set-compiler-release.apt.vm

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 Setting the --release of the Java Compiler
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 Mahmoud Anouti
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 2019-12-20
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~~ NOTE: For help with the syntax of this file, see:
~~ http://maven.apache.org/doxia/references/apt-format.html

Setting the <<<--release>>> of the Java Compiler

  Starting JDK 9, the <<<javac>>> executable can accept the <<<--release>>>
  option to specify against which Java SE release you want to build the project.
  For example, you have JDK 11 installed and used by Maven, but you want to
  build the project against Java 8.
  The <<<--release>>> option ensures that the code is compiled following the
  rules of the programming language of the specified release, and that generated
  classes target the release as well as the public API of that release. This
  means that, unlike the
  {{{../examples/set-compiler-source-and-target.html}<<<-source>>> and <<<-target>>> options}},
  the compiler will detect and generate an error when using APIs that don't exist
  in previous releases of Java SE.

  Since version 3.6 of the Compiler Plugin, this option can be provided either
  via a property:

+-----
<project>
  [...]
  <properties>
    <maven.compiler.release>8</maven.compiler.release>
  </properties>
  [...]
</project>
+-----

  or by configuring the plugin directly:

+-----
<project>
  [...]
  <build>
    [...]
    <plugins>
      <plugin>
        <groupId>org.apache.maven.plugins</groupId>
        <artifactId>maven-compiler-plugin</artifactId>
        <version>${project.version}</version>
        <configuration>
          <release>8</release>
        </configuration>
      </plugin>
    </plugins>
    [...]
  </build>
  [...]
</project>
+-----

  <<Note:>> The value in the <<<release>>> parameter follows the new version naming scheme adopted
  since Java 9. As such, the release number does not start with 1.x anymore. Also note that the
  supported <<<release>>> targets include the release of the currently used JDK plus a limited number
  of previous releases.

* Usage on JDK 8

  The <<<--release>>> option is not supported using JDK 8. To enable a project that targets Java 8
  to be built using JDK 8 and also JDK 9 or later requires the conditional usage of the
  <<<--release>>> option.

  Conditional parametrization is required for the Compiler Plugin version below <<<3.13.0>>>
  or compilerId different that <<<javac>>>.

  This may be done through the use of a profile:

+-----
<project>
  [...]
  <profiles>
    [...]
    <profile>
      <id>set-compiler-release</id>
      <activation>
        <jdk>[9,)</jdk>
      </activation>
      <properties>
        <maven.compiler.release>8</maven.compiler.release>
      </properties>
    </profile>
    [...]
  </profiles>
  [...]
</project>
+-----

* Usage on JDK 8 - automatic detection

  Since version <<<3.13.0>>> of the Compiler Plugin together with the default <<<javac>>> compilerId you don't need conditional parametrisation of `release`.
  The <<<release>>> parameter will only be effective for Java 9 or above, otherwise the <<<source>>> and <<<target>>> will be passed to the compiler.

  So you can simply configure as:

+-----
<project>
 [...]
 <build>
   [...]
   <plugins>
     <plugin>
       <groupId>org.apache.maven.plugins</groupId>
       <artifactId>maven-compiler-plugin</artifactId>
       <version>${project.version}</version>
       <configuration>
         <release>8</release>
         <source>8</source>
         <target>8</target>
       </configuration>
     </plugin>
   </plugins>
   [...]
 </build>
 [...]
</project>
+-----