Older projects with module-info
Projects that want to be compatible with older versions of Java (i.e 1.8 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:
- the
module-info.java
must be compiled withrelease=9
, - while the rest of the sources must be compiled with the lower expected compatibility version of
source
/target
.
The preferred way to do this is by having 2 execution blocks, as described below:
- default
default-compile
execution withrelease=9
, - additional custom
base-compile
execution with expected target compatibility.
Notice that, in addition, 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 for the 2 executions. With toolchains configuration, it is quite easy to achieve this, even if a little bit more complex.
Java 6 to 8 Compatibility
In case you want the project to be Java 6, 7 or 8 compatible, you can simply use JDK 9 for both execution blocks.
The easiest way is to use Java 9 as the runtime for Maven, by setting JAVA_HOME=/path/to/jdk-9
before running mvn
. But if you want to use an older Java runtime for Maven, you can use the maven-toolchain-plugin to specify the shared JDK or a custom jdkToolchain and refer to the JDK 9 installation on your system.
<project> [...] <build> [...] <plugins> <plugin> <groupId>org.apache.maven.plugins</groupId> <artifactId>maven-compiler-plugin</artifactId> <version>3.12.0</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> <!-- recompile everything for target VM except the module-info.java --> <excludes> <exclude>module-info.java</exclude> </excludes> </configuration> </execution> </executions> <!-- defaults for compile and testCompile --> <configuration> <release>6</release><!-- or 7 or 8 depending on compatibility expectations --> <!-- Only required when Maven runtime JAVA_HOME isn't at least Java 9 and when haven't configured the maven-toolchains-plugin --> <jdkToolchain> <version>9</version> </jdkToolchain> </configuration> </plugin> </plugins> [...] </build> [...] </project>
Java 5 or below Compatibility
Given that Maven 3 requires newer Java release at runtime, you'll absolutely need to use Toolchains to use a JDK different from Maven runtime.
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>3.12.0</version> <executions> <execution> <id>default-compile</id> <configuration> <release>9</release> <!-- no excludes: compile everything to ensure module-info contains right entries --> <!-- toolchain required when JAVA_HOME is JDK 8 or below --> <jdkToolchain> <version>9</version> </jdkToolchain> </configuration> </execution> <execution> <id>base-compile</id> <goals> <goal>compile</goal> </goals> <configuration> <!-- recompile everything for target VM except the module-info.java --> <excludes> <exclude>module-info.java</exclude> </excludes> </configuration> </execution> </executions> <!-- defaults for compile and testCompile --> <configuration> <source>1.5</source> <target>1.5</target> <!-- jdkToolchain required when JAVA_HOME is JDK 9 or above --> <jdkToolchain> <version>[1.5,9)</version> </jdkToolchain> </configuration> </plugin> </plugins> [...] </build> [...] </project>