Java tutorial
/* * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. * * This code is free software; you can redistribute it and/or modify it * under the terms of the GNU General Public License version 2 only, as * published by the Free Software Foundation. Oracle designates this * particular file as subject to the "Classpath" exception as provided * by Oracle in the LICENSE file that accompanied this code. * * This code is distributed in the hope that it will be useful, but WITHOUT * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License * version 2 for more details (a copy is included in the LICENSE file that * accompanied this code). * * You should have received a copy of the GNU General Public License version * 2 along with this work; if not, write to the Free Software Foundation, * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. * * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA * or visit www.oracle.com if you need additional information or have any * questions. */ /* * This file is available under and governed by the GNU General Public * License version 2 only, as published by the Free Software Foundation. * However, the following notice accompanied the original version of this * file: * * Written by Doug Lea, Bill Scherer, and Michael Scott with * assistance from members of JCP JSR-166 Expert Group and released to * the public domain, as explained at * http://creativecommons.org/publicdomain/zero/1.0/ */ package java.util.concurrent; import java.lang.invoke.MethodHandles; import java.lang.invoke.VarHandle; import java.util.AbstractQueue; import java.util.Collection; import java.util.Collections; import java.util.Iterator; import java.util.Objects; import java.util.Spliterator; import java.util.Spliterators; import java.util.concurrent.locks.LockSupport; import java.util.concurrent.locks.ReentrantLock; /** * A {@linkplain BlockingQueue blocking queue} in which each insert * operation must wait for a corresponding remove operation by another * thread, and vice versa. A synchronous queue does not have any * internal capacity, not even a capacity of one. You cannot * {@code peek} at a synchronous queue because an element is only * present when you try to remove it; you cannot insert an element * (using any method) unless another thread is trying to remove it; * you cannot iterate as there is nothing to iterate. The * <em>head</em> of the queue is the element that the first queued * inserting thread is trying to add to the queue; if there is no such * queued thread then no element is available for removal and * {@code poll()} will return {@code null}. For purposes of other * {@code Collection} methods (for example {@code contains}), a * {@code SynchronousQueue} acts as an empty collection. This queue * does not permit {@code null} elements. * * <p>Synchronous queues are similar to rendezvous channels used in * CSP and Ada. They are well suited for handoff designs, in which an * object running in one thread must sync up with an object running * in another thread in order to hand it some information, event, or * task. * * <p>This class supports an optional fairness policy for ordering * waiting producer and consumer threads. By default, this ordering * is not guaranteed. However, a queue constructed with fairness set * to {@code true} grants threads access in FIFO order. * * <p>This class and its iterator implement all of the <em>optional</em> * methods of the {@link Collection} and {@link Iterator} interfaces. * * <p>This class is a member of the * <a href="{@docRoot}/java.base/java/util/package-summary.html#CollectionsFramework"> * Java Collections Framework</a>. * * @since 1.5 * @author Doug Lea and Bill Scherer and Michael Scott * @param <E> the type of elements held in this queue */ public class SynchronousQueue<E> extends AbstractQueue<E> implements BlockingQueue<E>, java.io.Serializable { private static final long serialVersionUID = -3223113410248163686L; /* * This class implements extensions of the dual stack and dual * queue algorithms described in "Nonblocking Concurrent Objects * with Condition Synchronization", by W. N. Scherer III and * M. L. Scott. 18th Annual Conf. on Distributed Computing, * Oct. 2004 (see also * http://www.cs.rochester.edu/u/scott/synchronization/pseudocode/duals.html). * The (Lifo) stack is used for non-fair mode, and the (Fifo) * queue for fair mode. The performance of the two is generally * similar. Fifo usually supports higher throughput under * contention but Lifo maintains higher thread locality in common * applications. * * A dual queue (and similarly stack) is one that at any given * time either holds "data" -- items provided by put operations, * or "requests" -- slots representing take operations, or is * empty. A call to "fulfill" (i.e., a call requesting an item * from a queue holding data or vice versa) dequeues a * complementary node. The most interesting feature of these * queues is that any operation can figure out which mode the * queue is in, and act accordingly without needing locks. * * Both the queue and stack extend abstract class Transferer * defining the single method transfer that does a put or a * take. These are unified into a single method because in dual * data structures, the put and take operations are symmetrical, * so nearly all code can be combined. The resulting transfer * methods are on the long side, but are easier to follow than * they would be if broken up into nearly-duplicated parts. * * The queue and stack data structures share many conceptual * similarities but very few concrete details. For simplicity, * they are kept distinct so that they can later evolve * separately. * * The algorithms here differ from the versions in the above paper * in extending them for use in synchronous queues, as well as * dealing with cancellation. The main differences include: * * 1. The original algorithms used bit-marked pointers, but * the ones here use mode bits in nodes, leading to a number * of further adaptations. * 2. SynchronousQueues must block threads waiting to become * fulfilled. * 3. Support for cancellation via timeout and interrupts, * including cleaning out cancelled nodes/threads * from lists to avoid garbage retention and memory depletion. * * Blocking is mainly accomplished using LockSupport park/unpark, * except that nodes that appear to be the next ones to become * fulfilled first spin a bit (on multiprocessors only). On very * busy synchronous queues, spinning can dramatically improve * throughput. And on less busy ones, the amount of spinning is * small enough not to be noticeable. * * Cleaning is done in different ways in queues vs stacks. For * queues, we can almost always remove a node immediately in O(1) * time (modulo retries for consistency checks) when it is * cancelled. But if it may be pinned as the current tail, it must * wait until some subsequent cancellation. For stacks, we need a * potentially O(n) traversal to be sure that we can remove the * node, but this can run concurrently with other threads * accessing the stack. * * While garbage collection takes care of most node reclamation * issues that otherwise complicate nonblocking algorithms, care * is taken to "forget" references to data, other nodes, and * threads that might be held on to long-term by blocked * threads. In cases where setting to null would otherwise * conflict with main algorithms, this is done by changing a * node's link to now point to the node itself. This doesn't arise * much for Stack nodes (because blocked threads do not hang on to * old head pointers), but references in Queue nodes must be * aggressively forgotten to avoid reachability of everything any * node has ever referred to since arrival. */ /** * Shared internal API for dual stacks and queues. */ abstract static class Transferer<E> { /** * Performs a put or take. * * @param e if non-null, the item to be handed to a consumer; * if null, requests that transfer return an item * offered by producer. * @param timed if this operation should timeout * @param nanos the timeout, in nanoseconds * @return if non-null, the item provided or received; if null, * the operation failed due to timeout or interrupt -- * the caller can distinguish which of these occurred * by checking Thread.interrupted. */ abstract E transfer(E e, boolean timed, long nanos); } /** * The number of times to spin before blocking in timed waits. * The value is empirically derived -- it works well across a * variety of processors and OSes. Empirically, the best value * seems not to vary with number of CPUs (beyond 2) so is just * a constant. */ static final int MAX_TIMED_SPINS = (Runtime.getRuntime().availableProcessors() < 2) ? 0 : 32; /** * The number of times to spin before blocking in untimed waits. * This is greater than timed value because untimed waits spin * faster since they don't need to check times on each spin. */ static final int MAX_UNTIMED_SPINS = MAX_TIMED_SPINS * 16; /** * The number of nanoseconds for which it is faster to spin * rather than to use timed park. A rough estimate suffices. */ static final long SPIN_FOR_TIMEOUT_THRESHOLD = 1000L; /** Dual stack */ static final class TransferStack<E> extends Transferer<E> { /* * This extends Scherer-Scott dual stack algorithm, differing, * among other ways, by using "covering" nodes rather than * bit-marked pointers: Fulfilling operations push on marker * nodes (with FULFILLING bit set in mode) to reserve a spot * to match a waiting node. */ /* Modes for SNodes, ORed together in node fields */ /** Node represents an unfulfilled consumer */ static final int REQUEST = 0; /** Node represents an unfulfilled producer */ static final int DATA = 1; /** Node is fulfilling another unfulfilled DATA or REQUEST */ static final int FULFILLING = 2; /** Returns true if m has fulfilling bit set. */ static boolean isFulfilling(int m) { return (m & FULFILLING) != 0; } /** Node class for TransferStacks. */ static final class SNode { volatile SNode next; // next node in stack volatile SNode match; // the node matched to this volatile Thread waiter; // to control park/unpark Object item; // data; or null for REQUESTs int mode; // Note: item and mode fields don't need to be volatile // since they are always written before, and read after, // other volatile/atomic operations. SNode(Object item) { this.item = item; } boolean casNext(SNode cmp, SNode val) { return cmp == next && SNEXT.compareAndSet(this, cmp, val); } /** * Tries to match node s to this node, if so, waking up thread. * Fulfillers call tryMatch to identify their waiters. * Waiters block until they have been matched. * * @param s the node to match * @return true if successfully matched to s */ boolean tryMatch(SNode s) { if (match == null && SMATCH.compareAndSet(this, null, s)) { Thread w = waiter; if (w != null) { // waiters need at most one unpark waiter = null; LockSupport.unpark(w); } return true; } return match == s; } /** * Tries to cancel a wait by matching node to itself. */ void tryCancel() { SMATCH.compareAndSet(this, null, this); } boolean isCancelled() { return match == this; } // VarHandle mechanics private static final VarHandle SMATCH; private static final VarHandle SNEXT; static { try { MethodHandles.Lookup l = MethodHandles.lookup(); SMATCH = l.findVarHandle(SNode.class, "match", SNode.class); SNEXT = l.findVarHandle(SNode.class, "next", SNode.class); } catch (ReflectiveOperationException e) { throw new ExceptionInInitializerError(e); } } } /** The head (top) of the stack */ volatile SNode head; boolean casHead(SNode h, SNode nh) { return h == head && SHEAD.compareAndSet(this, h, nh); } /** * Creates or resets fields of a node. Called only from transfer * where the node to push on stack is lazily created and * reused when possible to help reduce intervals between reads * and CASes of head and to avoid surges of garbage when CASes * to push nodes fail due to contention. */ static SNode snode(SNode s, Object e, SNode next, int mode) { if (s == null) s = new SNode(e); s.mode = mode; s.next = next; return s; } /** * Puts or takes an item. */ @SuppressWarnings("unchecked") E transfer(E e, boolean timed, long nanos) { /* * Basic algorithm is to loop trying one of three actions: * * 1. If apparently empty or already containing nodes of same * mode, try to push node on stack and wait for a match, * returning it, or null if cancelled. * * 2. If apparently containing node of complementary mode, * try to push a fulfilling node on to stack, match * with corresponding waiting node, pop both from * stack, and return matched item. The matching or * unlinking might not actually be necessary because of * other threads performing action 3: * * 3. If top of stack already holds another fulfilling node, * help it out by doing its match and/or pop * operations, and then continue. The code for helping * is essentially the same as for fulfilling, except * that it doesn't return the item. */ SNode s = null; // constructed/reused as needed int mode = (e == null) ? REQUEST : DATA; for (;;) { SNode h = head; if (h == null || h.mode == mode) { // empty or same-mode if (timed && nanos <= 0L) { // can't wait if (h != null && h.isCancelled()) casHead(h, h.next); // pop cancelled node else return null; } else if (casHead(h, s = snode(s, e, h, mode))) { SNode m = awaitFulfill(s, timed, nanos); if (m == s) { // wait was cancelled clean(s); return null; } if ((h = head) != null && h.next == s) casHead(h, s.next); // help s's fulfiller return (E) ((mode == REQUEST) ? m.item : s.item); } } else if (!isFulfilling(h.mode)) { // try to fulfill if (h.isCancelled()) // already cancelled casHead(h, h.next); // pop and retry else if (casHead(h, s = snode(s, e, h, FULFILLING | mode))) { for (;;) { // loop until matched or waiters disappear SNode m = s.next; // m is s's match if (m == null) { // all waiters are gone casHead(s, null); // pop fulfill node s = null; // use new node next time break; // restart main loop } SNode mn = m.next; if (m.tryMatch(s)) { casHead(s, mn); // pop both s and m return (E) ((mode == REQUEST) ? m.item : s.item); } else // lost match s.casNext(m, mn); // help unlink } } } else { // help a fulfiller SNode m = h.next; // m is h's match if (m == null) // waiter is gone casHead(h, null); // pop fulfilling node else { SNode mn = m.next; if (m.tryMatch(h)) // help match casHead(h, mn); // pop both h and m else // lost match h.casNext(m, mn); // help unlink } } } } /** * Spins/blocks until node s is matched by a fulfill operation. * * @param s the waiting node * @param timed true if timed wait * @param nanos timeout value * @return matched node, or s if cancelled */ SNode awaitFulfill(SNode s, boolean timed, long nanos) { /* * When a node/thread is about to block, it sets its waiter * field and then rechecks state at least one more time * before actually parking, thus covering race vs * fulfiller noticing that waiter is non-null so should be * woken. * * When invoked by nodes that appear at the point of call * to be at the head of the stack, calls to park are * preceded by spins to avoid blocking when producers and * consumers are arriving very close in time. This can * happen enough to bother only on multiprocessors. * * The order of checks for returning out of main loop * reflects fact that interrupts have precedence over * normal returns, which have precedence over * timeouts. (So, on timeout, one last check for match is * done before giving up.) Except that calls from untimed * SynchronousQueue.{poll/offer} don't check interrupts * and don't wait at all, so are trapped in transfer * method rather than calling awaitFulfill. */ final long deadline = timed ? System.nanoTime() + nanos : 0L; Thread w = Thread.currentThread(); int spins = shouldSpin(s) ? (timed ? MAX_TIMED_SPINS : MAX_UNTIMED_SPINS) : 0; for (;;) { if (w.isInterrupted()) s.tryCancel(); SNode m = s.match; if (m != null) return m; if (timed) { nanos = deadline - System.nanoTime(); if (nanos <= 0L) { s.tryCancel(); continue; } } if (spins > 0) { Thread.onSpinWait(); spins = shouldSpin(s) ? (spins - 1) : 0; } else if (s.waiter == null) s.waiter = w; // establish waiter so can park next iter else if (!timed) LockSupport.park(this); else if (nanos > SPIN_FOR_TIMEOUT_THRESHOLD) LockSupport.parkNanos(this, nanos); } } /** * Returns true if node s is at head or there is an active * fulfiller. */ boolean shouldSpin(SNode s) { SNode h = head; return (h == s || h == null || isFulfilling(h.mode)); } /** * Unlinks s from the stack. */ void clean(SNode s) { s.item = null; // forget item s.waiter = null; // forget thread /* * At worst we may need to traverse entire stack to unlink * s. If there are multiple concurrent calls to clean, we * might not see s if another thread has already removed * it. But we can stop when we see any node known to * follow s. We use s.next unless it too is cancelled, in * which case we try the node one past. We don't check any * further because we don't want to doubly traverse just to * find sentinel. */ SNode past = s.next; if (past != null && past.isCancelled()) past = past.next; // Absorb cancelled nodes at head SNode p; while ((p = head) != null && p != past && p.isCancelled()) casHead(p, p.next); // Unsplice embedded nodes while (p != null && p != past) { SNode n = p.next; if (n != null && n.isCancelled()) p.casNext(n, n.next); else p = n; } } // VarHandle mechanics private static final VarHandle SHEAD; static { try { MethodHandles.Lookup l = MethodHandles.lookup(); SHEAD = l.findVarHandle(TransferStack.class, "head", SNode.class); } catch (ReflectiveOperationException e) { throw new ExceptionInInitializerError(e); } } } /** Dual Queue */ static final class TransferQueue<E> extends Transferer<E> { /* * This extends Scherer-Scott dual queue algorithm, differing, * among other ways, by using modes within nodes rather than * marked pointers. The algorithm is a little simpler than * that for stacks because fulfillers do not need explicit * nodes, and matching is done by CAS'ing QNode.item field * from non-null to null (for put) or vice versa (for take). */ /** Node class for TransferQueue. */ static final class QNode { volatile QNode next; // next node in queue volatile Object item; // CAS'ed to or from null volatile Thread waiter; // to control park/unpark final boolean isData; QNode(Object item, boolean isData) { this.item = item; this.isData = isData; } boolean casNext(QNode cmp, QNode val) { return next == cmp && QNEXT.compareAndSet(this, cmp, val); } boolean casItem(Object cmp, Object val) { return item == cmp && QITEM.compareAndSet(this, cmp, val); } /** * Tries to cancel by CAS'ing ref to this as item. */ void tryCancel(Object cmp) { QITEM.compareAndSet(this, cmp, this); } boolean isCancelled() { return item == this; } /** * Returns true if this node is known to be off the queue * because its next pointer has been forgotten due to * an advanceHead operation. */ boolean isOffList() { return next == this; } // VarHandle mechanics private static final VarHandle QITEM; private static final VarHandle QNEXT; static { try { MethodHandles.Lookup l = MethodHandles.lookup(); QITEM = l.findVarHandle(QNode.class, "item", Object.class); QNEXT = l.findVarHandle(QNode.class, "next", QNode.class); } catch (ReflectiveOperationException e) { throw new ExceptionInInitializerError(e); } } } /** Head of queue */ transient volatile QNode head; /** Tail of queue */ transient volatile QNode tail; /** * Reference to a cancelled node that might not yet have been * unlinked from queue because it was the last inserted node * when it was cancelled. */ transient volatile QNode cleanMe; TransferQueue() { QNode h = new QNode(null, false); // initialize to dummy node. head = h; tail = h; } /** * Tries to cas nh as new head; if successful, unlink * old head's next node to avoid garbage retention. */ void advanceHead(QNode h, QNode nh) { if (h == head && QHEAD.compareAndSet(this, h, nh)) h.next = h; // forget old next } /** * Tries to cas nt as new tail. */ void advanceTail(QNode t, QNode nt) { if (tail == t) QTAIL.compareAndSet(this, t, nt); } /** * Tries to CAS cleanMe slot. */ boolean casCleanMe(QNode cmp, QNode val) { return cleanMe == cmp && QCLEANME.compareAndSet(this, cmp, val); } /** * Puts or takes an item. */ @SuppressWarnings("unchecked") E transfer(E e, boolean timed, long nanos) { /* Basic algorithm is to loop trying to take either of * two actions: * * 1. If queue apparently empty or holding same-mode nodes, * try to add node to queue of waiters, wait to be * fulfilled (or cancelled) and return matching item. * * 2. If queue apparently contains waiting items, and this * call is of complementary mode, try to fulfill by CAS'ing * item field of waiting node and dequeuing it, and then * returning matching item. * * In each case, along the way, check for and try to help * advance head and tail on behalf of other stalled/slow * threads. * * The loop starts off with a null check guarding against * seeing uninitialized head or tail values. This never * happens in current SynchronousQueue, but could if * callers held non-volatile/final ref to the * transferer. The check is here anyway because it places * null checks at top of loop, which is usually faster * than having them implicitly interspersed. */ QNode s = null; // constructed/reused as needed boolean isData = (e != null); for (;;) { QNode t = tail; QNode h = head; if (t == null || h == null) // saw uninitialized value continue; // spin if (h == t || t.isData == isData) { // empty or same-mode QNode tn = t.next; if (t != tail) // inconsistent read continue; if (tn != null) { // lagging tail advanceTail(t, tn); continue; } if (timed && nanos <= 0L) // can't wait return null; if (s == null) s = new QNode(e, isData); if (!t.casNext(null, s)) // failed to link in continue; advanceTail(t, s); // swing tail and wait Object x = awaitFulfill(s, e, timed, nanos); if (x == s) { // wait was cancelled clean(t, s); return null; } if (!s.isOffList()) { // not already unlinked advanceHead(t, s); // unlink if head if (x != null) // and forget fields s.item = s; s.waiter = null; } return (x != null) ? (E) x : e; } else { // complementary-mode QNode m = h.next; // node to fulfill if (t != tail || m == null || h != head) continue; // inconsistent read Object x = m.item; if (isData == (x != null) || // m already fulfilled x == m || // m cancelled !m.casItem(x, e)) { // lost CAS advanceHead(h, m); // dequeue and retry continue; } advanceHead(h, m); // successfully fulfilled LockSupport.unpark(m.waiter); return (x != null) ? (E) x : e; } } } /** * Spins/blocks until node s is fulfilled. * * @param s the waiting node * @param e the comparison value for checking match * @param timed true if timed wait * @param nanos timeout value * @return matched item, or s if cancelled */ Object awaitFulfill(QNode s, E e, boolean timed, long nanos) { /* Same idea as TransferStack.awaitFulfill */ final long deadline = timed ? System.nanoTime() + nanos : 0L; Thread w = Thread.currentThread(); int spins = (head.next == s) ? (timed ? MAX_TIMED_SPINS : MAX_UNTIMED_SPINS) : 0; for (;;) { if (w.isInterrupted()) s.tryCancel(e); Object x = s.item; if (x != e) return x; if (timed) { nanos = deadline - System.nanoTime(); if (nanos <= 0L) { s.tryCancel(e); continue; } } if (spins > 0) { --spins; Thread.onSpinWait(); } else if (s.waiter == null) s.waiter = w; else if (!timed) LockSupport.park(this); else if (nanos > SPIN_FOR_TIMEOUT_THRESHOLD) LockSupport.parkNanos(this, nanos); } } /** * Gets rid of cancelled node s with original predecessor pred. */ void clean(QNode pred, QNode s) { s.waiter = null; // forget thread /* * At any given time, exactly one node on list cannot be * deleted -- the last inserted node. To accommodate this, * if we cannot delete s, we save its predecessor as * "cleanMe", deleting the previously saved version * first. At least one of node s or the node previously * saved can always be deleted, so this always terminates. */ while (pred.next == s) { // Return early if already unlinked QNode h = head; QNode hn = h.next; // Absorb cancelled first node as head if (hn != null && hn.isCancelled()) { advanceHead(h, hn); continue; } QNode t = tail; // Ensure consistent read for tail if (t == h) return; QNode tn = t.next; if (t != tail) continue; if (tn != null) { advanceTail(t, tn); continue; } if (s != t) { // If not tail, try to unsplice QNode sn = s.next; if (sn == s || pred.casNext(s, sn)) return; } QNode dp = cleanMe; if (dp != null) { // Try unlinking previous cancelled node QNode d = dp.next; QNode dn; if (d == null || // d is gone or d == dp || // d is off list or !d.isCancelled() || // d not cancelled or (d != t && // d not tail and (dn = d.next) != null && // has successor dn != d && // that is on list dp.casNext(d, dn))) // d unspliced casCleanMe(dp, null); if (dp == pred) return; // s is already saved node } else if (casCleanMe(null, pred)) return; // Postpone cleaning s } } // VarHandle mechanics private static final VarHandle QHEAD; private static final VarHandle QTAIL; private static final VarHandle QCLEANME; static { try { MethodHandles.Lookup l = MethodHandles.lookup(); QHEAD = l.findVarHandle(TransferQueue.class, "head", QNode.class); QTAIL = l.findVarHandle(TransferQueue.class, "tail", QNode.class); QCLEANME = l.findVarHandle(TransferQueue.class, "cleanMe", QNode.class); } catch (ReflectiveOperationException e) { throw new ExceptionInInitializerError(e); } } } /** * The transferer. Set only in constructor, but cannot be declared * as final without further complicating serialization. Since * this is accessed only at most once per public method, there * isn't a noticeable performance penalty for using volatile * instead of final here. */ private transient volatile Transferer<E> transferer; /** * Creates a {@code SynchronousQueue} with nonfair access policy. */ public SynchronousQueue() { this(false); } /** * Creates a {@code SynchronousQueue} with the specified fairness policy. * * @param fair if true, waiting threads contend in FIFO order for * access; otherwise the order is unspecified. */ public SynchronousQueue(boolean fair) { transferer = fair ? new TransferQueue<E>() : new TransferStack<E>(); } /** * Adds the specified element to this queue, waiting if necessary for * another thread to receive it. * * @throws InterruptedException {@inheritDoc} * @throws NullPointerException {@inheritDoc} */ public void put(E e) throws InterruptedException { if (e == null) throw new NullPointerException(); if (transferer.transfer(e, false, 0) == null) { Thread.interrupted(); throw new InterruptedException(); } } /** * Inserts the specified element into this queue, waiting if necessary * up to the specified wait time for another thread to receive it. * * @return {@code true} if successful, or {@code false} if the * specified waiting time elapses before a consumer appears * @throws InterruptedException {@inheritDoc} * @throws NullPointerException {@inheritDoc} */ public boolean offer(E e, long timeout, TimeUnit unit) throws InterruptedException { if (e == null) throw new NullPointerException(); if (transferer.transfer(e, true, unit.toNanos(timeout)) != null) return true; if (!Thread.interrupted()) return false; throw new InterruptedException(); } /** * Inserts the specified element into this queue, if another thread is * waiting to receive it. * * @param e the element to add * @return {@code true} if the element was added to this queue, else * {@code false} * @throws NullPointerException if the specified element is null */ public boolean offer(E e) { if (e == null) throw new NullPointerException(); return transferer.transfer(e, true, 0) != null; } /** * Retrieves and removes the head of this queue, waiting if necessary * for another thread to insert it. * * @return the head of this queue * @throws InterruptedException {@inheritDoc} */ public E take() throws InterruptedException { E e = transferer.transfer(null, false, 0); if (e != null) return e; Thread.interrupted(); throw new InterruptedException(); } /** * Retrieves and removes the head of this queue, waiting * if necessary up to the specified wait time, for another thread * to insert it. * * @return the head of this queue, or {@code null} if the * specified waiting time elapses before an element is present * @throws InterruptedException {@inheritDoc} */ public E poll(long timeout, TimeUnit unit) throws InterruptedException { E e = transferer.transfer(null, true, unit.toNanos(timeout)); if (e != null || !Thread.interrupted()) return e; throw new InterruptedException(); } /** * Retrieves and removes the head of this queue, if another thread * is currently making an element available. * * @return the head of this queue, or {@code null} if no * element is available */ public E poll() { return transferer.transfer(null, true, 0); } /** * Always returns {@code true}. * A {@code SynchronousQueue} has no internal capacity. * * @return {@code true} */ public boolean isEmpty() { return true; } /** * Always returns zero. * A {@code SynchronousQueue} has no internal capacity. * * @return zero */ public int size() { return 0; } /** * Always returns zero. * A {@code SynchronousQueue} has no internal capacity. * * @return zero */ public int remainingCapacity() { return 0; } /** * Does nothing. * A {@code SynchronousQueue} has no internal capacity. */ public void clear() { } /** * Always returns {@code false}. * A {@code SynchronousQueue} has no internal capacity. * * @param o the element * @return {@code false} */ public boolean contains(Object o) { return false; } /** * Always returns {@code false}. * A {@code SynchronousQueue} has no internal capacity. * * @param o the element to remove * @return {@code false} */ public boolean remove(Object o) { return false; } /** * Returns {@code false} unless the given collection is empty. * A {@code SynchronousQueue} has no internal capacity. * * @param c the collection * @return {@code false} unless given collection is empty */ public boolean containsAll(Collection<?> c) { return c.isEmpty(); } /** * Always returns {@code false}. * A {@code SynchronousQueue} has no internal capacity. * * @param c the collection * @return {@code false} */ public boolean removeAll(Collection<?> c) { return false; } /** * Always returns {@code false}. * A {@code SynchronousQueue} has no internal capacity. * * @param c the collection * @return {@code false} */ public boolean retainAll(Collection<?> c) { return false; } /** * Always returns {@code null}. * A {@code SynchronousQueue} does not return elements * unless actively waited on. * * @return {@code null} */ public E peek() { return null; } /** * Returns an empty iterator in which {@code hasNext} always returns * {@code false}. * * @return an empty iterator */ public Iterator<E> iterator() { return Collections.emptyIterator(); } /** * Returns an empty spliterator in which calls to * {@link Spliterator#trySplit() trySplit} always return {@code null}. * * @return an empty spliterator * @since 1.8 */ public Spliterator<E> spliterator() { return Spliterators.emptySpliterator(); } /** * Returns a zero-length array. * @return a zero-length array */ public Object[] toArray() { return new Object[0]; } /** * Sets the zeroth element of the specified array to {@code null} * (if the array has non-zero length) and returns it. * * @param a the array * @return the specified array * @throws NullPointerException if the specified array is null */ public <T> T[] toArray(T[] a) { if (a.length > 0) a[0] = null; return a; } /** * Always returns {@code "[]"}. * @return {@code "[]"} */ public String toString() { return "[]"; } /** * @throws UnsupportedOperationException {@inheritDoc} * @throws ClassCastException {@inheritDoc} * @throws NullPointerException {@inheritDoc} * @throws IllegalArgumentException {@inheritDoc} */ public int drainTo(Collection<? super E> c) { Objects.requireNonNull(c); if (c == this) throw new IllegalArgumentException(); int n = 0; for (E e; (e = poll()) != null; n++) c.add(e); return n; } /** * @throws UnsupportedOperationException {@inheritDoc} * @throws ClassCastException {@inheritDoc} * @throws NullPointerException {@inheritDoc} * @throws IllegalArgumentException {@inheritDoc} */ public int drainTo(Collection<? super E> c, int maxElements) { Objects.requireNonNull(c); if (c == this) throw new IllegalArgumentException(); int n = 0; for (E e; n < maxElements && (e = poll()) != null; n++) c.add(e); return n; } /* * To cope with serialization strategy in the 1.5 version of * SynchronousQueue, we declare some unused classes and fields * that exist solely to enable serializability across versions. * These fields are never used, so are initialized only if this * object is ever serialized or deserialized. */ @SuppressWarnings("serial") static class WaitQueue implements java.io.Serializable { } static class LifoWaitQueue extends WaitQueue { private static final long serialVersionUID = -3633113410248163686L; } static class FifoWaitQueue extends WaitQueue { private static final long serialVersionUID = -3623113410248163686L; } private ReentrantLock qlock; private WaitQueue waitingProducers; private WaitQueue waitingConsumers; /** * Saves this queue to a stream (that is, serializes it). * @param s the stream * @throws java.io.IOException if an I/O error occurs */ private void writeObject(java.io.ObjectOutputStream s) throws java.io.IOException { boolean fair = transferer instanceof TransferQueue; if (fair) { qlock = new ReentrantLock(true); waitingProducers = new FifoWaitQueue(); waitingConsumers = new FifoWaitQueue(); } else { qlock = new ReentrantLock(); waitingProducers = new LifoWaitQueue(); waitingConsumers = new LifoWaitQueue(); } s.defaultWriteObject(); } /** * Reconstitutes this queue from a stream (that is, deserializes it). * @param s the stream * @throws ClassNotFoundException if the class of a serialized object * could not be found * @throws java.io.IOException if an I/O error occurs */ private void readObject(java.io.ObjectInputStream s) throws java.io.IOException, ClassNotFoundException { s.defaultReadObject(); if (waitingProducers instanceof FifoWaitQueue) transferer = new TransferQueue<E>(); else transferer = new TransferStack<E>(); } static { // Reduce the risk of rare disastrous classloading in first call to // LockSupport.park: https://bugs.openjdk.java.net/browse/JDK-8074773 Class<?> ensureLoaded = LockSupport.class; } }