Add support for specifying the Java toolchain for invoking R8

It would be awesome if this:

defaulted to the project-wide java toolchain
one could override if needed

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

There's a comment in Gr8Task as well that this could be helpful -

gr8/plugin-common/src/main/kotlin/com/gradleup/gr8/Gr8Task.kt

Line 86 in c1a4086

    
                  * We might need an option to override that as if you're running newer versions of Java to run the task

Good callout 👍 . Any idea what options to expose override this? I guess it should take a path?

gr8 {
  javaHome.set("path/to/java/home")
}

ideally should expose setting the toolchain launcher itself, while defaulting to the toolchain provided by the Java extension by default, check the section "Toolchains for JVM projects" here:

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

that way if someone does something like

java {
  sourceCompatibility = JavaVersion.VERSION_11
  targetCompatibility = JavaVersion.VERSION_11

  toolchain {
    languageVersion.set(JavaLanguageVersion.of(11))
    vendor.set(JvmVendorSpec.AZUL)
  }
}

you can get that toolchain picked up automatically.

If you have a Property<JavaLauncher> you can turn it into a java.home File with something like:

val javaHome = launcher.get().executablePath.asFile.parentFile.parentFile

Property<JavaLauncher>

Thanks that sounds perfect 👍

Do we want to be able to run R8 using e.g. Java 20 while still passing it Java 11 runtime jars? That'd save launching a new JVM if your main build is using 20. But maybe it's not worth it?

I forked the task (only needed a handy R8 invoke task :)), that's exactly what I'm doing (invoking on Gradle the JVM Gradle is running on) while runtime jars are passed in from the launcher. I guess it would make sense to name it differently or actually use it for launching?

For invoking afaik the Worker APIs had nice facilities to create isolated tasks that run on a different JVM.

invoking on Gradle the JVM Gradle is running on while runtime jars are passed in from the launcher

Nice. I think that's a good default, no need to downgrade the JVM just to get the runtime jars.

I can make a PR for this later in the evening today

Awesome, thanks!

Closed with #16