Today I searched for a Maven plugin that would filter some of my source files before compiling them. An equivalent to the resources plugin with filtering turned on, but for Java sources, that would replace occurences of, say, ${token.name} with somevalue wherever that string occurs in the source files.

I could not find such a plugin and I think I understand why. When the resources plugin processes the resources, it simply copies them from src/main/resources to target/classes, optionally filtering them. In the packaging phase, Maven simply zips up everything he finds in target/classes to the target jarfile.

But with source code you cannot simply copy the .java files to, say, target/generated after filtering them, and expect Maven to compile these filtered source files. You would have .java source files with the exact same name and same package declaration in src/main/java and in target/generated. So source file filtering cannot work in Maven unless you tell the compiler to change the directory from which to compile files. Well, I don’t have that much experience with Maven and I don’t know how to do that. I know the build-helper plugin can add a directory to the compile path, but I don’t know how to remove a directory from it.

On my project I needed to process only one single file. Let’s call it mypackage.Locator.java. I declared that class in a file named src/main/java/mypackage/_Locator.java (note the underscore). Then I configured an antrun task to copy that file over to target/generated/mypackage/Locator.java, a directory that build-helper had added to the compile path. I then told the compiler plugin to exclude all source files whose names begin with an underscore.

The most important parts of my pom.xml file look like this:

  <build>
    <plugins>

      <plugin>
        <groupId>org.apache.maven.plugins</groupId>
        <artifactId>maven-compiler-plugin</artifactId>
        <configuration>
          <excludes>
            <exclude>**/_*.java</exclude>
          </excludes>
        </configuration>
      </plugin>

      <plugin>
        <artifactId>maven-antrun-plugin</artifactId>
        <executions>
          <execution>
            <id>generate-locator</id>
            <phase>process-sources</phase>
            <configuration>
              <tasks>
                <filter token="token.name" value="somevalue"/>
                <copy file="src/main/java/mypackage/_Locator.java"
                      tofile="target/generated/mypackage/Locator.java"
                      filtering="true"/>
              </tasks>
            </configuration>
            <goals>
              <goal>run</goal>
            </goals>
          </execution>
        </executions>
      </plugin>

    </plugins>
  </build>

Note that you now let Ant filter the source files, so you must use Ant-like tokens (e.g. @token.name@). This works reasonably well, but if I can make some time for it I would really like to know how to remove the default source directory from the compile path.