add support for Java toolchains

[madisp]

Add support for specifying the Java toolchain to use for the R8 invoke.
The JAVA_HOME of the specified toolchain will be passed in to the R8 invoke as library jars. If no toolchain is specified then the default one from Java plugin extension is used.

NOTE: requires bumping Gradle api to 6.7, potentially breaking change for downstream consumers?

[martinbonnin]

Very cool, thanks 👍 !

In the current state, usage should look like this, right?

gr8 {
  create("relocated") {
    toolchain(java.toolchain {
      languageVersion.set(JavaLanguageVersion.of(11))
    })
  }
}

Would it be possible to use a more java-like way to set it like so?

gr8 {
  create("relocated") {
    toolchain {
      languageVersion.set(JavaLanguageVersion.of(11))
    }
  }
}

requires bumping Gradle api to 6.7, potentially breaking change for downstream consumers?

I think this is fine. Users can stay on older version if really needed but I would expect most Gradle plugin developers to use more recent versions of Gradle.

[madisp]

Very cool, thanks 👍 !

In the current state, usage should look like this, right?

gr8 {
  create("relocated") {
    toolchain(java.toolchain {
      languageVersion.set(JavaLanguageVersion.of(11))
    })
  }
}

Would it be possible to use a more java-like way to set it like so?

gr8 {
  create("relocated") {
    toolchain {
      languageVersion.set(JavaLanguageVersion.of(11))
    }
  }
}

hm, I checked the docs and a better way is actually to ask for Launcher (or in this case Compiler) specifically:

https://docs.gradle.org/current/userguide/toolchains.html#toolchains_for_tasks

invoking java.toolchain {} would also modify your build toolchain. So perhaps something like this:

gr8 {
  create("relocated") {
    toolchain(javaToolchains.compilerFor {
        languageVersion.set(JavaLanguageVersion.of(11))
    })
  }
}

and a convenience method that takes the Action<JavaToolchainSpec> like so:

gr8 {
  create("relocated") {
    toolchain {
      languageVersion.set(JavaLanguageVersion.of(11))
    }
  }
}

Looking at your comment we could easily hide the former and expose the latter.

one thing that slightly bugs me here, the name toolchain in that case isn't very great and compiler wouldn't be straightforward as well. Would be ok to name it something like systemClassesToolchain instead? A bit longer but much more descriptive.

[martinbonnin]

invoking java.toolchain {} would also modify your build toolchain

Right, makes sense 👍

Would be ok to name it something like systemClassesToolchain instead?

Agreed, this is better 👍

[martinbonnin]

@madisp any objection to merging this as is? The plugin is still pre-1.0 so we can always change later and this is a nice change to ship

[madisp]

@martinbonnin no objections now :D

I updated to use the JavaCompiler instead of JavaLauncher, changed the configuration block to accept compiler directly and added a convenience method taking an Action<JavaToolchainSpec>.

Added a note at the end of the README.md as well.

[martinbonnin]

Thanks for the quick turnaround !