Cleanup logic in JavaInstanceRunnable close method

[srkukarni]

Motivation

For functions running in Threadruntime, javainstanceRunnable is potentially called at two places. Once inside the run loop of javainstancerunnable itself in the finally clause, and one outside in the threadruntime. The logic exists to make sure that its called atleast once. However often it ends up being called in two places. This causes some potential errors while doing source/sink closes since their classloading is unloaded in close, thus the second close potentially causes a classNotFoundError.
This pr modifies the logic of close to put back all variables to null, as well as adding a sync clause to make sure that these two calls don't step on each other.

Modifications

Describe the modifications you've done.

Verifying this change

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This change is a trivial rework / code cleanup without any test coverage.

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This change is already covered by existing tests, such as (please describe tests).

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• Added integration tests for end-to-end deployment with large payloads (10MB)
• Extended integration test for recovery after broker failure

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• Dependencies (does it add or upgrade a dependency): (yes / no)
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Documentation

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[jerrypeng]

can you add a comment to why this method needs to be synchronized for future reference

[srkukarni]

added comments

[merlimat]

Merged in 2.3.1 at
d1c17cd