Java 9+ projects with module-info

If a project with a module-info.java file does not need to be compatible with Java 8 or earlier environment, there is nothing special to do.

Java 8 projects with module-info

Projects that want to be compatible with older versions of Java (i.e, 8 or below bytecode and API), but also want to provide a module-info.java for use on Java 9+ runtime, must be aware that they need to call javac twice:

1. the module-info.java must be compiled with release set to 9 or later,
2. the rest of the sources must be compiled with the lower expected compatibility version of source/target.

A way to do this is by having two execution blocks, as described below:

1. default default-compile execution with release set to 9 or later,
2. additional custom base-compile execution with expected target compatibility.

The following snippet gives an example. This snippet assumes that the JDK used by Maven supports the --release 8 option. It may not be the case for all JDKs, as newer JDKs may drop the support of Java versions that are too old.

<project>
  [...]
  <build>
    [...]
    <plugins>
      <plugin>
        <groupId>org.apache.maven.plugins</groupId>
        <artifactId>maven-compiler-plugin</artifactId>
        <version>...</version>
        <executions>
          <execution>
            <id>default-compile</id>
            <configuration>
              <release>9</release>
              <!-- no excludes: compile everything to ensure module-info contains right entries -->
            </configuration>
          </execution>
          <execution>
            <id>base-compile</id>
            <goals>
              <goal>compile</goal>
            </goals>
            <configuration>
              <release>8</release>
            </configuration>
          </execution>
        </executions>
      </plugin>
    </plugins>
    [...]
  </build>
  [...]
</project>

If the JDK used by Maven does not support the --release 8 option, then projects which want to be compatible with old Java versions need to use two different JDKs for the two executions. Using a toolchains configuration, it is possible to achieve this, even if more complex. In above snippet, the following fragment:

              <release>8</release>

can be replaced by:

              <source>1.8</source>
              <target>1.8</target>
              <jdkToolchain>
                <version>1.8</version>
              </jdkToolchain>