Get rid of powermock

[tisonkun]

In this comment #16884 (comment) I wrote:

I don't like powermock as it lacks maintenance. It seems all usage of powermock is Whitebox. We can get rid of it with simple reflections. This can be a separate issue, though.

Thus I file this issue to investigate whether we can remove powermock from our testing framework.

WDTY @merlimat @lhotari @codelipenghui @nicoloboschi?

[shoothzj]

I think we can simply modify the access modifiers, and mark @VisableForTesting .We have used this in many codes.

[tisonkun]

@shoothzj IIUC do you suggest a replacement of powermock here?

[shoothzj]

@tisonkun Yes, I suggest to replace powermock. Using @VisableForTesting instead of reflections

[tisonkun]

@shoothzj thank you. I'll consider this approach when protptyping. There're some cases we try to hijack third-party lib's field, like eventLoopGroup in bookkeeper code. In those situations, we should still use reflections. (... or patching upstream, or change testing logics)

But yes, we can use multiple alternatives properly in each concrete case.

[tisonkun]

I draft an incompleted patch on #16997.

However, I notice that there's some code we hack in "set final" field, like:

pulsar/pulsar-broker/src/test/java/org/apache/pulsar/client/api/SimpleProducerConsumerTest.java

Lines 1071 to 1072 in 21634ed

	EntryCache entryCache = spy((EntryCache) Whitebox.getInternalState(ledger, "entryCache"));
	Whitebox.setInternalState(ledger, "entryCache", entryCache);

.. or even "set static final" field, like:

pulsar/pulsar-functions/runtime/src/test/java/org/apache/pulsar/functions/runtime/thread/ThreadRuntimeFactoryTest.java

Line 111 in 21634ed

Whitebox.setInternalState(DirectMemoryUtils.class, "JVM_MAX_DIRECT_MEMORY", 1024L);

It seems powermock-reflect is the best library existing to handle these cases since JDK12+ it's banned changing final field instead you have to use Unsafe. I tend not to maintain such unsafe hacks within Pulsar codebase. And perhaps we should gradually refactor code to avoid depending on such manners.

cc @shoothzj @eolivelli @nicoloboschi