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. 

  1. <project>
  2.   [...]
  3.   <build>
  4.     [...]
  5.     <plugins>
  6.       <plugin>
  7.         <groupId>org.apache.maven.plugins</groupId>
  8.         <artifactId>maven-compiler-plugin</artifactId>
  9.         <version>3.6.1</version>
  10.         <executions>
  11.           <execution>
  12.             <id>default-compile</id>
  13.             <configuration>
  14.               <!-- compile everything to ensure module-info contains right entries -->
  15.               <!-- required when JAVA_HOME is JDK 8 or below -->
  16.               <jdkToolchain>
  17.                 <version>9</version>
  18.               </jdkToolchain>
  19.               <release>9</release>
  20.             </configuration>
  21.           </execution>
  22.           <execution>
  23.             <id>base-compile</id>
  24.             <goals>
  25.               <goal>compile</goal>
  26.             </goals>
  27.             <!-- recompile everything for target VM except the module-info.java
  28.             <configuration>
  29.               <excludes>
  30.                 <exclude>module-info.java</exclude>
  31.               </excludes>
  32.             </configuration>
  33.           </execution>
  34.         </executions>
  35.         <!-- defaults for compile and testCompile -->
  36.         <configuration>
  37.           <!-- jdkToolchain required when JAVA_HOME is JDK 9 or above -->
  38.           <jdkToolchain>
  39.             <version>[1.5,9)</version>
  40.           </jdkToolchain>
  41.           <source>1.5</source>
  42.           <target>1.5</target>
  43.         </configuration>
  44.       </plugin>
  45.     </plugins>
  46.     [...]
  47.   </build>
  48.   [...]
  49. </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. 

  1. <project>
  2.   [...]
  3.   <build>
  4.     [...]
  5.     <plugins>
  6.       <plugin>
  7.         <groupId>org.apache.maven.plugins</groupId>
  8.         <artifactId>maven-compiler-plugin</artifactId>
  9.         <version>3.6.1</version>
  10.         <executions>
  11.           <execution>
  12.             <id>default-compile</id>
  13.             <configuration>
  14.               <!-- compile everything to ensure module-info contains right entries -->
  15.               <release>9</release>
  16.             </configuration>
  17.           </execution>
  18.           <execution>
  19.             <id>base-compile</id>
  20.             <goals>
  21.               <goal>compile</goal>
  22.             </goals>
  23.             <!-- recompile everything for target VM except the module-info.java
  24.             <configuration>
  25.               <excludes>
  26.                 <exclude>module-info.java</exclude>
  27.               </excludes>
  28.             </configuration>
  29.           </execution>
  30.         </executions>
  31.         <!-- defaults for compile and testCompile -->
  32.         <configuration>
  33.           <!-- Only required when JAVA_HOME isn't at least Java 9 and when haven't configured the maven-toolchains-plugin -->
  34.           <jdkToolchain>
  35.             <version>9</version>
  36.           </jdkToolchain>
  37.           <release>6</release>
  38.         </configuration>
  39.       </plugin>
  40.     </plugins>
  41.     [...]
  42.   </build>
  43.   [...]
  44. </project>