Add mechanism for different package managers to contribute their installed JVM toolchains

[big-guy]

Instead of embedding multiple implementations for each type of package manager (ASDF, SDKMAN!, etc) into Gradle itself, like repositories, users should be able to configure their build to look in different locations (or providers) for installed JVM toolchains.

As part of this, we would introduce a mechanism for users to configure this, deprecate the built-in mechanisms and eventually remove them.

[dkowis]

Good plan! Looking forward to being able to contribute something to do this :D

[kravemir]

Ideally, convention over configuration, something that gradle (wrapper) could automatically pick up from environment, without a need to modify anything in gradle files, i.e. not having to adjust gradle.properties to set org.gradle.java.installations.fromEnv=JDK8,JRE17.

Something like Linux's PATH environment variable would be nice to have - comma delimited paths. Maybe JAVA_HOMES, JAVA_PATH(S), GRADLE_JAVA_HOMES, GRADLE_JAVA_PATH(S) or else.

Regarding #29214 attempt for Nix. Per-directory setup can be achieved with tool like direnv with use nix, that would just set gradle's default env variable, and wrapper should work okay. For NixOS (#29214), the env var can be set globally.

[dkowis]

I would also love a way to do it via a gradle settings plugin, so that I don't have to rely on Environment variables that only get updated when the shell changes. As well, on OSX, where I do have to spend most of my professional time, it'd be nice if an update to things could be reflected immediately, rather than having to do shell commands.

Just some thoughts on this, because I'm missing it pretty hard right now.

[koppor]

@big-guy I would really like to test things here; we got an external PR from an OpenJDK developer, which relies on JDK-ea. Following more constraints it turns down to require this issue to be implemented to "easily" use an EA build of OpenJDK.

[koppor]

For reference: The currently available supported toolchains are described at https://docs.gradle.org/current/userguide/toolchains.html#sec:auto_detection