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copied!<p>Java's <a href="http://java.sun.com/javase/7/docs/api/java/util/concurrent/locks/AbstractQueuedSynchronizer.html" rel="nofollow noreferrer"><code>AbstractQueuedSynchronizer</code></a> (and its sibling <a href="http://java.sun.com/javase/7/docs/api/java/util/concurrent/locks/AbstractQueuedLongSynchronizer.html" rel="nofollow noreferrer"><code>AbstractQueuedLongSynchronizer</code></a>) works similarly, or at least it <em>could</em> be implemented similarly. These types form the basis for several concurrency primitives in the Java library, such as <a href="http://java.sun.com/javase/7/docs/api/java/util/concurrent/locks/ReentrantLock.html" rel="nofollow noreferrer"><code>ReentrantLock</code></a> and <a href="http://java.sun.com/javase/7/docs/api/java/util/concurrent/FutureTask.html" rel="nofollow noreferrer"><code>FutureTask</code></a>.</p> <p>It works by way of using an atomic integer to represent state. A lock may define the value 0 as unlocked, and 1 as locked. Any thread wishing to acquire the lock attempts to change the lock state from 0 to 1 via an atomic <em>compare-and-set</em> operation; if the attempt fails, the current state is not 0, which means that the lock is owned by some other thread.</p> <p><code>AbstractQueuedSynchronizer</code> also facilitates waiting on locks and notification of <em>conditions</em> by maintaining <em>CLH queues</em>, which are lock-free linked lists representing the line of threads waiting either to acquire the lock or to receive notification via a condition. Such notification moves one or all of the threads waiting on the condition to the head of the queue of those waiting to acquire the related lock.</p> <p><em>Most</em> of this machinery can be implemented in terms of an atomic integer representing the state as well as a couple of atomic pointers for each waiting queue. The actual scheduling of which threads will contend to inspect and change the state variable (via, say, <a href="http://java.sun.com/javase/7/docs/api/java/util/concurrent/locks/AbstractQueuedSynchronizer.html#tryAcquire%28int%29" rel="nofollow noreferrer"><code>AbstractQueuedSynchronizer#tryAcquire(int)</code></a>) is outside the scope of such a library and falls to the host system's scheduler.</p>

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