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<TITLE>Creating a Bounded Work Queue [5.0] (Java Developers Almanac Example)
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    <B><A HREF="../java.util.concurrent/pkg.html">java.util.concurrent</A></B><font color="#666666" class="xsmall-font">
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  <h3>e1076. Creating a Bounded Work Queue [5.0]</h3>

A work queue is used to coordinate work between a <font color="#0066ff"><i>producer</i></font> and
a set of worker threads.  When some work needs to be performed, the
producer adds an object containing the work information to the work
queue.  One of the worker threads then removes the object from the
work queue and acts upon the information.

<P> If a bounded work queue is full, the producer will block when
it adds to the queue.  It remains blocked until a worker thread
removes an object from the queue. If the queue empty, a worker thread
retrieving work will block until an object is made available by the
producer.  See also <a href="../java.lang/WorkQueue.html" class="eglink"><b>e102</b> Implementing an Unbounded Work Queue</a>.


<pre>    // Create a bounded blocking queue of integers
    final int capacity = <font color="#0066ff"><i>10</i></font>;
    BlockingQueue&lt;Integer&gt; queue = new ArrayBlockingQueue&lt;Integer&gt;(capacity);
    
    // Create a set of worker threads
    final int numWorkers = <font color="#0066ff"><i>2</i></font>;
    Worker[] workers = new Worker[numWorkers];
    for (int i=0; i&lt;workers.length; i++) {
        workers[i] = new Worker(queue);
        workers[i].start();
    }
    
    try {
        // Add some work to the queue; block if the queue is full.
        // Note that null cannot be added to a blocking queue.
        for (int i=0; i&lt;100; i++) {
            queue.put(i);
        }
    
        // Add special end-of-stream markers to terminate the workers
        for (int i=0; i&lt;workers.length; i++) {
            queue.put(Worker.NO_MORE_WORK);
        }
    } catch (InterruptedException e) {
    }
</pre>
This worker thread removes an integer from the work queue and
calculates its square.

<pre>    class Worker extends Thread {
        // Special end-of-stream marker. If a worker retrieves
        // an Integer that equals this marker, the worker will terminate.
        static final Integer NO_MORE_WORK = new Integer(0);
    
        BlockingQueue&lt;Integer&gt; q;
    
        Worker(BlockingQueue&lt;Integer&gt; q) {
            this.q = q;
        }
        public void run() {
            try {
                while (true) {
                    // Retrieve an integer; block if the queue is empty
                    Integer x = q.take();
    
                    // Terminate if the end-of-stream marker was retrieved
                    if (x == NO_MORE_WORK) {
                        break;
                    }
    
                    // Compute the square of x
                    int y = x * x;
                }
            } catch (InterruptedException e) {
            }
        }
    }
</pre>

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