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

    /**
     * toString contains toStrings of elements
     */
    @Test
    public void testToString() {
        BlockingQueue q = populatedQueue(SIZE);
        String s = q.toString();
        for (int i = 0; i < SIZE; ++i) {
            assertTrue(s.contains(String.valueOf(i)));
        }
    }

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    /**
     * offer transfers elements across Executor tasks
     */
    @Test
    public void testOfferInExecutor() {
        final BlockingQueue q = new LocalConcurrentBlockingObjectQueue(2);
        q.add(one);
        q.add(two);
        ExecutorService executor = Executors.newFixedThreadPool(2);
        final CheckedBarrier threadsStarted = new CheckedBarrier(2);
        executor.execute(new CheckedRunnable() {
            public void realRun() throws InterruptedException {
                assertFalse(q.offer(three));
                threadsStarted.await();
                assertTrue(q.offer(three, LONG_DELAY_MS, MILLISECONDS));
                assertEquals(0, q.remainingCapacity());
            }
        });


        executor.execute(new CheckedRunnable() {
            public void realRun() throws InterruptedException {
                threadsStarted.await();
                assertEquals(0, q.remainingCapacity());
                assertSame(one, q.take());
            }
        });


        joinPool(executor);
    }

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    /**
     * timed poll retrieves elements across Executor threads
     */
    @Test
    public void testPollInExecutor() {
        final BlockingQueue q = new LocalConcurrentBlockingObjectQueue(2);
        final CheckedBarrier threadsStarted = new CheckedBarrier(2);
        ExecutorService executor = Executors.newFixedThreadPool(2);
        executor.execute(new CheckedRunnable() {
            public void realRun() throws InterruptedException {
                assertNull(q.poll());
                threadsStarted.await();
                assertSame(one, q.poll(LONG_DELAY_MS, MILLISECONDS));
                checkEmpty(q);
            }
        });


        executor.execute(new CheckedRunnable() {
            public void realRun() throws InterruptedException {
                threadsStarted.await();
                q.put(one);
            }
        });


        joinPool(executor);
    }

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     * drainTo(c) empties queue into another collection c
     */
    @Test
    @Ignore
    public void testDrainTo() {
        BlockingQueue q = populatedQueue(SIZE);
        ArrayList l = new ArrayList();
        q.drainTo(l);
        assertEquals(0, q.size());
        assertEquals(SIZE, l.size());
        for (int i = 0; i < SIZE; ++i)
            assertEquals(l.get(i), new Integer(i));
        q.add(zero);
        q.add(one);
        assertFalse(q.isEmpty());
        assertTrue(q.contains(zero));
        assertTrue(q.contains(one));
        l.clear();
        q.drainTo(l);
        assertEquals(0, q.size());
        assertEquals(2, l.size());
        for (int i = 0; i < 2; ++i)
            assertEquals(l.get(i), new Integer(i));
    }

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     * drainTo empties full queue, unblocking a waiting put.
     */
    @Ignore
    @Test
    public void testDrainToWithActivePut() throws InterruptedException {
        final BlockingQueue q = populatedQueue(SIZE);
        Thread t = new Thread(new CheckedRunnable() {
            public void realRun() throws InterruptedException {
                q.put(new Integer(SIZE + 1));
            }
        });


        t.start();
        ArrayList l = new ArrayList();
        q.drainTo(l);
        assertTrue(l.size() >= SIZE);
        for (int i = 0; i < SIZE; ++i)
            assertEquals(l.get(i), new Integer(i));
        t.join();
        assertTrue(q.size() + l.size() >= SIZE);
    }

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     * drainTo(c, n) empties first min(n, size) elements of queue into c
     */
    @Ignore
    @Test
    public void testDrainToN() {
        BlockingQueue q = new LocalConcurrentBlockingObjectQueue(SIZE * 2);
        for (int i = 0; i < SIZE + 2; ++i) {
            for (int j = 0; j < SIZE; j++)
                assertTrue(q.offer(new Integer(j)));
            ArrayList l = new ArrayList();
            q.drainTo(l, i);
            int k = (i < SIZE) ? i : SIZE;
            assertEquals(k, l.size());
            assertEquals(SIZE - k, q.size());
            for (int j = 0; j < k; ++j)
                assertEquals(l.get(j), new Integer(j));
            while (q.poll() != null) ;
        }
    }

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    /**
     * Queue transitions from empty to full when elements added
     */
    @Test
    public void testEmptyFull() {
        BlockingQueue q = new LocalConcurrentBlockingObjectQueue(2);
        assertTrue(q.isEmpty());
        assertEquals(2, q.remainingCapacity());
        q.add(one);
        assertFalse(q.isEmpty());
        q.add(two);
        assertFalse(q.isEmpty());
        assertEquals(0, q.remainingCapacity());
        assertFalse(q.offer(three));
    }

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    /**
     * timed offer(null) throws NullPointerException
     */
    public void testTimedOfferNull() throws InterruptedException {
        final BlockingQueue q = emptyCollection();
        long startTime = System.nanoTime();
        try {
            q.offer(null, LONG_DELAY_MS, MILLISECONDS);
            shouldThrow();
        } catch (NullPointerException success) {
        }
        assertTrue(millisElapsedSince(startTime) < LONG_DELAY_MS);
    }

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    /**
     * remainingCapacity decreases on add, increases on remove
     */
    @Test
    public void testRemainingCapacity() {
        BlockingQueue q = populatedQueue(SIZE);
        for (int i = 0; i < SIZE; ++i) {
            assertEquals(i, q.remainingCapacity());
            assertEquals(SIZE - i, q.size());
            q.remove();
        }
        for (int i = 0; i < SIZE; ++i) {
            assertEquals(SIZE - i, q.remainingCapacity());
            assertEquals(i, q.size());
            q.add(new Integer(i));
        }
    }

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    /**
     * put(null) throws NullPointerException
     */
    public void testPutNull() throws InterruptedException {
        final BlockingQueue q = emptyCollection();
        try {
            q.put(null);
            shouldThrow();
        } catch (NullPointerException success) {
        }
    }

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