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Hibernate JPA 2 Metamodel Generator

Reference Guide

by Hardy Ferentschik
1. Introduction
1.1. What is it about?
1.2. Canonical Metamodel
2. Usage
2.1. Usage from the command line
2.1.1. Usage with Ant
2.1.2. Usage with Maven
2.2. Usage within the IDE
2.2.1. Idea
2.2.2. Eclipse
2.2.3. NetBeans
2.3. Processor specific options
A. Further information
References
1.1. What is it about?
1.2. Canonical Metamodel

JPA 2 defines a new typesafe Criteria API which allows criteria queries to be constructed in a strongly-typed manner, using metamodel objects to provide type safety. For developers it is important that the task of the metamodel generation can be automated. Hibernate Static Metamodel Generator is an annotation processor based on the [Pluggable Annotation Processing API] with the task of creating JPA 2 static metamodel classes. The following example show two JPA 2 entities Order and Item, together with the metamodel class Order_ and a typesafe query.

Example 1.1. JPA 2 annotated entities Order and Item



@Entity

public class Order {

    @Id 

    @GeneratedValue

    Integer id;

    

    @ManyToOne 

    Customer customer;

    

    @OneToMany 

    Set<Item> items;

    BigDecimal totalCost;

    

    // standard setter/getter methods

    ...

}


@Entity  

public class Item { 

    @Id

    @GeneratedValue

      Integer id;

    

      int quantity;

        

    @ManyToOne

    Order order;

    

    // standard setter/getter methods

    ...

}

Example 1.2. Metamodel class Order_



@StaticMetamodel(Order.class)

public class Order_ {

    public static volatile SingularAttribute<Order, Integer> id;

    public static volatile SingularAttribute<Order, Customer> customer;

    public static volatile SetAttribute<Order, Item> items;

    public static volatile SingularAttribute<Order, BigDecimal> totalCost;

}

       

Example 1.3. Typesafe citeria query



CriteriaBuilder cb = entityManager.getCriteriaBuilder();

CriteriaQuery<Order> cq = cb.createQuery(Order.class);

SetJoin<Order, Item> itemNode = cq.from(Order.class).join(Order_.items);

cq.where( cb.equal(itemNode.get(Item_.id), 5 ) ).distinct(true);

The structure of the metamodel classes is described in the [JPA 2 Specification], but for completeness the definition is repeated in the following paragraphs. Feel free to skip ahead to Chapter 2, Usage if you are not interested into the gory details.

The annotation processor produces for every managed class in the persistence unit a metamodel class based on these rules:

  • For each managed class X in package p, a metamodel class X_ in package p is created.

  • The name of the metamodel class is derived from the name of the managed class by appending "_" to the name of the managed class.

  • The metamodel class X_ must be annotated with the javax.persistence.StaticMetamodel annotation.

  • If class X extends another class S, where S is the most derived managed class (i.e., entity or mapped superclass) extended by X, then class X_ must extend class S_, where S_ is the metamodel class created for S.

  • For every persistent non-collection-valued attribute y declared by class X, where the type of y is Y, the metamodel class must contain a declaration as follows: 

    public static volatile SingularAttribute<X, Y> y;

  • For every persistent collection-valued attribute z declared by class X, where the element type of z is Z, the metamodel class must contain a declaration as follows:

    • if the collection type of z is java.util.Collection, then

      public static volatile CollectionAttribute<X, Z> z;

    • if the collection type of z is java.util.Set, then

      public static volatile SetAttribute<X, Z> z;

    • if the collection type of z is java.util.List, then

      public static volatile ListAttribute<X, Z> z;

    • if the collection type of z is java.util.Map, then 

      public static volatile MapAttribute<X, K, Z> z;

      where K is the type of the key of the map in class X

Import statements must be included for the needed javax.persistence.metamodel types as appropriate and all classes X, Y, Z, and K.

2.1. Usage from the command line
2.1.1. Usage with Ant
2.1.2. Usage with Maven
2.2. Usage within the IDE
2.2.1. Idea
2.2.2. Eclipse
2.2.3. NetBeans
2.3. Processor specific options

The jar file for the annotation processor can be found in the JBoss Maven repository using Example 2.1, “Maven dependency ”.

Example 2.1. Maven dependency 


<dependency>

    <groupId>org.hibernate</groupId>

    <artifactId>hibernate-jpamodelgen</artifactId>

    <version>1.0.0</version>

</dependency>

Alternatively, a full distribution package can be downloaded from SourceForge.

In most cases the annotation processor will automatically run provided the processor jar is added to the classpath and a JDK 6 is used. This happens due to Java's Service Provider contract and the fact the the Hibernate Static Metamodel Generator jar files contains the file javax.annotation.processing.Processor in the META-INF/services directory. The fully qualified name of the processor itself is: org.hibernate.jpamodelgen.JPAMetaModelEntityProcessor.

Note

The use of a Java 6 compiler is a prerequisite.

As mentioned before, the annotation processor will run automatically each time the Java compiler is called, provided the jar file is on the classpath. Sometimes, however, it is useful to control the annotation processing in more detail, for example if you exclusively want to run the processor without compiling any other source files. Example 2.2, “Javac Task configuration” shows how Ant's Javac Task can be configured to just run annotation processing.

Example 2.2. Javac Task configuration


<javac srcdir="${src.dir}"

    destdir="${target.dir}"

    failonerror="false"

    fork="true"

    classpath="${classpath}">

    <compilerarg value="-proc:only"/>

</javac>

The option -proc:only instructs the compiler to just run the annotation processing. You can also completely disable processing by specifying -proc:none.

Tip

Run 'javac -help' to see which other annotation processor relevant options can be specified.

There are several ways of running the annotation processor as part of a Maven build. Again, it will automatically run if you are using a JDK 6 compiler and the annotation processor jar is on the classpath. In case you have more than one annotation processors on your classpath you can explicitly pass the processor option to the compiler plugin:

Example 2.3. Maven compiler plugin configuration - direct execution


<plugin>

    <artifactId>maven-compiler-plugin</artifactId>

    <configuration>

        <source>1.6</source>

        <target>1.6</target>

        <compilerArguments>

            <processor>org.hibernate.jpamodelgen.JPAMetaModelEntityProcessor</processor>

        </compilerArguments>

    </configuration>

</plugin>

The maven-compiler-plugin approach has the disadvantage that the maven compiler plugin does currently not allow to specify multiple compiler arguments (MCOMPILER-62) and that messages from the Messenger API are suppressed (MCOMPILER-66). A better approach is to disable annotation processing for the compiler plugin as seen in Example 2.4, “Maven compiler plugin configuration - indirect execution”.

Example 2.4. Maven compiler plugin configuration - indirect execution


<plugin>

    <artifactId>maven-compiler-plugin</artifactId>

    <configuration>

        <source>1.6</source>

        <target>1.6</target>

        <compilerArgument>-proc:none</compilerArgument>

    </configuration>

</plugin>

Once disabled, the maven-annotation-plugin for annotation processing (you will need the following additional maven repositories: maven-annotation-plugin and jfrog) can be used. The configuration can be seen in Example 2.5, “Configuration with maven-annotation-plugin”.

Example 2.5. Configuration with maven-annotation-plugin


<plugin>

    <groupId>org.bsc.maven</groupId>

    <artifactId>maven-processor-plugin</artifactId>

    <executions>

        <execution>

            <id>process</id>

            <goals>

                <goal>process</goal>

            </goals>

            <phase>generate-sources</phase>

            <configuration>

                <!-- source output directory -->

                <outputDirectory>target/metamodel</outputDirectory>

            </configuration>

        </execution>

    </executions>

</plugin>

<plugin>

    <groupId>org.codehaus.mojo</groupId>

    <artifactId>build-helper-maven-plugin</artifactId>

    <version>1.3</version>

    <executions>

        <execution>

            <id>add-source</id>

            <phase>generate-sources</phase>

            <goals>

                <goal>add-source</goal>

            </goals>

            <configuration>

                <sources>

                    <source>target/metamodel</source>

                </sources>

            </configuration>

        </execution>

    </executions>

</plugin>

Of course you also want to have annotation processing available in your favorite IDE. The following paragraphs and screenshots show you how to enable the Hibernate Static Metamodel Generator within your IDE.

Intellij Idea contains from version 9.x onwards a specifc configuration section for annotation processing under the project settings window. The screenshots show you how to configure the Hibernate Static Metamodel Generator.

In Eclipse, from the Galileo release onwards, exists an additional configuration section under Java Compiler. There you can configure all kinds of aspects of annotation processing. Just check the "Enable annotation processing" option, configure the directory for the generated sources and finally add the Hibernate Static Metamodel Generator and JPA 2 jar files to the factory path.

Netbeans support for annotation processors is at the time of this wrinting still in the making. Refer to NetBeans issues 111065, 111293 and 111294.

The Hibernate Static Metamodel Generator accepts a series of custom options which can be passed to the processor in the format -A[property]=[value]. The supported properties are:

Table 2.1. Annotation processor options (passed via -A[property]=[value])

Option nameOption value and usage
debugif set to true additional trace information will be outputted by the processor
persistenceXmlPer default the processor looks in /META-INF for persistence.xml. Specifying this option a persitence.xml file from a different location can be specified (has to be on the classpath)
ormXmlAllows to specify additional entity mapping files. The specified value for this option is a comma separated string of mapping file names. Even when this option is specified /META-INF/orm.xml is implicit.
lazyXmlParsingPossible values are true or false. If set to true the annotation processor tries to determine whether any of the xml files has changed between invocations and if unchanged skips the xml parsing. This feature is experimental and contains the risk of wron results in some cases of mixed mode configurations. To determine wether a file has been modified a temporary file Hibernate-Static-Metamodel-Generator.tmp is used. This file gets created in the java.io.tmpdir directory.


For further usage question or problems consult the Hibernate Forum. For bug reports use the METAGEN project in the Hibernate Jira instance. Feedback is always welcome.

[Pluggable Annotation Processing API] JSR 269: Pluggable Annotation Processing API. Copyright © 2006 SUN MICROSYSTEMS, INC.. JSR 269 JCP Page .

[JPA 2 Specification] JSR 317: Java™ Persistence API, Version 2.0 . Java Persistence 2.0 Expert Group. . Copyright © 2009 SUN MICROSYSTEMS, INC.. JSR 317 JCP Page .

Copyright © 2010 Red Hat Inc.