Generating sources by running a project's java class in Maven

You can execute the maven-compile-plugin in the generate-sources phase. Just add another execution before the existing execution and configure it so that it just picks up the sources for the generator.

Or split the project in two: build the generator with a separate POM and include the generator library as a dependency to the POM that's generating the sources.

Personally I would split the project. Keeps the build files cleaner and easier to maintain.

I didn't want to have 2 different projects, so I tried to setup Maven for adding the generated compiled code to the final jar package.

This is the working solution I've used:

• In process-classes phase (executed just after the compile phase):
  • exec-maven-plugin for executing a main class able to generate my source files in target/generated-sources/java folder (in my specific case I used the Roaster library for source code generation);
  • build-helper-maven-plugin for adding the generated sources in the correct location
• In prepare-package phase:
  • maven-compiler-plugin, in order to detect the changes and recompile the module
  • maven-jar-plugin for producing the jar package

This is my pom.xml:

<build>
    <plugins>

        <plugin>
            <groupId>org.codehaus.mojo</groupId>
            <artifactId>exec-maven-plugin</artifactId>
            <version>1.6.0</version>
            <executions>
                <execution>
                    <phase>process-classes</phase>
                    <goals>
                        <goal>java</goal>
                    </goals>
                    <configuration>
                        <mainClass>com.example.MyClassWriter</mainClass>
                        <arguments>
                            <argument>${project.basedir}</argument>
                            <argument>${project.build.directory}</argument>
                        </arguments>
                    </configuration>
                </execution>
            </executions>
        </plugin>

        <plugin>
            <groupId>org.codehaus.mojo</groupId>
            <artifactId>build-helper-maven-plugin</artifactId>
            <version>3.0.0</version>
            <executions>
                <execution>
                    <phase>process-classes</phase>
                    <goals>
                        <goal>add-source</goal>
                    </goals>
                    <configuration>
                        <sources>
                            <source>${project.build.directory}/generated-sources/java</source>
                        </sources>
                    </configuration>
                </execution>
            </executions>
        </plugin>

        <plugin>
            <groupId>org.apache.maven.plugins</groupId>
            <artifactId>maven-compiler-plugin</artifactId>
            <version>3.6.1</version>
            <executions>
                <execution>
                    <phase>prepare-package</phase>
                    <goals>
                        <goal>compile</goal>
                    </goals>
                </execution>
            </executions>
        </plugin>

        <plugin>
            <groupId>org.apache.maven.plugins</groupId>
            <artifactId>maven-jar-plugin</artifactId>
            <version>3.0.2</version>
            <executions>
                <execution>
                    <phase>prepare-package</phase>
                </execution>
            </executions>
        </plugin>

    </plugins>
</build>

In order to do this in one project, there are 3 steps:

1. Compile generator code

   We can do it in generate-sources phase, using maven-compiler-plugin. You can also exclude other source files.

2. Run generator to generate code

   We can do it in process-sources phase, using exec-maven-plugin.

3. Compile project

Below is the key part of pom.xml

  <plugin>
    <groupId>org.apache.maven.plugins</groupId>
    <artifactId>maven-compiler-plugin</artifactId>
    <version>3.8.0</version>
    <configuration>
      <source>1.8</source>
      <target>1.8</target>
    </configuration>
    <executions>
        <execution>
            <id>compile-generator</id>
            <phase>generate-sources</phase>
            <goals>
              <goal>compile</goal>
            </goals>
            <configuration>
              <includes>
                <include>source/file/of/generator/*.java</include>
              </includes>
              <excludes>
                <exclude>other/source/files/*.java</exclude>
              </excludes>
            </configuration>
        </execution>
    </executions>
  </plugin>   
  <plugin>
    <groupId>org.codehaus.mojo</groupId>
    <artifactId>exec-maven-plugin</artifactId>
    <version>1.6.0</version>
    <executions>
        <execution>
            <id>generate-codes</id>
            <goals>
                <goal>java</goal>
            </goals>
            <phase>process-sources</phase>
            <configuration>
                <mainClass>your.main.class.of.generator</mainClass>
            </configuration>
        </execution>
    </executions>
  </plugin>