Re: Yet another ConcurrentModificationException question

Eric Sosman <esosman@ieee-dot-org.invalid>
Wed, 20 Aug 2008 08:26:06 -0400
Heiner K?cker wrote:

Eric Sosman schrieb

Heiner K?cker wrote:

Andy Chambers schrieb

Apologies for asking about this FAQ. I have looked up the relevant
java tutorial but I just want to check my understanding with the
following example.

Iterator oIterator = m_oCollection.iterator();
while( oIterator.hasNext()) {
           Element oElement = (Element);
           String sOIDValue =
           if( sOIDValue != null && sOIDValue.equals(sElementOID) ) {
               // We have a match
               return oElement;

If I know that neither getAttribute() or getOIDKey() can alter the
"structure" of m_oCollection, is it true to say that a
ConcurrentModificationException thrown from within the body of the
while loop above, *must* have been caused by another thread altering
the structure of m_oCollection?

So to prevent this from happening, I can make m_oCollection a
synchronized Vector (currently it is just a normal vector), and put
this while loop inside a synchronized block.

Use a for loop , no iterator.

    If you mean

for (Object obj : m_oCollection) {
    Element oElement = (Element)obj;
... there *is* an Iterator, hidden by the syntax but present anyhow.

I dont mean the for each loop.

Use the for loop:

for ( int i = 0; i < vector.size() ; i++ )
   final Element oElement;
    * synchronized block for check
    * vector size and get element in
    * one atomar operation
   synchronized( vector )
       if ( i < vector.size() )
           oElement = (Element)vector.get( i );

 > [...]

     Thanks for the clarification. I wouldn't recommend this
pattern, though, because other threads can still interfere
and there'll be no ConcurrentModificationException to alert
you to the interference.

     Here's the kind of scenario I'm thinking of: Suppose the
Vector holds "Alice", "Betty", "Carol", in that order. Now
consider two threads, T1 running the code above and T2 making
modifications to the Vector:

    T1: i = 0, synchronize, i<3 -> true, desynchronize
    T1: synchronize, i<3 -> true, get "Alice", desynchronize
    T2: synchronize, delete "Alice" and slide the
        remainder leftward, desynchronize
    T1: i = 1, synchronize, i<2 -> true, desynchronize
    T1: synchronize, i<2 ->true, get "Carol", desynchronize
    T1: i = 2, synchronize, i<2 -> false, desynchronize,
        loop exits

Observe that T1 has completely missed "Betty", even though
"Betty" was in the Vector the whole time and is in fact
still there.

     A related problem arises if T2 inserts "Adelaide" at the
start of the Vector after T1 has processed that position: T1
would process "Alice" twice. (With enough insertions and
unlucky timing, T1 could process "Alice" N times.)

     ConcurrentModificationException is not the end of the world,
but a red flag that warns you of a race condition in your code,
something that needs to be fixed. Suppressing the exception
means the code won't suddenly die, but that doesn't mean it's
living well ...

Eric Sosman

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