Re: NPE in PriorityQueue.poll()

"Daniel Pitts" <>
16 Nov 2006 11:41:59 -0800
Twisted wrote:

This is a strange one. App just recovered gracefully from:

    at java.util.PriorityQueue.siftDownComparable(
    at java.util.PriorityQueue.siftDown(
    at java.util.PriorityQueue.poll(

The line of my own code that's involved is basically

Foo bar = baz.poll();

with baz an instance of PriorityQueue<Foo> and definitely not itself
null (and besides, the stack trace would have consisted of only the
last line if it were).

Looks like a library bug. JDK 1.6.0 -server -incgc -Xmx256 under WinXP
in case it matters, with the 1.6.0 standard library (including
PriorityQueue implementation).

Is the poll in a synchronized block like your add is?
Foo bar;
synchronized (queue) {
   bar = baz.poll()

You need to make sure all access to baz is syncronized if it is access
in more than one thread.

BTW, from the Javadoc (in 1.5.0):
 * <p> <strong>Note that this implementation is not
 * Multiple threads should not access a <tt>PriorityQueue</tt>
 * instance concurrently if any of the threads modifies the list
 * structurally. Instead, use the thread-safe {@link
 * java.util.concurrent.PriorityBlockingQueue} class.

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