Testing no memory leak occurs via references

[Aleksei Ivanov]

Hello,

In clientlibs, there's occasionally a need to verify an object isn't 
leaked. For this purpose, WeakReference or PhantomReference is used.

Then, we need to make the reference object be cleared, so a GC cycle 
need to be triggered. The common approach is generating 
OutOfMemoryError, catching it and verifying whether the reference is 
cleared.

Some tests use a utility method regtesthelpers/Util.generateOOME [1].

For example, these tests follow the above approach:
https://github.com/openjdk/jdk/blob/master/test/jdk/javax/swing/border/TestTitledBorderLeak.java
https://github.com/openjdk/jdk/blob/master/test/jdk/java/awt/List/ListGarbageCollectionTest/AwtListGarbageCollectionTest.java


The AwtListGarbageCollectionTest.java test started to fail pretty often 
in the end of January 2023.

I followed a piece of advice provided in a JBS comment for JDK-8300727 
[2] and replaced generating OOME with a simple call to System.gc() along 
with adding a loop for re-trying.

The specification for System.gc() [3] mentions that this call can be 
ignored, which started a discussion in the PR #12594 [4] that 
System.gc() should not be used, at the very least without generating 
OOME in addition to invoking System.gc().

At the same time, many tests for Reference objects, such as 
ReferenceEnqueue.java [5] and PhantomReferentClearing.java [6], rely 
solely on System.gc.


What would be your recommendation? Are there best practices in core-libs 
and hotspot for testing for memory leaks that clientlibs should follow?


-- 
Regards,
Alexey

[1] 
https://github.com/openjdk/jdk/blob/29ee7c3b70ded8cd124ca5b4a38a2aee7c39068b/test/jdk/javax/swing/regtesthelpers/Util.java#L87
[2] https://bugs.openjdk.org/browse/JDK-8300727
[3] 
https://docs.oracle.com/en/java/javase/17/docs/api/java.base/java/lang/System.html#gc()
[4] https://github.com/openjdk/jdk/pull/12594
[5] 
https://github.com/openjdk/jdk/blob/f612dcfebea7ffd4390f833646ad45d6f0ebd04f/test/jdk/java/lang/ref/ReferenceEnqueue.java#L54-L60
[6] 
https://github.com/openjdk/jdk/blob/f612dcfebea7ffd4390f833646ad45d6f0ebd04f/test/jdk/java/lang/ref/PhantomReferentClearing.java#L85-L92
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