Maven 3 - How to add annotation processor dependency?

Question

I need to run an annotation processor on my project's sources. The annotation processor should not become a transitive dependency of the project since it's only needed for annotation processing and nothing else.

Here is the complete (non-working) test pom I use for this:

<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/maven-v4_0_0.xsd">
  <modelVersion>4.0.0</modelVersion>
  <groupId>test</groupId>
  <artifactId>test</artifactId>
  <packaging>jar</packaging>
  <version>1.0-SNAPSHOT</version>
  <name>Test annotations</name>
  <properties>
    <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
    <hibernate-jpamodelgen.version>1.2.0.Final</hibernate-jpamodelgen.version>
  </properties>
  <dependencies>
    <dependency>
      <groupId>javax</groupId>
      <artifactId>javaee-api</artifactId>
      <version>6.0</version>
      <scope>provided</scope>
    </dependency>
  </dependencies>
  <build>
    <plugins>
      <plugin>
        <groupId>org.apache.maven.plugins</groupId>
        <artifactId>maven-compiler-plugin</artifactId>
        <version>3.0</version>
        <configuration>
          <annotationProcessors>
            <annotationProcessor>
              org.hibernate.jpamodelgen.JPAMetaModelEntityProcessor</annotationProcessor>
          </annotationProcessors>
          <debug>true</debug>
          <optimize>true</optimize>
          <source>1.6</source>
          <target>1.6</target>
          <compilerArguments>
            <AaddGeneratedAnnotation>true</AaddGeneratedAnnotation>
            <Adebug>true</Adebug>
          </compilerArguments>
        </configuration>
        <dependencies>
          <dependency>
            <groupId>org.hibernate</groupId>
            <artifactId>hibernate-jpamodelgen</artifactId>
            <version>${hibernate-jpamodelgen.version}</version>
          </dependency>
        </dependencies>
      </plugin>
    </plugins>
  </build>
</project>

I explicitly defined org.hibernate.jpamodelgen.JPAMetaModelEntityProcessor as an annotation processor in the plugin configuration for tests and I know it shouldn't be required.

The problem I'm encountering is that the hibernate-jpamodelgen dependency is not added to the compiler classpath so the annotation processor is not found and the build fails.

As per this answer, I tried adding the dependency as a build extension (not sure I understand what those are supposed to be!) like so:

<extensions>
  <extension>
    <groupId>org.hibernate</groupId>
    <artifactId>hibernate-jpamodelgen</artifactId>
    <version>${hibernate-jpamodelgen.version}</version>
  </extension>
</extensions>

This also doesn't add hibernate-jpamodelgen to the compiler classpath.

The only thing I found which works so far is adding the dependency to the project in the <dependencies> section. This has the unfortunate side-effect of adding hibernate-jpamodelgen as a transitive dependency afterwards which I want to avoid.

My previous working setup uses the maven-processor-plugin plugin to achieve what I want. However, this plugin is not supported by eclipse m2e and the latest version of the maven-compiler-plugin now handles multiple compiler arguments properly so I'd rather use the latter.

Answer 1

Add the dependency as an optional dependency (<optional>true</optional>). This will add the dependency under compilation, but will prevent it for being a transitive dependency:

<dependency>
  <groupId>org.hibernate</groupId>
  <artifactId>hibernate-jpamodelgen</artifactId>
  <version>${hibernate-jpamodelgen.version}</version>
  <optional>true</optional>
</dependency>

If you're creating an artifact in this module with all your dependencies in it (like a .war), you may use the <scope>provided</scope> instead. This both prevents the dependency to be transitive and to be included in the artifact the module produces.