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copied!<p>If waiting for all tasks in the <code>ExecutorService</code> to finish isn't precisely your goal, but rather waiting until a <em>specific batch of tasks</em> has completed, you can use a <a href="http://docs.oracle.com/javase/8/docs/api/java/util/concurrent/CompletionService.html" rel="noreferrer"><code>CompletionService</code></a> — specifically, an <a href="http://docs.oracle.com/javase/8/docs/api/java/util/concurrent/ExecutorCompletionService.html" rel="noreferrer"><code>ExecutorCompletionService</code></a>.</p> <p>The idea is to create an <code>ExecutorCompletionService</code> wrapping your <code>Executor</code>, <a href="http://docs.oracle.com/javase/8/docs/api/java/util/concurrent/ExecutorCompletionService.html#submit(java.util.concurrent.Callable)" rel="noreferrer">submit</a> some <em>known number</em> of tasks through the <code>CompletionService</code>, then draw that <em>same number</em> of results from the <em>completion queue</em> using either <a href="http://docs.oracle.com/javase/8/docs/api/java/util/concurrent/ExecutorCompletionService.html#take()" rel="noreferrer"><code>take()</code></a> (which blocks) or <a href="http://docs.oracle.com/javase/8/docs/api/java/util/concurrent/ExecutorCompletionService.html#poll()" rel="noreferrer"><code>poll()</code></a> (which does not). Once you've drawn all the expected results corresponding to the tasks you submitted, you know they're all done.</p> <p>Let me state this one more time, because it's not obvious from the interface: You must know how many things you put into the <code>CompletionService</code> in order to know how many things to try to draw out. This matters especially with the <code>take()</code> method: call it one time too many and it will block your calling thread until some other thread submits another job to the same <code>CompletionService</code>.</p> <p>There are <a href="http://jcip.net.s3-website-us-east-1.amazonaws.com/listings/Renderer.java" rel="noreferrer">some examples showing how to use <code>CompletionService</code></a> in the book <a href="http://jcip.net/" rel="noreferrer"><em>Java Concurrency in Practice</em></a>.</p>

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