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EclipseLink Solutions Guide for EclipseLink
Release 2.5
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Main Tasks

Complete these tasks to migrate an application that uses Hibernate as its persistence provider to EclipseLink.

Task 1: Prerequisites

EclipseLink 2.4 or later.

Download EclipseLink from http://www.eclipse.org/eclipselink/downloads/.

Task 1: Convert the Hibernate Entity Annotation

The Hibernate entity annotation, defined by the org.hibernate.annotations.Entity class, adds additional metadata beyond what is defined by the JPA standard @Entity annotation.

Example 8-1 illustrates a sample Hibernate entity annotation. The example uses the selectBeforeUpdate, dynamicInsert, dynamicUpdate, optimisticLock, and polymophism attributes. Note that the Hibernate entity annotation also defines mutable and persister attributes, which are not used in this example.

Example 8-1 Sample Hibernate Entity Annotation

@org.hibernate.annotations.Entity(
     selectBeforeUpdate = true,
     dynamicInsert = true, 
     dynamicUpdate = true,
     optimisticLock = OptimisticLockType.ALL,
     polymorphism = PolymorphismType.EXPLICIT)

The following sections describe how EclipseLink handles selects, locks, polymorphism, and dynamic updates and inserts. For more information, see "EclipseLink/Examples/JPA/Migration/Hibernate/V3Annotations" in the Eclipselink documentation, at:

http://wiki.eclipse.org/EclipseLink/Examples/JPA/Migration/Hibernate/V3Annotations

Convert the SelectBeforeUpdate, dynamicInsert and dynamicUpdate Attributes

In Hibernate, the selectBeforeUpdate attribute specifies that Hibernate should never perform a SQL update unless it is certain that an object is actually modified. The dynamicInsert attribute specifies that the INSERT SQL statement should be generated at runtime and contain only the columns whose values are not null. The dynamicUpdate attribute specifies that the UPDATE SQL statement should be generated at runtime and can contain only those columns whose values have changed.

By default, EclipseLink will always insert all mapped columns and will update only the columns that have changed. If alternative operations are required, then the queries used for these operations can be customized by using Java code, SQL, or stored procedures.

Convert the OptimisticLock Attribute

In Hibernate, the optimisticLock attribute determines the optimistic locking strategy.

EclipseLink's optimistic locking functionality supports all of the Hibernate locking types and more. Table 8-1 translates locking types from Hibernate's @Entity(optimisticLock) attributes into EclipseLink locking policies. These policies can be configured either with the EclipseLink @OptimisticLocking annotation or in the EclipseLink orm.xml file. For more information, see @OptimisticLocking.

Table 8-1 Translating Hibernate's OptimisticLock to EclipseLink's OptimisticLocking

Hibernate's OptimisticLock Type Description EclipseLink OptimisticLocking

NONE

No optimistic locking

EclipseLink defaults to no optimistic locking.

VERSION

Use a column version

Use the JPA @Version annotation or the EclipseLink annotation:

@OptimisticLocking(type = OptimisticLockingType.VERSION_COLUMN)

DIRTY

Changed columns are compared

Use the JPA @Version annotation or the EclipseLink annotation:

@OptimisticLocking(type = OptimisticLockingType.CHANGED_COLUMNS)

ALL

All columns are compared

Use the EclipseLink annotation:

@OptimisticLocking(type = OptimisticLockingType.ALL_COLUMNS)


Additionally, EclipseLink allows you to compare a specific set of selected columns using the OptimisticLockingType.SELECTED_COLUMNS annotation. This allows you to select the critical columns that should be compared if the CHANGED or ALL strategies do not meet your needs.

Task 2: Convert the Hibernate Custom Sequence Generator Annotation

In Hibernate, the @GeneratedValue annotation defines the identifier generation strategy. The @GenericGenerator allows you to define a Hibernate-specific ID generator. Example 8-2 illustrates a custom generator for sequence values.

Example 8-2 Custom Generator for Sequence Values

.
.
.
@Id
   @GeneratedValue(generator = "system-uuid")
   @GenericGenerator(name = "system-uuid", strategy = "mypackage.UUIDGenerator")
   public String getTransactionGuid()
.
.
.
 

In EclipseLink, a custom sequence generator can be implemented and registered by using the @GeneratedValue annotation. For more information, see "How to use Custom Sequencing" in the EclipseLink documentation, at:

http://wiki.eclipse.org/EclipseLink/Examples/JPA/CustomSequencing

Task 3: Convert Hibernate Mapping Annotations

The following sections describe how to convert various Hibernate annotations to EclipseLink annotations.

Convert the @ForeignKey Annotation

In Hibernate, the @ForeignKey annotation allows you to define the name of the foreign key to be used during schema generation.

EclipseLink does generate reasonable names for foreign keys, but does not provide an annotation or eclipselink-orm.xml support for specifying the name to use. When migrating, the recommended solution is to have EclipseLink generate the schema (DDL) commands to a script file instead of directly on the database. The script can then be customized to use different names prior to being executed.


NoteNote:

The foreign key name is not used by EclipseLink at runtime, but is required if EclipseLink attempts to drop the schema. In this case, the drop script should be generated to a file and customized to match the foreign key names used during creation.


Convert the @Cache Annotation

In Hibernate, the @Cache annotation configures the caching of entities and relationships. Because EclipseLink uses an entity cache instead of a data cache, the relationships are automatically cached. In these cases, the @Cache annotation should be removed during migration.When the @Cache annotation is used on an entity, its behavior is similar to the EclipseLink @Cache annotation. For more information about the @Cache annotation and equivalent eclipselink-orm.xml configuration values, see Java Persistence API (JPA) Extensions Reference for Oracle TopLink.

Task 4: Modify the persistence.xml File

The persistence.xml file is the deployment descriptor file for JPA persistence. It specifies the persistence units, and declares the managed persistence classes, the object-relational mapping, and the database connection details. Example 8-3 illustrates a persistence.xml file for an application that uses Hibernate. Hibernate-specific values appear in bold font.

Example 8-3 Persistence File for an Application that Uses Hibernate

<persistence>
   <persistence-unit name="helloworld">
      <provider>org.hibernate.ejb.HibernatePersistence</provider>
      <jta-data-source>java:/DefaultDS</jta-data-source>
      <properties>
         <property name="hibernate.dialect" value="org.hibernate.dialect.HSQLDialect"/>
         <property name="hibernate.hbm2ddl.auto" value="create-drop"/>
      </properties>
   </persistence-unit>
</persistence>

Modified persistence.xml File

Example 8-4 illustrates a persistence.xml file modified for an application that uses EclipseLink. Key differences include the value for the persistence provider. For EclipseLink, this value is org.eclipse.persistence.jpa.PersistenceProvider. The names of EclipseLink-specific properties are typically be prefixed by eclipselink, for example, eclipselink.target-database. EclipseLink-specific values appear in bold font.

Example 8-4 Persistence File Modified for EclipseLink

<xml version="1.0" encoding="UTF-8"?>
<persistence version="1.0" xmlns="http://java.sun.com/xml/ns/persistence" 
  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
  xsi:schemaLocation="http://java.sun.com/xml/ns/persistence http://java.sun.com/xml/ns/persistence/persistence_1_0.xsd">
  <persistence-unit name="helloworld">
    <provider>org.eclipse.persistence.jpa.PersistenceProvider</provider>
    <jta-data-source>java:/DefaultDS</jta-data-source>
    <!-- For Java SE applications, entity classes must be specified for EclipseLink weaving. For Java EE applications, the classes are found automatically. -->
    <class>Todo</class>
    <properties>
      <property name="eclipselink.ddl-generation" value="drop-and-create-tables"/>
      <property name="eclipselink.ddl-generation.output-mode" value="database"/>
      <property name="eclipselink.logging.level" value="FINE"/>
    </properties>
  </persistence-unit>
</persistence> 

Drop and Create the Database Tables

For production environments, you would usually have the schema set up on the database. The following properties defined in the persistence unit are more suitable for examples and demonstrations. These properties will instruct EclipseLink to automatically drop and create database tables. Any previously existing tables will be removed.

To use the Drop and Create Database Tables feature, add the following properties to the persistence.xml file.

<property name="eclipselink.ddl-generation" value="drop-and-create-tables"/>
 <property name="eclipselink.ddl-generation.output-mode" value="database"/>

For more information on this feature, see the drop-and-create-tables entry in "ddl-generation" in Java Persistence API (JPA) Extensions Reference for Oracle TopLink.

Create or Extend Database Tables

The Create or Extend Database Tables feature allows you match the database schema to the object model by creating new database tables or by modifying existing tables. You can modify existing tables by specifying field name changes and by add and removing fields.


NoteNote:

In the current release, the Create or Extend Database Tables feature will not rename or delete existing columns. It will only add missing table columns.


The Create or Extend Database Tables feature reduces the need to repopulate test data. You avoid the need to use the Drop and Create Database Tables feature when the schema changes, due to changes in the object model. The Create or Extend Database Tables feature can also be used with extensibility to add table columns.

To use the Create or Extend Database Tables feature, add the following properties to the persistence.xml file. When the context is loaded, EclipseLink will query the database for each table required in the persistence unit and use the results to determine if the table needs to be created or extended.

<property name="eclipselink.ddl-generation" value="create-or-extend-tables" />
 <property name="eclipselink.ddl-generation.output-mode" value="database" />

For more information on this feature, see the create-or-extend-tables entry in "ddl-generation" in Java Persistence API (JPA) Extensions Reference for Oracle TopLink.

Task 5: Convert Hibernate API to EclipseLink API

Table 8-2 describes the Hibernate classes that are commonly used in a JPA project and their equivalent EclipseLink (JPA) interfaces. All of the Hibernate classes are in the org.hibernate package. All of the JPA interfaces (and the Persistence class) are in the javax.persistence package.

For information about the EclipseLink API, see Java API Reference for EclipseLink.

Table 8-2 Hibernate Classes and Equivalent JPA Interfaces

org.hibernate javax.persistence Description

cfg.Configuration

Persistence

Provides a bootstrap class that configures the session factory (in Hibernate) or the entity manager factory (in JPA). It is generally used to create a single session (or entity manager) factory for the JVM.

SessionFactory

EntityManagerFactory

Provides APIs to open Hibernate sessions (or JPA entity managers) to process a user request. Generally, a session (or entity manager) is opened per thread processing client requests.

Session

EntityManager

Provides APIs to store and load entities to and from the database. It also provides APIs to get a transaction and create a query.

Transaction

EntityTransaction

Provides APIs to manage transactions.

Query

Query

Provides APIs to execute queries.


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