TisonKun opened a new pull request #7302: [FLINK-11156] [tests] Reconcile powermock with JDK 9 URL: https://github.com/apache/flink/pull/7302 …Test ## What is the purpose of the change The issue is caused because powermock hasn't fully supported JDK 9+ yet. Specifically, when we `@PrepareForTest` on a class(`ZooKeeperCompletedCheckpointStore`) has a static final field initialized by `Comparator#comparing`, it causes `IncompatibleClassChangeError`. We can fallback to use the previous syntax as a workaround. Given https://github.com/powermock/powermock/issues/901 the author of powermock said "Right now the project is in frozen state because I do not have enough time.", we'd better fix the issue different from waiting to bump version. Here we have to use powermock for hijacking the constructor of `ZooKeeperStateHandleStore`. An alternative for resolving this issue is get rid of powermock, I have tried but ended with adding a testing constructor of `ZooKeeperStateHandleStore`, which looks quite hack for me. It may be better to introduce a builder for classes depending on `powemock#whennew` so we can get rid of it. ## Brief change log - fallback the implementation to previous style as a workaround ## Verifying this change This pull request itself is the test. Run `mvn test -Pjava9 -DfailIfNoTests=false -Dcheckstyle.skip=true -Dtest=org.apache.flink.runtime.checkpoint.ZooKeeperCompletedCheckpointStoreMockitoTest` for testing locally. ## Does this pull request potentially affect one of the following parts: - Dependencies (does it add or upgrade a dependency): (**no**) - The public API, i.e., is any changed class annotated with `@Public(Evolving)`: (**no**) - The serializers: (**no**) - The runtime per-record code paths (performance sensitive): (**no**) - Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Yarn/Mesos, ZooKeeper: (**no**) - The S3 file system connector: (**no**) ## Documentation - Does this pull request introduce a new feature? (**no**) - If yes, how is the feature documented? (**not applicable**) cc @GJL
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