CODEWITHSUNDEEP
javagarbage-collection
Jimilian
Jimilian

Reputation: 3919

When does GC remove objects that have only weak references?

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).

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.

Upvotes: 4

Views: 995

Answers (2)

user207421
user207421

Reputation: 310957

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.

Upvotes: 1

Alain O'Dea
Alain O'Dea

Reputation: 21696

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

Upvotes: 1

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