JDK16 support in Gradle

[rieske]

We have an experimental pipeline set up that tests Gradle against an early access release of JDK16.

So far the Groovy build scripts are failing to compile using JDK16. The errors look to be coming from the asm library that is bundled in our groovy-all distribution.
asm library does not yet support JDK16. From this commit it looks like the support is being worked on and will come out with version 9.
We should keep an eye for the release and probably coordinate with Groovy team to see if they could incorporate the update in the next iteration of Groovy 2.5

This is the first issue that needs to be overcome to get visibility on other potential breakages.

[rieske]

A beta of asm version 9 with JDK16 support is now available: https://asm.ow2.io/versions.html

[dreis2211]

@rieske Since September ASM 9 should be available

[msgilligan]

Can JDK 16 support be added to Gradle 6.x?

[ljacomet]

See #15538 for existing problem with Java compilation

[big-guy]

We're waiting for the Groovy 3 upgrade #15566

[tbroyer]

Fwiw, besides Groovy 3 (already merged and available in 7.0-M2 AFAICT), there are other issues due to https://openjdk.java.net/jeps/396:

• using ProjectBuilder: https://scans.gradle.com/s/yzcxrlwrb5ote/tests/:test/net.ltgt.gradle.errorprone.ErrorProneOptionsTest/classMethod
• using configuration-cache: https://scans.gradle.com/s/yzcxrlwrb5ote/tests/:test/net.ltgt.gradle.errorprone.ErrorPronePluginIntegrationTest/is%20configuration-cache%20friendly (I use an AtomicBoolean in gradle-errorprone-plugin)

I was able to workaround the first one by passing --illegal-access=permit in the test task's jvmArgs: https://scans.gradle.com/s/dy7knrif4vt3w/tests (other failures besides "is configuration-cache friendly" are currently expected)