Add support for Java 16

[dreis2211]

Hi,

I want to raise this issue to collect/track the needed things in order for JDK 16 to work.

• Add JavaVersion.SIXTEEN
• Adjust TestJarCreator multi-release jar tests
• Add build pipeline for JDK 16

Related/Required issues:

• Spring-Framework: Compatibility with JDK 16 spring-framework#25264
• Gradle: Support Java 16 gradle/gradle#13481

I would volunteer do this again for JDK 16 after working on JDK 14 & JDK 15 in the past. I would start - probably tomorrow - with fixing the first two points on this list. The pipeline task requires Gradle support for JDK 16, so this would need to wait a bit.

Let me know what you think.
Cheers,
Christoph

[snicoll]

Ah ah, @dreis2211 this is too funny. We've discussed that very topic on the team meeting yesterday :) Sure, that sounds great and thanks a lot for your efforts!

[wilkinsona]

I'd like to see us use Gradle's toolchain support so that we don't have to wait for their Java 16 support. We can run Gradle itself on Java 8 and compile and test with Java 16. This'll require our Java 16 CI image to contain JDK 8 and JDK 16.

[dreis2211]

I talked with @snicoll earlier. I'll see what I can come up with. My current plan is to essentially replace & adjust the functionality that I once introduced for a custom buildJavaHome property. E.g. introduce a new parameter toolchainVersion, which - if available - configures the toolchain stuff inside JavaConventions. I might need a little help in the end for the CI image stuff though.