Java extension support

[fwcd]

Support vscode-java, so Java can see Kotlin classes and vice versa.

[mrjones2014]

@fwcd any suggestions on where to start for a PR for this? in its current state, how does the language server locate compiled files?

[fwcd]

@mrjones2014 Sure.

Currently, the language server uses Maven or Gradle to find a set of JAR paths:
https://github.com/fwcd/KotlinLanguageServer/blob/d9dee813be0e4cc1a1c9fc15a2f7ed8da99b15e2/src/main/kotlin/org/javacs/kt/classpath/findClassPath.kt#L46-L56

Although vscode-java is compatible with Maven and Gradle too, it still uses an Eclipse project model under the hood (because the Java language server is based upon Eclipse JDT), thus resulting is a project structure like this when using Gradle:

root
├───build | target         
│   └───classes            << Gradle's/Maven's class files (these are used by the Kotlin LS)
├───bin                    << Compiled class files (these are used by the Java LS)
├───build.gradle | pom.xml << Build file
├───.classpath             << Eclipse project classpath (used by the Java LS)
├───.project               << Eclipse project config (used by the Java LS)
└───src
    └───main
        ├───kotlin         << Kotlin sources
        └───java           << Java sources

To allow Kotlin to see Java classes, we need to add the bin directory somehow to the Kotlin LS classpath without interfering too much with the Maven/Gradle-specific logic.

Allowing Java to see Kotlin classes is probably trickier because we would need to modify the .classpath file in order for the Java LS to recognize the compiled Kotlin classes. Additionally, .kt-files that contain more than one top-level class can be split up in multiple .class-files after compilation, which is something the Java language server might have trouble with.

[mrjones2014]

@fwcd could this help?
https://github.com/Ruin0x11/intellij-lsp-server

[fwcd]

@mrjones2014 Although this project looks interesting, the main issue with intellij-lsp-server is that it requires IntelliJ to run side-by-side. Maybe it would be possible to isolate the IntelliJ-specific code, but that might add a lot of unnecessary complexity to the project.

[mrjones2014]

@fwcd hm yeah, good point. I'm really busy at work at the moment but eventually I'm gonna try to get around to making that PR.

[khe817]

@fwcd, any news on this? What can I do manually in the meanwhile to make Java see Kotlin?

Thanks,

[fwcd]

@khe817 Using Java classes from Kotlin will hopefully be possible soon (I've successfully experimented with this over at the kotlin-debug-adapter).

The other way around is a bit trickier, unfortunately, since it requires integration with the Java side and its project model (Eclipse .project and .classpath files).