Developing JPA 2.x applications for a Java EE environment

Containers in the application server can provide many of the necessary functions for the Java™ Persistence API (JPA) in a Java Enterprise Edition (Java EE) environment. The application server also provides JPA command tools to assist you with developing applications in a Java EE environment.

About this task

Attention: When you use these JPA command tools, run them from the <profile_root>/bin directory, rather than from the app_server_root/bin directory to make sure that you have the latest version of the commands for your release level.

For this task, you must specify the com.ibm.ws.jpa-2.1.thinclient_9.0.jar stand-alone Java archive (JAR) file in your classpath for compiling against JPA 2.1 interfaces. To compile against JPA 2.0 interfaces, specify the com.ibm.ws.jpa-2.0.thinclient_9.0.jar. This stand-alone JAR file is available from the installation images. The location of these files on the server installation image is in the ${app_server_root}/runtimes directory.

Important: JPA applications require different configuration techniques from applications that use container-managed persistence (CMP) or bean-managed persistence (BMP). They do not follow the typical deployment techniques that are associated with applications that implement CMP or BMP. In JPA applications, you must define a persistence unit and configure the appropriate properties to ensure that the applications can run in a Java EE environment.

The container supports all necessary injections to ensure that applications run in the Java EE environment. For example, the container can inject the @PersistenceUnit and @PersistenceContext for your applications.

Procedure

  1. Generate your entities classes. These are Plain Old Java Object (POJO) entities. Depending upon your development model, you might use some or all of the JPA tools:
    • Top-down mapping: You start from scratch with the entity definitions and the object-relational mappings, and then you derive the database schemas from that data. If you use this approach, you are most likely concerned with creating the architecture of your object model and then writing your entity classes. These entity classes would eventually drive the creation of your database model. If you are using a top-down mapping of the object model to the relational model, develop the entity classes and then use Your JPA provider's functionality to generate the database tables that are based on the entity classes. The wsmapping tool helps with this approach when you use OpenJPA as your persistence provider.
    • Bottom-up mapping: You start with your data model, which are the database schemas, and then you work upwards to your entity classes. The wsreversemapping tool helps with this approach when you use OpenJPA as your persistence provider.
    • Meet in the middle mapping: probably the most common development model. You have a combination of the data model and the object model partially complete. Depending on the goals and requirements, you must negotiate the relationships to resolve any differences. Both the wsmapping tool and the wsreversemapping tool help with this approach when you use OpenJPA as your persistence provider.
    The JPA solution for the application server provides several tools that help with developing JPA applications. Combining these tools with IBM® Rational® Application Developer provides a solid development environment for either Java EE or Java SE applications. Rational Application Developer includes GUI tools to insert annotations, a customized persistence.xml file editor, a database explorer, and other features. Another alternative is the Eclipse Dali project. More information about Rational Application Developer or the Eclipse Dali plug-in can be found at their respective websites.
  2. Compile the entity classes.

    Compile the entities as you would any Java class, unless you are using the Criteria API. If you are using the Criteria API, you must also generate the Criteria API metamodel classes.

    The following are examples of how you use this option.

    Using EclipseLink, provide the following arguments with the javac command:
    -processor 
    org.eclipse.persistence.internal.jpa.modelgen.CanonicalModelProcessor 
    -Aeclipselink.persistencexml= persistence.xml_location 
    app_server_root/java/bin/javac 
    -classpath app_server_root/runtimes/com.ibm.ws.jpa-2.1.thinclient_9.0.0.jar 
    -processor
    org.eclipse.persistence.internal.jpa.modelgen.CanonicalModelProcessor 
    -Aeclipselink.persistencexml=app_location/src/META-INF/persistence.xml mypackage\MyEntity.java 
    Using OpenJPA or WSJPA, provide the following arguments with the javac command:
    -Aopenjpa.metamodel=true 
    app_server_root/java/bin/javac 
    -Aopenjpa.metamodel=true 
    -classpath app_server_root/runtimes/com.ibm.ws.jpa-2.0.thinclient_9.0.0.jar  
    mypackage/MyEntity.java 
  3. Enhance the entity classes using the JPA enhancer tool, eclenhancer for EclipseLink and wsenhancer for OpenJPA. An enhancer is a tool that adds provider-specific integration bytecode to your persistent classes after you have written them. The enhancer post-processes the bytecode that is generated by the Java compiler and adds the fields and methods that are necessary to implement the persistence features. Although the application server’s persistence provider can automatically enhance the entities at run time, you obtain better performance if you can enhance your entities when you build the application. The application does not attempt to enhance entities that are already enhanced.

    For examples of how to use the eclenhancer tool, see the eclenhancer command topic. For examples of how to use the wsenhancer tool, see the wsenhancer command topic.

  4. Optional: If you are not using the development model for bottom-up mapping, generate or update your database tables automatically or by using the eclenhancer or wsmapping tool.
    • By default, the object-relational mapping does not occur automatically, but you can configure the application server to provide that mapping with the eclipselink.ddl-generation property for EclipseLink, or the openjpa.jdbc.SynchronizeMappings property for OpenJPA. These properties can accelerate development by automatically ensuring that the database tables match the object model. To enable automatic mapping, include the following line in the persistence.xml file:
      For EclipseLink:
      <property name="eclipselink.ddl-generation" value =“create-tables”/> 
      For OpenJPA:
      <property name="openjpa.jdbc.SynchronizeMappings" value="buildSchema(ForeignKeys=true)"/>  
      JPA 2.1 standard:
      <property name="javax.persistence.schema-generation.database.action" value="create"/>  
      避免困難 避免困難: To enable automatic object-relational mapping at run time, all of your persistent classes must be listed in the Java .class file, mapping file, and Java archive (JAR) file elements in XML format.gotcha
    • To manually update or generate your database tables, run the JPA mapping tool for the application server from the command line to create the tables in the database. For examples on how to run the eclenhancer tool, see the eclenhancer command topic. For examples of how to use the wsmapping tool, see the wsmapping command topic.
  5. Optional: If you are using DB2® and want to use static Structured Query Language (SQL), run the wsdbgen command. In order to use the wsdbgen command, IBM Optim pureQuery Run time must be installed. The wsdbgen command creates the persistence_unit_name.pdqxml file under the same META-INF directory where your persistence.xml file is located. If you have multiple persistence units, the wsdbgen command must be run for each persistence unit.

    When multiple pdqxml files are referenced by an application, use the Merge utility to combine them into a single pdqxml file. Specify the combined pdqxml file as pureQueryXml property of pdqProperties. Refer to the Merge utility documentation in the IBM Integrated Data Management information center.

    避免困難 避免困難: Applications that implement the JPA and are configured to run static SQL can experience various exceptions. These exceptions might occur with the wsdbgen command, which you can use to prepare the application, or when the application is running and calls a JPA method. To resolve this problem, complete the following steps:
    1. Install the program temporary fixes (PTF) for the iSeries JDBC Driver V5R4. Install PTF numbers SI32561 and SI32562. You can find the PTFs through the IBM System i® Support: PTF Cover Letters website.
    2. If you use DB2 Universal Database™ for iSeries V6R1 or V5R3, visit the fix website for the appropriate release.
    3. Install the required level of pureQuery, which is Version 1.3.100 or later. For more information, see the IBM Optim pureQuery Runtime website. Install the latest JCC driver, which is Version 3.52.95 or later, with the fix for APAR PK65069. The latest JCC drivers are part of the IBM DB2 software package.
    4. For DB2 on a z/OS® server, install PTF UK39204 for the V8 alternate driver or PTF UK39205 for V9, and install the fix for APAR PK67706.
    gotcha

    For examples on how to run this command, see the topic, wsdbgen command.

  6. Optional: If you are using OpenJPA and application-managed identity, generate an application-managed identity class with the wsappid tool. When you use an application-managed identity, one or more of the fields must be an identity field. Use an identity class if your entity has multiple identity fields and at least one of the fields is related to another entity. The application-managed identity tool generates Java code that uses the identity class for any persistent type that implements application-managed identity.

    For examples on how to use the wsappid tool, see the topic wsappid command.

Example

The following is an example of a persistence.xml file:
<?xml version="1.0" encoding="UTF-8"?> 
<persistence xmlns="http://java.sun.com/xml/ns/persistence" 
             xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" version="2.1" 
             xsi:schemaLocation="http://java.sun.com/xml/ns/persistence 
http://xmlns.jcp.org/xml/ns/persistence/persistence_2_1.xsd” ">
 	  
<persistence-unit name="TheWildZooPU" transaction-type="JTA">
        <jta-data-source>jdbc/DataSourceJNDI</jta-data-source>
        <!-- additional Mapping file, in addition to orm.xml>
        <mapping-file>META-INF/JPAorm.xml</mapping-file>

        <class>com.company.bean.jpa.PersistebleObjectImpl</class>
        <class>com.company.bean.jpa.Animal</class>
        <class>com.company.bean.jpa.Dog</class>
        <class>com.company.bean.jpa.Cat</class>

        <properties>
             <property name=”eclipselink.ddl-generation”
                       value=”create-tables”/> 
			
        </properties>

     </persistence-unit>
</persistence>

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