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From "Xiao-Feng Li (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HARMONY-3883) [classlib]WeakHashMap.keySet().toArray() intermittently throws NoSuchElementException in java.lang.ThreadTest of DRLVM kernel test
Date Wed, 23 May 2007 07:34:16 GMT

    [ https://issues.apache.org/jira/browse/HARMONY-3883?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12498142
] 

Xiao-Feng Li commented on HARMONY-3883:
---------------------------------------

To correct the behavior, probably we can loop in the addNext() to find
the first non-NULL value. When a NULL is met, just loop back to get
next entry. Something like:
         public boolean addNext() {
             if (expectedModCount == modCount) {
                 while (hasNext()) {
                     currentEntry = nextEntry;
                     nextEntry = currentEntry.next;
                     R result = type.get(currentEntry);
                     // free the key
                     if(result == null) continue;   // <--- add this line
                     nextKey = null;
                     return TRUE;
                 }
                 return FALSE
             }
             throw new ConcurrentModificationException();
         }

Note the code is only for example of the idea, not tested.

I am not sure if we want to modify Next() similarly.

Thanks,
xiaofeng



-- 
http://xiao-feng.blogspot.com


> [classlib]WeakHashMap.keySet().toArray() intermittently throws NoSuchElementException
in java.lang.ThreadTest of DRLVM kernel test
> ----------------------------------------------------------------------------------------------------------------------------------
>
>                 Key: HARMONY-3883
>                 URL: https://issues.apache.org/jira/browse/HARMONY-3883
>             Project: Harmony
>          Issue Type: Bug
>          Components: Classlib
>         Environment: win/linux 32/64
>            Reporter: Li-Gang Wang
>         Attachments: H-3883.patch, H-3883_add_alt1.patch, H-3883_add_alt2.patch, WeakHashmapTest.java
>
>
> I found there is a design bug in WeakHashMap.keySet().toArray() .
> WeakHashMap.keySet().toArray() inherits AbstractCollection.toArray(). Its implementation
is as follows:
> 	public Object[] toArray() {
> 		int size = size(), index = 0;
> 		Iterator<?> it = iterator();
> 		Object[] array = new Object[size];
> 		while (index < size) {
>             array[index++] = it.next();
>         }
> 		return array;
> 	}
> After assigning 'size()' to 'size', the 'while' loop expects 'size' elements in the iterator
'it'. But actually GC might happen and clear some weak keys of the WeakHashMap instance in
the meantime. The iterator will skip the cleared weak keys in 'it.hasNext()' called by 'it.next()',
so the actual number of the elements in 'it' is smaller than 'size'. When 'it' runs out its
elements, 'it.next()' will throw a NoSuchElementException. Parts of HashIterator implementation
in WeakHashMap is as follows:
>         public boolean hasNext() {
>             if (nextEntry != null) {
>                 return true;
>             }
>             while (true) {
>                 if (nextEntry == null) {
>                     while (position < elementData.length) {
>                         if ((nextEntry = elementData[position++]) != null) {
>                             break;
>                         }
>                     }
>                     if (nextEntry == null) {
>                         return false;
>                     }
>                 }
>                 // ensure key of next entry is not gc'ed
>                 nextKey = nextEntry.get();
>                 if (nextKey != null || nextEntry.isNull) {
>                     return true;
>                 }
>                 nextEntry = nextEntry.next;
>             }
>         }
>         public R next() {
>             if (expectedModCount == modCount) {
>                 if (hasNext()) {
>                     currentEntry = nextEntry;
>                     nextEntry = currentEntry.next;
>                     R result = type.get(currentEntry);
>                     // free the key
>                     nextKey = null;
>                     return result;
>                 }
>                 throw new NoSuchElementException();
>             }
>             throw new ConcurrentModificationException();
>         }
> I suspect the intermittent failure in java.lang.ThreadTest may disappear if this bug
is fixed.

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