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


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

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    /**
     * drainTo(this) throws IllegalArgumentException
     */
    public void testDrainToSelf() {
        final BlockingQueue q = emptyCollection();
        try {
            q.drainTo(q);
            shouldThrow();
        } catch (IllegalArgumentException success) {
        }
    }

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    /**
     * Offer succeeds if not full; fails if full
     */
    @Test
    public void testOffer() {
        BlockingQueue q = new LocalConcurrentBlockingObjectQueue(1);
        assertTrue(q.offer(zero));
        assertFalse(q.offer(one));
    }

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    /**
     * drainTo(null, n) throws NullPointerException
     */
    public void testDrainToNullN() {
        final BlockingQueue q = emptyCollection();
        try {
            q.drainTo(null, 0);
            shouldThrow();
        } catch (NullPointerException success) {
        }
    }

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     * add succeeds if not full; throws ISE if full
     */
    @Test
    public void testAdd() {
        try {
            BlockingQueue q = new LocalConcurrentBlockingObjectQueue(SIZE);
            for (int i = 0; i < SIZE; ++i) {
                assertTrue(q.add(new Integer(i)));
            }
            assertEquals(0, q.remainingCapacity());
            q.add(new Integer(SIZE));
            shouldThrow();
        } catch (IllegalStateException success) {
        }
    }

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    /**
     * drainTo(this, n) throws IllegalArgumentException
     */
    public void testDrainToSelfN() {
        final BlockingQueue q = emptyCollection();
        try {
            q.drainTo(q, 0);
            shouldThrow();
        } catch (IllegalArgumentException success) {
        }
    }

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    /**
     * drainTo(c, n) returns 0 and does nothing when n <= 0
     */
    public void testDrainToNonPositiveMaxElements() {
        final BlockingQueue q = emptyCollection();
        final int[] ns = {0, -1, -42, Integer.MIN_VALUE};
        for (int n : ns)
            assertEquals(0, q.drainTo(new ArrayList(), n));
        if (q.remainingCapacity() > 0) {
            // Not SynchronousQueue, that is
            Object one = makeElement(1);
            q.add(one);
            ArrayList c = new ArrayList();
            for (int n : ns)
                assertEquals(0, q.drainTo(new ArrayList(), n));
            assertEquals(1, q.size());
            assertSame(one, q.poll());
            assertTrue(c.isEmpty());
        }
    }

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     * addAll(this) throws IAE
     */
    @Test
    public void testAddAllSelf() {
        try {
            BlockingQueue q = populatedQueue(SIZE);
            q.addAll(q);
            shouldThrow();
        } catch (IllegalArgumentException success) {
        }
    }

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    /**
     * timed poll before a delayed offer times out; after offer succeeds;
     * on interruption throws
     */
    public void testTimedPollWithOffer() throws InterruptedException {
        final BlockingQueue q = emptyCollection();
        final CheckedBarrier barrier = new CheckedBarrier(2);
        final Object zero = makeElement(0);
        Thread t = newStartedThread(new CheckedRunnable() {
            public void realRun() throws InterruptedException {
                long startTime = System.nanoTime();
                assertNull(q.poll(timeoutMillis(), MILLISECONDS));
                assertTrue(millisElapsedSince(startTime) >= timeoutMillis());


                barrier.await();


                assertSame(zero, q.poll(LONG_DELAY_MS, MILLISECONDS));


                Thread.currentThread().interrupt();
                try {
                    q.poll(LONG_DELAY_MS, MILLISECONDS);
                    shouldThrow();
                } catch (InterruptedException success) {
                }
                assertFalse(Thread.interrupted());


                barrier.await();
                try {
                    q.poll(LONG_DELAY_MS, MILLISECONDS);
                    shouldThrow();
                } catch (InterruptedException success) {
                }
                assertFalse(Thread.interrupted());
            }
        });


        barrier.await();
        long startTime = System.nanoTime();
        assertTrue(q.offer(zero, LONG_DELAY_MS, MILLISECONDS));
        assertTrue(millisElapsedSince(startTime) < LONG_DELAY_MS);


        barrier.await();
        assertThreadStaysAlive(t);
        t.interrupt();

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     * possibly adding some elements
     */
    @Test
    public void testAddAll3() {
        try {
            BlockingQueue q = new LocalConcurrentBlockingObjectQueue(SIZE);
            Integer[] ints = new Integer[SIZE];
            for (int i = 0; i < SIZE - 1; ++i)
                ints[i] = new Integer(i);
            q.addAll(Arrays.asList(ints));
            shouldThrow();
        } catch (NullPointerException success) {
        }
    }

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