How to run exec-maven-plugin only if a file is not up-to-date

In my Maven build, I use exec-maven-plugin to run an external tool for Java code generation. In this case the flatbuffers compiler (flatc).

By default, flatc always runs, so you always get a new .java file. This will always trigger Java compiler to run, which will in turn always trigger jar packaging to run, etc. I would like flatc to only run if it’s source file is updated, so I can get a properly incremental build and avoid needlessly recompiling.

flatc itself is not Java based, it does not support conditional execution, and there is no specific Maven plugin wrapping this tool. (There are specific Maven plugins for some other similar tools, like protoc.)

I was able to do it by using build-helper-maven-plugin, like this:

<plugin>
  <groupId>org.codehaus.mojo</groupId>
  <artifactId>build-helper-maven-plugin</artifactId>
  <version>3.0.0</version>
  <executions>
    <execution>
      <id>fbs-register-sources</id>
      <phase>generate-sources</phase>
      <goals>
        <goal>add-source</goal>
      </goals>
      <configuration>
        <sources>
          <source>${project.build.directory}/generated-sources/flatc</source>
        </sources>
      </configuration>
    </execution>
    <execution>
      <id>check-fbs-compile</id>
      <phase>generate-sources</phase>
      <goals>
        <goal>uptodate-property</goal>
      </goals>
      <configuration>
        <name>flatc.notRequired</name>
        <fileSet>
          <directory>${project.basedir}/src/main/fbs</directory>
          <outputDirectory>${project.build.directory}/generated-sources/flatc</outputDirectory>
          <includes>
            <include>my.fbs</include>
          </includes>
          <mapper>
            <type>merge</type>
            <to>My.java</to>
          </mapper>
        </fileSet>
      </configuration>
    </execution>
  </executions>
</plugin>
<plugin>
  <groupId>org.codehaus.mojo</groupId>
  <artifactId>exec-maven-plugin</artifactId>
  <executions>
    <execution>
      <id>fbs-compile</id>
      <phase>generate-sources</phase>
      <goals>
        <goal>exec</goal>
      </goals>
      <configuration>
        <skip>${flatc.notRequired}</skip>
        <!-- ... -->