java8ify.gradle

import java.util.jar.JarFile;
import java.util.jar.JarOutputStream;
import org.gradle.api.artifacts.transform.TransformParameters;

def java8ified = Attribute.of('java8ified', Boolean)

configurations.all {
    attributes.attribute(java8ified, true)
}

dependencies {
    attributesSchema.attribute(java8ified)
    artifactTypes.getByName("jar") {
        attributes.attribute(java8ified, false)
    }

    registerTransform(Java8ify) {
        from.attribute(java8ified, false)
        to.attribute(java8ified, true)
    }
}

abstract class Java8ify implements TransformAction<TransformParameters.None> {
    @InputArtifact
    abstract Provider<FileSystemLocation> getInputArtifact()

    @Override
    void transform(TransformOutputs outputs) {
        def inputFile = inputArtifact.get().asFile
        def inputJar = new JarFile(inputFile)
        // Skip JARs without module info files
        if (!inputJar.stream().filter({entry -> entry.name.endsWith("module-info.class")}).findAny().isPresent()) {
            outputs.file(inputArtifact)
            return
        }
        println "java8ifying ${inputFile.name}"
        def outputFile = outputs.file("java8ified/${inputFile.name}")
        outputFile.withOutputStream { out ->
             def outputJar = new JarOutputStream(out)
             inputJar.entries().each { entry ->
                 if (entry.name.endsWith('module-info.class') || entry.name.startsWith('META-INF/versions/9'))
                     return
                outputJar.putNextEntry(entry)
                outputJar << inputJar.getInputStream(entry)
                outputJar.closeEntry()
             }
             outputJar.finish()
        }
    }
}