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Diffstat (limited to 'gcc-4.4.3/libjava/classpath/external/jsr166/java/util/concurrent/SynchronousQueue.java')
| -rw-r--r-- | gcc-4.4.3/libjava/classpath/external/jsr166/java/util/concurrent/SynchronousQueue.java | 1127 |
1 files changed, 1127 insertions, 0 deletions
diff --git a/gcc-4.4.3/libjava/classpath/external/jsr166/java/util/concurrent/SynchronousQueue.java b/gcc-4.4.3/libjava/classpath/external/jsr166/java/util/concurrent/SynchronousQueue.java new file mode 100644 index 000000000..e47e0401c --- /dev/null +++ b/gcc-4.4.3/libjava/classpath/external/jsr166/java/util/concurrent/SynchronousQueue.java @@ -0,0 +1,1127 @@ +/* + * 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/licenses/publicdomain + */ + +package java.util.concurrent; +import java.util.concurrent.locks.*; +import java.util.concurrent.atomic.*; +import java.util.*; + +/** + * 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 + * <tt>peek</tt> 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 + * <tt>poll()</tt> will return <tt>null</tt>. For purposes of other + * <tt>Collection</tt> methods (for example <tt>contains</tt>), a + * <tt>SynchronousQueue</tt> acts as an empty collection. This queue + * does not permit <tt>null</tt> 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 <tt>true</tt> 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}/../technotes/guides/collections/index.html"> + * 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 collection + */ +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. + */ + static abstract class Transferer { + /** + * 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 Object transfer(Object e, boolean timed, long nanos); + } + + /** The number of CPUs, for spin control */ + static final int NCPUS = Runtime.getRuntime().availableProcessors(); + + /** + * 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 maxTimedSpins = (NCPUS < 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 maxUntimedSpins = maxTimedSpins * 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 spinForTimeoutThreshold = 1000L; + + /** Dual stack */ + static final class TransferStack extends Transferer { + /* + * 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; + + /** Return 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; + } + + static final AtomicReferenceFieldUpdater<SNode, SNode> + nextUpdater = AtomicReferenceFieldUpdater.newUpdater + (SNode.class, SNode.class, "next"); + + boolean casNext(SNode cmp, SNode val) { + return (cmp == next && + nextUpdater.compareAndSet(this, cmp, val)); + } + + static final AtomicReferenceFieldUpdater<SNode, SNode> + matchUpdater = AtomicReferenceFieldUpdater.newUpdater + (SNode.class, SNode.class, "match"); + + /** + * 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 && + matchUpdater.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() { + matchUpdater.compareAndSet(this, null, this); + } + + boolean isCancelled() { + return match == this; + } + } + + /** The head (top) of the stack */ + volatile SNode head; + + static final AtomicReferenceFieldUpdater<TransferStack, SNode> + headUpdater = AtomicReferenceFieldUpdater.newUpdater + (TransferStack.class, SNode.class, "head"); + + boolean casHead(SNode h, SNode nh) { + return h == head && headUpdater.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. + */ + Object transfer(Object 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 <= 0) { // 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 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 (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. + */ + long lastTime = (timed)? System.nanoTime() : 0; + Thread w = Thread.currentThread(); + SNode h = head; + int spins = (shouldSpin(s)? + (timed? maxTimedSpins : maxUntimedSpins) : 0); + for (;;) { + if (w.isInterrupted()) + s.tryCancel(); + SNode m = s.match; + if (m != null) + return m; + if (timed) { + long now = System.nanoTime(); + nanos -= now - lastTime; + lastTime = now; + if (nanos <= 0) { + s.tryCancel(); + continue; + } + } + if (spins > 0) + 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 > spinForTimeoutThreshold) + 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; + } + } + } + + /** Dual Queue */ + static final class TransferQueue extends Transferer { + /* + * 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; + } + + static final AtomicReferenceFieldUpdater<QNode, QNode> + nextUpdater = AtomicReferenceFieldUpdater.newUpdater + (QNode.class, QNode.class, "next"); + + boolean casNext(QNode cmp, QNode val) { + return (next == cmp && + nextUpdater.compareAndSet(this, cmp, val)); + } + + static final AtomicReferenceFieldUpdater<QNode, Object> + itemUpdater = AtomicReferenceFieldUpdater.newUpdater + (QNode.class, Object.class, "item"); + + boolean casItem(Object cmp, Object val) { + return (item == cmp && + itemUpdater.compareAndSet(this, cmp, val)); + } + + /** + * Tries to cancel by CAS'ing ref to this as item. + */ + void tryCancel(Object cmp) { + itemUpdater.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; + } + } + + /** 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 cancelled. + */ + transient volatile QNode cleanMe; + + TransferQueue() { + QNode h = new QNode(null, false); // initialize to dummy node. + head = h; + tail = h; + } + + static final AtomicReferenceFieldUpdater<TransferQueue, QNode> + headUpdater = AtomicReferenceFieldUpdater.newUpdater + (TransferQueue.class, QNode.class, "head"); + + /** + * 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 && headUpdater.compareAndSet(this, h, nh)) + h.next = h; // forget old next + } + + static final AtomicReferenceFieldUpdater<TransferQueue, QNode> + tailUpdater = AtomicReferenceFieldUpdater.newUpdater + (TransferQueue.class, QNode.class, "tail"); + + /** + * Tries to cas nt as new tail. + */ + void advanceTail(QNode t, QNode nt) { + if (tail == t) + tailUpdater.compareAndSet(this, t, nt); + } + + static final AtomicReferenceFieldUpdater<TransferQueue, QNode> + cleanMeUpdater = AtomicReferenceFieldUpdater.newUpdater + (TransferQueue.class, QNode.class, "cleanMe"); + + /** + * Tries to CAS cleanMe slot. + */ + boolean casCleanMe(QNode cmp, QNode val) { + return (cleanMe == cmp && + cleanMeUpdater.compareAndSet(this, cmp, val)); + } + + /** + * Puts or takes an item. + */ + Object transfer(Object 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 <= 0) // 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)? 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)? 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, Object e, boolean timed, long nanos) { + /* Same idea as TransferStack.awaitFulfill */ + long lastTime = (timed)? System.nanoTime() : 0; + Thread w = Thread.currentThread(); + int spins = ((head.next == s) ? + (timed? maxTimedSpins : maxUntimedSpins) : 0); + for (;;) { + if (w.isInterrupted()) + s.tryCancel(e); + Object x = s.item; + if (x != e) + return x; + if (timed) { + long now = System.nanoTime(); + nanos -= now - lastTime; + lastTime = now; + if (nanos <= 0) { + s.tryCancel(e); + continue; + } + } + if (spins > 0) + --spins; + else if (s.waiter == null) + s.waiter = w; + else if (!timed) + LockSupport.park(this); + else if (nanos > spinForTimeoutThreshold) + 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 + } + } + } + + /** + * 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 transferer; + + /** + * Creates a <tt>SynchronousQueue</tt> with nonfair access policy. + */ + public SynchronousQueue() { + this(false); + } + + /** + * Creates a <tt>SynchronousQueue</tt> 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() : new TransferStack(); + } + + /** + * 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 o) throws InterruptedException { + if (o == null) throw new NullPointerException(); + if (transferer.transfer(o, 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 <tt>true</tt> if successful, or <tt>false</tt> if the + * specified waiting time elapses before a consumer appears. + * @throws InterruptedException {@inheritDoc} + * @throws NullPointerException {@inheritDoc} + */ + public boolean offer(E o, long timeout, TimeUnit unit) + throws InterruptedException { + if (o == null) throw new NullPointerException(); + if (transferer.transfer(o, 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 <tt>true</tt> if the element was added to this queue, else + * <tt>false</tt> + * @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 { + Object e = transferer.transfer(null, false, 0); + if (e != null) + return (E)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 <tt>null</tt> if the + * specified waiting time elapses before an element is present. + * @throws InterruptedException {@inheritDoc} + */ + public E poll(long timeout, TimeUnit unit) throws InterruptedException { + Object e = transferer.transfer(null, true, unit.toNanos(timeout)); + if (e != null || !Thread.interrupted()) + return (E)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 <tt>null</tt> if no + * element is available. + */ + public E poll() { + return (E)transferer.transfer(null, true, 0); + } + + /** + * Always returns <tt>true</tt>. + * A <tt>SynchronousQueue</tt> has no internal capacity. + * + * @return <tt>true</tt> + */ + public boolean isEmpty() { + return true; + } + + /** + * Always returns zero. + * A <tt>SynchronousQueue</tt> has no internal capacity. + * + * @return zero. + */ + public int size() { + return 0; + } + + /** + * Always returns zero. + * A <tt>SynchronousQueue</tt> has no internal capacity. + * + * @return zero. + */ + public int remainingCapacity() { + return 0; + } + + /** + * Does nothing. + * A <tt>SynchronousQueue</tt> has no internal capacity. + */ + public void clear() { + } + + /** + * Always returns <tt>false</tt>. + * A <tt>SynchronousQueue</tt> has no internal capacity. + * + * @param o the element + * @return <tt>false</tt> + */ + public boolean contains(Object o) { + return false; + } + + /** + * Always returns <tt>false</tt>. + * A <tt>SynchronousQueue</tt> has no internal capacity. + * + * @param o the element to remove + * @return <tt>false</tt> + */ + public boolean remove(Object o) { + return false; + } + + /** + * Returns <tt>false</tt> unless the given collection is empty. + * A <tt>SynchronousQueue</tt> has no internal capacity. + * + * @param c the collection + * @return <tt>false</tt> unless given collection is empty + */ + public boolean containsAll(Collection<?> c) { + return c.isEmpty(); + } + + /** + * Always returns <tt>false</tt>. + * A <tt>SynchronousQueue</tt> has no internal capacity. + * + * @param c the collection + * @return <tt>false</tt> + */ + public boolean removeAll(Collection<?> c) { + return false; + } + + /** + * Always returns <tt>false</tt>. + * A <tt>SynchronousQueue</tt> has no internal capacity. + * + * @param c the collection + * @return <tt>false</tt> + */ + public boolean retainAll(Collection<?> c) { + return false; + } + + /** + * Always returns <tt>null</tt>. + * A <tt>SynchronousQueue</tt> does not return elements + * unless actively waited on. + * + * @return <tt>null</tt> + */ + public E peek() { + return null; + } + + static class EmptyIterator<E> implements Iterator<E> { + public boolean hasNext() { + return false; + } + public E next() { + throw new NoSuchElementException(); + } + public void remove() { + throw new IllegalStateException(); + } + } + + /** + * Returns an empty iterator in which <tt>hasNext</tt> always returns + * <tt>false</tt>. + * + * @return an empty iterator + */ + public Iterator<E> iterator() { + return new EmptyIterator<E>(); + } + + /** + * Returns a zero-length array. + * @return a zero-length array + */ + public Object[] toArray() { + return new Object[0]; + } + + /** + * Sets the zeroeth element of the specified array to <tt>null</tt> + * (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; + } + + /** + * @throws UnsupportedOperationException {@inheritDoc} + * @throws ClassCastException {@inheritDoc} + * @throws NullPointerException {@inheritDoc} + * @throws IllegalArgumentException {@inheritDoc} + */ + public int drainTo(Collection<? super E> c) { + if (c == null) + throw new NullPointerException(); + if (c == this) + throw new IllegalArgumentException(); + int n = 0; + E e; + while ( (e = poll()) != null) { + c.add(e); + ++n; + } + return n; + } + + /** + * @throws UnsupportedOperationException {@inheritDoc} + * @throws ClassCastException {@inheritDoc} + * @throws NullPointerException {@inheritDoc} + * @throws IllegalArgumentException {@inheritDoc} + */ + public int drainTo(Collection<? super E> c, int maxElements) { + if (c == null) + throw new NullPointerException(); + if (c == this) + throw new IllegalArgumentException(); + int n = 0; + E e; + while (n < maxElements && (e = poll()) != null) { + c.add(e); + ++n; + } + 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. + */ + + 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; + + /** + * Save the state to a stream (that is, serialize it). + * + * @param s the stream + */ + 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(); + } + + private void readObject(final java.io.ObjectInputStream s) + throws java.io.IOException, ClassNotFoundException { + s.defaultReadObject(); + if (waitingProducers instanceof FifoWaitQueue) + transferer = new TransferQueue(); + else + transferer = new TransferStack(); + } + +} |