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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 15:00:32 GMT

    [ https://issues.apache.org/jira/browse/HARMONY-3883?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12498262
] 

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

In that case, the java.lang.ThreadGroup.activeCount() should check if
the thread object is NULL, before invoking its isAlive() method.

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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