RFR: 8280555: serviceability/sa/TestObjectMonitorIterate.java is failing due to ObjectMonitor referencing a null Object

[Chris Plummer]

This test is failing in the loom repo when using -Xcomp. The reason is because loom introduced doing a full GC in the codecache sweeper, which causes some of the Objects referenced by ObjectMonitors to be GC'd. The fix is to check for the null Objects so we don't get an NPE.

I'm choosing to fix this in the jdk repo rather than the loom repo since it is a latent bug that theoretically could occur even without the loom changes, and also to help reduce the amount of changes to be reviewed when loom is integrated into jdk.

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Commit messages:
 - Fix test to handle ObjectMonitors that reference a null Object.

Changes: https://git.openjdk.java.net/jdk/pull/7238/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7238&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8280555
  Stats: 7 lines in 1 file changed: 4 ins; 0 del; 3 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7238.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7238/head:pull/7238

PR: https://git.openjdk.java.net/jdk/pull/7238