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POHow to ensure garbage collection of a FutureTask that is submitted to a ThreadPoolExecutor and then cancelled?
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copied!<p>I am submitting <code>Callable</code> objects to a <code>ThreadPoolExecutor</code> and they seem to be sticking around in memory.</p> <p>Looking at the heap dump with the MAT tool for Eclipse see that the <code>Callable</code> objects are being referenced by a <code>FutureTask$Sync</code>'s <strong>callable</strong> variable. That <code>FutureTask$Sync</code> is referenced by a <code>FutureTask</code>'s <strong>sync</strong> variable. That <code>FutureTask</code> is referenced by the <code>FutureTask$Sync</code>'s <strong>this$0</strong> variable.</p> <p>I have read around about this (<a href="http://stochastyk.blogspot.com/2006/12/memory-issues-using-java.html" rel="noreferrer">here</a>, <a href="http://markmail.org/message/3r3z6oxjkkiimjn6#query:+page:1+mid:45ouqntsrs72hyn6+state:results" rel="noreferrer">here</a>, and on SO) and it seems like the <code>FutureTask</code> that the callable is wrapped in upon the <code>ThreadPoolExecutor</code>'s submit() holds a reference to the callable forever.</p> <p>What I am confused about is how to ensure that the <code>FutureTask</code> gets garbage collected so it doesn't continue to hold the callable in memory, and hold anything the callable might be holding in memory?</p> <p>Just to give more details about my particular situation, I am trying to implement the <code>ThreadPoolExecutor</code> in a way that allows all of the submitted tasks to be canceled if needed. I have tried several different methods I found on SO and elsewhere, such as completely shutting the executor down (with <code>shutdown()</code>, <code>shutdownNow()</code> etc) and also keeping a list of the futures return by <code>submit()</code> and calling cancel on all them and then clearing the list of futures. Ideally I would like not to have to shut it down, and just <code>cancel()</code> and clear out when needed.</p> <p>All of these methods don't seem to make a difference. If I submit a callable to the pool, there is a good chance it will end up sticking around.</p> <p>What am I doing wrong?</p> <p>Thanks.</p> <p><strong>Edit:</strong></p> <p>As requested, here is the constructor for the ThreadPoolExecutor.</p> <pre><code>public ThreadPoolExecutor(int corePoolSize, int maximumPoolSize, long keepAliveTime, TimeUnit unit, BlockingQueue<Runnable> workQueue) { super(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue); } </code></pre> <p>After further testing I can see that if I let the tasks that have been submitted to the ThreadPoolExecutor finish, then there is no leak. If I try to cancel them in anyway such as:</p> <pre><code>shutdownNow() </code></pre> <p>Or saving a reference to the future and calling cancel on it later:</p> <pre><code>Future referenceToCancelLater = submit(task); ... referenceToCancelLater.cancel(false); </code></pre> <p>Or by removing them from the queue with methods like:</p> <pre><code>getQueue.drainTo(someList) </code></pre> <p>or</p> <pre><code>getQueue.clear() </code></pre> <p>or Looping through saved references to the futures and calling:</p> <pre><code>getQueue.remove(task) </code></pre> <p>Any of those cases causes the FutureTask to stick around as described above.</p> <p>So the real question in all of this is how to I properly cancel or remove items from a ThreadPoolExecutor so that the FutureTask is garbage collected and not leaked forever?</p>

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