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From "Mikhail Markov (JIRA)" <j...@apache.org>
Subject [jira] Updated: (HARMONY-3883) [classlib]WeakHashMap.keySet().toArray() intermittently throws NoSuchElementException in java.lang.ThreadTest of DRLVM kernel test
Date Fri, 18 May 2007 09:25:16 GMT

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

Mikhail Markov updated HARMONY-3883:
------------------------------------

    Attachment: H-3883_add_alt2.patch
                H-3883_add_alt1.patch

I agree with your concerns.
Actually  I was thinking about this gap while preparing the previous patch but thought that
it's so smal that atomicity will never be broken.

I have 2 alternatives of fixing this:
1) The simplest and most obvious one: just add try/catch block to the toArray() methods (see
H-3883_add_alt1.patch)
2) Avoid catching exceptions by adding special private method ensuring operation atomicity
but not throwing NoSuchElementException (see H-3883_add_alt2.patch)

Which one do we prefer? (I personally like the 2nd one).


> [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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