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Older projects with module-info

For projects that want to be compatible with older versions of Java (i.e 1.8 or below), but also want to provide a module-info.java for Java 9 projects must be aware that they need to call javac twice: the module-info.java must be compiled with release=9, while the rest of the sources must be compiled with a lower version of source/target.

The preferred way to do this is by having 2 execution blocks are described below. JDK 9 only supports compilations for Java 6 and above, so projects wanting to be compatible with Java 5 or below need to use two different JDKs. With toolchains it is quite easy to achieve this. Be aware that you will need at least Maven 3.3.1 to specify a custom jdkToolchain in your plugin configuration. You could add a jdkToolchain to do base-compile execution-block as well referring to JDK 5. 

<project>
  [...]
  <build>
    [...]
    <plugins>
      <plugin>
        <groupId>org.apache.maven.plugins</groupId>
        <artifactId>maven-compiler-plugin</artifactId>
        <version>3.8.0</version>
        <executions>
          <execution>
            <id>default-compile</id>
            <configuration>
              <!-- compile everything to ensure module-info contains right entries -->
              <!-- required when JAVA_HOME is JDK 8 or below -->
              <jdkToolchain>
                <version>9</version>
              </jdkToolchain>
              <release>9</release>
            </configuration>
          </execution>
          <execution>
            <id>base-compile</id>
            <goals>
              <goal>compile</goal>
            </goals>
            <!-- recompile everything for target VM except the module-info.java -->
            <configuration>
              <excludes>
                <exclude>module-info.java</exclude>
              </excludes>
            </configuration>
          </execution>
        </executions>
        <!-- defaults for compile and testCompile -->
        <configuration>
          <!-- jdkToolchain required when JAVA_HOME is JDK 9 or above -->
          <jdkToolchain>
            <version>[1.5,9)</version>
          </jdkToolchain>
          <source>1.5</source>
          <target>1.5</target>
        </configuration>
      </plugin>
    </plugins>
    [...]
  </build>
  [...]
</project>

In case you want the project to be Java 6 compatible, the easiest to do this is to use Java 9 for both execution blocks. You can use the maven-toolchain-plugin to specify the shared JDK (supported since Maven 2.0.9) or a custom jdkToolchain (supported since Maven 3.3.1) and refer to the JDK 9 installation on your system. Or simply use Java 9 as the runtime for Maven by setting JAVA_HOME=/path/to/jdk-9. 

<project>
  [...]
  <build>
    [...]
    <plugins>
      <plugin>
        <groupId>org.apache.maven.plugins</groupId>
        <artifactId>maven-compiler-plugin</artifactId>
        <version>3.8.0</version>
        <executions>
          <execution>
            <id>default-compile</id>
            <configuration>
              <!-- compile everything to ensure module-info contains right entries -->
              <release>9</release>
            </configuration>
          </execution>
          <execution>
            <id>base-compile</id>
            <goals>
              <goal>compile</goal>
            </goals>
            <!-- recompile everything for target VM except the module-info.java -->
            <configuration>
              <excludes>
                <exclude>module-info.java</exclude>
              </excludes>
            </configuration>
          </execution>
        </executions>
        <!-- defaults for compile and testCompile -->
        <configuration>
          <!-- Only required when JAVA_HOME isn't at least Java 9 and when haven't configured the maven-toolchains-plugin -->
          <jdkToolchain>
            <version>9</version>
          </jdkToolchain>
          <release>6</release>
        </configuration>
      </plugin>
    </plugins>
    [...]
  </build>
  [...]
</project>