Multi-Release jars in gradle

Question

I have been scratching my head for awhile trying to set up my library to use multi-release jars to use java 9+ features with backup java 8 implementations. However, it is only needed for a module of my project.

My current build.gradle for the module looks like this:

apply plugin: 'java'
apply plugin: 'java-library'

dependencies {
    compile project(":common")

    compile 'org.ow2.asm:asm:6.2'
    compile 'org.ow2.asm:asm-util:6.2'

    testCompile "junit:junit:$junit_version"
    testCompileOnly "com.google.auto.service:auto-service:$autoservice_version"

    java9Implementation files(sourceSets.main.output.classesDirs) { builtBy compileJava }
}

compileJava {
    sourceCompatibility = 8
    targetCompatibility = 8
    options.compilerArgs.addAll(['--release', '8'])
}

compileJava9Java {
    sourceCompatibility = 9
    targetCompatibility = 9
    options.compilerArgs.addAll(['--release', '9'])
}

sourceSets {
    java9 {
        java {
            srcDirs = ['src/main/java9']
        }
    }

}

jar {
    into('META-INF/versions/9') {
        from sourceSets.java9.output
    }

    manifest {
        attributes(
                "Manifest-Version": "1.0",
                "Multi-Release": true
        )
    }
}

However refreshing my build.gradle in intellij is getting me this error: Could not find method java9Implementation() for arguments [file collection] on object of type org.gradle.api.internal.artifacts.dsl.dependencies.DefaultDependencyHandler.

I should also note this is on gradle 4.8.1.

Answer 1

You're problem is, that your sourceSet is defined after the dependencies block. Put it above dependencies and it should work.

The reference you used also explains it:

A source set represents a set of sources that are going to be compiled together. A jar is built from the result of the compilation of one or more source sets. For each source set, Gradle will automatically create a corresponding compile task that you can configure. This means that if we have sources for Java 8 and sources for Java 9, then they should live in separate source sets. That’s what we do by creating a Java 9 specific source set that will contain the specialized version of our class.

So before you can use 'java9'Implementation, gradle needs to know what 'java9' is supposed to mean.