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I have a java application "with history" and it uses WeakReferences for caching. I made several heapdumps and saw that all of them contains a lot of objects with weak references (10%-15% of heap size, ~1.2GB).

  • Does it mean that weak references produce memory pressure on JVM?
  • And forces a FullGC with stop-the-world pauses?

P.S. I know that WeakReference produces performance penalty for GC for CMS, because it makes algorithm harder. But did somebody see a paper or some official information about it? I found only this SO post.

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Object Computing, Inc. did a presentation on this that I've found useful in the past. Here's an excerpt:

Kinds of Object References

  • Strong references
  • SoftReference
  • GC’ed any time after there are no strong references to the referent, but is typically retained until memory is low
  • can be used to implement caches of objects that can be recreated if needed

WeakRefernence

  • GC’ed any time after there are no strong or soft references to the referent
  • often used for “canonical mappings” where each object has a unique identifier (one-to-one), and in collections of “listeners”

"For soft and weak references, the get returns null method when the referent object has been GC’ed."

SOURCE: http://java.ociweb.com/mark/other-presentations/JavaGC.pdf

That seems to suggest SoftReference is the go to choice for cached objects.

In practice I have used Guava caching APIs and let it manage the details: https://github.com/google/guava/wiki/CachesExplained

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Thanks for your answer, but it doesn't answer on my question :) For cache I prefer Encache, because it has a lot of options, Beans, statistics, etc. But in case of this topic I'm interesting in question when GC removes objects with WeakReference.
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Does it mean that weak references produce memory pressure on JVM?

No. Weak references don't affect GC at all. They just give you a way to track it.

And forces a FullGC with stop-the-world pauses?

No.

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So from GC performance perspective there is no difference between using and not using WeakReference? And removing WeakReference doesn't increase performance?
And do you know when GC cleans up object with WeakReference? For example, in CMS algorithm.
I've already answered your first question. The answer to the second question is basically 'whenever it likes'.

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