Include maven dependencies built with Java9 or Java10 in a Kotlin project

Question

Is it possible to include Java9 (or 10) dependencies? As Kotlin is only able to compile to Java 8 so far. I'm getting an java.lang.UnsupportedClassVersionError error obviously stating that a class in the JAR/dependency I'm including has been compiled by a more recent version of the Java Runtime. However am I not able to specify a newer JDK, which is able to run classes compiled with older versions (the Kotlin stuff?).

<plugin>
  <groupId>org.apache.maven.plugins</groupId>
  <artifactId>maven-compiler-plugin</artifactId>
  <version>3.7.0</version>
  <configuration>
    <release>9</release>
    <source>${maven.compiler.source}</source>
    <target>${maven.compiler.target}</target>
    <encoding>${project.build.sourceEncoding}</encoding>
  </configuration>
  <dependencies>
    <dependency>
      <groupId>org.ow2.asm</groupId>
      <artifactId>asm</artifactId>
      <version>6.1</version> <!-- Use newer version of ASM -->
    </dependency>
  </dependencies>
</plugin>

With the properties

<maven.compiler.source>9</maven.compiler.source>
<maven.compiler.target>9</maven.compiler.target>

But now I'm getting the error: "Error:Kotlin: Unknown JVM target version: 1.9 Supported versions: 1.6, 1.8"

Kotlin version is 1.2.41 , so far I can always switch to the newest version until it's released...

It's okay with the following XML fragment. However the problem with java.lang.UnsupportedClassVersionError remains.

        <plugin>
            <groupId>org.apache.maven.plugins</groupId>
            <artifactId>maven-compiler-plugin</artifactId>
            <version>3.5.1</version>
            <configuration>
                <source>9</source>
                <target>9</target>
            </configuration>
            <executions>
                <!-- Replacing default-compile as it is treated specially by maven -->
                <execution>
                    <id>default-compile</id>
                    <phase>none</phase>
                </execution>
                <!-- Replacing default-testCompile as it is treated specially by maven -->
                <execution>
                    <id>default-testCompile</id>
                    <phase>none</phase>
                </execution>
                <execution>
                    <id>java-compile</id>
                    <phase>compile</phase>
                    <goals> <goal>compile</goal> </goals>
                </execution>
                <execution>
                    <id>java-test-compile</id>
                    <phase>test-compile</phase>
                    <goals> <goal>testCompile</goal> </goals>
                </execution>
            </executions>
        </plugin>

I'm trying to build a Kotlin module/maven bundle for a (currently) Java9 project of mine.

The Maven-Kotlin Plugin configuration is:

      <plugin>
            <groupId>org.jetbrains.kotlin</groupId>
            <artifactId>kotlin-maven-plugin</artifactId>
            <version>${kotlin.version}</version>
            <configuration>
                <jvmTarget>1.8</jvmTarget>
            </configuration>
            <executions>
                <execution>
                    <id>compile</id>
                    <phase>compile</phase>
                    <goals>
                        <goal>compile</goal>
                    </goals>
                    <configuration>
                        <sourceDirs>
                            <source>src/main/java</source>
                            <source>src/main/kotlin</source>
                        </sourceDirs>
                    </configuration>
                </execution>
                <execution>
                    <id>test-compile</id>
                    <phase>test-compile</phase>
                    <goals>
                        <goal>test-compile</goal>
                    </goals>
                    <configuration>
                        <sourceDirs>
                            <source>src/test/java</source>
                            <source>src/test/kotlin</source>
                        </sourceDirs>
                    </configuration>
                </execution>
            </executions>
        </plugin>

My whole pom.xml:

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>

    <groupId>restful-web</groupId>
    <artifactId>restful-web</artifactId>
    <version>1.0-SNAPSHOT</version>

    <properties>
        <kotlin.version>1.2.41</kotlin.version>
        <kotlin.compiler.jvmTarget>1.8</kotlin.compiler.jvmTarget>
        <sirix.version>0.8.9-SNAPSHOT</sirix.version>
        <main.verticle>org.sirix.rest.SirixVerticle</main.verticle>
        <maven.compiler.source>9</maven.compiler.source>
        <maven.compiler.target>9</maven.compiler.target>
    </properties>

    <dependencies>
        <dependency>
            <groupId>io.vertx</groupId>
            <artifactId>vertx-core</artifactId>
            <version>3.5.1</version>
        </dependency>
        <dependency>
            <groupId>io.vertx</groupId>
            <artifactId>vertx-web</artifactId>
            <version>3.5.1</version>
        </dependency>
        <dependency>
            <groupId>io.vertx</groupId>
            <artifactId>vertx-lang-kotlin</artifactId>
            <version>3.5.1</version>
        </dependency>
        <dependency>
            <groupId>org.jetbrains.kotlin</groupId>
            <artifactId>kotlin-stdlib-jdk8</artifactId>
            <version>${kotlin.version}</version>
        </dependency>
        <dependency>
            <groupId>com.github.sirixdb.sirix</groupId>
            <artifactId>sirix-core</artifactId>
            <version>${sirix.version}</version>
        </dependency>
        <dependency>
            <groupId>com.github.sirixdb.sirix</groupId>
            <artifactId>sirix-xquery</artifactId>
            <version>${sirix.version}</version>
        </dependency>
    </dependencies>

    <build>
        <plugins>
            <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-compiler-plugin</artifactId>
                <version>3.5.1</version>
                <configuration>
                    <source>${maven.compiler.source}</source>
                    <target>${maven.compiler.target}</target>
                </configuration>
                <executions>
                    <!-- Replacing default-compile as it is treated specially by maven -->
                    <execution>
                        <id>default-compile</id>
                        <phase>none</phase>
                    </execution>
                    <!-- Replacing default-testCompile as it is treated specially by maven -->
                    <execution>
                        <id>default-testCompile</id>
                        <phase>none</phase>
                    </execution>
                    <execution>
                        <id>java-compile</id>
                        <phase>compile</phase>
                        <goals> <goal>compile</goal> </goals>
                    </execution>
                    <execution>
                        <id>java-test-compile</id>
                        <phase>test-compile</phase>
                        <goals> <goal>testCompile</goal> </goals>
                    </execution>
                </executions>
            </plugin>
            <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-shade-plugin</artifactId>
                <version>2.3</version>
                <executions>
                    <execution>
                        <phase>package</phase>
                        <goals>
                            <goal>shade</goal>
                        </goals>
                        <configuration>
                            <transformers>
                                <transformer implementation="org.apache.maven.plugins.shade.resource.ManifestResourceTransformer">
                                    <manifestEntries>
                                        <Main-Class>io.vertx.core.Launcher</Main-Class>
                                        <Main-Verticle>${main.verticle}</Main-Verticle>
                                    </manifestEntries>
                                </transformer>
                                <transformer implementation="org.apache.maven.plugins.shade.resource.AppendingTransformer">
                                    <resource>META-INF/services/io.vertx.core.spi.VerticleFactory</resource>
                                </transformer>
                            </transformers>
                            <artifactSet>
                            </artifactSet>
                            <outputFile>${project.build.directory}/${project.artifactId}-${project.version}-fat.jar</outputFile>
                        </configuration>
                    </execution>
                </executions>
            </plugin>
            <plugin>
                <groupId>org.jetbrains.kotlin</groupId>
                <artifactId>kotlin-maven-plugin</artifactId>
                <version>${kotlin.version}</version>
                <configuration>
                    <jvmTarget>1.8</jvmTarget>
                </configuration>
                <executions>
                    <execution>
                        <id>compile</id>
                        <phase>compile</phase>
                        <goals>
                            <goal>compile</goal>
                        </goals>
                        <configuration>
                            <sourceDirs>
                                <source>src/main/java</source>
                                <source>src/main/kotlin</source>
                            </sourceDirs>
                        </configuration>
                    </execution>
                    <execution>
                        <id>test-compile</id>
                        <phase>test-compile</phase>
                        <goals>
                            <goal>test-compile</goal>
                        </goals>
                        <configuration>
                            <sourceDirs>
                                <source>src/test/java</source>
                                <source>src/test/kotlin</source>
                            </sourceDirs>
                        </configuration>
                    </execution>
                </executions>
            </plugin>
        </plugins>
    </build>

    <pluginRepositories>
        <pluginRepository>
            <id>kotlin-bintray</id>
            <name>Kotlin Bintray</name>
            <url>http://dl.bintray.com/kotlin/kotlin-dev</url>
            <releases>
                <enabled>true</enabled>
            </releases>
            <snapshots>
                <enabled>false</enabled>
            </snapshots>
        </pluginRepository>
    </pluginRepositories>

    <repositories>
        <repository>
            <id>sonatype-nexus-snapshots</id>
            <name>Sonatype Nexus Snapshots</name>
            <url>https://oss.sonatype.org/content/repositories/snapshots</url>
            <releases>
                <enabled>false</enabled>
            </releases>
            <snapshots>
                <enabled>true</enabled>
            </snapshots>
        </repository>
    </repositories>

</project>

The error message / relevant part of the stacktrace is:

java.lang.UnsupportedClassVersionError: org/sirix/access/conf/DatabaseConfiguration has been compiled by a more recent version of the Java Runtime (class file version 53.0), this version of the Java Runtime only recognizes class file versions up to 52.0

Answer 1

I suspect that the problem is in the Kotlin Maven plugin, which infers the -jvm-target argument value directly from maven.compiler.source / maven.compiler.target properties, not taking into account the fact that the Kotlin compiler only supports versions 1.6 and 1.8. The workaround would be to specify the JVM bytecode target version explicitly to 1.8, for example (note that 1.8-targeted bytecode will run on Java 9 just fine):

<plugin>
    <artifactId>kotlin-maven-plugin</artifactId>
    <groupId>org.jetbrains.kotlin</groupId>
    <version>${kotlin.version}</version>
    ...
    <configuration>
        <jvmTarget>1.8</jvmTarget>
    </configuration>
</plugin>

That said, I couldn't reproduce your issue on a small sample project, so if you have a minute, I'd very much appreciate if you reported an issue at https://kotl.in/issue