Destination dir of compileJava changed too eagerly if Kotlin plugin applied

[tlinkowski]

I have a project where the Kotlin plugin is applied but there are no main Kotlin sources (only test Kotlin sources). However, applying this module plugin causes the destination directory of compileJava to be changed to that of compileKotlin which breaks my build.

The culprit in this plugin resides here:

gradle-modules-plugin/src/main/java/org/javamodularity/moduleplugin/tasks/CompileJavaTaskMutator.java

Lines 44 to 46 in 4014f02

	// https://github.com/java9-modularity/gradle-modules-plugin/issues/45
	helper().findTask(COMPILE_KOTLIN_TASK_NAME, AbstractCompile.class)
	.ifPresent(compileKotlin -> javaCompile.setDestinationDir(compileKotlin.getDestinationDir()));

And this is how I worked around this problem in my project:
https://github.com/tlinkowski/tlinkowski-superpom/blob/4a692ac44ba0a78dd838aacdb7055e50112f6dbf/subprojects/my-superpom-gradle-plugin/src/main/kotlin/pl/tlinkowski/gradle/my/superpom/internal/plugin/EagerDestinationDirModificationFixPlugin.kt#L51-L67

To sum up, it seems to me that it'd be safest if this plugin changed the destination dir of compileJava only if:

• compileKotlin is present (like now), and
• there are some sources in src/main/kotlin, and
• there's only module-info.java in src/main/java

[paulbakker]

What was the reason to change the java compile output in the first place?

[tlinkowski]

So that during the compilation of module-info.java, the Java compiler sees the packages exported by Kotlin (otherwise, javac complains on the first exports clause, claiming there's no such package).

However, in #99 and #101, we've established that we won't be changing the destination dir of compileJava (or any other task) at all. Instead, we'll add the destination dirs to modulepath (for compilation), or produce a special "merged" dir with outputs (for run/test).