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Message-ID: <9623c9a50705230756x783c64a5te80d2cd5ba937720@mail.gmail.com>
Date: Wed, 23 May 2007 22:56:37 +0800
From: "Xiao-Feng Li" <xiaofeng.li@gmail.com>
To: dev@harmony.apache.org, "Mikhail Markov (JIRA)" <jira@apache.org>
Subject: Re: [jira] Commented: (HARMONY-3883)
 [classlib]WeakHashMap.keySet().toArray() intermittently throws
 NoSuchElementException in java.lang.ThreadTest of DRLVM kernel test
Cc: commits@harmony.apache.org
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References: <27580776.1179382216322.JavaMail.jira@brutus>
	 <20936777.1179931757727.JavaMail.jira@brutus>

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

Thanks,
xiaofeng

On 5/23/07, Mikhail Markov (JIRA) <jira@apache.org> wrote:
>
>     [ https://issues.apache.org/jira/browse/HARMONY-3883?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12498255 ]
>
> Mikhail Markov commented on HARMONY-3883:
> -----------------------------------------
>
> Hmm... null entries are allowed in the WeakHashMap. See the following valid code:
> import java.util.WeakHashMap;
>
> public class Test {
>     public static void main(String[] args)  throws Exception {
>         WeakHashMap map = new WeakHashMap();
>         map.put("1", null);
>         map.put(null, null);
>         map.put("2", null);
>         Object[] keys = map.keySet().toArray();
>
>         for (int i = 0; i < keys.length; ++i) {
>             System.out.println("key[" + i + "] = " + keys[i]);
>         }
>     }
> }
>
> So, the proposed idea does not work as it will remove valid null entry ...
>
> > [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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> -
> You can reply to this email to add a comment to the issue online.
>
>


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