How does mixedJavaRelease work in a library?

[tobiasdiez]

I tried to add modularity.mixedJavaRelease 8 to my gradle.build file, but outgoingVariants still reports org.gradle.jvm.version = 14, and indeed the compiled version is not usable in JDK < 14. What I'm doing wrong?
See https://github.com/tobiasdiez/EasyBind/blob/45106f173f638a74320bf9a9df26d527a11f133a/build.gradle#L18-L23

Setting source/targetCompatibility to jdk 9 instead of using mixedJavaRelease works as expected (but of course doesn't provide jdk 8 support). Both cannot be combined since then I get the error "sourceCompatibility should not be set together with --release option".

[siordache]

I suppose that outgoingVariants is not able to detect the compile flags added or changed by the gradle-modules-plugin, so the reported versions are inaccurate. To check the versions of the classes in the generated JAR, I added the following task to build.gradle:

task printClassVersion(dependsOn: jar) {
    doLast {
        copy {
            from zipTree(tasks.jar.archiveFile.asFile)
            into "$buildDir/exploded"
        }
        file("$buildDir/exploded").eachFileRecurse(groovy.io.FileType.FILES) { f ->
            if(f.name.endsWith(".class")) {
                def dis = new DataInputStream(new FileInputStream(f))
                dis.skipBytes(6)
                def majorVersion = dis.readUnsignedShort()
                println "JavaVersion: ${majorVersion - 44} - $f"
            }
        }
    }
}

When using modularity.mixedJavaRelease 8, the above task reports JavaVersion: 8 for all classes in the JAR except module-info.class, which is 9. Therefore, I think that mixedJavaRelease works as expected.

However, you said that the compiled version is not usable in JDK < 14. Could this be because your library depends on JavaFX 14?

[tobiasdiez]

Thanks for looking into this.

When the library is used with JDK 12, the following error message is displayed

- Incompatible attribute:
                  - Required org.gradle.jvm.version '12' and found incompatible value '14'.

So this looks like gradle really thinks the library was compiled for jdk 14.

[siordache]

Not sure if it helps, but put this in your library's build.gradle:

configurations.each { cfg ->
    cfg.attributes {
        attribute(TargetJvmVersion.TARGET_JVM_VERSION_ATTRIBUTE, 8)
    }
}

[weisJ]

I too ran into this issue. This also confuses my IDE as it forces the language level to be set to the newest supported version of the compiler (not very helpful when trying to stay compatible with the chosen target).