Using antrun maven plugin to execute a jar from the plugin classpath

Question

I have a build step that uses antrun plugin to execute a code generator. The code generator is available as maven plugin and only needed for the build step.

We had the code generator dependency as the dependency of the application and wanted to exclude it from packaging. I figured out that the code generator is not a dependency of the application and hence it should be a dependency of the plugin.

This used to work.

    <dependencies>
        <dependency>
            <groupId>com.myCompany</groupId>
            <artifactId>codeGenerator</artifactId>
            <version>1.0</version>
        </dependency>
    </dependencies>
    <build>
        <plugins>
            <plugin>
                <artifactId>maven-antrun-plugin</artifactId>
                <executions>
                    <execution>
                        <id>codeGeneration</id>
                        <phase>initialize</phase>
                        <configuration>
                            <target>
                                <echo message="jar location:  ${com.myCompany:codeGenerator:jar}"/>
                                <java jar="${com.myCompany:codeGenerator:jar}"
                                      fork="true"
                                      failonerror="true">
                                    <arg value="--input"/>
                                    <arg value=..."/>
                                </java>
                </executions>
            </plugin>
        </plugins>
    </build>

The jar from the maven dependency get's executed. The jar file is resolved successfully.

[WARNING] [echo] jar location: /home/.../.m2/repository/com/mycompany/codegenerator/1.0/codegenerator.jar

I tried to put the dependency into the plugin definition and use the maven.plugin.classpath instead of maven.dependency.classpath.

    <build>
        <plugins>
            <plugin>
                <artifactId>maven-antrun-plugin</artifactId>
                <executions>
                    <execution>
                        <id>codeGeneration</id>
                        <phase>initialize</phase>
                        <configuration>
                            <target>
                                <property name="plugin_classpath" refid="maven.plugin.classpath"/>
                                <echo message="plugin classpath:  ${plugin_classpath}"/>
                                <echo message="jar location:  ${com.mycompany:codeGenerator:jar}"/>
                                <java jar="${com.myCompany:codeGenerator:jar}"
                                      fork="true"
                                      failonerror="true"
                                      classpathref="maven.plugin.classpath">
                                    <arg value="--input"/>
                                    <arg value=".../>
                                </java>
                            </target>
                        </configuration>
                        <goals>
                            <goal>run</goal>
                        </goals>
                    </execution>
                </executions>
                <dependencies>
                    <dependency>
                        <groupId>com.myCompany</groupId>
                        <artifactId>codeGenerator</artifactId>
                        <version></version>
                    </dependency>
                </dependencies>
            </plugin>
        </plugins>
    </build>

The classpath looks good, the generator is included but the reference of the jar in <java jar="${com.myCompany:codeGenerator:jar} doesn't work anymore as soon as I remove the code generator from the dependencies of the application.

[WARNING] [echo] jar location: ${com.myCompany:codeGenerator:jar} [INFO] [java] Error: Unable to access jarfile /home/... /${com.myCompany:codeGenerator:jar}

Why does the resolution of the jar stop working? Is it somehow dependent on the dependency classpath? Is there something that I can do to make it work with the plugin classpath?

P.S.: I would prefer to have the code generation in a separate maven module, but I guess that would just move the problem but not solve it.

Workaround: Define a property to construct the path manually

    <codegenerator-jar>
        <!-- Workaround: Normal jar resolution with ${groupId:artifactId:jar} only works if the dependency is in the dependency classpath. The code generator is only on the plugin classpath -->
        ${settings.localRepository}/com/mycompany/codegenerator/${sdk-version}/codegenerator-${sdk-version}.jar
    </codegenerator-jar>