Support for multi-release JARs

[zyxist]

Multi-release JARs allow packaging multiple variants of individual classes that target different JDK versions. Currently, no build system supports them, and there is no agreed convention on a directory structure layout.

A closer look at similar Maven plugins shows that they favor the following layout:

• /src/main/java - base JDK,
• /src/main/java9 - only the classes with JDK9-specific implementation,
• /src/main/java10 - only the classes with JDK10-specific implementation,
• etc.

The support would have to look like this:

• running the compilation several times:
  • once for the base JDK,
  • then, once for each supported Java version.
• each compiler run would have to take the combined effect of all the previous compilations, and compile only the classes that have their own variants in the given JDK version,
• hooking into jar plugin to pack everything correctly,
• adding the necessary entry to the manifest,

The open question is: what to do with unit tests?

[sormuras]

Hi Tomasz, who needs unit tests?!

As mr-jars are only fully supported when they are packaged (i.e. not exploded in directory tree structure) one must execute test after packaging mr-jars.

Here is an example how to organize black box unit tests after the package phase is done: https://github.com/sormuras/testing-in-the-modular-world
Note that the module foo may have /src/main/java<version> folders.