It appears to me that the way I build mixed Java/Scala projects w/ Gradle is somewhat wrong.
I start off with a multimodule project with two submodules, named foo and bar.
In foo I have created a Java interface (in foo/src/main/java/a/b/Foo.java) and
I've created some Scala code which makes use of that Java interface
(in foo/src/main/scala/a/b/Foo.scala).
More specifically Foo.scala contains one trait, named FooComponent, and one object,
named FooComponentImpl.
In bar I have just some ScalaTest/JUnit testing code
(in bar/src/test/scala/a/b/FooComponentTest.scala).
Finally, this project can be both built w/ SBT, as follows:
$ sbt clean test package
and w/ Gradle (and in this case you'll spot immediately an issue), as follows:
$ gradle clean test assemble
In the Gradle case the test cannot be compiled as it cannot find the FooComponent
trait that it needs to mix-in. This is due to the fact that there's no Scala bytecode
which managed to get into the resulting .jar file (which can be located in
foo/build/libs/foo.jar).
Have a try with this:
$ mv foo/src/main/scala/a/b/Foo.scala foo/src/main/scala/a/b/Foos.scala
and then again:
$ gradle clean test assemble
:)