Look for javac instead of java

[arlaneenalra]

There are some situations where using bin/java as a heuristic to find JAVA_HOME within a JDK package can result in mistakenly finding a JRE root instead. Looking for bin/javac fixes this since JREs do not have javac.

This should fix #97

Before this PR

The findJavaHome method could mistakenly find a JRE inside of the JDK packages instead of the JDK resulting in an unusable JAVA_HOME. This main impacted some older JDKs.

After this PR

==COMMIT_MSG==
Use bin/javac to find JAVA_HOME instead of bin/java to avoid accidentally installing a JRE instead of a JDK.
==COMMIT_MSG==

Possible downsides?

[palantirtech]

Thanks for your interest in palantir/gradle-jdks, @arlaneenalra! Before we can accept your pull request, you need to sign our contributor license agreement - just visit https://cla.palantir.com/ and follow the instructions. Once you sign, I'll automatically update this pull request.

[changelog-app]

Generate changelog in changelog/@unreleased

Type

• Feature
• Improvement
• Fix
• Break
• Deprecation
• Manual task
• Migration

Description

Use bin/javac to find JAVA_HOME instead of bin/java to avoid accidentally installing a JRE instead of a JDK.

Check the box to generate changelog(s)

• Generate changelog entry

[arlaneenalra]

Note: I'll sign the CLA .. but the rest of those items seem a bit much ..

[arlaneenalra]

Bump: What is needed to get this reviewed?

[jonahbeckford]

My error is different from the summary's linked issue #97, but I think this PR would fix the wrong selection of the JRE rather than the JDK when I'm running into with Java 8:

> Task :ffi-java:compileJava FAILED
Compiling with JDK Java compiler API.
FAILURE: Build failed with an exception.
* What went wrong:
Execution failed for task ':ffi-java:compileJava'.
> Could not find tools.jar. Please check that /root/.gradle/gradle-jdks/azul-zulu-8.72.0.17-8.0.382-569852f9b038db1a contains a valid JDK installation.

@arlaneenalra: It looks like you have to press two checkmark boxes ("Type: Fix" and "Check the box to generate changelog(s)"). Is that possible?

[arlaneenalra]

@arlaneenalra: It looks like you have to press two checkmark boxes ("Type: Fix" and "Check the box to generate changelog(s)"). Is that possible?

Maybe .. I have no idea how that changelog-app thing works and can't find the documentation in my cursory glance online .. I'll take a crack at adding what I think might be the correct file ...