Override IndyBootstrapDispatcher class module

[eyalkoren]

What does this PR do?

Closes #1415
Supersedes #1465

Checklist

• This is a bugfix
  • I have updated CHANGELOG.asciidoc
  • I have added tests that would fail without this fix - we currently only have integration tests for J9 with Java 8
  • I added a unit test for the module override functionality
  • I tested manually that the fix works on the adoptopenjdk/openjdk11-openj9:jdk-11.0.1.13-alpine-slim docker image with Spring PetClinic
  • I tested manually that the fix works on the adoptopenjdk/openjdk9-openj9:jdk-9.0.4.12_openj9-0.9.0-alpine docker image with Spring PetClinic

[apmmachine]

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Build stats

• Build Cause: [Pull request #1468 updated]

• Start Time: 2020-11-04T14:43:23.735+0000

• Duration: 47 min 57 sec

Test stats 🧪

Test	Results
Failed	0
Passed	1618
Skipped	12
Total	1630

[SylvainJuge]

LGTM, only a few minor comments/questions.

[SylvainJuge]

[question] Would it be possible to have an exception thrown with an explicit message when this method is called and IndyBootstrapDispatcherModuleSetter hasn't been loaded from the right classloader ? If yes, that might help to diagnose any unexpected state in the field.

[eyalkoren]

This can't happen - the setJavaBaseModule method tries to load it from the bootstrap CL explicitly, so failing to inject it there will result with ClassNotFoundException (which I think is explanatory enough).
As you can see here, I do load it with the test's class loader, but its single method is never invoked on any class that is not loaded correctly.