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POBlocking and simultaneously releasing a variable number of threads in Java
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copied!<p>I'm building a buffer in Java that will collect write actions requested by many threads, and flush them as a group, say, once per second. I'd like to give it a method called <code>waitForFlush</code> that will block and thread that calls it until the next flush event completes. Meanwhile, a separate, standalone thread is flushing and sleeping in a loop. So I'm basically looking for a concurrency structure or pattern that will allow many threads to block at a particular point, and then release them all simultaneously, and I'm finding that none of Java's builtin concurrency primitives are a really close match for this. The best I've come up with so far is wait/notifyAll, as follows:</p> <pre><code>public class Buffer { private volatile long lastFlushTime = System.currentTimeMillis(); private final Object flushMonitor = new Object(); public void waitForFlush() { long entryTime = System.currentTimeMillis(); synchronized(flushMonitor) { while(lastFlushTime <= entryTime) { flushMonitor.wait(); } } } public void flush() { // do flush stuff here synchronized(flushMonitor) { lastFlushTime = System.currentTimeMillis(); flushMonitor.notifyAll(); } } } </code></pre> <p>Although I think this would work fine in practice, the synchronization block in <code>waitForNotify()</code> still feels somewhat imperfect to me. Ideally, for this use case, you would be able to call <code>wait()</code> without synchronizing on the associated Object, and all of the blocked threads would be released at the precise same moment when <code>notifyAll()</code> is called, instead of having to exit the synchronized block one-by-one.</p> <p>So, in general, is there a better way to block and simultaneously release a variable number of threads than I've outlined above (the Semaphore and CountDownLatch classes would only work for for a fixed number of threads, I think)?</p>

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