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copied!<p>The way an <code>ExecutorService</code> works is that when you call <code>invokeAll</code> it waits for all tasks to complete:</p> <blockquote> <p>Executes the given tasks, returning a list of Futures holding their status and results when all complete. Future.isDone() is true for each element of the returned list. <strong>Note that a completed task could have terminated either normally or by throwing an exception</strong>. The results of this method are undefined if the given collection is modified while this operation is in progress.<sup><a href="http://docs.oracle.com/javase/7/docs/api/java/util/concurrent/ExecutorService.html#invokeAll%28java.util.Collection%29">1</a></sup>(emphasis added)</p> </blockquote> <p>What that means is that your tasks are all done but some may have thrown an exception. This exception is part of the <code>Future</code> - calling <code>get</code> causes the exception to be rethrown wrapped in an <code>ExecutionException</code>.</p> <p>From you stacktrack</p> <pre><code>java.util.concurrent.ExecutionException: java.lang.NullPointerException at java.util.concurrent.FutureTask$Sync.innerGet(Unknown Source) at java.util.concurrent.FutureTask.get(Unknown Source) at ^^^ <-- from get </code></pre> <p>You can see that this is indeed the case. One of your tasks has failed with an NPE. The <code>ExecutorService</code> caught the exception and is telling you about it by throwing an <code>ExecutionException</code> when you call <code>Future.get</code>.</p> <p>Now, if you want to take tasks <em>as they complete</em> you need an <code>ExecutorCompletionService</code>. This acts as a <code>BlockingQueue</code> that will allow you to poll for tasks as and when they finish.</p> <pre><code>public static void main(String[] args) throws Exception { final ExecutorService executorService = Executors.newFixedThreadPool(10); final ExecutorCompletionService<String> completionService = new ExecutorCompletionService<>(executorService); executorService.submit(new Runnable() { @Override public void run() { for (int i = 0; i < 100; ++i) { try { final Future<String> myValue = completionService.take(); //do stuff with the Future final String result = myValue.get(); System.out.println(result); } catch (InterruptedException ex) { return; } catch (ExecutionException ex) { System.err.println("TASK FAILED"); } } } }); for (int i = 0; i < 100; ++i) { completionService.submit(new Callable<String>() { @Override public String call() throws Exception { if (Math.random() > 0.5) { throw new RuntimeException("FAILED"); } return "SUCCESS"; } }); } executorService.shutdown(); } </code></pre> <p>In this example I have one task that calls <code>take</code> on the <code>ExecutorCompletionService</code> which gets the <code>Future</code>s as they become available and then I submit tasks to the <code>ExecutorCompletionService</code>.</p> <p>This will allow you to get the failed task as soon as it fails rather than having to wait for all the tasks to either fail of succeed together.</p> <p>The only complication is that it is difficult to tell the polling thread that all the tasks are done as everything is now asynchronous. In this instance I have used the knowledge that 100 tasks will be submitted so that it only need to poll 100 times. A more general way would be to collect the <code>Future</code>s from the <code>submit</code> method as well and then loop over them to see if everything is completed.</p>

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