IssueTracker

Since Gradle 2.1, incremental compilation of Java source is now supported [1].  It can be enabled/tested with:

    afterEvaluate {
        tasks.withType(JavaCompile) {
            println "Enabling " + name + ".options.incremental = true"
            options.incremental = true
            options.listFiles = true
        }
    }

When it's enabled on one of the Android javaCompile tasks, with e.g., "compileDebugJava.options.incremental=true", the gradle JavaCompile task reports:

    :compileDebugJava - is not incremental. Unable to infer the source directories.

Looking at the Gradle source, this happens because:

    if (!sourceDirs.areSourceDirsKnown()) {
        LOG.lifecycle("{} - is not incremental. Unable to infer the source directories.", displayName);
        return cleaningCompiler;
    }

And areSourceDirsKnown() returns false if *any* of the entries in compileDebugJava.source is not a SourceDirectorySet [2].  Inspecting `compileDebugJava.source` set reveals that it is a CompositeFileTree$FilteredFileTree, so the check for SourceDirectorySet fails, thus disabling incremental Java compilation.

A Robolectric task ("compileTestDebugJava", for example) shows that incremental support is working properly, as long as the JavaCompile task's source property is set with only SourceDirectorySet entries.

From the BasePlugin.groovy source, the task's Source is set with:

    compileTask.source = sourceList.toArray()

[1] http://www.gradle.org/docs/current/userguide/java_plugin.html#sec:incremental_compile
[2] https://github.com/gradle/gradle/blob/master/subprojects/language-java/src/main/java/org/gradle/api/internal/tasks/compile/incremental/CompilationSourceDirs.java#L45
[3] https://android.googlesource.com/platform/tools/build/+/master/gradle/src/main/groovy/com/android/build/gradle/BasePlugin.groovy#734