|false" dynamic-update="true|false" dynamic-insert="true|false" entity-name="EntityName" node="element-name" extends="SuperclassName"> <property .... /> ..... </subclass>
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Each subclass declares its own persistent properties and subclasses.
<version>
and <id>
properties
are assumed to be inherited from the root class. Each subclass in a hierarchy must
define a unique discriminator-value
. If this is not specified, the
fully qualified Java class name is used.
For information about inheritance mappings see Chapter 9, Inheritance mapping.
Each subclass can also be mapped to its own table. This is called the table-per-subclass
mapping strategy. An inherited state is retrieved by joining with the table of the
superclass. To do this you use the <joined-subclass>
element. For example:
<joined-subclass name="ClassName" table=
"tablename" proxy=
"ProxyInterface" lazy="
true|false" 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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A discriminator column is not 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 then 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. This is called the table-per-concrete-class strategy. Each table defines all
persistent states of the class, including the inherited state. In Hibernate, it is
not necessary to explicitly map such inheritance hierarchies. You
can 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. For example:
<union-subclass name="ClassName" table=
"tablename" proxy=
"ProxyInterface" lazy="
true|false" 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 that have a one-to-one relationship. For example:
<join table="tablename" schema
="owner" catalo
g="catalog" fetch=
"join|select" invers
e="true|false" option
al="true|false"> <key ... /> <property ... /> ... </join>
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For example, 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.
The <key>
element has featured a few times within this guide.
It appears anywhere the parent mapping element defines a join to
a new table that references
the primary key of the original table. It also defines the foreign key in the joined table:
<key column="columnname" on-del
ete="noaction|cascade" proper
ty-ref="propertyName" not-nu
ll="true|false" update
="true|false" unique
="true|false" />
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For systems where delete performance is important, we recommend that all keys should be
defined on-delete="cascade"
. Hibernate uses 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 association
to a non-nullable foreign key, you must declare the key column using
<key not-null="true">
.
Mapping elements which accept a column
attribute will alternatively
accept a <column>
subelement. Likewise, <formula>
is an alternative to the formula
attribute. For example:
<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"
read="SQL expression"
write="SQL expression"/>
<formula>SQL expression</formula>
Most of the attributes on column
provide a means of tailoring the
DDL during automatic schema generation. The read
and write
attributes allow you to specify custom SQL that Hibernate will use to access the column's value.
For more on this, see the discussion of
column read and write expressions.
The column
and formula
elements can 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>
If your application has two persistent classes with the same name, and you do not want to
specify the fully qualified package name in Hibernate queries, classes can be "imported"
explicitly, rather than relying upon auto-import="true"
. You can also import
classes and interfaces that are not explicitly mapped:
<import class="java.lang.Object" rename="Universe"/>
<import class="ClassName" rename
="ShortName" />
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There is one more type of property mapping. The <any>
mapping element
defines a polymorphic association to classes from multiple tables. This type of mapping
requires more than one column. The first column contains the type of the associated entity.
The remaining columns contain the identifier. It is impossible to specify a foreign key constraint
for this kind of association. This is not the usual way of mapping
polymorphic associations and you should use this only in special cases. For example, for audit logs,
user session data, etc.
The meta-type
attribute allows the application to specify a custom type that
maps database column values to persistent classes that 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" id-typ
e="idtypename" meta-t
ype="metatypename" cascad
e="cascade_style" access
="field|property|ClassName" optimi
stic-lock="true|false" > <meta-value ... /> <meta-value ... /> ..... <column .... /> <column .... /> ..... </any>
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In relation to the persistence service, Java language-level objects are classified 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. Saves and deletions, however, can be cascaded from a parent entity to its children. This is different from the ODMG model of object persistence by reachability and corresponds more closely to how application objects are usually used in large systems. Entities support circular and shared references. They can 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 is 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 cannot be independently versioned. Values have no independent identity, so they cannot be shared by two entities or collections.
Until now, we have been using the term "persistent class" to refer to
entities. We will continue to do that. Not all
user-defined classes with a persistent state, however, 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, all types (classes) provided
by the JDK have value type semantics in Java, while user-defined types can
be mapped with entity or value type semantics. This decision is up to the
application developer. An entity class in a domain model will normally have
shared references to a single instance of that class, while composition or
aggregation usually translates to a value type.
We will revisit both concepts throughout this reference guide.
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,
<class>
, <subclass>
and so on are used.
For value types we use <property>
,
<component>
etc., that usually have a type
attribute. The value of this attribute is the name of a Hibernate
mapping type. Hibernate provides a range of mappings for standard
JDK value types out of the box. You can write your own mapping types and implement your own
custom conversion strategies.
With the exception of collections, all built-in Hibernate types support null semantics.
The built-in basic mapping types can be roughly categorized into the following:
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
can 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 can be inconvenient for some
applications, since the blob or clob object cannot be reused outside of
a transaction. 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 considered mutable Java types. This is 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 can be of any basic type except
binary
, blob
and clob
.
Composite identifiers are also allowed. See below for more information.
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. 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. View
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 can 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 regularly use a certain UserType
, it is 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 can 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 rarely need to use a custom type, it is
considered good practice 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 be mapped
as a component. One reason for this is abstraction. With a custom type, your mapping
documents would be protected against changes to the way
monetary values are represented.
It is possible to provide more than one mapping for a particular persistent class. In this case, you must specify an entity name to 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>
Associations are now specified using entity-name
instead of
class
.
You can 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
. This is usually double quotes, but the SQL
Server uses brackets and MySQL uses backticks.
<class name="LineItem" table="`Line Item`">
<id name="id" column="`Item Id`"/><generator class="assigned"/></id>
<property name="itemNumber" column="`Item #`"/>
...
</class>
XML does not suit all users 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 do not cover this approach in this
reference guide since 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 website for more examples of XDoclet and Hibernate.
JDK 5.0 introduced XDoclet-style annotations at the language level that are type-safe and
checked at compile time. This mechanism 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
}
Support for JDK 5.0 Annotations (and JSR-220) is currently under development. Please refer to the Hibernate Annotations module for more details.
Generated properties are properties that have their values generated by the
database. Typically, Hibernate applications needed to refresh
objects that contain any properties for which the database was generating values.
Marking properties as generated, however, lets the application delegate this
responsibility to Hibernate. When Hibernate issues an SQL INSERT
or UPDATE for an entity that 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): the given property value
is not generated within the database.
insert
: the given property value is generated on
insert, but is not regenerated on subsequent updates. Properties like created-date
fall into this category. Even though
version and
timestamp properties can
be marked as generated, this option is not available.
always
: the property value is generated both
on insert and on update.
Hibernate allows you to customize the SQL it uses to read and write the values of columns mapped to simple properties. For example, if your database provides a set of data encryption functions, you can invoke them for individual columns like this:
<!-- XML : generated by JHighlight v1.0 (http://jhighlight.dev.java.net) --> <span class="xml_tag_symbols"><</span><span class="xml_tag_name">property</span><span class="xml_plain"> </span><span class="xml_attribute_name">name</span><span class="xml_tag_symbols">=</span><span class="xml_attribute_value">"creditCardNumber"</span><span class="xml_tag_symbols">></span><span class="xml_plain"></span><br /> <span class="xml_plain"> </span><span class="xml_tag_symbols"><</span><span class="xml_tag_name">column</span><span class="xml_plain"> </span><br /> <span class="xml_plain"> </span><span class="xml_attribute_name">name</span><span class="xml_tag_symbols">=</span><span class="xml_attribute_value">"credit_card_num"</span><span class="xml_plain"></span><br /> <span class="xml_plain"> </span><span class="xml_attribute_name">read</span><span class="xml_tag_symbols">=</span><span class="xml_attribute_value">"decrypt(credit_card_num)"</span><span class="xml_plain"></span><br /> <span class="xml_plain"> </span><span class="xml_attribute_name">write</span><span class="xml_tag_symbols">=</span><span class="xml_attribute_value">"encrypt(?)"</span><span class="xml_tag_symbols">/></span><span class="xml_plain"></span><br /> <span class="xml_tag_symbols"></</span><span class="xml_tag_name">property</span><span class="xml_tag_symbols">></span><span class="xml_plain"></span><br />
Hibernate applies the custom expressions automatically whenever the property is
referenced in a query. This functionality is similar to a derived-property
formula
with two differences:
The property is backed by one or more columns that are exported as part of automatic schema generation.
The property is read-write, not read-only.
The write
expression, if specified, must contain exactly one '?' placeholder
for the value.
Auxiliary database objects allow for the CREATE and DROP of arbitrary database objects. In conjunction with
Hibernate's schema evolution tools, they have 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, any
SQL command that can be run via a java.sql.Statement.execute()
method is valid (for example, 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 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 that constructs 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 so that they only apply when certain dialects are used.
<hibernate-mapping>
...
<database-object>
<definition class="MyTriggerDefinition"/>
<dialect-scope name="org.hibernate.dialect.Oracle9iDialect"/>
<dialect-scope name="org.hibernate.dialect.Oracle10gDialect"/>
</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
("anything you like" means you will have to write an implementation of
org.hibernate.usertype.UserCollectionType
.)
Notice how the instance variable was initialized 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
.
Be aware of the following errors:
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 on the interface type.
Collections instances have the usual behavior of value types. They are automatically persisted when referenced by a persistent object and are 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 cannot 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.
Use persistent collections the same way you use ordinary Java collections. However, please ensure you understand the semantics of bidirectional associations (these are discussed later).
There are quite a range of mappings that can be generated for collections that cover many common relational models. We suggest you experiment with the schema generation tool so that you understand how various mapping declarations translate to database tables.
The Hibernate mapping element used for mapping a collection depends upon
the type of 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" table="tab
le_name" schema="sc
hema_name" lazy="true
|extra|false" inverse="t
rue|false" cascade="a
ll|none|save-update|delete|all-delete-orphan|delete-orphan" sort="unso
rted|natural|comparatorClass" order-by="
column_name asc|desc" where="arb
itrary sql where condition" fetch="joi
n|select|subselect" batch-size
="N" access="fi
eld|property|ClassName" optimistic
-lock="true|false" mutable="t
rue|false" 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 can 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 may need to specify
not-null="true"
.
<key column="productSerialNumber" not-null="true"/>
The foreign key constraint can use ON DELETE CASCADE
.
<key column="productSerialNumber" on-delete="cascade"/>
See the previous chapter for a full definition of the <key>
element.
Collections can contain almost any other Hibernate type, including: basic types, custom types, components and 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 a 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. An index column is 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 can be an entity reference mapped with
<map-key-many-to-many>
, or it can 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 that are numbered from zero by default.
<list-index column="column_name" base="
0|1|..."/>
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<map-key column="column_name" formul
a="any SQL expression" type="
type_name" node="@attribute-name" length="N"/>
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<map-key-many-to-many column="column_name" formul
a="any SQL expression" class="ClassName" />
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If your table does not have an index column, and you still wish to use List
as the property type, you can map the property as a Hibernate <bag>.
A bag does not retain its order when it is retrieved from the database, but it can be
optionally sorted or ordered.
Any collection of values or many-to-many associations 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 use the <element>
tag. For example:
<element column="column_name" formul
a="any SQL expression" type="
typename" 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" formul
a="any SQL expression" class=
"ClassName" fetch=
"select|join" unique
="true|false" not-fo
und="ignore|exception" entity
-name="EntityName" proper
ty-ref="propertyNameFromAssociatedClass" node="element-name" embed-xml="true|false" />
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Here are some examples.
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 (this is 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 cannot belong to more than one instance of the collection.
An instance of the contained entity class cannot appear at more than one value of the collection index.
An association from Product
to Part
requires
the 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" not-fo
und="ignore|exception" entity
-name="EntityName" node="element-name" embed-xml="true|false" />
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|
The <one-to-many>
element does not need to
declare any columns. Nor is it necessary to specify the table
name anywhere.
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 for more information.
The following 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 and is implemented using LinkedHashSet
or
LinkedHashMap
. This performs the ordering in the SQL query and
not in the 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>
The value of the order-by
attribute is an SQL ordering, not
an HQL ordering.
Associations can even be sorted by 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 and single-valued at the other
set or bag valued at both ends
You can specify a bidirectional many-to-many association by mapping two many-to-many associations to the same database table and declaring one end as inverse. You cannot select an indexed collection.
Here is an example of a bidirectional many-to-many association that illustrates how 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 a many-to-many relationship in Javais created:
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 can 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"
does not
affect the operation of cascades as 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 that maps to the index column you can use
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>
If there is no such property on the child class, the association cannot be considered
truly bidirectional. That is, there is information available at one end of the association that is
not available at the other end. In this case, you cannot map the collection
inverse="true"
. Instead, you 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.
There are three possible approaches to mapping a ternary association. One approach 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 remodel the association as an entity class. This is the most common approach.
A final alternative is to use composite elements, which will be discussed later.
The majority of the many-to-many associations and collections of values shown previously all map to tables with composite keys, even though it has been have suggested that entities should have synthetic identifiers (surrogate keys). A pure association table does not seem to benefit much from a surrogate key, although a collection of composite values might. It is for this reason that 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. For example:
<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>
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, however, provide any mechanism for discovering the surrogate key value
of a particular row.
The update performance of an <idbag>
supersedes a regular <bag>
.
Hibernate can locate individual rows efficiently and update or delete them
individually, similar to a list, map or set.
In the current implementation, the native
identifier generation
strategy is not supported for <idbag>
collection identifiers.
This section covers collection examples.
The following class has a collection of Child
instances:
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; }
....
....
}
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 this association must 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 has 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 explanation of a parent/child relationship mapping, see Chapter 22, Example: Parent/Child for more information.
Even more complex association mappings are covered in the next chapter.
Association mappings are often the most difficult thing to implement correctly. In
this section we examine some canonical cases one by one, starting
with unidirectional mappings and then bidirectional cases.
We will use Person
and Address
in all
the examples.
Associations will be classified by multiplicity and whether or not they map to an intervening join table.
Nullable foreign keys are not considered to be good practice in traditional data modelling, so our examples do not use nullable foreign keys. This is not a requirement of Hibernate, and the mappings will 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 In this example, however, we have reversed the direction of the association:
<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 an unusual case, and is not 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 )
You should instead use a join table for this kind of association.
A unidirectional one-to-many association on a join table
is the preferred option. Specifying unique="true"
,
changes 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 common when the association is optional. For example:
<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 possible, but extremely unusual.
<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, here is an example of 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. The following example illustrates 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,
set the key
column of the foreign key to not null
.
Hibernate will 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>
If the underlying foreign key column is NOT NULL
, it
is important that you define not-null="true"
on the
<key>
element of the collection mapping.
Do not 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 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 )
The following is an example of a bidirectional one-to-many association on a join table.
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 possible, but extremely unusual.
<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 )