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    <p>Just because you call <code>cancel()</code> on <code>Future</code> doesn't mean that the task will stop automatically. You have to do some work within the task to make sure that it will stop:</p> <ul> <li>Use <code>cancel(true)</code> so that an interrupt is sent to the task.</li> <li>Handle <code>InterruptedException</code>. If a function in your task throws an <code>InterruptedException</code>, make sure you exit gracefully as soon as possible upon catching the exception.</li> <li>Periodically check <code>Thread.currentThread().isInterrupted()</code> if the task does continuous computation.</li> </ul> <p>For example:</p> <pre><code>class LongTask implements Callable&lt;Double&gt; { public Double call() { // Sleep for a while; handle InterruptedException appropriately try { Thread.sleep(10000); } catch (InterruptedException ex) { System.out.println("Exiting gracefully!"); return null; } // Compute for a while; check Thread.isInterrupted() periodically double sum = 0.0; for (long i = 0; i &lt; 10000000; i++) { sum += 10.0 if (Thread.currentThread().isInterrupted()) { System.out.println("Exiting gracefully"); return null; } } return sum; } } </code></pre> <p>Also, as other posts have mentioned: <code>ConcurrentModificationException</code> can be thrown even if using the thread-safe <code>Vector</code> class, because iterators you obtain from <code>Vector</code> are not thread-safe, and thus need to be synchronized. The advanced for-loop uses iterators, so watch out:</p> <pre><code>final Vector&lt;Double&gt; vector = new Vector&lt;Double&gt;(); vector.add(1.0); vector.add(2.0); // Not thread safe! If another thread modifies "vector" during the loop, then // a ConcurrentModificationException will be thrown. for (Double num : vector) { System.out.println(num); } // You can try this as a quick fix, but it might not be what you want: synchronized (vector) { // "vector" must be final for (Double num : vector) { System.out.println(num); } } </code></pre>
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    1. USMatt Fichman
    1. USMatt Fichman
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    1. COExcellent - somehow I never came across Thread.interrupted() - I can use that tomorrow!
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      1. USKevin Day
    2. COFirst, calling future.cancel(true) does absolutely nothing. The contract of invokeAll states that it will cancel the tasks before returning, and the implementation uses a final block to ensure it. Second, do not ever call Thread.interrupted(), doing so clears the interrupted state of the thread. Most implementations would want to use Thread.isInterrupted(). Clearing the flag should be scrutinized. Third, he doesn't have to handle the InterruptedException unless he is using blocking methods such as lock acquisition, and then the compiler ensures that he is. The FutureTask will catch exceptions.
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      1. USTim Bender
    3. CO@Tim Bender: You're right: future.cancel(true) does nothing, tested by myself. But I haven't understood what you think I should do..
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