compilation failure for spring boot application with java 9 and maven

Question

I am trying to build spring-boot application that uses java-9 and would be deployed to heroku. As a build tool I use maven

Answer 1

Rightly pointed out by @Robert you need to update your maven-compiler-plugin. To add to it, also as stated in the Maven/Jigsaw+9 over the maven plugins. The minimum compatible version of maven-compiler-plugin with current jdk-9+181 is 3.6.2 which can be used as :

<!--Minimum compatible version required-->
<plugin>
    <groupId>org.apache.maven.plugins</groupId>
    <artifactId>maven-compiler-plugin</artifactId>
    <version>3.6.2</version>
    <configuration>
        <jdkToolchain>
            <version>9</version>
        </jdkToolchain>
    </configuration>
</plugin>

---

And to quote from one of the examples of maven-compiler-plugin with module-info.java here:

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.

which can be achieved using (JAVA_HOME set as 1.8.x) :

<plugin>
    <groupId>org.apache.maven.plugins</groupId>
    <artifactId>maven-compiler-plugin</artifactId>
    <version>3.6.2</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>

Few things to note there though are:

... you will need at least Maven 3.3.1 to specify a custom jdkToolchain in your plugin configuration

---

Or you can also configure the JAVA_HOME as /jdk9/Contents/Home/bin and use the following configuration:

<plugin>
    <groupId>org.apache.maven.plugins</groupId>
    <artifactId>maven-compiler-plugin</artifactId>
    <version>3.6.2</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>

Answer 2

maven-compiler-plugin 3.6.0 is based on the first signature of module-info.class, which has changed a couple of times. It is not compatible with current Java 9 signature. You should use 3.6.2 when using most recent versions of JDK 9.

Answer 3

Looks like your toolchains.xml does not include an entry for Java 9. Try this:

<toolchain>
    <type>jdk</type>
    <provides>
        <version>1.9</version>
        <vendor>oracle</vendor>
    </provides>
    <configuration>
        <jdkHome>/path/to/jdk/9</jdkHome>
    </configuration>
</toolchain>