ThreadLocal usage in JsonFactory may cause memory leaks

[soldierkam]

JsonFactory assign BufferRecycler to thread local but does not remove it. Very often jackson-core is loaded by web application classloader so we have situation where this classloader is referenced from thread object. As a result, after undeploying web app, classes wouldn't be unloaded (or wil be unloaded after some time because of the SoftReference).

[cowtowncoder]

Right, SoftReference is intended as the mechanism for removing the dependency.
Reuse of buffers is a significant performance improvement for common usage, so I do not want to remove functionality itself.
But I am open to improvement ideas.

One possibility I could consider is adding a JsonFactory.Feature that would allow disabling this recycling. If this makes sense, it could be added in 2.6.

[cowtowncoder]

Actually, even if class eviction is delayed a bit, how big of a problem should this be? BufferRecycler only refers to native JDK types (byte and char arrays) and has no references to other Jackson classes.
So at most it would seem like it'd be able to retain BufferRecycler class and nothing else?

[cowtowncoder]

Added JsonFactory.Feature.USE_THREAD_LOCAL_FOR_BUFFER_RECYCLING, disabling of which will prevent recycling of BufferRecycler. Note that doing this is likely to have negative performance impact, especially for reading/writing small json documents.

[JasonHuangHuster]

Hi , i found my java server had frequent full gc these days .

1. i 've dump the heap ( jmap -histo:live) and found lots of char array objects in the memory (the whole live heap is 500M and char[] objects is 60% of it, that's 300M)
2. then i checked some char[] objects , they're json datas i put on zookeeper which is parsed by Jackson . and those objects are refferenced by ThreadLocal , and the content inside is the same . my server had 500 threads ,so the total char[] is quite a big usage .

IMHO , those json file are read only in my server, so singleton instance of each is fit in my case .
would you give me some tips for how to dealing with this ? tks advance ! 😃

[cowtowncoder]

@JasonHuangHuster for questions please use mailing lists; this is a closed issue.

As to memory usage, SoftReferences are automatically reclaimed by GCs, and not just OldGen ones so I don't think this usage is related.
jmap would include soft-reachable values but that doesn't tell much: they get evicted if there is any memory pressue.

But if you think it could help, try disabling JsonFactory.Feature.USE_THREAD_LOCAL_FOR_BUFFER_RECYCLING on JsonFactory used (and construct ObjectMapper with factory configured this way).