java/scala interoperability

[barlevalon]

Currently, as I understand, the only way to build/package mixed java/scala projects is into two separate jars (with separate targets) for each language.
This means that dependencies between the two can only be one-way. This is currently a major hurdle for us at Wix as we try to adopt Bazel. We have many mixed java-scala projects with bi-directional dependencies, currently being built with Maven.
Perhaps I'm missing something and there is a way to build the java and scala classes "together"? If not, any plans to increase interoperability in the future? I'd be glad to work on it myself, just need to be pointed in the right direction.

[laszlocsomor]

Do you mean you'd want a library rule with both .java and .scala files in the srcs?

You've probably seen these, but just in case:

• Example use of the Scala rules and how they to interop with java_* rules: https://github.com/bazelbuild/bazel/blob/15e9a506118b4067655e5e7f72ff852be5b797ca/tools/build_defs/scala/test/BUILD
• Implementation of Scala rules: https://github.com/bazelbuild/bazel/blob/15e9a506118b4067655e5e7f72ff852be5b797ca/tools/build_defs/scala/scala.bzl
• My first guess where you'll need to start tinkering with the Scala rules' implementation:

  bazel/tools/build_defs/scala/scala.bzl

  Line 32 in 15e9a50

  jar cmf {manifest} {out} -C {out}_tmp .

[damienmg]

As I suggested in another issue, we could wrap scala file into a java_import or do something similar than the protobuf rules to avoid having too much of the java support to copy

[iirina]

I'll close this as duplicate of #970.