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POWaiting on a condition in a reentrant lock
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  1. POWaiting on a condition in a reentrant lock
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    copied!<p>The following code is taken from the <a href="http://docs.oracle.com/javase/1.5.0/docs/api/java/util/concurrent/locks/Condition.html" rel="noreferrer">JavaDoc of <code>Condition</code></a>:</p> <pre><code>class BoundedBuffer { final Lock lock = new ReentrantLock(); final Condition notFull = lock.newCondition(); final Condition notEmpty = lock.newCondition(); final Object[] items = new Object[100]; int putptr, takeptr, count; public void put(Object x) throws InterruptedException { lock.lock(); try { while (count == items.length) notFull.await(); items[putptr] = x; if (++putptr == items.length) putptr = 0; ++count; notEmpty.signal(); } finally { lock.unlock(); } } public Object take() throws InterruptedException { lock.lock(); try { while (count == 0) notEmpty.await(); Object x = items[takeptr]; if (++takeptr == items.length) takeptr = 0; --count; notFull.signal(); return x; } finally { lock.unlock(); } } } </code></pre> <p>Imagine 2 threads, <em>Consumer</em> and <em>Producer</em>, one using <code>take</code>, one <code>put</code> on a single instance of <code>BoundedBuffer</code>. </p> <p>Let's say <em>Consumer</em> goes first, runs <code>take()</code> in which he locks the <code>lock</code> and now loops on <code>notEmpty.await();</code>. </p> <p>How can now <em>Producer</em> possibly get into the <code>put()</code> method past locking the <code>lock</code>, which is already held by the <em>Consumer</em>?</p> <p>What am I missing here? Is the <code>lock</code> "temporarily released" while the thread is waiting on one of its conditions? And what does the <em>reentrancy</em> of the lock mean, exactly?</p>
 

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