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From "Jimmy,Jing Lv" <firep...@gmail.com>
Subject Re: [Luni][WeakReference]Avoid using strong-referenced objects as keys of WeakHashMap
Date Thu, 29 May 2008 11:05:22 GMT
Hi,

2008/5/28 Alexey Varlamov <alexey.v.varlamov@gmail.com>:
> Generally speaking, using WeakReference for caching is just wrong -
> cache entry will be removed as long as it's key has no more strong
> references thus such cache does not fulfil it's destination.
> SoftReference is more sensible for implementing caches as it will
> survive until memory is short (well, AFAIR drlvm does not obey this
> contract yet).
>

Agreed. However we have a WeakHashMap (but no SoftHashMap), so just
pick up and use.
And just a guess, I think this may be a kind of cache that keeps a
balance of memory and performance?
it lost its contents faster than SoftReference so that the heap won't
grow too large, and cache
the contents while its key is still strong-referenced (possibility of
re-using is still high).

> FYI, there is handy org.apache.harmony.luni.util.ThreadLocalCache
> which also should help to avoid synchronization overhead.
>
> Regards,
> Alexey
>
> 2008/5/28, Jimmy,Jing Lv <firepure@gmail.com>:
>> Hi,
>>
>>    I see a memory leak using Harmony Beans, looking into heap there
>> are many objects of WeakHashMap.Entry, and in hours they were never
>> GCed. And at last I find it was in java.beans.Introspector,  using a
>> WeakHashMap as a cache:
>>
>>    private static Map<Class<?>, StandardBeanInfo> theCache =
>> Collections.synchronizedMap(new WeakHashMap<Class<?>,
>> StandardBeanInfo>(DEFAULT_CAPACITY));
>>
>>    Unfortunately, we see it uses Class objects as WeakHashMap.Entry
>> key, which is always referenced by vm (only can be GCed when a class
>> is unloaded).  but WeakHashMap.Entry extends
>> java.lang.ref.WeakReference, which requires a weak reference as its
>> key, as a result, using Class object shall always keep Entries in
>> heap, cause a memory in that way.
>>    A quick search I also find in java.util.ResourceBundle, it defines
>> its cache as:
>>
>>    private static final WeakHashMap<Object, Hashtable<String,
>> ResourceBundle>> cache = new WeakHashMap<Object, Hashtable<String,
>> ResourceBundle>>();
>>
>>    and in code it uses ClassLoader objects as key.
>>    However as we know, ClassLoader objects are handled by vm as well
>> and may not be released, this as well may cause memory leak.
>>
>>    I suggest if we want to use WeakHashMap(and don't want to cause a
>> memory leak), we may set its key carefully and avoid using objects
>> that are referenced in other area (especially vm) that we can not
>> handle, like Class, etc. And I have a a little approach for String
>> objects, we know they are handled by vm with a pool, so directly use
>> String as key seems  also cause a leak, however if we put it into
>> WeakHashMap as:
>>
>>     WeakHashMap.put(new String(oldString), valueObject);
>>
>>    Run a few minutes I see no leak. Maybe some VM guru can tell us
>> what's going on with the string pool do in VM?
>>
>>    To fix these 2 problem, I suggest create a new inner class and use
>> it as Key of WeakHashMap (don't know if "new String(oldString)" should
>> work? ).
>>    And anyone saw the same problem in Harmony code? Any
>> comments/suggests? Thanks!
>>
>> --
>>
>> Best Regards!
>>
>> Jimmy, Jing Lv
>> China Software Development Lab, IBM
>>
>



-- 

Best Regards!

Jimmy, Jing Lv
China Software Development Lab, IBM

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