Examples of java.util.concurrent.BlockingQueue

java.util.concurrent.BlockingQueue

A {@link java.util.Queue} that additionally supports operationsthat wait for the queue to become non-empty when retrieving an element, and wait for space to become available in the queue when storing an element.

{@code BlockingQueue} methods come in four forms, with different waysof handling operations that cannot be satisfied immediately, but may be satisfied at some point in the future: one throws an exception, the second returns a special value (either {@code null} or {@code false}, depending on the operation), the third blocks the current thread indefinitely until the operation can succeed, and the fourth blocks for only a given maximum time limit before giving up. These methods are summarized in the following table:

Summary of BlockingQueue methods
	Throws exception	Special value	Blocks	Times out
Insert	{@link #add add(e)}	{@link #offer offer(e)}	{@link #put put(e)}	{@link #offer(Object,long,TimeUnit) offer(e, time, unit)}
Remove	{@link #remove remove()}	{@link #poll poll()}	{@link #take take()}	{@link #poll(long,TimeUnit) poll(time, unit)}
Examine	{@link #element element()}	{@link #peek peek()}	not applicable	not applicable

A {@code BlockingQueue} does not accept {@code null} elements.Implementations throw {@code NullPointerException} on attemptsto {@code add}, {@code put} or {@code offer} a {@code null}. A {@code null} is used as a sentinel value to indicate failure of{@code poll} operations.

A {@code BlockingQueue} may be capacity bounded. At any giventime it may have a {@code remainingCapacity} beyond which noadditional elements can be {@code put} without blocking.A {@code BlockingQueue} without any intrinsic capacity constraints alwaysreports a remaining capacity of {@code Integer.MAX_VALUE}.

{@code BlockingQueue} implementations are designed to be usedprimarily for producer-consumer queues, but additionally support the {@link java.util.Collection} interface. So, for example, it ispossible to remove an arbitrary element from a queue using {@code remove(x)}. However, such operations are in general not performed very efficiently, and are intended for only occasional use, such as when a queued message is cancelled.

{@code BlockingQueue} implementations are thread-safe. Allqueuing methods achieve their effects atomically using internal locks or other forms of concurrency control. However, the bulk Collection operations {@code addAll}, {@code containsAll}, {@code retainAll} and {@code removeAll} arenot necessarily performed atomically unless specified otherwise in an implementation. So it is possible, for example, for {@code addAll(c)} to fail (throwing an exception) after addingonly some of the elements in {@code c}.

A {@code BlockingQueue} does not intrinsically supportany kind of "close" or "shutdown" operation to indicate that no more items will be added. The needs and usage of such features tend to be implementation-dependent. For example, a common tactic is for producers to insert special end-of-stream or poison objects, that are interpreted accordingly when taken by consumers.

Usage example, based on a typical producer-consumer scenario. Note that a {@code BlockingQueue} can safely be used with multipleproducers and multiple consumers.

  {@code}class Producer implements Runnable  private final BlockingQueue queue; Producer(BlockingQueue q) { queue = q; } public void run() { try { while (true) { queue.put(produce()); } } catch (InterruptedException ex) { ... handle ...} } Object produce() { ... } } class Consumer implements Runnable { private final BlockingQueue queue; Consumer(BlockingQueue q) { queue = q; } public void run() { try { while (true) { consume(queue.take()); } } catch (InterruptedException ex) { ... handle ...} } void consume(Object x) { ... } } class Setup { void main() { BlockingQueue q = new SomeQueueImplementation(); Producer p = new Producer(q); Consumer c1 = new Consumer(q); Consumer c2 = new Consumer(q); new Thread(p).start(); new Thread(c1).start(); new Thread(c2).start(); } }}

Memory consistency effects: As with other concurrent collections, actions in a thread prior to placing an object into a {@code BlockingQueue}happen-before actions subsequent to the access or removal of that element from the {@code BlockingQueue} in another thread.

This interface is a member of the Java Collections Framework. @since 1.5 @author Doug Lea @param < E> the type of elements held in this collection

    /**
     * clear removes all elements
     */
    @Test
    public void testClear() {
        BlockingQueue q = populatedQueue(SIZE);
        q.clear();
        assertTrue(q.isEmpty());
        assertEquals(0, q.size());
        assertEquals(SIZE, q.remainingCapacity());
        q.add(one);
        assertFalse(q.isEmpty());
        assertTrue(q.contains(one));
        q.clear();
        assertTrue(q.isEmpty());
    }

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    /**
     * containsAll(c) is true when c contains a subset of elements
     */
    @Test
    public void testContainsAll() {
        BlockingQueue q = populatedQueue(SIZE);
        LocalConcurrentBlockingObjectQueue p = new LocalConcurrentBlockingObjectQueue(SIZE);
        for (int i = 0; i < SIZE; ++i) {
            assertTrue(q.containsAll(p));
            assertFalse(p.containsAll(q));
            p.add(new Integer(i));
        }
        assertTrue(p.containsAll(q));
    }

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    /**
     * retainAll(c) retains only those elements of c and reports true if changed
     */
    @Test
    public void testRetainAll() {
        BlockingQueue q = populatedQueue(SIZE);
        BlockingQueue p = populatedQueue(SIZE);
        for (int i = 0; i < SIZE; ++i) {
            boolean changed = q.retainAll(p);
            if (i == 0)
                assertFalse(changed);
            else
                assertTrue(changed);


            assertTrue(q.containsAll(p));
            assertEquals(SIZE - i, q.size());
            p.remove();
        }
    }

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     * removeAll(c) removes only those elements of c and reports true if changed
     */
    @Test
    public void testRemoveAll() {
        for (int i = 1; i < SIZE; ++i) {
            BlockingQueue q = populatedQueue(SIZE);
            BlockingQueue p = populatedQueue(i);
            assertTrue(q.removeAll(p));
            assertEquals(SIZE - i, q.size());
            for (int j = 0; j < i; ++j) {
                Integer I = (Integer) (p.remove());
                assertFalse(q.contains(I));
            }
        }
    }

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    /**
     * toArray() contains all elements in FIFO order
     */
    @Test
    public void testToArray() {
        BlockingQueue q = new LocalConcurrentBlockingObjectQueue(SIZE);
        for (int i = 0; i < SIZE; i++) {
            checkToArray(q);
            q.add(i);
        }
        // Provoke wraparound
        for (int i = 0; i < SIZE; i++) {
            checkToArray(q);
            assertEquals(i, q.poll());
            checkToArray(q);
            q.add(SIZE + i);
        }
        for (int i = 0; i < SIZE; i++) {
            checkToArray(q);
            assertEquals(SIZE + i, q.poll());
        }
    }

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     * toArray(a) contains all elements in FIFO order
     */
    @Ignore
    @Test
    public void testToArray2() {
        BlockingQueue q = new LocalConcurrentBlockingObjectQueue(SIZE);
        for (int i = 0; i < SIZE; i++) {
            checkToArray2(q);
            q.add(i);
        }
        // Provoke wraparound
        for (int i = 0; i < SIZE; i++) {
            checkToArray2(q);
            assertEquals(i, q.poll());
            checkToArray2(q);
            q.add(SIZE + i);
        }
        for (int i = 0; i < SIZE; i++) {
            checkToArray2(q);
            assertEquals(SIZE + i, q.poll());
        }
    }

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    /**
     * toArray(incompatible array type) throws ArrayStoreException
     */
    @Test
    public void testToArray1_BadArg() {
        BlockingQueue q = populatedQueue(SIZE);
        try {
            q.toArray(new String[10]);
            shouldThrow();
        } catch (ArrayStoreException success) {
        }
    }

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    /**
     * iterator iterates through all elements
     */
    @Test
    public void testIterator() throws InterruptedException {
        BlockingQueue q = populatedQueue(SIZE);
        Iterator it = q.iterator();
        while (it.hasNext()) {
            assertEquals(it.next(), q.take());
        }
    }

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     * iterator.remove removes current element
     */
    @Ignore
    @Test
    public void testIteratorRemove() {
        final BlockingQueue q = new LocalConcurrentBlockingObjectQueue(3);
        q.add(two);
        q.add(one);
        q.add(three);


        Iterator it = q.iterator();
        it.next();
        it.remove();


        it = q.iterator();
        assertSame(it.next(), one);
        assertSame(it.next(), three);
        assertFalse(it.hasNext());
    }

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     * put blocks interruptibly waiting for take when full
     */
    @Test
    public void testPutWithTake() throws Exception {
        final int capacity = 2;
        final BlockingQueue q = new SharedConcurrentBlockingObjectQueue<Integer>(capacity, Integer.class);
        final CountDownLatch pleaseTake = new CountDownLatch(1);
        final CountDownLatch pleaseInterrupt = new CountDownLatch(1);
        Thread t = newStartedThread(new CheckedRunnable() {
            public void realRun() throws Exception {
                for (int i = 0; i < capacity; i++)
                    q.put(i);
                pleaseTake.countDown();
                q.put(86);


                pleaseInterrupt.countDown();
                try {
                    q.put(99);
                    shouldThrow();
                } catch (InterruptedException success) {
                }
                assertFalse(Thread.interrupted());
            }
        });


        await(pleaseTake);
        assertEquals(0, q.remainingCapacity());
        assertEquals(0, q.take());


        await(pleaseInterrupt);
        assertThreadStaysAlive(t);
        t.interrupt();
        awaitTermination(t);
        assertEquals(0, q.remainingCapacity());
    }

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