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HIBERNATE - Relational Persistence for Idiomatic Java

Hibernate Reference Documentation

4.0.1.Final

Legal Notice

2012-01-11


Preface
1. Tutorial
1.1. Part 1 - The first Hibernate Application
1.1.1. Setup
1.1.2. The first class
1.1.3. The mapping file
1.1.4. Hibernate configuration
1.1.5. Building with Maven
1.1.6. Startup and helpers
1.1.7. Loading and storing objects
1.2. Part 2 - Mapping associations
1.2.1. Mapping the Person class
1.2.2. A unidirectional Set-based association
1.2.3. Working the association
1.2.4. Collection of values
1.2.5. Bi-directional associations
1.2.6. Working bi-directional links
1.3. Part 3 - The EventManager web application
1.3.1. Writing the basic servlet
1.3.2. Processing and rendering
1.3.3. Deploying and testing
1.4. Summary
2. Architecture
2.1. Overview
2.1.1. Minimal architecture
2.1.2. Comprehensive architecture
2.1.3. Basic APIs
2.2. JMX Integration
2.3. Contextual sessions
3. Configuration
3.1. Programmatic configuration
3.2. Obtaining a SessionFactory
3.3. JDBC connections
3.4. Optional configuration properties
3.4.1. SQL Dialects
3.4.2. Outer Join Fetching
3.4.3. Binary Streams
3.4.4. Second-level and query cache
3.4.5. Query Language Substitution
3.4.6. Hibernate statistics
3.5. Logging
3.6. Implementing a NamingStrategy
3.7. Implementing a PersisterClassProvider
3.8. XML configuration file
3.9. Java EE Application Server integration
3.9.1. Transaction strategy configuration
3.9.2. JNDI-bound SessionFactory
3.9.3. Current Session context management with JTA
3.9.4. JMX deployment
4. Persistent Classes
4.1. A simple POJO example
4.1.1. Implement a no-argument constructor
4.1.2. Provide an identifier property
4.1.3. Prefer non-final classes (semi-optional)
4.1.4. Declare accessors and mutators for persistent fields (optional)
4.2. Implementing inheritance
4.3. Implementing equals() and hashCode()
4.4. Dynamic models
4.5. Tuplizers
4.6. EntityNameResolvers
5. Basic O/R Mapping
5.1. Mapping declaration
5.1.1. Entity
5.1.2. Identifiers
5.1.3. Optimistic locking properties (optional)
5.1.4. Property
5.1.5. Embedded objects (aka components)
5.1.6. Inheritance strategy
5.1.7. Mapping one to one and one to many associations
5.1.8. Natural-id
5.1.9. Any
5.1.10. Properties
5.1.11. Some hbm.xml specificities
5.2. Hibernate types
5.2.1. Entities and values
5.2.2. Basic value types
5.2.3. Custom value types
5.3. Mapping a class more than once
5.4. SQL quoted identifiers
5.5. Generated properties
5.6. Column transformers: read and write expressions
5.7. Auxiliary database objects
6. Types
6.1. Value types
6.1.1. Basic value types
6.1.2. Composite types
6.1.3. Collection types
6.2. Entity types
6.3. Significance of type categories
6.4. Custom types
6.4.1. Custom types using org.hibernate.type.Type
6.4.2. Custom types using org.hibernate.usertype.UserType
6.4.3. Custom types using org.hibernate.usertype.CompositeUserType
6.5. Type registry
7. Collection mapping
7.1. Persistent collections
7.2. How to map collections
7.2.1. Collection foreign keys
7.2.2. Indexed collections
7.2.3. Collections of basic types and embeddable objects
7.3. Advanced collection mappings
7.3.1. Sorted collections
7.3.2. Bidirectional associations
7.3.3. Bidirectional associations with indexed collections
7.3.4. Ternary associations
7.3.5. Using an <idbag>
7.4. Collection examples
8. Association Mappings
8.1. Introduction
8.2. Unidirectional associations
8.2.1. Many-to-one
8.2.2. One-to-one
8.2.3. One-to-many
8.3. Unidirectional associations with join tables
8.3.1. One-to-many
8.3.2. Many-to-one
8.3.3. One-to-one
8.3.4. Many-to-many
8.4. Bidirectional associations
8.4.1. one-to-many / many-to-one
8.4.2. One-to-one
8.5. Bidirectional associations with join tables
8.5.1. one-to-many / many-to-one
8.5.2. one to one
8.5.3. Many-to-many
8.6. More complex association mappings
9. Component Mapping
9.1. Dependent objects
9.2. Collections of dependent objects
9.3. Components as Map indices
9.4. Components as composite identifiers
9.5. Dynamic components
10. Inheritance mapping
10.1. The three strategies
10.1.1. Table per class hierarchy
10.1.2. Table per subclass
10.1.3. Table per subclass: using a discriminator
10.1.4. Mixing table per class hierarchy with table per subclass
10.1.5. Table per concrete class
10.1.6. Table per concrete class using implicit polymorphism
10.1.7. Mixing implicit polymorphism with other inheritance mappings
10.2. Limitations
11. Working with objects
11.1. Hibernate object states
11.2. Making objects persistent
11.3. Loading an object
11.4. Querying
11.4.1. Executing queries
11.4.2. Filtering collections
11.4.3. Criteria queries
11.4.4. Queries in native SQL
11.5. Modifying persistent objects
11.6. Modifying detached objects
11.7. Automatic state detection
11.8. Deleting persistent objects
11.9. Replicating object between two different datastores
11.10. Flushing the Session
11.11. Transitive persistence
11.12. Using metadata
12. Read-only entities
12.1. Making persistent entities read-only
12.1.1. Entities of immutable classes
12.1.2. Loading persistent entities as read-only
12.1.3. Loading read-only entities from an HQL query/criteria
12.1.4. Making a persistent entity read-only
12.2. Read-only affect on property type
12.2.1. Simple properties
12.2.2. Unidirectional associations
12.2.3. Bidirectional associations
13. Transactions and Concurrency
13.1. Session and transaction scopes
13.1.1. Unit of work
13.1.2. Long conversations
13.1.3. Considering object identity
13.1.4. Common issues
13.2. Database transaction demarcation
13.2.1. Non-managed environment
13.2.2. Using JTA
13.2.3. Exception handling
13.2.4. Transaction timeout
13.3. Optimistic concurrency control
13.3.1. Application version checking
13.3.2. Extended session and automatic versioning
13.3.3. Detached objects and automatic versioning
13.3.4. Customizing automatic versioning
13.4. Pessimistic locking
13.5. Connection release modes
14. Interceptors and events
14.1. Interceptors
14.2. Event system
14.3. Hibernate declarative security
15. Batch processing
15.1. Batch inserts
15.2. Batch updates
15.3. The StatelessSession interface
15.4. DML-style operations
16. HQL: The Hibernate Query Language
16.1. Case Sensitivity
16.2. The from clause
16.3. Associations and joins
16.4. Forms of join syntax
16.5. Referring to identifier property
16.6. The select clause
16.7. Aggregate functions
16.8. Polymorphic queries
16.9. The where clause
16.10. Expressions
16.11. The order by clause
16.12. The group by clause
16.13. Subqueries
16.14. HQL examples
16.15. Bulk update and delete
16.16. Tips & Tricks
16.17. Components
16.18. Row value constructor syntax
17. Criteria Queries
17.1. Creating a Criteria instance
17.2. Narrowing the result set
17.3. Ordering the results
17.4. Associations
17.5. Dynamic association fetching
17.6. Components
17.7. Collections
17.8. Example queries
17.9. Projections, aggregation and grouping
17.10. Detached queries and subqueries
17.11. Queries by natural identifier
18. Native SQL
18.1. Using a SQLQuery
18.1.1. Scalar queries
18.1.2. Entity queries
18.1.3. Handling associations and collections
18.1.4. Returning multiple entities
18.1.5. Returning non-managed entities
18.1.6. Handling inheritance
18.1.7. Parameters
18.2. Named SQL queries
18.2.1. Using return-property to explicitly specify column/alias names
18.2.2. Using stored procedures for querying
18.3. Custom SQL for create, update and delete
18.4. Custom SQL for loading
19. Filtering data
19.1. Hibernate filters
20. XML Mapping
20.1. Working with XML data
20.1.1. Specifying XML and class mapping together
20.1.2. Specifying only an XML mapping
20.2. XML mapping metadata
20.3. Manipulating XML data
21. Improving performance
21.1. Fetching strategies
21.1.1. Working with lazy associations
21.1.2. Tuning fetch strategies
21.1.3. Single-ended association proxies
21.1.4. Initializing collections and proxies
21.1.5. Using batch fetching
21.1.6. Using subselect fetching
21.1.7. Fetch profiles
21.1.8. Using lazy property fetching
21.2. The Second Level Cache
21.2.1. Cache mappings
21.2.2. Strategy: read only
21.2.3. Strategy: read/write
21.2.4. Strategy: nonstrict read/write
21.2.5. Strategy: transactional
21.2.6. Cache-provider/concurrency-strategy compatibility
21.3. Managing the caches
21.4. The Query Cache
21.4.1. Enabling query caching
21.4.2. Query cache regions
21.5. Understanding Collection performance
21.5.1. Taxonomy
21.5.2. Lists, maps, idbags and sets are the most efficient collections to update
21.5.3. Bags and lists are the most efficient inverse collections
21.5.4. One shot delete
21.6. Monitoring performance
21.6.1. Monitoring a SessionFactory
21.6.2. Metrics
22. Toolset Guide
22.1. Automatic schema generation
22.1.1. Customizing the schema
22.1.2. Running the tool
22.1.3. Properties
22.1.4. Using Ant
22.1.5. Incremental schema updates
22.1.6. Using Ant for incremental schema updates
22.1.7. Schema validation
22.1.8. Using Ant for schema validation
23. Additional modules
23.1. Bean Validation
23.1.1. Adding Bean Validation
23.1.2. Configuration
23.1.3. Catching violations
23.1.4. Database schema
23.2. Hibernate Search
23.2.1. Description
23.2.2. Integration with Hibernate Annotations
24. Example: Parent/Child
24.1. A note about collections
24.2. Bidirectional one-to-many
24.3. Cascading life cycle
24.4. Cascades and unsaved-value
24.5. Conclusion
25. Example: Weblog Application
25.1. Persistent Classes
25.2. Hibernate Mappings
25.3. Hibernate Code
26. Example: Various Mappings
26.1. Employer/Employee
26.2. Author/Work
26.3. Customer/Order/Product
26.4. Miscellaneous example mappings
26.4.1. "Typed" one-to-one association
26.4.2. Composite key example
26.4.3. Many-to-many with shared composite key attribute
26.4.4. Content based discrimination
26.4.5. Associations on alternate keys
27. Best Practices
28. Database Portability Considerations
28.1. Portability Basics
28.2. Dialect
28.3. Dialect resolution
28.4. Identifier generation
28.5. Database functions
28.6. Type mappings
References

Working with both Object-Oriented software and Relational Databases can be cumbersome and time consuming. Development costs are significantly higher due to a paradigm mismatch between how data is represented in objects versus relational databases. Hibernate is an Object/Relational Mapping solution for Java environments. The term Object/Relational Mapping refers to the technique of mapping data from an object model representation to a relational data model representation (and visa versa). See http://en.wikipedia.org/wiki/Object-relational_mapping for a good high-level discussion.

Note

While having a strong background in SQL is not required to use Hibernate, having a basic understanding of the concepts can greatly help you understand Hibernate more fully and quickly. Probably the single best background is an understanding of data modeling principles. You might want to consider these resources as a good starting point:

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 significantly reduce development time otherwise spent with manual data handling in SQL and JDBC. Hibernate’s design goal is to relieve the developer from 95% of common data persistence-related programming tasks by eliminating the need for manual, hand-crafted data processing using SQL and JDBC. However, unlike many other persistence solutions, Hibernate does not hide the power of SQL from you and guarantees that your investment in relational technology and knowledge is as valid as always.

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:

  1. 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.

  2. Read Chapter 2, Architecture to understand the environments where Hibernate can be used.

  3. 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.

  4. 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].

  5. FAQs are answered on the Hibernate website.

  6. Links to third party demos, examples, and tutorials are maintained on the Hibernate website.

  7. The Community Area on the Hibernate website is a good resource for design patterns and various integration solutions (Tomcat, JBoss AS, Struts, EJB, etc.).

There are a number of ways to become involved in the Hibernate community, including

  • Trying stuff out and reporting bugs. See http://hibernate.org/issuetracker.html details.

  • Trying your hand at fixing some bugs or implementing enhancements. Again, see http://hibernate.org/issuetracker.html details.

  • http://hibernate.org/community.html list a few ways to engage in the community.

    • There are forums for users to ask questions and receive help from the community.

    • There are also IRC channels for both user and developer discussions.

  • Helping improve or translate this documentation. Contact us on the developer mailing list if you have interest.

  • Evangelizing Hibernate within your organization.

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.

Important

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.

Note

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.

Note

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>

Tip

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://www.hibernate.org/dtd/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).

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.

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.

Note

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.

Tip

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 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.

Caution

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://www.hibernate.org/dtd/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.internal.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>

Note

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.

Tip

In most cases, Hibernate is able to properly determine which dialect to use. See Section 28.3, “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.

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 13, 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"

Note

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 16, 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.

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.

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="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.

Note

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.

The diagram below provides a high-level view of the Hibernate architecture:

Unfortunately we cannot provide a detailed view of all possible runtime architectures. Hibernate is sufficiently flexible to be used in a number of ways in many, many architectures. We will, however, illustrate 2 specifically since they are extremes.

Here are quick discussions about some of the API objects depicted in the preceding diagrams (you will see them again in more detail in later chapters).

SessionFactory (org.hibernate.SessionFactory)

A thread-safe, immutable cache of compiled mappings for a single database. A factory for org.hibernate.Session instances. A client of org.hibernate.connection.ConnectionProvider. Optionally maintains a second level cache of data that is reusable between transactions at a process or cluster level.

Session (org.hibernate.Session)

A single-threaded, short-lived object representing a conversation between the application and the persistent store. Wraps a JDBC java.sql.Connection. Factory for org.hibernate.Transaction. Maintains a first level cache of persistent the application's persistent objects and collections; this cache is used when navigating the object graph or looking up objects by identifier.

Persistent objects and collections

Short-lived, single threaded objects containing persistent state and business function. These can be ordinary JavaBeans/POJOs. They are associated with exactly one org.hibernate.Session. Once the org.hibernate.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). Chapter 11, Working with objects discusses transient, persistent and detached object states.

Transient and detached objects and collections

Instances of persistent classes that are not currently associated with a org.hibernate.Session. They may have been instantiated by the application and not yet persisted, or they may have been instantiated by a closed org.hibernate.Session. Chapter 11, Working with objects discusses transient, persistent and detached object states.

Transaction (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 org.hibernate.Session might span several org.hibernate.Transactions in some cases. However, transaction demarcation, either using the underlying API or org.hibernate.Transaction, is never optional.

ConnectionProvider (org.hibernate.connection.ConnectionProvider)

(Optional) A factory for, and pool of, JDBC connections. It abstracts the application from underlying javax.sql.DataSource or java.sql.DriverManager. It is not exposed to application, but it can be extended and/or implemented by the developer.

TransactionFactory (org.hibernate.TransactionFactory)

(Optional) A factory for org.hibernate.Transaction instances. It is not exposed to the application, but it can be extended and/or implemented by the developer.

Extension Interfaces

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.

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.spi.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.spi.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:

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 13, Transactions and Concurrency for more information and code examples.

The hibernate.current_session_context_class configuration parameter defines which org.hibernate.context.spi.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.internal.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:

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.

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:


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:


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.

Table 3.3. Hibernate Configuration Properties

Property namePurpose
hibernate.dialectThe classname of a Hibernate org.hibernate.dialect.Dialect which allows Hibernate to generate SQL optimized for a particular relational database.

e.g. full.classname.of.Dialect

In most cases Hibernate will actually be able to choose the correct org.hibernate.dialect.Dialect implementation based on the JDBC metadata returned by the JDBC driver.

hibernate.show_sqlWrite all SQL statements to console. This is an alternative to setting the log category org.hibernate.SQL to debug.

e.g. true | false

hibernate.format_sqlPretty print the SQL in the log and console.

e.g. true | false

hibernate.default_schemaQualify unqualified table names with the given schema/tablespace in generated SQL.

e.g. SCHEMA_NAME

hibernate.default_catalogQualifies unqualified table names with the given catalog in generated SQL.

e.g. CATALOG_NAME

hibernate.session_factory_nameThe org.hibernate.SessionFactory will be automatically bound to this name in JNDI after it has been created.

e.g. jndi/composite/name

hibernate.max_fetch_depthSets 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 0 and 3

hibernate.default_batch_fetch_sizeSets a default size for Hibernate batch fetching of associations.

e.g. recommended values 4, 8, 16

hibernate.default_entity_modeSets a default mode for entity representation for all sessions opened from this SessionFactory

dynamic-map, dom4j, pojo

hibernate.order_updatesForces 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. true | false

hibernate.generate_statisticsIf enabled, Hibernate will collect statistics useful for performance tuning.

e.g. true | false

hibernate.use_identifier_rollbackIf enabled, generated identifier properties will be reset to default values when objects are deleted.

e.g. true | false

hibernate.use_sql_commentsIf turned on, Hibernate will generate comments inside the SQL, for easier debugging, defaults to false.

e.g. true | false

hibernate.id.new_generator_mappingsSetting is relevant when using @GeneratedValue. It indicates whether or not the new IdentifierGenerator implementations are used for javax.persistence.GenerationType.AUTO, javax.persistence.GenerationType.TABLE and javax.persistence.GenerationType.SEQUENCE. Default to false to keep backward compatibility.

e.g. true | false


Note

We recommend all new projects which make use of to use @GeneratedValue to also set hibernate.id.new_generator_mappings=true as the new generators are more efficient and closer to the JPA 2 specification semantic. However they are not backward compatible with existing databases (if a sequence or a table is used for id generation).

Table 3.4. Hibernate JDBC and Connection Properties

Property namePurpose
hibernate.jdbc.fetch_sizeA non-zero value determines the JDBC fetch size (calls Statement.setFetchSize()).
hibernate.jdbc.batch_sizeA non-zero value enables use of JDBC2 batch updates by Hibernate.

e.g. recommended values between 5 and 30

hibernate.jdbc.batch_versioned_dataSet 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. true | false

hibernate.jdbc.factory_classSelect a custom org.hibernate.jdbc.Batcher. Most applications will not need this configuration property.

e.g. classname.of.BatcherFactory

hibernate.jdbc.use_scrollable_resultsetEnables 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. true | false

hibernate.jdbc.use_streams_for_binaryUse streams when writing/reading binary or serializable types to/from JDBC. *system-level property*

e.g. true | false

hibernate.jdbc.use_get_generated_keysEnables 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. true|false

hibernate.connection.provider_classThe classname of a custom org.hibernate.connection.ConnectionProvider which provides JDBC connections to Hibernate.

e.g. classname.of.ConnectionProvider

hibernate.connection.isolationSets 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. 1, 2, 4, 8

hibernate.connection.autocommitEnables autocommit for JDBC pooled connections (it is not recommended).

e.g. true | false

hibernate.connection.release_modeSpecifies 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. auto (default) | on_close | after_transaction | after_statement

This setting only affects Sessions returned from SessionFactory.openSession. For Sessions obtained through SessionFactory.getCurrentSession, the CurrentSessionContext implementation configured for use controls the connection release mode for those Sessions. See Section 2.3, “Contextual sessions”

hibernate.connection.<propertyName>Pass the JDBC property <propertyName> to DriverManager.getConnection().
hibernate.jndi.<propertyName>Pass the property <propertyName> to the JNDI InitialContextFactory.



Table 3.7. Miscellaneous Properties

Property namePurpose
hibernate.current_session_context_classSupply a custom strategy for the scoping of the "current" Session. See Section 2.3, “Contextual sessions” for more information about the built-in strategies.

e.g. jta | thread | managed | custom.Class

hibernate.query.factory_classChooses the HQL parser implementation.

e.g. org.hibernate.hql.internal.ast.ASTQueryTranslatorFactory or org.hibernate.hql.internal.classic.ClassicQueryTranslatorFactory

hibernate.query.substitutionsIs used to map from tokens in Hibernate queries to SQL tokens (tokens might be function or literal names, for example).

e.g. hqlLiteral=SQL_LITERAL, hqlFunction=SQLFUNC

hibernate.hbm2ddl.autoAutomatically 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. validate | update | create | create-drop

hibernate.hbm2ddl.import_files

Comma-separated names of the optional files containing SQL DML statements executed during the SessionFactory creation. This is useful for testing or demoing: by adding INSERT statements for example you can populate your database with a minimal set of data when it is deployed.

File order matters, the statements of a give file are executed before the statements of the following files. These statements are only executed if the schema is created ie if hibernate.hbm2ddl.auto is set to create or create-drop.

e.g. /humans.sql,/dogs.sql

hibernate.hbm2ddl.import_files_sql_extractor

The classname of a custom ImportSqlCommandExtractor (defaults to the built-in SingleLineSqlCommandExtractor). This is useful for implementing dedicated parser that extracts single SQL statements from each import file. Hibernate provides also MultipleLinesSqlCommandExtractor which supports instructions/comments and quoted strings spread over multiple lines (mandatory semicolon at the end of each statement).

e.g. classname.of.ImportSqlCommandExtractor

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 hibernate.cfg.xml. Reflection can sometimes be useful when troubleshooting. Hibernate always requires either CGLIB or javassist even if you turn off the optimizer.

e.g. true | false

hibernate.bytecode.provider

Both javassist or cglib can be used as byte manipulation engines; the default is javassist.

e.g. javassist | cglib


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.


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:


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.

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://www.hibernate.org/dtd/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:

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:

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:


A JNDI-bound Hibernate SessionFactory can simplify the lookup function of the factory and create new Sessions. 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 2.3, “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 Sessions 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 Sessions 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.internal.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). The term "persistent" here means that the classes are able to be persisted, not that they are in the persistent state (see Section 11.1, “Hibernate object states” for discussion).

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, Hibernate assumes very little about the nature of your persistent objects. You can express a domain model in other ways (using trees of java.util.Map instances, for example).

Example 4.1. Simple POJO representing a cat

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 property named id. This property maps to the primary key column(s) of the underlying database table. The type of the identifier property can be any "basic" type (see ???). See Section 9.4, “Components as composite identifiers” for information on mapping composite (multi-column) identifiers.

Note

Identifiers do not necessarily need to identify column(s) in the database physically defined as a primary key. They should just identify columns that can be used to uniquely identify rows in the underlying table.

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 (lazy loading), 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. To persist a final class which does not implement a "full" interface you must disable proxy generation. See Example 4.2, “Disabling proxies in hbm.xml and Example 4.3, “Disabling proxies in annotations”.



If the final class does implement a proper interface, you could alternatively tell Hibernate to use the interface instead when generating the proxies. See Example 4.4, “Proxying an interface in hbm.xml and Example 4.5, “Proxying an interface in annotations”.



You should also avoid declaring public final methods as this will again limit the ability to generate proxies from this class. If you want to use a class with public final methods, you must explicitly disable proxying. Again, see Example 4.2, “Disabling proxies in hbm.xml and Example 4.3, “Disabling proxies in annotations”.

You have to override the equals() and hashCode() methods if you:

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 Sets.

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 13.1.3, “Considering object identity”). Immutable or unique properties are usually good candidates for a business key.

Persistent entities do not necessarily have to be represented as POJO classes or as JavaBean objects at runtime. Hibernate also supports dynamic models (using Maps of Maps 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 Maps. 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 Maps of Maps:

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 20, 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, how to extract values from such a data structure and how to 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:

Users can also plug in their own tuplizers. Perhaps you require that 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, Example 4.6, “Specify custom tuplizers in annotations” shows how to specify a custom org.hibernate.tuple.entity.EntityTuplizer using annotations while Example 4.7, “Specify custom tuplizers in hbm.xml shows how to do the same in hbm.xml



org.hibernate.EntityNameResolver 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 implementations as is
 */
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:

  1. Implement a custom tuplizer (see Section 4.5, “Tuplizers”), implementing the getEntityNameResolvers method

  2. Register it with the org.hibernate.impl.SessionFactoryImpl (which is the implementation class for org.hibernate.SessionFactory) using the registerEntityNameResolver method.

Object/relational mappings can be defined in three approaches:

Annotations are split in two categories, the logical mapping annotations (describing the object model, the association between two entities etc.) and the physical mapping annotations (describing the physical schema, tables, columns, indexes, etc). We will mix annotations from both categories in the following code examples.

JPA annotations are in the javax.persistence.* package. Hibernate specific extensions are in org.hibernate.annotations.*. You favorite IDE can auto-complete annotations and their attributes for you (even without a specific "JPA" plugin, since JPA annotations are plain Java 5 annotations).

Here is an example of mapping

package eg;


@Entity 
@Table(name="cats") @Inheritance(strategy=SINGLE_TABLE)
@DiscriminatorValue("C") @DiscriminatorColumn(name="subclass", discriminatorType=CHAR)
public class Cat {
   
   @Id @GeneratedValue
   public Integer getId() { return id; }
   public void setId(Integer id) { this.id = id; }
   private Integer id;
   public BigDecimal getWeight() { return weight; }
   public void setWeight(BigDecimal weight) { this.weight = weight; }
   private BigDecimal weight;
   @Temporal(DATE) @NotNull @Column(updatable=false)
   public Date getBirthdate() { return birthdate; }
   public void setBirthdate(Date birthdate) { this.birthdate = birthdate; }
   private Date birthdate;
   @org.hibernate.annotations.Type(type="eg.types.ColorUserType")
   @NotNull @Column(updatable=false)
   public ColorType getColor() { return color; }
   public void setColor(ColorType color) { this.color = color; }
   private ColorType color;
   @NotNull @Column(updatable=false)
   public String getSex() { return sex; }
   public void setSex(String sex) { this.sex = sex; }
   private String sex;
   @NotNull @Column(updatable=false)
   public Integer getLitterId() { return litterId; }
   public void setLitterId(Integer litterId) { this.litterId = litterId; }
   private Integer litterId;
   @ManyToOne @JoinColumn(name="mother_id", updatable=false)
   public Cat getMother() { return mother; }
   public void setMother(Cat mother) { this.mother = mother; }
   private Cat mother;
   @OneToMany(mappedBy="mother") @OrderBy("litterId")
   public Set<Cat> getKittens() { return kittens; }
   public void setKittens(Set<Cat> kittens) { this.kittens = kittens; }
   private Set<Cat> kittens = new HashSet<Cat>();
}
@Entity @DiscriminatorValue("D")
public class DomesticCat extends Cat {
   public String getName() { return name; }
   public void setName(String name) { this.name = name }
   private String name;
}
@Entity
public class Dog { ... }

The legacy hbm.xml approach uses an XML schema 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://www.hibernate.org/dtd/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 concepts of the mapping documents (both annotations and XML). 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).

An entity is a regular Java object (aka POJO) which will be persisted by Hibernate.

To mark an object as an entity in annotations, use the @Entity annotation.

@Entity

public class Flight implements Serializable {
    Long id;
    @Id
    public Long getId() { return id; }
    public void setId(Long id) { this.id = id; }
}         

That's pretty much it, the rest is optional. There are however any options to tweak your entity mapping, let's explore them.

@Table lets you define the table the entity will be persisted into. If undefined, the table name is the unqualified class name of the entity. You can also optionally define the catalog, the schema as well as unique constraints on the table.

@Entity

@Table(name="TBL_FLIGHT", 
       schema="AIR_COMMAND", 
       uniqueConstraints=
           @UniqueConstraint(
               name="flight_number", 
               columnNames={"comp_prefix", "flight_number"} ) )
public class Flight implements Serializable {
    @Column(name="comp_prefix")
    public String getCompagnyPrefix() { return companyPrefix; }
    @Column(name="flight_number")
    public String getNumber() { return number; }
}

The constraint name is optional (generated if left undefined). The column names composing the constraint correspond to the column names as defined before the Hibernate NamingStrategy is applied.

@Entity.name lets you define the shortcut name of the entity you can used in JP-QL and HQL queries. It defaults to the unqualified class name of the class.

Hibernate goes beyond the JPA specification and provide additional configurations. Some of them are hosted on @org.hibernate.annotations.Entity:

Some entities are not mutable. They cannot be updated or deleted by the application. This allows Hibernate to make some minor performance optimizations.. Use the @Immutable annotation.

You can also alter how Hibernate deals with lazy initialization for this class. On @Proxy, use lazy=false to disable lazy fetching (not recommended). You can also specify an interface to use for lazy initializing proxies (defaults to the class itself): use proxyClass on @Proxy. Hibernate will initially return proxies (Javassist or CGLIB) that implement the named interface. The persistent object will load when a method of the proxy is invoked. See "Initializing collections and proxies" below.

@BatchSize specifies a "batch size" for fetching instances of this class by identifier. Not yet loaded instances are loaded batch-size at a time (default 1).

You can specific an arbitrary SQL WHERE condition to be used when retrieving objects of this class. Use @Where for that.

In the same vein, @Check lets you define an SQL expression used to generate a multi-row check constraint for automatic schema generation.

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 using @org.hibernate.annotations.Subselect:

@Entity

@Subselect("select item.name, max(bid.amount), count(*) "
        + "from item "
        + "join bid on bid.item_id = item.id "
        + "group by item.name")
@Synchronize( {"item", "bid"} ) //tables impacted
public class Summary {
    @Id
    public String getId() { return id; }
    ...
}

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.

We will now explore the same options using the hbm.xml structure. You can declare a persistent class using the class element. For example:

<class
        name="(1)ClassName"
        table=(2)"tableName"
        discri(3)minator-value="discriminator_value"
        mutabl(4)e="true|false"
        schema(5)="owner"
        catalo(6)g="catalog"
        proxy=(7)"ProxyInterface"
        dynami(8)c-update="true|false"
        dynami(9)c-insert="true|false"
        select(10)-before-update="true|false"
        polymo(11)rphism="implicit|explicit"
        where=(12)"arbitrary sql where condition"
        persis(13)ter="PersisterClass"
        batch-(14)size="N"
        optimi(15)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"
/>
1

name (optional): the fully qualified Java class name of the persistent class or interface. If this attribute is missing, it is assumed that the mapping is for a non-POJO entity.

2

table (optional - defaults to the unqualified class name): the name of its database table.

3

discriminator-value (optional - defaults to the class name): a value that distinguishes individual subclasses that is used for polymorphic behavior. Acceptable values include null and not null.

4

mutable (optional - defaults to true): specifies that instances of the class are (not) mutable.

5

schema (optional): overrides the schema name specified by the root <hibernate-mapping> element.

6

catalog (optional): overrides the catalog name specified by the root <hibernate-mapping> element.

7

proxy (optional): specifies an interface to use for lazy initializing proxies. You can specify the name of the class itself.

8

dynamic-update (optional - defaults to false): specifies that UPDATE SQL should be generated at runtime and can contain only those columns whose values have changed.

9

dynamic-insert (optional - defaults to false): specifies that INSERT SQL should be generated at runtime and contain only the columns whose values are not null.

10

select-before-update (optional - defaults to false): specifies that Hibernate should never perform an SQL UPDATE unless it is certain that an object is actually modified. Only when a transient object has been associated with a new session using update(), will Hibernate perform an extra SQL SELECT to determine if an UPDATE is actually required.

11

polymorphisms (optional - defaults to implicit): determines whether implicit or explicit query polymorphisms is used.

12

where (optional): specifies an arbitrary SQL WHERE condition to be used when retrieving objects of this class.

13

persister (optional): specifies a custom ClassPersister.

14

batch-size (optional - defaults to 1): specifies a "batch size" for fetching instances of this class by identifier.

15

optimistic-lock (optional - defaults to version): determines the optimistic locking strategy.

(16)

lazy (optional): lazy fetching can be disabled by setting lazy="false".

(17)

entity-name (optional - defaults to the class name): Hibernate3 allows a class to be mapped multiple times, potentially to different tables. It also allows entity mappings that are represented by Maps or XML at the Java level. In these cases, you should provide an explicit arbitrary name for the entity. See Section 4.4, “Dynamic models” and Chapter 20, XML Mapping for more information.

(18)

check (optional): an SQL expression used to generate a multi-row check constraint for automatic schema generation.

(19)

rowid (optional): Hibernate can use ROWIDs on databases. On Oracle, for example, Hibernate can use the rowid extra column for fast updates once this option has been set to rowid. A ROWID is an implementation detail and represents the physical location of a stored tuple.

(20)

subselect (optional): maps an immutable and read-only entity to a database subselect. This is useful if you want to have a view instead of a base table. See below for more information.

(21)

abstract (optional): is used to mark abstract superclasses in <union-subclass> hierarchies.

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.

Here is how to do a virtual view (subselect) in XML:


<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>

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.

Mark the identifier property with @Id.

@Entity

public class Person {
   @Id Integer getId() { ... }
   ...
}

In hbm.xml, use the <id> element which defines the mapping from that property to the primary key column.

<id
        name="(1)propertyName"
        type="(2)typename"
        column(3)="column_name"
        unsave(4)d-value="null|any|none|undefined|id_value"
        access(5)="field|property|ClassName">
        node="element-name|@attribute-name|element/@attribute|."

        <generator class="generatorClass"/>
</id>
1

name (optional): the name of the identifier property.

2

type (optional): a name that indicates the Hibernate type.

3

column (optional - defaults to the property name): the name of the primary key column.

4

unsaved-value (optional - defaults to a "sensible" value): an identifier property value that indicates an instance is newly instantiated (unsaved), distinguishing it from detached instances that were saved or loaded in a previous session.

5

access (optional - defaults to property): the strategy Hibernate should use for accessing the property value.

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 and indeed has no corresponding element in annotations.

You can also declare the identifier as a composite identifier. This allows access to legacy data with composite keys. Its use is strongly discouraged for anything else.

You can define a composite primary key through several syntaxes:

As you can see the last case is far from obvious. It has been inherited from the dark ages of EJB 2 for backward compatibilities and we recommend you not to use it (for simplicity sake).

Let's explore all three cases using examples.

Here is a simple example of @EmbeddedId.

@Entity

class User {
   @EmbeddedId
   @AttributeOverride(name="firstName", column=@Column(name="fld_firstname")
   UserId id;
   Integer age;
}
@Embeddable
class UserId implements Serializable {
   String firstName;
   String lastName;
}

You can notice that the UserId class is serializable. To override the column mapping, use @AttributeOverride.

An embedded id can itself contains the primary key of an associated entity.

@Entity

class Customer {
   @EmbeddedId CustomerId id;
   boolean preferredCustomer;
   @MapsId("userId")
   @JoinColumns({
      @JoinColumn(name="userfirstname_fk", referencedColumnName="firstName"),
      @JoinColumn(name="userlastname_fk", referencedColumnName="lastName")
   })
   @OneToOne User user;
}
@Embeddable
class CustomerId implements Serializable {
   UserId userId;
   String customerNumber;
   //implements equals and hashCode
}
@Entity 
class User {
   @EmbeddedId UserId id;
   Integer age;
}
@Embeddable
class UserId implements Serializable {
   String firstName;
   String lastName;
   //implements equals and hashCode
}

In the embedded id object, the association is represented as the identifier of the associated entity. But you can link its value to a regular association in the entity via the @MapsId annotation. The @MapsId value correspond to the property name of the embedded id object containing the associated entity's identifier. In the database, it means that the Customer.user and the CustomerId.userId properties share the same underlying column (user_fk in this case).

In practice, your code only sets the Customer.user property and the user id value is copied by Hibernate into the CustomerId.userId property.

While not supported in JPA, Hibernate lets you place your association directly in the embedded id component (instead of having to use the @MapsId annotation).

@Entity

class Customer {
   @EmbeddedId CustomerId id;
   boolean preferredCustomer;
}
@Embeddable
class CustomerId implements Serializable {
   @OneToOne
   @JoinColumns({
      @JoinColumn(name="userfirstname_fk", referencedColumnName="firstName"),
      @JoinColumn(name="userlastname_fk", referencedColumnName="lastName")
   }) 
   User user;
   String customerNumber;
   //implements equals and hashCode
}
@Entity 
class User {
   @EmbeddedId UserId id;
   Integer age;
}
@Embeddable
class UserId implements Serializable {
   String firstName;
   String lastName;
   //implements equals and hashCode
}

Let's now rewrite these examples using the hbm.xml syntax.


<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>

First a simple example:


<class name="User">
   <composite-id name="id" class="UserId">
      <key-property name="firstName" column="fld_firstname"/>
      <key-property name="lastName"/>
   </composite-id>
</class>

Then an example showing how an association can be mapped.


<class name="Customer">
   <composite-id name="id" class="CustomerId">
      <key-property name="firstName" column="userfirstname_fk"/>
      <key-property name="lastName" column="userfirstname_fk"/>
      <key-property name="customerNumber"/>
   </composite-id>

   <property name="preferredCustomer"/>

   <many-to-one name="user">
      <column name="userfirstname_fk" updatable="false" insertable="false"/>
      <column name="userlastname_fk" updatable="false" insertable="false"/>
   </many-to-one>
</class>

<class name="User">
   <composite-id name="id" class="UserId">
      <key-property name="firstName"/>
      <key-property name="lastName"/>
   </composite-id>

   <property name="age"/>
</class>

Notice a few things in the previous example:

The last example shows how to map association directly in the embedded id component.


<class name="Customer">
   <composite-id name="id" class="CustomerId">
      <key-many-to-one name="user">
         <column name="userfirstname_fk"/>
         <column name="userlastname_fk"/>
      </key-many-to-one>
      <key-property name="customerNumber"/>
   </composite-id>

   <property name="preferredCustomer"/>
</class>

<class name="User">
   <composite-id name="id" class="UserId">
      <key-property name="firstName"/>
      <key-property name="lastName"/>
   </composite-id>

   <property name="age"/>
</class>

This is the recommended approach to map composite identifier. The following options should not be considered unless some constraint are present.

Another, arguably more natural, approach is to place @Id on multiple properties of your entity. This approach is only supported by Hibernate (not JPA compliant) but does not require an extra embeddable component.

@Entity

class Customer implements Serializable {
   @Id @OneToOne
   @JoinColumns({
      @JoinColumn(name="userfirstname_fk", referencedColumnName="firstName"),
      @JoinColumn(name="userlastname_fk", referencedColumnName="lastName")
   })
   User user;
  
   @Id String customerNumber;
   boolean preferredCustomer;
   //implements equals and hashCode
}
@Entity 
class User {
   @EmbeddedId UserId id;
   Integer age;
}
@Embeddable
class UserId implements Serializable {
   String firstName;
   String lastName;
   //implements equals and hashCode
}

In this case Customer is its own identifier representation: it must implement Serializable and must implement equals() and hashCode().

In hbm.xml, the same mapping is:


<class name="Customer">
   <composite-id>
      <key-many-to-one name="user">
         <column name="userfirstname_fk"/>
         <column name="userlastname_fk"/>
      </key-many-to-one>
      <key-property name="customerNumber"/>
   </composite-id>

   <property name="preferredCustomer"/>
</class>

<class name="User">
   <composite-id name="id" class="UserId">
      <key-property name="firstName"/>
      <key-property name="lastName"/>
   </composite-id>

   <property name="age"/>
</class>

@IdClass on an entity points to the class (component) representing the identifier of the class. The properties marked @Id on the entity must have their corresponding property on the @IdClass. The return type of search twin property must be either identical for basic properties or must correspond to the identifier class of the associated entity for an association.

@Entity

@IdClass(CustomerId.class)
class Customer implements Serializable {
   @Id @OneToOne
   @JoinColumns({
      @JoinColumn(name="userfirstname_fk", referencedColumnName="firstName"),
      @JoinColumn(name="userlastname_fk", referencedColumnName="lastName")
   }) 
   User user;
  
   @Id String customerNumber;
   boolean preferredCustomer;
}
class CustomerId implements Serializable {
   UserId user;
   String customerNumber;
   //implements equals and hashCode
}
@Entity 
class User {
   @EmbeddedId UserId id;
   Integer age;
   //implements equals and hashCode
}
@Embeddable
class UserId implements Serializable {
   String firstName;
   String lastName;
   //implements equals and hashCode
}

Customer and CustomerId do have the same properties customerNumber as well as user. CustomerId must be Serializable and implement equals() and hashCode().

While not JPA standard, Hibernate let's you declare the vanilla associated property in the @IdClass.

@Entity

@IdClass(CustomerId.class)
class Customer implements Serializable {
   @Id @OneToOne
   @JoinColumns({
      @JoinColumn(name="userfirstname_fk", referencedColumnName="firstName"),
      @JoinColumn(name="userlastname_fk", referencedColumnName="lastName")
   }) 
   User user;
  
   @Id String customerNumber;
   boolean preferredCustomer;
}
class CustomerId implements Serializable {
   @OneToOne User user;
   String customerNumber;
   //implements equals and hashCode
}
@Entity 
class User {
   @EmbeddedId UserId id;
   Integer age;
   //implements equals and hashCode
}
@Embeddable
class UserId implements Serializable {
  String firstName;
  String lastName;
}

This feature is of limited interest though as you are likely to have chosen the @IdClass approach to stay JPA compliant or you have a quite twisted mind.

Here are the equivalent on hbm.xml files:


<class name="Customer">
   <composite-id class="CustomerId" mapped="true">
      <key-many-to-one name="user">
         <column name="userfirstname_fk"/>
         <column name="userlastname_fk"/>
      </key-many-to-one>
      <key-property name="customerNumber"/>
   </composite-id>

   <property name="preferredCustomer"/>
</class>

<class name="User">
   <composite-id name="id" class="UserId">
      <key-property name="firstName"/>
      <key-property name="lastName"/>
   </composite-id>

   <property name="age"/>
</class>

Hibernate can generate and populate identifier values for you automatically. This is the recommended approach over "business" or "natural" id (especially composite ids).

Hibernate offers various generation strategies, let's explore the most common ones first that happens to be standardized by JPA:

To mark an id property as generated, use the @GeneratedValue annotation. You can specify the strategy used (default to AUTO) by setting strategy.

@Entity

public class Customer {
   @Id @GeneratedValue
   Integer getId() { ... };
}
@Entity 
public class Invoice {
   @Id @GeneratedValue(strategy=GenerationType.IDENTITY)
   Integer getId() { ... };
}

SEQUENCE and TABLE require additional configurations that you can set using @SequenceGenerator and @TableGenerator:

  • name: name of the generator

  • table / sequenceName: name of the table or the sequence (defaulting respectively to hibernate_sequences and hibernate_sequence)

  • catalog / schema:

  • initialValue: the value from which the id is to start generating

  • allocationSize: the amount to increment by when allocating id numbers from the generator

In addition, the TABLE strategy also let you customize:

  • pkColumnName: the column name containing the entity identifier

  • valueColumnName: the column name containing the identifier value

  • pkColumnValue: the entity identifier

  • uniqueConstraints: any potential column constraint on the table containing the ids

To link a table or sequence generator definition with an actual generated property, use the same name in both the definition name and the generator value generator as shown below.

@Id 

@GeneratedValue(
    strategy=GenerationType.SEQUENCE, 
    generator="SEQ_GEN")
@javax.persistence.SequenceGenerator(
    name="SEQ_GEN",
    sequenceName="my_sequence",
    allocationSize=20
)
public Integer getId() { ... }        

The scope of a generator definition can be the application or the class. Class-defined generators are not visible outside the class and can override application level generators. Application level generators are defined in JPA's XML deployment descriptors (see XXXXXX ???):

<table-generator name="EMP_GEN"

            table="GENERATOR_TABLE"
            pk-column-name="key"
            value-column-name="hi"
            pk-column-value="EMP"
            allocation-size="20"/>
//and the annotation equivalent
@javax.persistence.TableGenerator(
    name="EMP_GEN",
    table="GENERATOR_TABLE",
    pkColumnName = "key",
    valueColumnName = "hi"
    pkColumnValue="EMP",
    allocationSize=20
)
<sequence-generator name="SEQ_GEN" 
    sequence-name="my_sequence"
    allocation-size="20"/>
//and the annotation equivalent
@javax.persistence.SequenceGenerator(
    name="SEQ_GEN",
    sequenceName="my_sequence",
    allocationSize=20
)
         

If a JPA XML descriptor (like META-INF/orm.xml) is used to define the generators, EMP_GEN and SEQ_GEN are application level generators.

Note

Package level definition is not supported by the JPA specification. However, you can use the @GenericGenerator at the package level (see ???).

These are the four standard JPA generators. Hibernate goes beyond that and provide additional generators or additional options as we will see below. You can also write your own custom identifier generator by implementing org.hibernate.id.IdentifierGenerator.

To define a custom generator, use the @GenericGenerator annotation (and its plural counter part @GenericGenerators) that describes the class of the identifier generator or its short cut name (as described below) and a list of key/value parameters. When using @GenericGenerator and assigning it via @GeneratedValue.generator, the @GeneratedValue.strategy is ignored: leave it blank.

@Id @GeneratedValue(generator="system-uuid")

@GenericGenerator(name="system-uuid", strategy = "uuid")
public String getId() {
@Id @GeneratedValue(generator="trigger-generated")
@GenericGenerator(
    name="trigger-generated", 
    strategy = "select",
    parameters = @Parameter(name="key", value = "socialSecurityNumber")
)
public String getId() {

The hbm.xml approach uses the optional <generator> child element inside <id>. 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

Generates a 128-bit UUID based on a custom algorithm. The value generated is represented as a string of 32 hexidecimal digits. Users can also configure it to use a separator (config parameter "separator") which separates the hexidecimal digits into 8{sep}8{sep}4{sep}8{sep}4. Note specifically that this is different than the IETF RFC 4122 representation of 8-4-4-4-12. If you need RFC 4122 compliant UUIDs, consider using "uuid2" generator discussed below.

uuid2

Generates a IETF RFC 4122 compliant (variant 2) 128-bit UUID. The exact "version" (the RFC term) generated depends on the pluggable "generation strategy" used (see below). Capable of generating values as java.util.UUID, java.lang.String or as a byte array of length 16 (byte[16]). The "generation strategy" is defined by the interface org.hibernate.id.UUIDGenerationStrategy. The generator defines 2 configuration parameters for defining which generation strategy to use:

Out of the box, comes with the following strategies:

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.

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:

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 5.1.2.3.1, “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 5.1.2.3, “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.

When using long transactions or conversations that span several database transactions, it is useful to store versioning data to ensure that if the same entity is updated by two conversations, the last to commit changes will be informed and not override the other conversation's work. It guarantees some isolation while still allowing for good scalability and works particularly well in read-often write-sometimes situations.

You can use two approaches: a dedicated version number or a timestamp.

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.

You can add optimistic locking capability to an entity using the @Version annotation:

@Entity

public class Flight implements Serializable {
...
    @Version
    @Column(name="OPTLOCK")
    public Integer getVersion() { ... }
}           

The version property will be mapped to the OPTLOCK column, and the entity manager will use it to detect conflicting updates (preventing lost updates you might otherwise see with the last-commit-wins strategy).

The version column may be a numeric. Hibernate supports any kind of type provided that you define and implement the appropriate UserVersionType.

The application must not alter the version number set up by Hibernate in any way. To artificially increase the version number, check in Hibernate Entity Manager's reference documentation LockModeType.OPTIMISTIC_FORCE_INCREMENT or LockModeType.PESSIMISTIC_FORCE_INCREMENT.

If the version number is generated by the database (via a trigger for example), make sure to use @org.hibernate.annotations.Generated(GenerationTime.ALWAYS).

To declare a version property in hbm.xml, use:

<version
        column(1)="version_column"
        name="(2)propertyName"
        type="(3)typename"
        access(4)="field|property|ClassName"
        unsave(5)d-value="null|negative|undefined"
        genera(6)ted="never|always"
        insert(7)="true|false"
        node="element-name|@attribute-name|element/@attribute|."
/>
1

column (optional - defaults to the property name): the name of the column holding the version number.

2

name: the name of a property of the persistent class.

3

type (optional - defaults to integer): the type of the version number.

4

access (optional - defaults to property): the strategy Hibernate uses to access the property value.

5

unsaved-value (optional - defaults to undefined): a version property value that indicates that an instance is newly instantiated (unsaved), distinguishing it from detached instances that were saved or loaded in a previous session. Undefined specifies that the identifier property value should be used.

6

generated (optional - defaults to never): specifies that this version property value is generated by the database. See the discussion of generated properties for more information.

7

insert (optional - defaults to true): specifies whether the version column should be included in SQL insert statements. It can be set to false if the database column is defined with a default value of 0.

Alternatively, you can use a timestamp. Timestamps are a less safe implementation of optimistic locking. However, sometimes an application might use the timestamps in other ways as well.

Simply mark a property of type Date or Calendar as @Version.

@Entity

public class Flight implements Serializable {
...
    @Version
    public Date getLastUpdate() { ... }
}           

When using timestamp versioning you can tell Hibernate where to retrieve the timestamp value from - database or JVM - by optionally adding the @org.hibernate.annotations.Source annotation to the property. Possible values for the value attribute of the annotation are org.hibernate.annotations.SourceType.VM and org.hibernate.annotations.SourceType.DB. The default is SourceType.DB which is also used in case there is no @Source annotation at all.

Like in the case of version numbers, the timestamp can also be generated by the database instead of Hibernate. To do that, use @org.hibernate.annotations.Generated(GenerationTime.ALWAYS).

In hbm.xml, use the <timestamp> element:

<timestamp
        column(1)="timestamp_column"
        name="(2)propertyName"
        access(3)="field|property|ClassName"
        unsave(4)d-value="null|undefined"
        source(5)="vm|db"
        genera(6)ted="never|always"
        node="element-name|@attribute-name|element/@attribute|."
/>
1

column (optional - defaults to the property name): the name of a column holding the timestamp.

2

name: the name of a JavaBeans style property of Java type Date or Timestamp of the persistent class.

3

access (optional - defaults to property): the strategy Hibernate uses for accessing the property value.

4

unsaved-value (optional - defaults to null): a version property value that indicates that an instance is newly instantiated (unsaved), distinguishing it from detached instances that were saved or loaded in a previous session. Undefined specifies that the identifier property value should be used.

5

source (optional - defaults to vm): Where should Hibernate retrieve the timestamp value from? From the database, or from the current JVM? Database-based timestamps incur an overhead because Hibernate must hit the database in order to determine the "next value". It is safer to use in clustered environments. Not all Dialects are known to support the retrieval of the database's current timestamp. Others may also be unsafe for usage in locking due to lack of precision (Oracle 8, for example).

6

generated (optional - defaults to never): specifies that this timestamp property value is actually generated by the database. See the discussion of generated properties for more information.

You need to decide which property needs to be made persistent in a given entity. This differs slightly between the annotation driven metadata and the hbm.xml files.

In the annotations world, every non static non transient property (field or method depending on the access type) of an entity is considered persistent, unless you annotate it as @Transient. Not having an annotation for your property is equivalent to the appropriate @Basic annotation.

The @Basic annotation allows you to declare the fetching strategy for a property. If set to LAZY, specifies that this property should be fetched lazily when the instance variable is first accessed. It requires build-time bytecode instrumentation, if your classes are not instrumented, property level lazy loading is silently ignored. The default is EAGER. You can also mark a property as not optional thanks to the @Basic.optional attribute. This will ensure that the underlying column are not nullable (if possible). Note that a better approach is to use the @NotNull annotation of the Bean Validation specification.

Let's look at a few examples:

public transient int counter; //transient property


private String firstname; //persistent property
@Transient
String getLengthInMeter() { ... } //transient property
String getName() {... } // persistent property
@Basic
int getLength() { ... } // persistent property
@Basic(fetch = FetchType.LAZY)
String getDetailedComment() { ... } // persistent property
@Temporal(TemporalType.TIME)
java.util.Date getDepartureTime() { ... } // persistent property           
@Enumerated(EnumType.STRING)
Starred getNote() { ... } //enum persisted as String in database

counter, a transient field, and lengthInMeter, a method annotated as @Transient, and will be ignored by the Hibernate. name, length, and firstname properties are mapped persistent and eagerly fetched (the default for simple properties). The detailedComment property value will be lazily fetched from the database once a lazy property of the entity is accessed for the first time. Usually you don't need to lazy simple properties (not to be confused with lazy association fetching). The recommended alternative is to use the projection capability of JP-QL (Java Persistence Query Language) or Criteria queries.

JPA support property mapping of all basic types supported by Hibernate (all basic Java types , their respective wrappers and serializable classes). Hibernate Annotations supports out of the box enum type mapping either into a ordinal column (saving the enum ordinal) or a string based column (saving the enum string representation): the persistence representation, defaulted to ordinal, can be overridden through the @Enumerated annotation as shown in the note property example.

In plain Java APIs, the temporal precision of time is not defined. When dealing with temporal data you might want to describe the expected precision in database. Temporal data can have DATE, TIME, or TIMESTAMP precision (ie the actual date, only the time, or both). Use the @Temporal annotation to fine tune that.

@Lob indicates that the property should be persisted in a Blob or a Clob depending on the property type: java.sql.Clob, Character[], char[] and java.lang.String will be persisted in a Clob. java.sql.Blob, Byte[], byte[] and Serializable type will be persisted in a Blob.

@Lob

public String getFullText() {
    return fullText;
}
@Lob
public byte[] getFullCode() {
    return fullCode;
}

If the property type implements java.io.Serializable and is not a basic type, and if the property is not annotated with @Lob, then the Hibernate serializable type is used.

You can also manually specify a type using the @org.hibernate.annotations.Type and some parameters if needed. @Type.type could be:

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.

@org.hibernate.annotations.TypeDef and @org.hibernate.annotations.TypeDefs allows you to declare type definitions. These annotations can be placed at the class or package level. Note that these definitions are global for the session factory (even when defined at the class level). If the type is used on a single entity, you can place the definition on the entity itself. Otherwise, it is recommended to place the definition at the package level. In the example below, when Hibernate encounters a property of class PhoneNumer, it delegates the persistence strategy to the custom mapping type PhoneNumberType. However, properties belonging to other classes, too, can delegate their persistence strategy to PhoneNumberType, by explicitly using the @Type annotation.

@TypeDef(

   name = "phoneNumber",
   defaultForType = PhoneNumber.class,
   typeClass = PhoneNumberType.class
)
@Entity
public class ContactDetails {
   [...]
   private PhoneNumber localPhoneNumber;
   @Type(type="phoneNumber")
   private OverseasPhoneNumber overseasPhoneNumber;
   [...]
}

The following example shows the usage of the parameters attribute to customize the TypeDef.

//in org/hibernate/test/annotations/entity/package-info.java

@TypeDefs(
    {
    @TypeDef(
        name="caster",
        typeClass = CasterStringType.class,
        parameters = {
            @Parameter(name="cast", value="lower")
        }
    )
    }
)
package org.hibernate.test.annotations.entity;
//in org/hibernate/test/annotations/entity/Forest.java
public class Forest {
    @Type(type="caster")
    public String getSmallText() {
    ...
}      

When using composite user type, you will have to express column definitions. The @Columns has been introduced for that purpose.

@Type(type="org.hibernate.test.annotations.entity.MonetaryAmountUserType")

@Columns(columns = {
    @Column(name="r_amount"),
    @Column(name="r_currency")
})
public MonetaryAmount getAmount() {
    return amount;
}
public class MonetaryAmount implements Serializable {
    private BigDecimal amount;
    private Currency currency;
    ...
}

By default the access type of a class hierarchy is defined by the position of the @Id or @EmbeddedId annotations. If these annotations are on a field, then only fields are considered for persistence and the state is accessed via the field. If there annotations are on a getter, then only the getters are considered for persistence and the state is accessed via the getter/setter. That works well in practice and is the recommended approach.

However in some situations, you need to:

The best use case is an embeddable class used by several entities that might not use the same access type. In this case it is better to force the access type at the embeddable class level.

To force the access type on a given class, use the @Access annotation as showed below:

@Entity

public class Order {
   @Id private Long id;
   public Long getId() { return id; }
   public void setId(Long id) { this.id = id; }
   @Embedded private Address address;
   public Address getAddress() { return address; }
   public void setAddress() { this.address = address; }
}
@Entity
public class User {
   private Long id;
   @Id public Long getId() { return id; }
   public void setId(Long id) { this.id = id; }
   private Address address;
   @Embedded public Address getAddress() { return address; }
   public void setAddress() { this.address = address; }
}
@Embeddable
@Access(AcessType.PROPERTY)
public class Address {
   private String street1;
   public String getStreet1() { return street1; }
   public void setStreet1() { this.street1 = street1; }
   private hashCode; //not persistent
}

You can also override the access type of a single property while keeping the other properties standard.

@Entity

public class Order {
   @Id private Long id;
   public Long getId() { return id; }
   public void setId(Long id) { this.id = id; }
   @Transient private String userId;
   @Transient private String orderId;
   @Access(AccessType.PROPERTY)
   public String getOrderNumber() { return userId + ":" + orderId; }
   public void setOrderNumber() { this.userId = ...; this.orderId = ...; }
}

In this example, the default access type is FIELD except for the orderNumber property. Note that the corresponding field, if any must be marked as @Transient or transient.

The column(s) used for a property mapping can be defined using the @Column annotation. Use it to override default values (see the JPA specification for more information on the defaults). You can use this annotation at the property level for properties that are:



@Entity
public class Flight implements Serializable {
...
@Column(updatable = false, name = "flight_name", nullable = false, length=50)
public String getName() { ... }
            

The name property is mapped to the flight_name column, which is not nullable, has a length of 50 and is not updatable (making the property immutable).

This annotation can be applied to regular properties as well as @Id or @Version properties.

@Column(
    name="colu(1)mnName";
    boolean un(2)ique() default false;
    boolean nu(3)llable() default true;
    boolean in(4)sertable() default true;
    boolean up(5)datable() default true;
    String col(6)umnDefinition() default "";
    String tab(7)le() default "";
    int length(8)() default 255;
    int precis(9)ion() default 0; // decimal precision
    int scale((10)) default 0; // decimal scale
1

name (optional): the column name (default to the property name)

2

unique (optional): set a unique constraint on this column or not (default false)

3

nullable (optional): set the column as nullable (default true).

4

insertable (optional): whether or not the column will be part of the insert statement (default true)

5

updatable (optional): whether or not the column will be part of the update statement (default true)

6

columnDefinition (optional): override the sql DDL fragment for this particular column (non portable)

7

table (optional): define the targeted table (default primary table)

8

length (optional): column length (default 255)

8

precision (optional): column decimal precision (default 0)

10

scale (optional): column decimal scale if useful (default 0)

The <property> element declares a persistent JavaBean style property of the class.

<property
        name="(1)propertyName"
        column(2)="column_name"
        type="(3)typename"
        update(4)="true|false"
        insert(4)="true|false"
        formul(5)a="arbitrary SQL expression"
        access(6)="field|property|ClassName"
        lazy="(7)true|false"
        unique(8)="true|false"
        not-nu(9)ll="true|false"
        optimi(10)stic-lock="true|false"
        genera(11)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"
/>
1

name: the name of the property, with an initial lowercase letter.

2

column (optional - defaults to the property name): the name of the mapped database table column. This can also be specified by nested <column> element(s).

3

type (optional): a name that indicates the Hibernate type.

4

update, insert (optional - defaults to true): specifies that the mapped columns should be included in SQL UPDATE and/or INSERT statements. Setting both to false allows a pure "derived" property whose value is initialized from some other property that maps to the same column(s), or by a trigger or other application.

5

formula (optional): an SQL expression that defines the value for a computed property. Computed properties do not have a column mapping of their own.

6

access (optional - defaults to property): the strategy Hibernate uses for accessing the property value.

7

lazy (optional - defaults to false): specifies that this property should be fetched lazily when the instance variable is first accessed. It requires build-time bytecode instrumentation.

8

unique (optional): enables the DDL generation of a unique constraint for the columns. Also, allow this to be the target of a property-ref.

9

not-null (optional): enables the DDL generation of a nullability constraint for the columns.

10

optimistic-lock (optional - defaults to true): specifies that updates to this property do or do not require acquisition of the optimistic lock. In other words, it determines if a version increment should occur when this property is dirty.

11

generated (optional - defaults to never): specifies that this property value is actually generated by the database. See the discussion of generated properties for more information.

typename could be:

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.

Embeddable objects (or components) are objects whose properties are mapped to the same table as the owning entity's table. Components can, in turn, declare their own properties, components or collections

It is possible to declare an embedded component inside an entity and even override its column mapping. Component classes have to be annotated at the class level with the @Embeddable annotation. It is possible to override the column mapping of an embedded object for a particular entity using the @Embedded and @AttributeOverride annotation in the associated property:

@Entity

public class Person implements Serializable {
    // Persistent component using defaults
    Address homeAddress;
    @Embedded
    @AttributeOverrides( {
            @AttributeOverride(name="iso2", column = @Column(name="bornIso2") ),
            @AttributeOverride(name="name", column = @Column(name="bornCountryName") )
    } )
    Country bornIn;
    ...
}          
@Embeddable

public class Address implements Serializable {
    String city;
    Country nationality; //no overriding here
}            
@Embeddable

public class Country implements Serializable {
    private String iso2;
    @Column(name="countryName") private String name;
    public String getIso2() { return iso2; }
    public void setIso2(String iso2) { this.iso2 = iso2; }
    
    public String getName() { return name; }
    public void setName(String name) { this.name = name; }
    ...
}            

An embeddable object inherits the access type of its owning entity (note that you can override that using the @Access annotation).

The Person entity has two component properties, homeAddress and bornIn. homeAddress property has not been annotated, but Hibernate will guess that it is a persistent component by looking for the @Embeddable annotation in the Address class. We also override the mapping of a column name (to bornCountryName) with the @Embedded and @AttributeOverride annotations for each mapped attribute of Country. As you can see, Country is also a nested component of Address, again using auto-detection by Hibernate and JPA defaults. Overriding columns of embedded objects of embedded objects is through dotted expressions.

    @Embedded

    @AttributeOverrides( {
            @AttributeOverride(name="city", column = @Column(name="fld_city") ),
            @AttributeOverride(name="nationality.iso2", column = @Column(name="nat_Iso2") ),
            @AttributeOverride(name="nationality.name", column = @Column(name="nat_CountryName") )
            //nationality columns in homeAddress are overridden
    } )
    Address homeAddress;

Hibernate Annotations supports something that is not explicitly supported by the JPA specification. You can annotate a embedded object with the @MappedSuperclass annotation to make the superclass properties persistent (see @MappedSuperclass for more informations).

You can also use association annotations in an embeddable object (ie @OneToOne, @ManyToOne, @OneToMany or @ManyToMany). To override the association columns you can use @AssociationOverride.

If you want to have the same embeddable object type twice in the same entity, the column name defaulting will not work as several embedded objects would share the same set of columns. In plain JPA, you need to override at least one set of columns. Hibernate, however, allows you to enhance the default naming mechanism through the NamingStrategy interface. You can write a strategy that prevent name clashing in such a situation. DefaultComponentSafeNamingStrategy is an example of this.

If a property of the embedded object points back to the owning entity, annotate it with the @Parent annotation. Hibernate will make sure this property is properly loaded with the entity reference.

In XML, use the <component> element.

<component
        name="(1)propertyName"
        class=(2)"className"
        insert(3)="true|false"
        update(4)="true|false"
        access(5)="field|property|ClassName"
        lazy="(6)true|false"
        optimi(7)stic-lock="true|false"
        unique(8)="true|false"
        node="element-name|."
>

        <property ...../>
        <many-to-one .... />
        ........
</component>
1

name: the name of the property.

2

class (optional - defaults to the property type determined by reflection): the name of the component (child) class.

3

insert: do the mapped columns appear in SQL INSERTs?

4

update: do the mapped columns appear in SQL UPDATEs?

5

access (optional - defaults to property): the strategy Hibernate uses for accessing the property value.

6

lazy (optional - defaults to false): specifies that this component should be fetched lazily when the instance variable is first accessed. It requires build-time bytecode instrumentation.

7

optimistic-lock (optional - defaults to true): specifies that updates to this component either do or do not require acquisition of the optimistic lock. It determines if a version increment should occur when this property is dirty.

8

unique (optional - defaults to false): specifies that a unique constraint exists upon all mapped columns of the component.

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 9.5, “Dynamic components” for more information. This feature is not supported in annotations.

Java is a language supporting polymorphism: a class can inherit from another. Several strategies are possible to persist a class hierarchy:

With this approach the properties of all the subclasses in a given mapped class hierarchy are stored in a single table.

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.

@Entity

@Inheritance(strategy=InheritanceType.SINGLE_TABLE)
@DiscriminatorColumn(
    name="planetype",
    discriminatorType=DiscriminatorType.STRING
)
@DiscriminatorValue("Plane")
public class Plane { ... }
@Entity
@DiscriminatorValue("A320")
public class A320 extends Plane { ... }          

In hbm.xml, for the table-per-class-hierarchy mapping strategy, the <subclass> declaration is used. For example:

<subclass
        name="(1)ClassName"
        discri(2)minator-value="discriminator_value"
        proxy=(3)"ProxyInterface"
        lazy="(4)true|false"
        dynamic-update="true|false"
        dynamic-insert="true|false"
        entity-name="EntityName"
        node="element-name"
        extends="SuperclassName">

        <property .... />
        .....
</subclass>
1

name: the fully qualified class name of the subclass.

2

discriminator-value (optional - defaults to the class name): a value that distinguishes individual subclasses.

3

proxy (optional): specifies a class or interface used for lazy initializing proxies.

4

lazy (optional - defaults to true): setting lazy="false" disables the use of lazy fetching.

For information about inheritance mappings see Chapter 10, Inheritance mapping.

Discriminators are 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. Hibernate Core supports the follwoing restricted set of types as discriminator column: string, character, integer, byte, short, boolean, yes_no, true_false.

Use the @DiscriminatorColumn to define the discriminator column as well as the discriminator type.

You can also use @DiscriminatorFormula to express in SQL a virtual discriminator column. This is particularly useful when the discriminator value can be extracted from one or more columns of the table. Both @DiscriminatorColumn and @DiscriminatorFormula are to be set on the root entity (once per persisted hierarchy).

@org.hibernate.annotations.DiscriminatorOptions allows to optionally specify Hibernate specific discriminator options which are not standardized in JPA. The available options are force and insert. The force attribute is useful if the table contains rows with "extra" discriminator values that are not mapped to a persistent class. This could for example occur when working with a legacy database. If force is set to true Hibernate will specify the allowed discriminator values in the SELECT query, even when retrieving all instances of the root class. The second option - insert - tells Hibernate whether or not to include the discriminator column in SQL INSERTs. Usually the column should be part of the INSERT statement, but if your discriminator column is also part of a mapped composite identifier you have to set this option to false.

Finally, use @DiscriminatorValue on each class of the hierarchy to specify the value stored in the discriminator column for a given entity. If you do not set @DiscriminatorValue on a class, the fully qualified class name is used.

@Entity

@Inheritance(strategy=InheritanceType.SINGLE_TABLE)
@DiscriminatorColumn(
    name="planetype",
    discriminatorType=DiscriminatorType.STRING
)
@DiscriminatorValue("Plane")
public class Plane { ... }
@Entity
@DiscriminatorValue("A320")
public class A320 extends Plane { ... }          

In hbm.xml, the <discriminator> element is used to define the discriminator column or formula:

<discriminator
        column(1)="discriminator_column"
        type="(2)discriminator_type"
        force=(3)"true|false"
        insert(4)="true|false"
        formul(5)a="arbitrary sql expression"
/>
1

column (optional - defaults to class): the name of the discriminator column.

2

type (optional - defaults to string): a name that indicates the Hibernate type

3

force (optional - defaults to false): "forces" Hibernate to specify the allowed discriminator values, even when retrieving all instances of the root class.

4

insert (optional - defaults to true): set this to false if your discriminator column is also part of a mapped composite identifier. It tells Hibernate not to include the column in SQL INSERTs.

5

formula (optional): an arbitrary SQL expression that is executed when a type has to be evaluated. It allows content-based discrimination.

Actual values of the discriminator column are specified by the discriminator-value attribute of the <class> and <subclass> elements.

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"/>

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. A discriminator column is not required for this mapping strategy. Each subclass must, however, declare a table column holding the object identifier. The primary key of this table is also a foreign key to the superclass table and described by the @PrimaryKeyJoinColumns or the <key> element.

@Entity @Table(name="CATS")

@Inheritance(strategy=InheritanceType.JOINED)
public class Cat implements Serializable { 
    @Id @GeneratedValue(generator="cat-uuid") 
    @GenericGenerator(name="cat-uuid", strategy="uuid")
    String getId() { return id; }
    ...
}
@Entity @Table(name="DOMESTIC_CATS")
@PrimaryKeyJoinColumn(name="CAT")
public class DomesticCat extends Cat { 
    public String getName() { return name; }
}            

In hbm.xml, use the <joined-subclass> element. For example:

<joined-subclass
        name="(1)ClassName"
        table=(2)"tablename"
        proxy=(3)"ProxyInterface"
        lazy="(4)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>
1

name: the fully qualified class name of the subclass.

2

table: the name of the subclass table.

3

proxy (optional): specifies a class or interface to use for lazy initializing proxies.

4

lazy (optional, defaults to true): setting lazy="false" disables the use of lazy fetching.

Use the <key> element to declare the primary key / foreign key column. 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://www.hibernate.org/dtd/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 10, Inheritance mapping.

This is sometimes useful to share common properties through a technical or a business superclass without including it as a regular mapped entity (ie no specific table for this entity). For that purpose you can map them as @MappedSuperclass.

@MappedSuperclass

public class BaseEntity {
    @Basic
    @Temporal(TemporalType.TIMESTAMP)
    public Date getLastUpdate() { ... }
    public String getLastUpdater() { ... }
    ...
}
@Entity class Order extends BaseEntity {
    @Id public Integer getId() { ... }
    ...
}

In database, this hierarchy will be represented as an Order table having the id, lastUpdate and lastUpdater columns. The embedded superclass property mappings are copied into their entity subclasses. Remember that the embeddable superclass is not the root of the hierarchy though.

You can override columns defined in entity superclasses at the root entity level using the @AttributeOverride annotation.

@MappedSuperclass

public class FlyingObject implements Serializable {
    public int getAltitude() {
        return altitude;
    }
    @Transient
    public int getMetricAltitude() {
        return metricAltitude;
    }
    @ManyToOne
    public PropulsionType getPropulsion() {
        return metricAltitude;
    }
    ...
}
@Entity
@AttributeOverride( name="altitude", column = @Column(name="fld_altitude") )
@AssociationOverride( 
   name="propulsion", 
   joinColumns = @JoinColumn(name="fld_propulsion_fk") 
)
public class Plane extends FlyingObject {
    ...
}

The altitude property will be persisted in an fld_altitude column of table Plane and the propulsion association will be materialized in a fld_propulsion_fk foreign key column.

You can define @AttributeOverride(s) and @AssociationOverride(s) on @Entity classes, @MappedSuperclass classes and properties pointing to an @Embeddable object.

In hbm.xml, simply map the properties of the superclass in the <class> element of the entity that needs to inherit them.

While not recommended for a fresh schema, some legacy databases force your to map a single entity on several tables.

Using the @SecondaryTable or @SecondaryTables class level annotations. To express that a column is in a particular table, use the table parameter of @Column or @JoinColumn.

@Entity

@Table(name="MainCat")
@SecondaryTables({
    @SecondaryTable(name="Cat1", pkJoinColumns={
        @PrimaryKeyJoinColumn(name="cat_id", referencedColumnName="id")
    ),
    @SecondaryTable(name="Cat2", uniqueConstraints={@UniqueConstraint(columnNames={"storyPart2"})})
})
public class Cat implements Serializable {
    private Integer id;
    private String name;
    private String storyPart1;
    private String storyPart2;
    @Id @GeneratedValue
    public Integer getId() {
        return id;
    }
    public String getName() {
        return name;
    }
    
    @Column(table="Cat1")
    public String getStoryPart1() {
        return storyPart1;
    }
    @Column(table="Cat2")
    public String getStoryPart2() {
        return storyPart2;
    }
}

In this example, name will be in MainCat. storyPart1 will be in Cat1 and storyPart2 will be in Cat2. Cat1 will be joined to MainCat using the cat_id as a foreign key, and Cat2 using id (ie the same column name, the MainCat id column has). Plus a unique constraint on storyPart2 has been set.

There is also additional tuning accessible via the @org.hibernate.annotations.Table annotation:

Make sure to use the secondary table name in the appliesto property

@Entity

@Table(name="MainCat")
@SecondaryTable(name="Cat1")
@org.hibernate.annotations.Table(
   appliesTo="Cat1",
   fetch=FetchMode.SELECT,
   optional=true)
public class Cat implements Serializable {
    private Integer id;
    private String name;
    private String storyPart1;
    private String storyPart2;
    @Id @GeneratedValue
    public Integer getId() {
        return id;
    }
    public String getName() {
        return name;
    }
    
    @Column(table="Cat1")
    public String getStoryPart1() {
        return storyPart1;
    }
    @Column(table="Cat2")
    public String getStoryPart2() {
        return storyPart2;
    }
}

In hbm.xml, use the <join> element.

<join
        table=(1)"tablename"
        schema(2)="owner"
        catalo(3)g="catalog"
        fetch=(4)"join|select"
        invers(5)e="true|false"
        option(6)al="true|false">

        <key ... />

        <property ... />
        ...
</join>
1

table: the name of the joined table.

2

schema (optional): overrides the schema name specified by the root <hibernate-mapping> element.

3

catalog (optional): overrides the catalog name specified by the root <hibernate-mapping> element.

4

fetch (optional - defaults to join): if set to join, the default, Hibernate will use an inner join to retrieve a <join> defined by a class or its superclasses. It will use an outer join for a <join> defined by a subclass. If set to select then Hibernate will use a sequential select for a <join> defined on a subclass. This will be issued only if a row represents an instance of the subclass. Inner joins will still be used to retrieve a <join> defined by the class and its superclasses.

5

inverse (optional - defaults to false): if enabled, Hibernate will not insert or update the properties defined by this join.

6

optional (optional - defaults to false): if enabled, Hibernate will insert a row only if the properties defined by this join are non-null. It will always use an outer join to retrieve the properties.

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.

To link one entity to an other, you need to map the association property as a to one association. In the relational model, you can either use a foreign key or an association table, or (a bit less common) share the same primary key value between the two entities.

To mark an association, use either @ManyToOne or @OnetoOne.

@ManyToOne and @OneToOne have a parameter named targetEntity which describes the target entity name. You usually don't need this parameter since the default value (the type of the property that stores the association) is good in almost all cases. However this is useful when you want to use interfaces as the return type instead of the regular entity.

Setting a value of the cascade attribute to any meaningful value other than nothing will propagate certain operations to the associated object. The meaningful values are divided into three categories.

By default, single point associations are eagerly fetched in JPA 2. You can mark it as lazily fetched by using @ManyToOne(fetch=FetchType.LAZY) in which case Hibernate will proxy the association and load it when the state of the associated entity is reached. You can force Hibernate not to use a proxy by using @LazyToOne(NO_PROXY). In this case, the property is fetched lazily when the instance variable is first accessed. This requires build-time bytecode instrumentation. lazy="false" specifies that the association will always be eagerly fetched.

With the default JPA options, single-ended associations are loaded with a subsequent select if set to LAZY, or a SQL JOIN is used for EAGER associations. You can however adjust the fetching strategy, ie how data is fetched by using @Fetch. FetchMode can be SELECT (a select is triggered when the association needs to be loaded) or JOIN (use a SQL JOIN to load the association while loading the owner entity). JOIN overrides any lazy attribute (an association loaded through a JOIN strategy cannot be lazy).

An ordinary association to another persistent class is declared using a

and a foreign key in one table is referencing the primary key column(s) of the target table.

@Entity

public class Flight implements Serializable {
    @ManyToOne( cascade = {CascadeType.PERSIST, CascadeType.MERGE} )
    @JoinColumn(name="COMP_ID")
    public Company getCompany() {
        return company;
    }
    ...
}            

The @JoinColumn attribute is optional, the default value(s) is the concatenation of the name of the relationship in the owner side, _ (underscore), and the name of the primary key column in the owned side. In this example company_id because the property name is company and the column id of Company is id.

@Entity

public class Flight implements Serializable {
    @ManyToOne( cascade = {CascadeType.PERSIST, CascadeType.MERGE}, targetEntity=CompanyImpl.class )
    @JoinColumn(name="COMP_ID")
    public Company getCompany() {
        return company;
    }
    ...
}
public interface Company {
    ...
}

You can also map a to one association through an association table. This association table described by the @JoinTable annotation will contains a foreign key referencing back the entity table (through @JoinTable.joinColumns) and a a foreign key referencing the target entity table (through @JoinTable.inverseJoinColumns).

@Entity

public class Flight implements Serializable {
    @ManyToOne( cascade = {CascadeType.PERSIST, CascadeType.MERGE} )
    @JoinTable(name="Flight_Company",
        joinColumns = @JoinColumn(name="FLIGHT_ID"),
        inverseJoinColumns = @JoinColumn(name="COMP_ID")
    )
    public Company getCompany() {
        return company;
    }
    ...
}       

You can mark an association as mandatory by using the optional=false attribute. We recommend to use Bean Validation's @NotNull annotation as a better alternative however. As a consequence, the foreign key column(s) will be marked as not nullable (if possible).

When Hibernate cannot resolve the association because the expected associated element is not in database (wrong id on the association column), an exception is raised. This might be inconvenient for legacy and badly maintained schemas. You can ask Hibernate to ignore such elements instead of raising an exception using the @NotFound annotation.


Sometimes you want to delegate to your database the deletion of cascade when a given entity is deleted. In this case Hibernate generates a cascade delete constraint at the database level.


Foreign key constraints, while generated by Hibernate, have a fairly unreadable name. You can override the constraint name using @ForeignKey.


Sometimes, you want to link one entity to an other not by the target entity primary key but by a different unique key. You can achieve that by referencing the unique key column(s) in @JoinColumn.referenceColumnName.

@Entity

class Person {
   @Id Integer personNumber;
   String firstName;
   @Column(name="I")
   String initial;
   String lastName;
}
@Entity
class Home {
   @ManyToOne
   @JoinColumns({
      @JoinColumn(name="first_name", referencedColumnName="firstName"),
      @JoinColumn(name="init", referencedColumnName="I"),
      @JoinColumn(name="last_name", referencedColumnName="lastName"),
   })
   Person owner
}

This is not encouraged however and should be reserved to legacy mappings.

In hbm.xml, mapping an association is similar. The main difference is that a @OneToOne is mapped as <many-to-one unique="true"/>, let's dive into the subject.

<many-to-one
        name="(1)propertyName"
        column(2)="column_name"
        class=(3)"ClassName"
        cascad(4)e="cascade_style"
        fetch=(5)"join|select"
        update(6)="true|false"
        insert(6)="true|false"
        proper(7)ty-ref="propertyNameFromAssociatedClass"
        access(8)="field|property|ClassName"
        unique(9)="true|false"
        not-nu(10)ll="true|false"
        optimi(11)stic-lock="true|false"
        lazy="(12)proxy|no-proxy|false"
        not-fo(13)und="ignore|exception"
        entity(14)-name="EntityName"
        formul(15)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"
/>
1

name: the name of the property.

2

column (optional): the name of the foreign key column. This can also be specified by nested <column> element(s).

3

class (optional - defaults to the property type determined by reflection): the name of the associated class.

4

cascade (optional): specifies which operations should be cascaded from the parent object to the associated object.

5

fetch (optional - defaults to select): chooses between outer-join fetching or sequential select fetching.

6

update, insert (optional - defaults to true): specifies that the mapped columns should be included in SQL UPDATE and/or INSERT statements. Setting both to false allows a pure "derived" association whose value is initialized from another property that maps to the same column(s), or by a trigger or other application.

7

property-ref (optional): the name of a property of the associated class that is joined to this foreign key. If not specified, the primary key of the associated class is used.

8

access (optional - defaults to property): the strategy Hibernate uses for accessing the property value.

9

unique (optional): enables the DDL generation of a unique constraint for the foreign-key column. By allowing this to be the target of a property-ref, you can make the association multiplicity one-to-one.

10

not-null (optional): enables the DDL generation of a nullability constraint for the foreign key columns.

11

optimistic-lock (optional - defaults to true): specifies that updates to this property do or do not require acquisition of the optimistic lock. In other words, it determines if a version increment should occur when this property is dirty.

12

lazy (optional - defaults to proxy): by default, single point associations are proxied. lazy="no-proxy" specifies that the property should be fetched lazily when the instance variable is first accessed. This requires build-time bytecode instrumentation. lazy="false" specifies that the association will always be eagerly fetched.

13

not-found (optional - defaults to exception): specifies how foreign keys that reference missing rows will be handled. ignore will treat a missing row as a null association.

14

entity-name (optional): the entity name of the associated class.

15

formula (optional): an SQL expression that defines the value for a computed foreign key.

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 11.11, “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"/>

The second approach is to ensure an entity and its associated entity share the same primary key. In this case the primary key column is also a foreign key and there is no extra column. These associations are always one to one.


Note

Many people got confused by these primary key based one to one associations. They can only be lazily loaded if Hibernate knows that the other side of the association is always present. To indicate to Hibernate that it is the case, use @OneToOne(optional=false).

In hbm.xml, use the following mapping.

<one-to-one
        name="(1)propertyName"
        class=(2)"ClassName"
        cascad(3)e="cascade_style"
        constr(4)ained="true|false"
        fetch=(5)"join|select"
        proper(6)ty-ref="propertyNameFromAssociatedClass"
        access(7)="field|property|ClassName"
        formul(8)a="any SQL expression"
        lazy="(9)proxy|no-proxy|false"
        entity(10)-name="EntityName"
        node="element-name|@attribute-name|element/@attribute|."
        embed-xml="true|false"
        foreign-key="foreign_key_name"
/>
1

name: the name of the property.

2

class (optional - defaults to the property type determined by reflection): the name of the associated class.

3

cascade (optional): specifies which operations should be cascaded from the parent object to the associated object.

4

constrained (optional): specifies that a foreign key constraint on the primary key of the mapped table and references the table of the associated class. This option affects the order in which save() and delete() are cascaded, and determines whether the association can be proxied. It is also used by the schema export tool.

5

fetch (optional - defaults to select): chooses between outer-join fetching or sequential select fetching.

6

property-ref (optional): the name of a property of the associated class that is joined to the primary key of this class. If not specified, the primary key of the associated class is used.

7

access (optional - defaults to property): the strategy Hibernate uses for accessing the property value.

8

formula (optional): almost all one-to-one associations map to the primary key of the owning entity. If this is not the case, you can specify another column, columns or expression to join on using an SQL formula. See org.hibernate.test.onetooneformula for an example.

9

lazy (optional - defaults to proxy): by default, single point associations are proxied. lazy="no-proxy" specifies that the property should be fetched lazily when the instance variable is first accessed. It requires build-time bytecode instrumentation. lazy="false" specifies that the association will always be eagerly fetched. Note that if constrained="false", proxying is impossible and Hibernate will eagerly fetch the association.

10

entity-name (optional): the entity name of the associated class.

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.

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 as @NaturalId or map them 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.

@Entity

public class Citizen {
    @Id
    @GeneratedValue
    private Integer id;
    private String firstname;
    private String lastname;
    
    @NaturalId
    @ManyToOne
    private State state;
    @NaturalId
    private String ssn;
    ...
}
//and later on query
List results = s.createCriteria( Citizen.class )
                .add( Restrictions.naturalId().set( "ssn", "1234" ).set( "state", ste ) )
                .list();

Or in XML,


<natural-id mutable="true|false"/>
        <property ... />
        <many-to-one ... />
        ......
</natural-id>

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.

There is one more type of property mapping. The @Any mapping 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 @Any annotation describes the column holding the metadata information. To link the value of the metadata information and an actual entity type, The @AnyDef and @AnyDefs annotations are used. The metaType 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 idType. You must specify the mapping from values of the metaType to class names.

@Any( metaColumn = @Column( name = "property_type" ), fetch=FetchType.EAGER )

@AnyMetaDef( 
    idType = "integer", 
    metaType = "string", 
    metaValues = {
        @MetaValue( value = "S", targetEntity = StringProperty.class ),
        @MetaValue( value = "I", targetEntity = IntegerProperty.class )
    } )
@JoinColumn( name = "property_id" )
public Property getMainProperty() {
    return mainProperty;
}

Note that @AnyDef can be mutualized and reused. It is recommended to place it as a package metadata in this case.

//on a package

@AnyMetaDef( name="property" 
    idType = "integer", 
    metaType = "string", 
    metaValues = {
        @MetaValue( value = "S", targetEntity = StringProperty.class ),
        @MetaValue( value = "I", targetEntity = IntegerProperty.class )
    } )
package org.hibernate.test.annotations.any;
//in a class
    @Any( metaDef="property", metaColumn = @Column( name = "property_type" ), fetch=FetchType.EAGER )
    @JoinColumn( name = "property_id" )
    public Property getMainProperty() {
        return mainProperty;
    }

The hbm.xml equivalent is:


<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="(1)propertyName"
        id-typ(2)e="idtypename"
        meta-t(3)ype="metatypename"
        cascad(4)e="cascade_style"
        access(5)="field|property|ClassName"
        optimi(6)stic-lock="true|false"
>
        <meta-value ... />
        <meta-value ... />
        .....
        <column .... />
        <column .... />
        .....
</any>
1

name: the property name.

2

id-type: the identifier type.

3

meta-type (optional - defaults to string): any type that is allowed for a discriminator mapping.

4

cascade (optional- defaults to none): the cascade style.

5

access (optional - defaults to property): the strategy Hibernate uses for accessing the property value.

6

optimistic-lock (optional - defaults to true): specifies that updates to this property either do or do not require acquisition of the optimistic lock. It defines whether a version increment should occur if this property is dirty.

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="(1)logicalName"
        insert(2)="true|false"
        update(3)="true|false"
        optimi(4)stic-lock="true|false"
        unique(5)="true|false"
>

        <property ...../>
        <many-to-one .... />
        ........
</properties>
1

name: the logical name of the grouping. It is not an actual property name.

2

insert: do the mapped columns appear in SQL INSERTs?

3

update: do the mapped columns appear in SQL UPDATEs?

4

optimistic-lock (optional - defaults to true): specifies that updates to these properties either do or do not require acquisition of the optimistic lock. It determines if a version increment should occur when these properties are dirty.

5

unique (optional - defaults to false): specifies that a unique constraint exists upon all mapped columns of the component.

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="owner"
         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.

The hbm.xml structure has some specificities naturally not present when using annotations, let's describe them briefly.

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.

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
         schem(1)a="schemaName"
         catal(2)og="catalogName"
         defau(3)lt-cascade="cascade_style"
         defau(4)lt-access="field|property|ClassName"
         defau(5)lt-lazy="true|false"
         auto-(6)import="true|false"
         packa(7)ge="package.name"
 />
1

schema (optional): the name of a database schema.

2

catalog (optional): the name of a database catalog.

3

default-cascade (optional - defaults to none): a default cascade style.

4

default-access (optional - defaults to property): the strategy Hibernate should use for accessing all properties. It can be a custom implementation of PropertyAccessor.

5

default-lazy (optional - defaults to true): the default value for unspecified lazy attributes of class and collection mappings.

6

auto-import (optional - defaults to true): specifies whether we can use unqualified class names of classes in this mapping in the query language.

7

package (optional): specifies a package prefix to use for unqualified class names in the mapping document.

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.

The <key> element is 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(1)="columnname"
        on-del(2)ete="noaction|cascade"
        proper(3)ty-ref="propertyName"
        not-nu(4)ll="true|false"
        update(5)="true|false"
        unique(6)="true|false"
/>
1

column (optional): the name of the foreign key column. This can also be specified by nested <column> element(s).

2

on-delete (optional - defaults to noaction): specifies whether the foreign key constraint has database-level cascade delete enabled.

3

property-ref (optional): specifies that the foreign key refers to columns that are not the primary key of the original table. It is provided for legacy data.

4

not-null (optional): specifies that the foreign key columns are not nullable. This is implied whenever the foreign key is also part of the primary key.

5

update (optional): specifies that the foreign key should never be updated. This is implied whenever the foreign key is also part of the primary key.

6

unique (optional): specifies that the foreign key should have a unique constraint. This is implied whenever the foreign key is also the primary key.

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">.

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 LONGVARCHAR or TEXT type.

image

Maps long byte arrays to a SQL LONGVARBINARY.

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.

materialized_clob

Maps long Java strings to a SQL CLOB type. When read, the CLOB value is immediately materialized into a Java string. Some drivers require the CLOB value to be read within a transaction. Once materialized, the Java string is available outside of the transaction.

materialized_blob

Maps long Java byte arrays to a SQL BLOB type. When read, the BLOB value is immediately materialized into a byte array. Some drivers require the BLOB value to be read within a transaction. Once materialized, the byte array is available outside of the transaction.

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