rt/emul/compact/src/main/java/java/util/concurrent/locks/AbstractQueuedSynchronizer.java
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31 * Written by Doug Lea with assistance from members of JCP JSR-166
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36 package java.util.concurrent.locks;
38 import java.util.concurrent.*;
39 import java.util.concurrent.atomic.*;
40 import sun.misc.Unsafe;
43 * Provides a framework for implementing blocking locks and related
44 * synchronizers (semaphores, events, etc) that rely on
45 * first-in-first-out (FIFO) wait queues. This class is designed to
46 * be a useful basis for most kinds of synchronizers that rely on a
47 * single atomic <tt>int</tt> value to represent state. Subclasses
48 * must define the protected methods that change this state, and which
49 * define what that state means in terms of this object being acquired
50 * or released. Given these, the other methods in this class carry
51 * out all queuing and blocking mechanics. Subclasses can maintain
52 * other state fields, but only the atomically updated <tt>int</tt>
53 * value manipulated using methods {@link #getState}, {@link
54 * #setState} and {@link #compareAndSetState} is tracked with respect
57 * <p>Subclasses should be defined as non-public internal helper
58 * classes that are used to implement the synchronization properties
59 * of their enclosing class. Class
60 * <tt>AbstractQueuedSynchronizer</tt> does not implement any
61 * synchronization interface. Instead it defines methods such as
62 * {@link #acquireInterruptibly} that can be invoked as
63 * appropriate by concrete locks and related synchronizers to
64 * implement their public methods.
66 * <p>This class supports either or both a default <em>exclusive</em>
67 * mode and a <em>shared</em> mode. When acquired in exclusive mode,
68 * attempted acquires by other threads cannot succeed. Shared mode
69 * acquires by multiple threads may (but need not) succeed. This class
70 * does not "understand" these differences except in the
71 * mechanical sense that when a shared mode acquire succeeds, the next
72 * waiting thread (if one exists) must also determine whether it can
73 * acquire as well. Threads waiting in the different modes share the
74 * same FIFO queue. Usually, implementation subclasses support only
75 * one of these modes, but both can come into play for example in a
76 * {@link ReadWriteLock}. Subclasses that support only exclusive or
77 * only shared modes need not define the methods supporting the unused mode.
79 * <p>This class defines a nested {@link ConditionObject} class that
80 * can be used as a {@link Condition} implementation by subclasses
81 * supporting exclusive mode for which method {@link
82 * #isHeldExclusively} reports whether synchronization is exclusively
83 * held with respect to the current thread, method {@link #release}
84 * invoked with the current {@link #getState} value fully releases
85 * this object, and {@link #acquire}, given this saved state value,
86 * eventually restores this object to its previous acquired state. No
87 * <tt>AbstractQueuedSynchronizer</tt> method otherwise creates such a
88 * condition, so if this constraint cannot be met, do not use it. The
89 * behavior of {@link ConditionObject} depends of course on the
90 * semantics of its synchronizer implementation.
92 * <p>This class provides inspection, instrumentation, and monitoring
93 * methods for the internal queue, as well as similar methods for
94 * condition objects. These can be exported as desired into classes
95 * using an <tt>AbstractQueuedSynchronizer</tt> for their
96 * synchronization mechanics.
98 * <p>Serialization of this class stores only the underlying atomic
99 * integer maintaining state, so deserialized objects have empty
100 * thread queues. Typical subclasses requiring serializability will
101 * define a <tt>readObject</tt> method that restores this to a known
102 * initial state upon deserialization.
106 * <p>To use this class as the basis of a synchronizer, redefine the
107 * following methods, as applicable, by inspecting and/or modifying
108 * the synchronization state using {@link #getState}, {@link
109 * #setState} and/or {@link #compareAndSetState}:
112 * <li> {@link #tryAcquire}
113 * <li> {@link #tryRelease}
114 * <li> {@link #tryAcquireShared}
115 * <li> {@link #tryReleaseShared}
116 * <li> {@link #isHeldExclusively}
119 * Each of these methods by default throws {@link
120 * UnsupportedOperationException}. Implementations of these methods
121 * must be internally thread-safe, and should in general be short and
122 * not block. Defining these methods is the <em>only</em> supported
123 * means of using this class. All other methods are declared
124 * <tt>final</tt> because they cannot be independently varied.
126 * <p>You may also find the inherited methods from {@link
127 * AbstractOwnableSynchronizer} useful to keep track of the thread
128 * owning an exclusive synchronizer. You are encouraged to use them
129 * -- this enables monitoring and diagnostic tools to assist users in
130 * determining which threads hold locks.
132 * <p>Even though this class is based on an internal FIFO queue, it
133 * does not automatically enforce FIFO acquisition policies. The core
134 * of exclusive synchronization takes the form:
138 * while (!tryAcquire(arg)) {
139 * <em>enqueue thread if it is not already queued</em>;
140 * <em>possibly block current thread</em>;
144 * if (tryRelease(arg))
145 * <em>unblock the first queued thread</em>;
148 * (Shared mode is similar but may involve cascading signals.)
150 * <p><a name="barging">Because checks in acquire are invoked before
151 * enqueuing, a newly acquiring thread may <em>barge</em> ahead of
152 * others that are blocked and queued. However, you can, if desired,
153 * define <tt>tryAcquire</tt> and/or <tt>tryAcquireShared</tt> to
154 * disable barging by internally invoking one or more of the inspection
155 * methods, thereby providing a <em>fair</em> FIFO acquisition order.
156 * In particular, most fair synchronizers can define <tt>tryAcquire</tt>
157 * to return <tt>false</tt> if {@link #hasQueuedPredecessors} (a method
158 * specifically designed to be used by fair synchronizers) returns
159 * <tt>true</tt>. Other variations are possible.
161 * <p>Throughput and scalability are generally highest for the
162 * default barging (also known as <em>greedy</em>,
163 * <em>renouncement</em>, and <em>convoy-avoidance</em>) strategy.
164 * While this is not guaranteed to be fair or starvation-free, earlier
165 * queued threads are allowed to recontend before later queued
166 * threads, and each recontention has an unbiased chance to succeed
167 * against incoming threads. Also, while acquires do not
168 * "spin" in the usual sense, they may perform multiple
169 * invocations of <tt>tryAcquire</tt> interspersed with other
170 * computations before blocking. This gives most of the benefits of
171 * spins when exclusive synchronization is only briefly held, without
172 * most of the liabilities when it isn't. If so desired, you can
173 * augment this by preceding calls to acquire methods with
174 * "fast-path" checks, possibly prechecking {@link #hasContended}
175 * and/or {@link #hasQueuedThreads} to only do so if the synchronizer
176 * is likely not to be contended.
178 * <p>This class provides an efficient and scalable basis for
179 * synchronization in part by specializing its range of use to
180 * synchronizers that can rely on <tt>int</tt> state, acquire, and
181 * release parameters, and an internal FIFO wait queue. When this does
182 * not suffice, you can build synchronizers from a lower level using
183 * {@link java.util.concurrent.atomic atomic} classes, your own custom
184 * {@link java.util.Queue} classes, and {@link LockSupport} blocking
187 * <h3>Usage Examples</h3>
189 * <p>Here is a non-reentrant mutual exclusion lock class that uses
190 * the value zero to represent the unlocked state, and one to
191 * represent the locked state. While a non-reentrant lock
192 * does not strictly require recording of the current owner
193 * thread, this class does so anyway to make usage easier to monitor.
194 * It also supports conditions and exposes
195 * one of the instrumentation methods:
198 * class Mutex implements Lock, java.io.Serializable {
200 * // Our internal helper class
201 * private static class Sync extends AbstractQueuedSynchronizer {
202 * // Report whether in locked state
203 * protected boolean isHeldExclusively() {
204 * return getState() == 1;
207 * // Acquire the lock if state is zero
208 * public boolean tryAcquire(int acquires) {
209 * assert acquires == 1; // Otherwise unused
210 * if (compareAndSetState(0, 1)) {
211 * setExclusiveOwnerThread(Thread.currentThread());
217 * // Release the lock by setting state to zero
218 * protected boolean tryRelease(int releases) {
219 * assert releases == 1; // Otherwise unused
220 * if (getState() == 0) throw new IllegalMonitorStateException();
221 * setExclusiveOwnerThread(null);
226 * // Provide a Condition
227 * Condition newCondition() { return new ConditionObject(); }
229 * // Deserialize properly
230 * private void readObject(ObjectInputStream s)
231 * throws IOException, ClassNotFoundException {
232 * s.defaultReadObject();
233 * setState(0); // reset to unlocked state
237 * // The sync object does all the hard work. We just forward to it.
238 * private final Sync sync = new Sync();
240 * public void lock() { sync.acquire(1); }
241 * public boolean tryLock() { return sync.tryAcquire(1); }
242 * public void unlock() { sync.release(1); }
243 * public Condition newCondition() { return sync.newCondition(); }
244 * public boolean isLocked() { return sync.isHeldExclusively(); }
245 * public boolean hasQueuedThreads() { return sync.hasQueuedThreads(); }
246 * public void lockInterruptibly() throws InterruptedException {
247 * sync.acquireInterruptibly(1);
249 * public boolean tryLock(long timeout, TimeUnit unit)
250 * throws InterruptedException {
251 * return sync.tryAcquireNanos(1, unit.toNanos(timeout));
256 * <p>Here is a latch class that is like a {@link CountDownLatch}
257 * except that it only requires a single <tt>signal</tt> to
258 * fire. Because a latch is non-exclusive, it uses the <tt>shared</tt>
259 * acquire and release methods.
262 * class BooleanLatch {
264 * private static class Sync extends AbstractQueuedSynchronizer {
265 * boolean isSignalled() { return getState() != 0; }
267 * protected int tryAcquireShared(int ignore) {
268 * return isSignalled() ? 1 : -1;
271 * protected boolean tryReleaseShared(int ignore) {
277 * private final Sync sync = new Sync();
278 * public boolean isSignalled() { return sync.isSignalled(); }
279 * public void signal() { sync.releaseShared(1); }
280 * public void await() throws InterruptedException {
281 * sync.acquireSharedInterruptibly(1);
289 public abstract class AbstractQueuedSynchronizer
290 extends AbstractOwnableSynchronizer
291 implements java.io.Serializable {
293 private static final long serialVersionUID = 7373984972572414691L;
296 * Creates a new <tt>AbstractQueuedSynchronizer</tt> instance
297 * with initial synchronization state of zero.
299 protected AbstractQueuedSynchronizer() { }
302 * Wait queue node class.
304 * <p>The wait queue is a variant of a "CLH" (Craig, Landin, and
305 * Hagersten) lock queue. CLH locks are normally used for
306 * spinlocks. We instead use them for blocking synchronizers, but
307 * use the same basic tactic of holding some of the control
308 * information about a thread in the predecessor of its node. A
309 * "status" field in each node keeps track of whether a thread
310 * should block. A node is signalled when its predecessor
311 * releases. Each node of the queue otherwise serves as a
312 * specific-notification-style monitor holding a single waiting
313 * thread. The status field does NOT control whether threads are
314 * granted locks etc though. A thread may try to acquire if it is
315 * first in the queue. But being first does not guarantee success;
316 * it only gives the right to contend. So the currently released
317 * contender thread may need to rewait.
319 * <p>To enqueue into a CLH lock, you atomically splice it in as new
320 * tail. To dequeue, you just set the head field.
322 * +------+ prev +-----+ +-----+
323 * head | | <---- | | <---- | | tail
324 * +------+ +-----+ +-----+
327 * <p>Insertion into a CLH queue requires only a single atomic
328 * operation on "tail", so there is a simple atomic point of
329 * demarcation from unqueued to queued. Similarly, dequeing
330 * involves only updating the "head". However, it takes a bit
331 * more work for nodes to determine who their successors are,
332 * in part to deal with possible cancellation due to timeouts
335 * <p>The "prev" links (not used in original CLH locks), are mainly
336 * needed to handle cancellation. If a node is cancelled, its
337 * successor is (normally) relinked to a non-cancelled
338 * predecessor. For explanation of similar mechanics in the case
339 * of spin locks, see the papers by Scott and Scherer at
340 * http://www.cs.rochester.edu/u/scott/synchronization/
342 * <p>We also use "next" links to implement blocking mechanics.
343 * The thread id for each node is kept in its own node, so a
344 * predecessor signals the next node to wake up by traversing
345 * next link to determine which thread it is. Determination of
346 * successor must avoid races with newly queued nodes to set
347 * the "next" fields of their predecessors. This is solved
348 * when necessary by checking backwards from the atomically
349 * updated "tail" when a node's successor appears to be null.
350 * (Or, said differently, the next-links are an optimization
351 * so that we don't usually need a backward scan.)
353 * <p>Cancellation introduces some conservatism to the basic
354 * algorithms. Since we must poll for cancellation of other
355 * nodes, we can miss noticing whether a cancelled node is
356 * ahead or behind us. This is dealt with by always unparking
357 * successors upon cancellation, allowing them to stabilize on
358 * a new predecessor, unless we can identify an uncancelled
359 * predecessor who will carry this responsibility.
361 * <p>CLH queues need a dummy header node to get started. But
362 * we don't create them on construction, because it would be wasted
363 * effort if there is never contention. Instead, the node
364 * is constructed and head and tail pointers are set upon first
367 * <p>Threads waiting on Conditions use the same nodes, but
368 * use an additional link. Conditions only need to link nodes
369 * in simple (non-concurrent) linked queues because they are
370 * only accessed when exclusively held. Upon await, a node is
371 * inserted into a condition queue. Upon signal, the node is
372 * transferred to the main queue. A special value of status
373 * field is used to mark which queue a node is on.
375 * <p>Thanks go to Dave Dice, Mark Moir, Victor Luchangco, Bill
376 * Scherer and Michael Scott, along with members of JSR-166
377 * expert group, for helpful ideas, discussions, and critiques
378 * on the design of this class.
380 static final class Node {
381 /** Marker to indicate a node is waiting in shared mode */
382 static final Node SHARED = new Node();
383 /** Marker to indicate a node is waiting in exclusive mode */
384 static final Node EXCLUSIVE = null;
386 /** waitStatus value to indicate thread has cancelled */
387 static final int CANCELLED = 1;
388 /** waitStatus value to indicate successor's thread needs unparking */
389 static final int SIGNAL = -1;
390 /** waitStatus value to indicate thread is waiting on condition */
391 static final int CONDITION = -2;
393 * waitStatus value to indicate the next acquireShared should
394 * unconditionally propagate
396 static final int PROPAGATE = -3;
399 * Status field, taking on only the values:
400 * SIGNAL: The successor of this node is (or will soon be)
401 * blocked (via park), so the current node must
402 * unpark its successor when it releases or
403 * cancels. To avoid races, acquire methods must
404 * first indicate they need a signal,
405 * then retry the atomic acquire, and then,
407 * CANCELLED: This node is cancelled due to timeout or interrupt.
408 * Nodes never leave this state. In particular,
409 * a thread with cancelled node never again blocks.
410 * CONDITION: This node is currently on a condition queue.
411 * It will not be used as a sync queue node
412 * until transferred, at which time the status
413 * will be set to 0. (Use of this value here has
414 * nothing to do with the other uses of the
415 * field, but simplifies mechanics.)
416 * PROPAGATE: A releaseShared should be propagated to other
417 * nodes. This is set (for head node only) in
418 * doReleaseShared to ensure propagation
419 * continues, even if other operations have
421 * 0: None of the above
423 * The values are arranged numerically to simplify use.
424 * Non-negative values mean that a node doesn't need to
425 * signal. So, most code doesn't need to check for particular
426 * values, just for sign.
428 * The field is initialized to 0 for normal sync nodes, and
429 * CONDITION for condition nodes. It is modified using CAS
430 * (or when possible, unconditional volatile writes).
432 volatile int waitStatus;
435 * Link to predecessor node that current node/thread relies on
436 * for checking waitStatus. Assigned during enqueing, and nulled
437 * out (for sake of GC) only upon dequeuing. Also, upon
438 * cancellation of a predecessor, we short-circuit while
439 * finding a non-cancelled one, which will always exist
440 * because the head node is never cancelled: A node becomes
441 * head only as a result of successful acquire. A
442 * cancelled thread never succeeds in acquiring, and a thread only
443 * cancels itself, not any other node.
448 * Link to the successor node that the current node/thread
449 * unparks upon release. Assigned during enqueuing, adjusted
450 * when bypassing cancelled predecessors, and nulled out (for
451 * sake of GC) when dequeued. The enq operation does not
452 * assign next field of a predecessor until after attachment,
453 * so seeing a null next field does not necessarily mean that
454 * node is at end of queue. However, if a next field appears
455 * to be null, we can scan prev's from the tail to
456 * double-check. The next field of cancelled nodes is set to
457 * point to the node itself instead of null, to make life
458 * easier for isOnSyncQueue.
463 * The thread that enqueued this node. Initialized on
464 * construction and nulled out after use.
466 volatile Thread thread;
469 * Link to next node waiting on condition, or the special
470 * value SHARED. Because condition queues are accessed only
471 * when holding in exclusive mode, we just need a simple
472 * linked queue to hold nodes while they are waiting on
473 * conditions. They are then transferred to the queue to
474 * re-acquire. And because conditions can only be exclusive,
475 * we save a field by using special value to indicate shared
481 * Returns true if node is waiting in shared mode
483 final boolean isShared() {
484 return nextWaiter == SHARED;
488 * Returns previous node, or throws NullPointerException if null.
489 * Use when predecessor cannot be null. The null check could
490 * be elided, but is present to help the VM.
492 * @return the predecessor of this node
494 final Node predecessor() throws NullPointerException {
497 throw new NullPointerException();
502 Node() { // Used to establish initial head or SHARED marker
505 Node(Thread thread, Node mode) { // Used by addWaiter
506 this.nextWaiter = mode;
507 this.thread = thread;
510 Node(Thread thread, int waitStatus) { // Used by Condition
511 this.waitStatus = waitStatus;
512 this.thread = thread;
517 * Head of the wait queue, lazily initialized. Except for
518 * initialization, it is modified only via method setHead. Note:
519 * If head exists, its waitStatus is guaranteed not to be
522 private transient volatile Node head;
525 * Tail of the wait queue, lazily initialized. Modified only via
526 * method enq to add new wait node.
528 private transient volatile Node tail;
531 * The synchronization state.
533 private volatile int state;
536 * Returns the current value of synchronization state.
537 * This operation has memory semantics of a <tt>volatile</tt> read.
538 * @return current state value
540 protected final int getState() {
545 * Sets the value of synchronization state.
546 * This operation has memory semantics of a <tt>volatile</tt> write.
547 * @param newState the new state value
549 protected final void setState(int newState) {
554 * Atomically sets synchronization state to the given updated
555 * value if the current state value equals the expected value.
556 * This operation has memory semantics of a <tt>volatile</tt> read
559 * @param expect the expected value
560 * @param update the new value
561 * @return true if successful. False return indicates that the actual
562 * value was not equal to the expected value.
564 protected final boolean compareAndSetState(int expect, int update) {
565 // See below for intrinsics setup to support this
566 return unsafe.compareAndSwapInt(this, stateOffset, expect, update);
572 * The number of nanoseconds for which it is faster to spin
573 * rather than to use timed park. A rough estimate suffices
574 * to improve responsiveness with very short timeouts.
576 static final long spinForTimeoutThreshold = 1000L;
579 * Inserts node into queue, initializing if necessary. See picture above.
580 * @param node the node to insert
581 * @return node's predecessor
583 private Node enq(final Node node) {
586 if (t == null) { // Must initialize
587 if (compareAndSetHead(new Node()))
591 if (compareAndSetTail(t, node)) {
600 * Creates and enqueues node for current thread and given mode.
602 * @param mode Node.EXCLUSIVE for exclusive, Node.SHARED for shared
603 * @return the new node
605 private Node addWaiter(Node mode) {
606 Node node = new Node(Thread.currentThread(), mode);
607 // Try the fast path of enq; backup to full enq on failure
611 if (compareAndSetTail(pred, node)) {
621 * Sets head of queue to be node, thus dequeuing. Called only by
622 * acquire methods. Also nulls out unused fields for sake of GC
623 * and to suppress unnecessary signals and traversals.
625 * @param node the node
627 private void setHead(Node node) {
634 * Wakes up node's successor, if one exists.
636 * @param node the node
638 private void unparkSuccessor(Node node) {
640 * If status is negative (i.e., possibly needing signal) try
641 * to clear in anticipation of signalling. It is OK if this
642 * fails or if status is changed by waiting thread.
644 int ws = node.waitStatus;
646 compareAndSetWaitStatus(node, ws, 0);
649 * Thread to unpark is held in successor, which is normally
650 * just the next node. But if cancelled or apparently null,
651 * traverse backwards from tail to find the actual
652 * non-cancelled successor.
655 if (s == null || s.waitStatus > 0) {
657 for (Node t = tail; t != null && t != node; t = t.prev)
658 if (t.waitStatus <= 0)
662 LockSupport.unpark(s.thread);
666 * Release action for shared mode -- signal successor and ensure
667 * propagation. (Note: For exclusive mode, release just amounts
668 * to calling unparkSuccessor of head if it needs signal.)
670 private void doReleaseShared() {
672 * Ensure that a release propagates, even if there are other
673 * in-progress acquires/releases. This proceeds in the usual
674 * way of trying to unparkSuccessor of head if it needs
675 * signal. But if it does not, status is set to PROPAGATE to
676 * ensure that upon release, propagation continues.
677 * Additionally, we must loop in case a new node is added
678 * while we are doing this. Also, unlike other uses of
679 * unparkSuccessor, we need to know if CAS to reset status
680 * fails, if so rechecking.
684 if (h != null && h != tail) {
685 int ws = h.waitStatus;
686 if (ws == Node.SIGNAL) {
687 if (!compareAndSetWaitStatus(h, Node.SIGNAL, 0))
688 continue; // loop to recheck cases
692 !compareAndSetWaitStatus(h, 0, Node.PROPAGATE))
693 continue; // loop on failed CAS
695 if (h == head) // loop if head changed
701 * Sets head of queue, and checks if successor may be waiting
702 * in shared mode, if so propagating if either propagate > 0 or
703 * PROPAGATE status was set.
705 * @param node the node
706 * @param propagate the return value from a tryAcquireShared
708 private void setHeadAndPropagate(Node node, int propagate) {
709 Node h = head; // Record old head for check below
712 * Try to signal next queued node if:
713 * Propagation was indicated by caller,
714 * or was recorded (as h.waitStatus) by a previous operation
715 * (note: this uses sign-check of waitStatus because
716 * PROPAGATE status may transition to SIGNAL.)
718 * The next node is waiting in shared mode,
719 * or we don't know, because it appears null
721 * The conservatism in both of these checks may cause
722 * unnecessary wake-ups, but only when there are multiple
723 * racing acquires/releases, so most need signals now or soon
726 if (propagate > 0 || h == null || h.waitStatus < 0) {
728 if (s == null || s.isShared())
733 // Utilities for various versions of acquire
736 * Cancels an ongoing attempt to acquire.
738 * @param node the node
740 private void cancelAcquire(Node node) {
741 // Ignore if node doesn't exist
747 // Skip cancelled predecessors
748 Node pred = node.prev;
749 while (pred.waitStatus > 0)
750 node.prev = pred = pred.prev;
752 // predNext is the apparent node to unsplice. CASes below will
753 // fail if not, in which case, we lost race vs another cancel
754 // or signal, so no further action is necessary.
755 Node predNext = pred.next;
757 // Can use unconditional write instead of CAS here.
758 // After this atomic step, other Nodes can skip past us.
759 // Before, we are free of interference from other threads.
760 node.waitStatus = Node.CANCELLED;
762 // If we are the tail, remove ourselves.
763 if (node == tail && compareAndSetTail(node, pred)) {
764 compareAndSetNext(pred, predNext, null);
766 // If successor needs signal, try to set pred's next-link
767 // so it will get one. Otherwise wake it up to propagate.
770 ((ws = pred.waitStatus) == Node.SIGNAL ||
771 (ws <= 0 && compareAndSetWaitStatus(pred, ws, Node.SIGNAL))) &&
772 pred.thread != null) {
773 Node next = node.next;
774 if (next != null && next.waitStatus <= 0)
775 compareAndSetNext(pred, predNext, next);
777 unparkSuccessor(node);
780 node.next = node; // help GC
785 * Checks and updates status for a node that failed to acquire.
786 * Returns true if thread should block. This is the main signal
787 * control in all acquire loops. Requires that pred == node.prev
789 * @param pred node's predecessor holding status
790 * @param node the node
791 * @return {@code true} if thread should block
793 private static boolean shouldParkAfterFailedAcquire(Node pred, Node node) {
794 int ws = pred.waitStatus;
795 if (ws == Node.SIGNAL)
797 * This node has already set status asking a release
798 * to signal it, so it can safely park.
803 * Predecessor was cancelled. Skip over predecessors and
807 node.prev = pred = pred.prev;
808 } while (pred.waitStatus > 0);
812 * waitStatus must be 0 or PROPAGATE. Indicate that we
813 * need a signal, but don't park yet. Caller will need to
814 * retry to make sure it cannot acquire before parking.
816 compareAndSetWaitStatus(pred, ws, Node.SIGNAL);
822 * Convenience method to interrupt current thread.
824 private static void selfInterrupt() {
825 Thread.currentThread().interrupt();
829 * Convenience method to park and then check if interrupted
831 * @return {@code true} if interrupted
833 private final boolean parkAndCheckInterrupt() {
834 LockSupport.park(this);
835 return Thread.interrupted();
839 * Various flavors of acquire, varying in exclusive/shared and
840 * control modes. Each is mostly the same, but annoyingly
841 * different. Only a little bit of factoring is possible due to
842 * interactions of exception mechanics (including ensuring that we
843 * cancel if tryAcquire throws exception) and other control, at
844 * least not without hurting performance too much.
848 * Acquires in exclusive uninterruptible mode for thread already in
849 * queue. Used by condition wait methods as well as acquire.
851 * @param node the node
852 * @param arg the acquire argument
853 * @return {@code true} if interrupted while waiting
855 final boolean acquireQueued(final Node node, int arg) {
856 boolean failed = true;
858 boolean interrupted = false;
860 final Node p = node.predecessor();
861 if (p == head && tryAcquire(arg)) {
863 p.next = null; // help GC
867 if (shouldParkAfterFailedAcquire(p, node) &&
868 parkAndCheckInterrupt())
878 * Acquires in exclusive interruptible mode.
879 * @param arg the acquire argument
881 private void doAcquireInterruptibly(int arg)
882 throws InterruptedException {
883 final Node node = addWaiter(Node.EXCLUSIVE);
884 boolean failed = true;
887 final Node p = node.predecessor();
888 if (p == head && tryAcquire(arg)) {
890 p.next = null; // help GC
894 if (shouldParkAfterFailedAcquire(p, node) &&
895 parkAndCheckInterrupt())
896 throw new InterruptedException();
905 * Acquires in exclusive timed mode.
907 * @param arg the acquire argument
908 * @param nanosTimeout max wait time
909 * @return {@code true} if acquired
911 private boolean doAcquireNanos(int arg, long nanosTimeout)
912 throws InterruptedException {
913 long lastTime = System.nanoTime();
914 final Node node = addWaiter(Node.EXCLUSIVE);
915 boolean failed = true;
918 final Node p = node.predecessor();
919 if (p == head && tryAcquire(arg)) {
921 p.next = null; // help GC
925 if (nanosTimeout <= 0)
927 if (shouldParkAfterFailedAcquire(p, node) &&
928 nanosTimeout > spinForTimeoutThreshold)
929 LockSupport.parkNanos(this, nanosTimeout);
930 long now = System.nanoTime();
931 nanosTimeout -= now - lastTime;
933 if (Thread.interrupted())
934 throw new InterruptedException();
943 * Acquires in shared uninterruptible mode.
944 * @param arg the acquire argument
946 private void doAcquireShared(int arg) {
947 final Node node = addWaiter(Node.SHARED);
948 boolean failed = true;
950 boolean interrupted = false;
952 final Node p = node.predecessor();
954 int r = tryAcquireShared(arg);
956 setHeadAndPropagate(node, r);
957 p.next = null; // help GC
964 if (shouldParkAfterFailedAcquire(p, node) &&
965 parkAndCheckInterrupt())
975 * Acquires in shared interruptible mode.
976 * @param arg the acquire argument
978 private void doAcquireSharedInterruptibly(int arg)
979 throws InterruptedException {
980 final Node node = addWaiter(Node.SHARED);
981 boolean failed = true;
984 final Node p = node.predecessor();
986 int r = tryAcquireShared(arg);
988 setHeadAndPropagate(node, r);
989 p.next = null; // help GC
994 if (shouldParkAfterFailedAcquire(p, node) &&
995 parkAndCheckInterrupt())
996 throw new InterruptedException();
1000 cancelAcquire(node);
1005 * Acquires in shared timed mode.
1007 * @param arg the acquire argument
1008 * @param nanosTimeout max wait time
1009 * @return {@code true} if acquired
1011 private boolean doAcquireSharedNanos(int arg, long nanosTimeout)
1012 throws InterruptedException {
1014 long lastTime = System.nanoTime();
1015 final Node node = addWaiter(Node.SHARED);
1016 boolean failed = true;
1019 final Node p = node.predecessor();
1021 int r = tryAcquireShared(arg);
1023 setHeadAndPropagate(node, r);
1024 p.next = null; // help GC
1029 if (nanosTimeout <= 0)
1031 if (shouldParkAfterFailedAcquire(p, node) &&
1032 nanosTimeout > spinForTimeoutThreshold)
1033 LockSupport.parkNanos(this, nanosTimeout);
1034 long now = System.nanoTime();
1035 nanosTimeout -= now - lastTime;
1037 if (Thread.interrupted())
1038 throw new InterruptedException();
1042 cancelAcquire(node);
1046 // Main exported methods
1049 * Attempts to acquire in exclusive mode. This method should query
1050 * if the state of the object permits it to be acquired in the
1051 * exclusive mode, and if so to acquire it.
1053 * <p>This method is always invoked by the thread performing
1054 * acquire. If this method reports failure, the acquire method
1055 * may queue the thread, if it is not already queued, until it is
1056 * signalled by a release from some other thread. This can be used
1057 * to implement method {@link Lock#tryLock()}.
1060 * implementation throws {@link UnsupportedOperationException}.
1062 * @param arg the acquire argument. This value is always the one
1063 * passed to an acquire method, or is the value saved on entry
1064 * to a condition wait. The value is otherwise uninterpreted
1065 * and can represent anything you like.
1066 * @return {@code true} if successful. Upon success, this object has
1068 * @throws IllegalMonitorStateException if acquiring would place this
1069 * synchronizer in an illegal state. This exception must be
1070 * thrown in a consistent fashion for synchronization to work
1072 * @throws UnsupportedOperationException if exclusive mode is not supported
1074 protected boolean tryAcquire(int arg) {
1075 throw new UnsupportedOperationException();
1079 * Attempts to set the state to reflect a release in exclusive
1082 * <p>This method is always invoked by the thread performing release.
1084 * <p>The default implementation throws
1085 * {@link UnsupportedOperationException}.
1087 * @param arg the release argument. This value is always the one
1088 * passed to a release method, or the current state value upon
1089 * entry to a condition wait. The value is otherwise
1090 * uninterpreted and can represent anything you like.
1091 * @return {@code true} if this object is now in a fully released
1092 * state, so that any waiting threads may attempt to acquire;
1093 * and {@code false} otherwise.
1094 * @throws IllegalMonitorStateException if releasing would place this
1095 * synchronizer in an illegal state. This exception must be
1096 * thrown in a consistent fashion for synchronization to work
1098 * @throws UnsupportedOperationException if exclusive mode is not supported
1100 protected boolean tryRelease(int arg) {
1101 throw new UnsupportedOperationException();
1105 * Attempts to acquire in shared mode. This method should query if
1106 * the state of the object permits it to be acquired in the shared
1107 * mode, and if so to acquire it.
1109 * <p>This method is always invoked by the thread performing
1110 * acquire. If this method reports failure, the acquire method
1111 * may queue the thread, if it is not already queued, until it is
1112 * signalled by a release from some other thread.
1114 * <p>The default implementation throws {@link
1115 * UnsupportedOperationException}.
1117 * @param arg the acquire argument. This value is always the one
1118 * passed to an acquire method, or is the value saved on entry
1119 * to a condition wait. The value is otherwise uninterpreted
1120 * and can represent anything you like.
1121 * @return a negative value on failure; zero if acquisition in shared
1122 * mode succeeded but no subsequent shared-mode acquire can
1123 * succeed; and a positive value if acquisition in shared
1124 * mode succeeded and subsequent shared-mode acquires might
1125 * also succeed, in which case a subsequent waiting thread
1126 * must check availability. (Support for three different
1127 * return values enables this method to be used in contexts
1128 * where acquires only sometimes act exclusively.) Upon
1129 * success, this object has been acquired.
1130 * @throws IllegalMonitorStateException if acquiring would place this
1131 * synchronizer in an illegal state. This exception must be
1132 * thrown in a consistent fashion for synchronization to work
1134 * @throws UnsupportedOperationException if shared mode is not supported
1136 protected int tryAcquireShared(int arg) {
1137 throw new UnsupportedOperationException();
1141 * Attempts to set the state to reflect a release in shared mode.
1143 * <p>This method is always invoked by the thread performing release.
1145 * <p>The default implementation throws
1146 * {@link UnsupportedOperationException}.
1148 * @param arg the release argument. This value is always the one
1149 * passed to a release method, or the current state value upon
1150 * entry to a condition wait. The value is otherwise
1151 * uninterpreted and can represent anything you like.
1152 * @return {@code true} if this release of shared mode may permit a
1153 * waiting acquire (shared or exclusive) to succeed; and
1154 * {@code false} otherwise
1155 * @throws IllegalMonitorStateException if releasing would place this
1156 * synchronizer in an illegal state. This exception must be
1157 * thrown in a consistent fashion for synchronization to work
1159 * @throws UnsupportedOperationException if shared mode is not supported
1161 protected boolean tryReleaseShared(int arg) {
1162 throw new UnsupportedOperationException();
1166 * Returns {@code true} if synchronization is held exclusively with
1167 * respect to the current (calling) thread. This method is invoked
1168 * upon each call to a non-waiting {@link ConditionObject} method.
1169 * (Waiting methods instead invoke {@link #release}.)
1171 * <p>The default implementation throws {@link
1172 * UnsupportedOperationException}. This method is invoked
1173 * internally only within {@link ConditionObject} methods, so need
1174 * not be defined if conditions are not used.
1176 * @return {@code true} if synchronization is held exclusively;
1177 * {@code false} otherwise
1178 * @throws UnsupportedOperationException if conditions are not supported
1180 protected boolean isHeldExclusively() {
1181 throw new UnsupportedOperationException();
1185 * Acquires in exclusive mode, ignoring interrupts. Implemented
1186 * by invoking at least once {@link #tryAcquire},
1187 * returning on success. Otherwise the thread is queued, possibly
1188 * repeatedly blocking and unblocking, invoking {@link
1189 * #tryAcquire} until success. This method can be used
1190 * to implement method {@link Lock#lock}.
1192 * @param arg the acquire argument. This value is conveyed to
1193 * {@link #tryAcquire} but is otherwise uninterpreted and
1194 * can represent anything you like.
1196 public final void acquire(int arg) {
1197 if (!tryAcquire(arg) &&
1198 acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
1203 * Acquires in exclusive mode, aborting if interrupted.
1204 * Implemented by first checking interrupt status, then invoking
1205 * at least once {@link #tryAcquire}, returning on
1206 * success. Otherwise the thread is queued, possibly repeatedly
1207 * blocking and unblocking, invoking {@link #tryAcquire}
1208 * until success or the thread is interrupted. This method can be
1209 * used to implement method {@link Lock#lockInterruptibly}.
1211 * @param arg the acquire argument. This value is conveyed to
1212 * {@link #tryAcquire} but is otherwise uninterpreted and
1213 * can represent anything you like.
1214 * @throws InterruptedException if the current thread is interrupted
1216 public final void acquireInterruptibly(int arg)
1217 throws InterruptedException {
1218 if (Thread.interrupted())
1219 throw new InterruptedException();
1220 if (!tryAcquire(arg))
1221 doAcquireInterruptibly(arg);
1225 * Attempts to acquire in exclusive mode, aborting if interrupted,
1226 * and failing if the given timeout elapses. Implemented by first
1227 * checking interrupt status, then invoking at least once {@link
1228 * #tryAcquire}, returning on success. Otherwise, the thread is
1229 * queued, possibly repeatedly blocking and unblocking, invoking
1230 * {@link #tryAcquire} until success or the thread is interrupted
1231 * or the timeout elapses. This method can be used to implement
1232 * method {@link Lock#tryLock(long, TimeUnit)}.
1234 * @param arg the acquire argument. This value is conveyed to
1235 * {@link #tryAcquire} but is otherwise uninterpreted and
1236 * can represent anything you like.
1237 * @param nanosTimeout the maximum number of nanoseconds to wait
1238 * @return {@code true} if acquired; {@code false} if timed out
1239 * @throws InterruptedException if the current thread is interrupted
1241 public final boolean tryAcquireNanos(int arg, long nanosTimeout)
1242 throws InterruptedException {
1243 if (Thread.interrupted())
1244 throw new InterruptedException();
1245 return tryAcquire(arg) ||
1246 doAcquireNanos(arg, nanosTimeout);
1250 * Releases in exclusive mode. Implemented by unblocking one or
1251 * more threads if {@link #tryRelease} returns true.
1252 * This method can be used to implement method {@link Lock#unlock}.
1254 * @param arg the release argument. This value is conveyed to
1255 * {@link #tryRelease} but is otherwise uninterpreted and
1256 * can represent anything you like.
1257 * @return the value returned from {@link #tryRelease}
1259 public final boolean release(int arg) {
1260 if (tryRelease(arg)) {
1262 if (h != null && h.waitStatus != 0)
1270 * Acquires in shared mode, ignoring interrupts. Implemented by
1271 * first invoking at least once {@link #tryAcquireShared},
1272 * returning on success. Otherwise the thread is queued, possibly
1273 * repeatedly blocking and unblocking, invoking {@link
1274 * #tryAcquireShared} until success.
1276 * @param arg the acquire argument. This value is conveyed to
1277 * {@link #tryAcquireShared} but is otherwise uninterpreted
1278 * and can represent anything you like.
1280 public final void acquireShared(int arg) {
1281 if (tryAcquireShared(arg) < 0)
1282 doAcquireShared(arg);
1286 * Acquires in shared mode, aborting if interrupted. Implemented
1287 * by first checking interrupt status, then invoking at least once
1288 * {@link #tryAcquireShared}, returning on success. Otherwise the
1289 * thread is queued, possibly repeatedly blocking and unblocking,
1290 * invoking {@link #tryAcquireShared} until success or the thread
1292 * @param arg the acquire argument
1293 * This value is conveyed to {@link #tryAcquireShared} but is
1294 * otherwise uninterpreted and can represent anything
1296 * @throws InterruptedException if the current thread is interrupted
1298 public final void acquireSharedInterruptibly(int arg)
1299 throws InterruptedException {
1300 if (Thread.interrupted())
1301 throw new InterruptedException();
1302 if (tryAcquireShared(arg) < 0)
1303 doAcquireSharedInterruptibly(arg);
1307 * Attempts to acquire in shared mode, aborting if interrupted, and
1308 * failing if the given timeout elapses. Implemented by first
1309 * checking interrupt status, then invoking at least once {@link
1310 * #tryAcquireShared}, returning on success. Otherwise, the
1311 * thread is queued, possibly repeatedly blocking and unblocking,
1312 * invoking {@link #tryAcquireShared} until success or the thread
1313 * is interrupted or the timeout elapses.
1315 * @param arg the acquire argument. This value is conveyed to
1316 * {@link #tryAcquireShared} but is otherwise uninterpreted
1317 * and can represent anything you like.
1318 * @param nanosTimeout the maximum number of nanoseconds to wait
1319 * @return {@code true} if acquired; {@code false} if timed out
1320 * @throws InterruptedException if the current thread is interrupted
1322 public final boolean tryAcquireSharedNanos(int arg, long nanosTimeout)
1323 throws InterruptedException {
1324 if (Thread.interrupted())
1325 throw new InterruptedException();
1326 return tryAcquireShared(arg) >= 0 ||
1327 doAcquireSharedNanos(arg, nanosTimeout);
1331 * Releases in shared mode. Implemented by unblocking one or more
1332 * threads if {@link #tryReleaseShared} returns true.
1334 * @param arg the release argument. This value is conveyed to
1335 * {@link #tryReleaseShared} but is otherwise uninterpreted
1336 * and can represent anything you like.
1337 * @return the value returned from {@link #tryReleaseShared}
1339 public final boolean releaseShared(int arg) {
1340 if (tryReleaseShared(arg)) {
1347 // Queue inspection methods
1350 * Queries whether any threads are waiting to acquire. Note that
1351 * because cancellations due to interrupts and timeouts may occur
1352 * at any time, a {@code true} return does not guarantee that any
1353 * other thread will ever acquire.
1355 * <p>In this implementation, this operation returns in
1358 * @return {@code true} if there may be other threads waiting to acquire
1360 public final boolean hasQueuedThreads() {
1361 return head != tail;
1365 * Queries whether any threads have ever contended to acquire this
1366 * synchronizer; that is if an acquire method has ever blocked.
1368 * <p>In this implementation, this operation returns in
1371 * @return {@code true} if there has ever been contention
1373 public final boolean hasContended() {
1374 return head != null;
1378 * Returns the first (longest-waiting) thread in the queue, or
1379 * {@code null} if no threads are currently queued.
1381 * <p>In this implementation, this operation normally returns in
1382 * constant time, but may iterate upon contention if other threads are
1383 * concurrently modifying the queue.
1385 * @return the first (longest-waiting) thread in the queue, or
1386 * {@code null} if no threads are currently queued
1388 public final Thread getFirstQueuedThread() {
1389 // handle only fast path, else relay
1390 return (head == tail) ? null : fullGetFirstQueuedThread();
1394 * Version of getFirstQueuedThread called when fastpath fails
1396 private Thread fullGetFirstQueuedThread() {
1398 * The first node is normally head.next. Try to get its
1399 * thread field, ensuring consistent reads: If thread
1400 * field is nulled out or s.prev is no longer head, then
1401 * some other thread(s) concurrently performed setHead in
1402 * between some of our reads. We try this twice before
1403 * resorting to traversal.
1407 if (((h = head) != null && (s = h.next) != null &&
1408 s.prev == head && (st = s.thread) != null) ||
1409 ((h = head) != null && (s = h.next) != null &&
1410 s.prev == head && (st = s.thread) != null))
1414 * Head's next field might not have been set yet, or may have
1415 * been unset after setHead. So we must check to see if tail
1416 * is actually first node. If not, we continue on, safely
1417 * traversing from tail back to head to find first,
1418 * guaranteeing termination.
1422 Thread firstThread = null;
1423 while (t != null && t != head) {
1424 Thread tt = t.thread;
1433 * Returns true if the given thread is currently queued.
1435 * <p>This implementation traverses the queue to determine
1436 * presence of the given thread.
1438 * @param thread the thread
1439 * @return {@code true} if the given thread is on the queue
1440 * @throws NullPointerException if the thread is null
1442 public final boolean isQueued(Thread thread) {
1444 throw new NullPointerException();
1445 for (Node p = tail; p != null; p = p.prev)
1446 if (p.thread == thread)
1452 * Returns {@code true} if the apparent first queued thread, if one
1453 * exists, is waiting in exclusive mode. If this method returns
1454 * {@code true}, and the current thread is attempting to acquire in
1455 * shared mode (that is, this method is invoked from {@link
1456 * #tryAcquireShared}) then it is guaranteed that the current thread
1457 * is not the first queued thread. Used only as a heuristic in
1458 * ReentrantReadWriteLock.
1460 final boolean apparentlyFirstQueuedIsExclusive() {
1462 return (h = head) != null &&
1463 (s = h.next) != null &&
1469 * Queries whether any threads have been waiting to acquire longer
1470 * than the current thread.
1472 * <p>An invocation of this method is equivalent to (but may be
1473 * more efficient than):
1475 * getFirstQueuedThread() != Thread.currentThread() &&
1476 * hasQueuedThreads()}</pre>
1478 * <p>Note that because cancellations due to interrupts and
1479 * timeouts may occur at any time, a {@code true} return does not
1480 * guarantee that some other thread will acquire before the current
1481 * thread. Likewise, it is possible for another thread to win a
1482 * race to enqueue after this method has returned {@code false},
1483 * due to the queue being empty.
1485 * <p>This method is designed to be used by a fair synchronizer to
1486 * avoid <a href="AbstractQueuedSynchronizer#barging">barging</a>.
1487 * Such a synchronizer's {@link #tryAcquire} method should return
1488 * {@code false}, and its {@link #tryAcquireShared} method should
1489 * return a negative value, if this method returns {@code true}
1490 * (unless this is a reentrant acquire). For example, the {@code
1491 * tryAcquire} method for a fair, reentrant, exclusive mode
1492 * synchronizer might look like this:
1495 * protected boolean tryAcquire(int arg) {
1496 * if (isHeldExclusively()) {
1497 * // A reentrant acquire; increment hold count
1499 * } else if (hasQueuedPredecessors()) {
1502 * // try to acquire normally
1506 * @return {@code true} if there is a queued thread preceding the
1507 * current thread, and {@code false} if the current thread
1508 * is at the head of the queue or the queue is empty
1511 public final boolean hasQueuedPredecessors() {
1512 // The correctness of this depends on head being initialized
1513 // before tail and on head.next being accurate if the current
1514 // thread is first in queue.
1515 Node t = tail; // Read fields in reverse initialization order
1519 ((s = h.next) == null || s.thread != Thread.currentThread());
1523 // Instrumentation and monitoring methods
1526 * Returns an estimate of the number of threads waiting to
1527 * acquire. The value is only an estimate because the number of
1528 * threads may change dynamically while this method traverses
1529 * internal data structures. This method is designed for use in
1530 * monitoring system state, not for synchronization
1533 * @return the estimated number of threads waiting to acquire
1535 public final int getQueueLength() {
1537 for (Node p = tail; p != null; p = p.prev) {
1538 if (p.thread != null)
1545 * Returns a collection containing threads that may be waiting to
1546 * acquire. Because the actual set of threads may change
1547 * dynamically while constructing this result, the returned
1548 * collection is only a best-effort estimate. The elements of the
1549 * returned collection are in no particular order. This method is
1550 * designed to facilitate construction of subclasses that provide
1551 * more extensive monitoring facilities.
1553 * @return the collection of threads
1555 public final Collection<Thread> getQueuedThreads() {
1556 ArrayList<Thread> list = new ArrayList<Thread>();
1557 for (Node p = tail; p != null; p = p.prev) {
1558 Thread t = p.thread;
1566 * Returns a collection containing threads that may be waiting to
1567 * acquire in exclusive mode. This has the same properties
1568 * as {@link #getQueuedThreads} except that it only returns
1569 * those threads waiting due to an exclusive acquire.
1571 * @return the collection of threads
1573 public final Collection<Thread> getExclusiveQueuedThreads() {
1574 ArrayList<Thread> list = new ArrayList<Thread>();
1575 for (Node p = tail; p != null; p = p.prev) {
1576 if (!p.isShared()) {
1577 Thread t = p.thread;
1586 * Returns a collection containing threads that may be waiting to
1587 * acquire in shared mode. This has the same properties
1588 * as {@link #getQueuedThreads} except that it only returns
1589 * those threads waiting due to a shared acquire.
1591 * @return the collection of threads
1593 public final Collection<Thread> getSharedQueuedThreads() {
1594 ArrayList<Thread> list = new ArrayList<Thread>();
1595 for (Node p = tail; p != null; p = p.prev) {
1597 Thread t = p.thread;
1606 * Returns a string identifying this synchronizer, as well as its state.
1607 * The state, in brackets, includes the String {@code "State ="}
1608 * followed by the current value of {@link #getState}, and either
1609 * {@code "nonempty"} or {@code "empty"} depending on whether the
1612 * @return a string identifying this synchronizer, as well as its state
1614 public String toString() {
1616 String q = hasQueuedThreads() ? "non" : "";
1617 return super.toString() +
1618 "[State = " + s + ", " + q + "empty queue]";
1622 // Internal support methods for Conditions
1625 * Returns true if a node, always one that was initially placed on
1626 * a condition queue, is now waiting to reacquire on sync queue.
1627 * @param node the node
1628 * @return true if is reacquiring
1630 final boolean isOnSyncQueue(Node node) {
1631 if (node.waitStatus == Node.CONDITION || node.prev == null)
1633 if (node.next != null) // If has successor, it must be on queue
1636 * node.prev can be non-null, but not yet on queue because
1637 * the CAS to place it on queue can fail. So we have to
1638 * traverse from tail to make sure it actually made it. It
1639 * will always be near the tail in calls to this method, and
1640 * unless the CAS failed (which is unlikely), it will be
1641 * there, so we hardly ever traverse much.
1643 return findNodeFromTail(node);
1647 * Returns true if node is on sync queue by searching backwards from tail.
1648 * Called only when needed by isOnSyncQueue.
1649 * @return true if present
1651 private boolean findNodeFromTail(Node node) {
1663 * Transfers a node from a condition queue onto sync queue.
1664 * Returns true if successful.
1665 * @param node the node
1666 * @return true if successfully transferred (else the node was
1667 * cancelled before signal).
1669 final boolean transferForSignal(Node node) {
1671 * If cannot change waitStatus, the node has been cancelled.
1673 if (!compareAndSetWaitStatus(node, Node.CONDITION, 0))
1677 * Splice onto queue and try to set waitStatus of predecessor to
1678 * indicate that thread is (probably) waiting. If cancelled or
1679 * attempt to set waitStatus fails, wake up to resync (in which
1680 * case the waitStatus can be transiently and harmlessly wrong).
1683 int ws = p.waitStatus;
1684 if (ws > 0 || !compareAndSetWaitStatus(p, ws, Node.SIGNAL))
1685 LockSupport.unpark(node.thread);
1690 * Transfers node, if necessary, to sync queue after a cancelled
1691 * wait. Returns true if thread was cancelled before being
1693 * @param current the waiting thread
1694 * @param node its node
1695 * @return true if cancelled before the node was signalled
1697 final boolean transferAfterCancelledWait(Node node) {
1698 if (compareAndSetWaitStatus(node, Node.CONDITION, 0)) {
1703 * If we lost out to a signal(), then we can't proceed
1704 * until it finishes its enq(). Cancelling during an
1705 * incomplete transfer is both rare and transient, so just
1708 while (!isOnSyncQueue(node))
1714 * Invokes release with current state value; returns saved state.
1715 * Cancels node and throws exception on failure.
1716 * @param node the condition node for this wait
1717 * @return previous sync state
1719 final int fullyRelease(Node node) {
1720 boolean failed = true;
1722 int savedState = getState();
1723 if (release(savedState)) {
1727 throw new IllegalMonitorStateException();
1731 node.waitStatus = Node.CANCELLED;
1735 // Instrumentation methods for conditions
1738 * Queries whether the given ConditionObject
1739 * uses this synchronizer as its lock.
1741 * @param condition the condition
1742 * @return <tt>true</tt> if owned
1743 * @throws NullPointerException if the condition is null
1745 public final boolean owns(ConditionObject condition) {
1746 if (condition == null)
1747 throw new NullPointerException();
1748 return condition.isOwnedBy(this);
1752 * Queries whether any threads are waiting on the given condition
1753 * associated with this synchronizer. Note that because timeouts
1754 * and interrupts may occur at any time, a <tt>true</tt> return
1755 * does not guarantee that a future <tt>signal</tt> will awaken
1756 * any threads. This method is designed primarily for use in
1757 * monitoring of the system state.
1759 * @param condition the condition
1760 * @return <tt>true</tt> if there are any waiting threads
1761 * @throws IllegalMonitorStateException if exclusive synchronization
1763 * @throws IllegalArgumentException if the given condition is
1764 * not associated with this synchronizer
1765 * @throws NullPointerException if the condition is null
1767 public final boolean hasWaiters(ConditionObject condition) {
1768 if (!owns(condition))
1769 throw new IllegalArgumentException("Not owner");
1770 return condition.hasWaiters();
1774 * Returns an estimate of the number of threads waiting on the
1775 * given condition associated with this synchronizer. Note that
1776 * because timeouts and interrupts may occur at any time, the
1777 * estimate serves only as an upper bound on the actual number of
1778 * waiters. This method is designed for use in monitoring of the
1779 * system state, not for synchronization control.
1781 * @param condition the condition
1782 * @return the estimated number of waiting threads
1783 * @throws IllegalMonitorStateException if exclusive synchronization
1785 * @throws IllegalArgumentException if the given condition is
1786 * not associated with this synchronizer
1787 * @throws NullPointerException if the condition is null
1789 public final int getWaitQueueLength(ConditionObject condition) {
1790 if (!owns(condition))
1791 throw new IllegalArgumentException("Not owner");
1792 return condition.getWaitQueueLength();
1796 * Returns a collection containing those threads that may be
1797 * waiting on the given condition associated with this
1798 * synchronizer. Because the actual set of threads may change
1799 * dynamically while constructing this result, the returned
1800 * collection is only a best-effort estimate. The elements of the
1801 * returned collection are in no particular order.
1803 * @param condition the condition
1804 * @return the collection of threads
1805 * @throws IllegalMonitorStateException if exclusive synchronization
1807 * @throws IllegalArgumentException if the given condition is
1808 * not associated with this synchronizer
1809 * @throws NullPointerException if the condition is null
1811 public final Collection<Thread> getWaitingThreads(ConditionObject condition) {
1812 if (!owns(condition))
1813 throw new IllegalArgumentException("Not owner");
1814 return condition.getWaitingThreads();
1818 * Condition implementation for a {@link
1819 * AbstractQueuedSynchronizer} serving as the basis of a {@link
1820 * Lock} implementation.
1822 * <p>Method documentation for this class describes mechanics,
1823 * not behavioral specifications from the point of view of Lock
1824 * and Condition users. Exported versions of this class will in
1825 * general need to be accompanied by documentation describing
1826 * condition semantics that rely on those of the associated
1827 * <tt>AbstractQueuedSynchronizer</tt>.
1829 * <p>This class is Serializable, but all fields are transient,
1830 * so deserialized conditions have no waiters.
1832 public class ConditionObject implements Condition, java.io.Serializable {
1833 private static final long serialVersionUID = 1173984872572414699L;
1834 /** First node of condition queue. */
1835 private transient Node firstWaiter;
1836 /** Last node of condition queue. */
1837 private transient Node lastWaiter;
1840 * Creates a new <tt>ConditionObject</tt> instance.
1842 public ConditionObject() { }
1847 * Adds a new waiter to wait queue.
1848 * @return its new wait node
1850 private Node addConditionWaiter() {
1851 Node t = lastWaiter;
1852 // If lastWaiter is cancelled, clean out.
1853 if (t != null && t.waitStatus != Node.CONDITION) {
1854 unlinkCancelledWaiters();
1857 Node node = new Node(Thread.currentThread(), Node.CONDITION);
1861 t.nextWaiter = node;
1867 * Removes and transfers nodes until hit non-cancelled one or
1868 * null. Split out from signal in part to encourage compilers
1869 * to inline the case of no waiters.
1870 * @param first (non-null) the first node on condition queue
1872 private void doSignal(Node first) {
1874 if ( (firstWaiter = first.nextWaiter) == null)
1876 first.nextWaiter = null;
1877 } while (!transferForSignal(first) &&
1878 (first = firstWaiter) != null);
1882 * Removes and transfers all nodes.
1883 * @param first (non-null) the first node on condition queue
1885 private void doSignalAll(Node first) {
1886 lastWaiter = firstWaiter = null;
1888 Node next = first.nextWaiter;
1889 first.nextWaiter = null;
1890 transferForSignal(first);
1892 } while (first != null);
1896 * Unlinks cancelled waiter nodes from condition queue.
1897 * Called only while holding lock. This is called when
1898 * cancellation occurred during condition wait, and upon
1899 * insertion of a new waiter when lastWaiter is seen to have
1900 * been cancelled. This method is needed to avoid garbage
1901 * retention in the absence of signals. So even though it may
1902 * require a full traversal, it comes into play only when
1903 * timeouts or cancellations occur in the absence of
1904 * signals. It traverses all nodes rather than stopping at a
1905 * particular target to unlink all pointers to garbage nodes
1906 * without requiring many re-traversals during cancellation
1909 private void unlinkCancelledWaiters() {
1910 Node t = firstWaiter;
1913 Node next = t.nextWaiter;
1914 if (t.waitStatus != Node.CONDITION) {
1915 t.nextWaiter = null;
1919 trail.nextWaiter = next;
1932 * Moves the longest-waiting thread, if one exists, from the
1933 * wait queue for this condition to the wait queue for the
1936 * @throws IllegalMonitorStateException if {@link #isHeldExclusively}
1937 * returns {@code false}
1939 public final void signal() {
1940 if (!isHeldExclusively())
1941 throw new IllegalMonitorStateException();
1942 Node first = firstWaiter;
1948 * Moves all threads from the wait queue for this condition to
1949 * the wait queue for the owning lock.
1951 * @throws IllegalMonitorStateException if {@link #isHeldExclusively}
1952 * returns {@code false}
1954 public final void signalAll() {
1955 if (!isHeldExclusively())
1956 throw new IllegalMonitorStateException();
1957 Node first = firstWaiter;
1963 * Implements uninterruptible condition wait.
1965 * <li> Save lock state returned by {@link #getState}.
1966 * <li> Invoke {@link #release} with
1967 * saved state as argument, throwing
1968 * IllegalMonitorStateException if it fails.
1969 * <li> Block until signalled.
1970 * <li> Reacquire by invoking specialized version of
1971 * {@link #acquire} with saved state as argument.
1974 public final void awaitUninterruptibly() {
1975 Node node = addConditionWaiter();
1976 int savedState = fullyRelease(node);
1977 boolean interrupted = false;
1978 while (!isOnSyncQueue(node)) {
1979 LockSupport.park(this);
1980 if (Thread.interrupted())
1983 if (acquireQueued(node, savedState) || interrupted)
1988 * For interruptible waits, we need to track whether to throw
1989 * InterruptedException, if interrupted while blocked on
1990 * condition, versus reinterrupt current thread, if
1991 * interrupted while blocked waiting to re-acquire.
1994 /** Mode meaning to reinterrupt on exit from wait */
1995 private static final int REINTERRUPT = 1;
1996 /** Mode meaning to throw InterruptedException on exit from wait */
1997 private static final int THROW_IE = -1;
2000 * Checks for interrupt, returning THROW_IE if interrupted
2001 * before signalled, REINTERRUPT if after signalled, or
2002 * 0 if not interrupted.
2004 private int checkInterruptWhileWaiting(Node node) {
2005 return Thread.interrupted() ?
2006 (transferAfterCancelledWait(node) ? THROW_IE : REINTERRUPT) :
2011 * Throws InterruptedException, reinterrupts current thread, or
2012 * does nothing, depending on mode.
2014 private void reportInterruptAfterWait(int interruptMode)
2015 throws InterruptedException {
2016 if (interruptMode == THROW_IE)
2017 throw new InterruptedException();
2018 else if (interruptMode == REINTERRUPT)
2023 * Implements interruptible condition wait.
2025 * <li> If current thread is interrupted, throw InterruptedException.
2026 * <li> Save lock state returned by {@link #getState}.
2027 * <li> Invoke {@link #release} with
2028 * saved state as argument, throwing
2029 * IllegalMonitorStateException if it fails.
2030 * <li> Block until signalled or interrupted.
2031 * <li> Reacquire by invoking specialized version of
2032 * {@link #acquire} with saved state as argument.
2033 * <li> If interrupted while blocked in step 4, throw InterruptedException.
2036 public final void await() throws InterruptedException {
2037 if (Thread.interrupted())
2038 throw new InterruptedException();
2039 Node node = addConditionWaiter();
2040 int savedState = fullyRelease(node);
2041 int interruptMode = 0;
2042 while (!isOnSyncQueue(node)) {
2043 LockSupport.park(this);
2044 if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
2047 if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
2048 interruptMode = REINTERRUPT;
2049 if (node.nextWaiter != null) // clean up if cancelled
2050 unlinkCancelledWaiters();
2051 if (interruptMode != 0)
2052 reportInterruptAfterWait(interruptMode);
2056 * Implements timed condition wait.
2058 * <li> If current thread is interrupted, throw InterruptedException.
2059 * <li> Save lock state returned by {@link #getState}.
2060 * <li> Invoke {@link #release} with
2061 * saved state as argument, throwing
2062 * IllegalMonitorStateException if it fails.
2063 * <li> Block until signalled, interrupted, or timed out.
2064 * <li> Reacquire by invoking specialized version of
2065 * {@link #acquire} with saved state as argument.
2066 * <li> If interrupted while blocked in step 4, throw InterruptedException.
2069 public final long awaitNanos(long nanosTimeout)
2070 throws InterruptedException {
2071 if (Thread.interrupted())
2072 throw new InterruptedException();
2073 Node node = addConditionWaiter();
2074 int savedState = fullyRelease(node);
2075 long lastTime = System.nanoTime();
2076 int interruptMode = 0;
2077 while (!isOnSyncQueue(node)) {
2078 if (nanosTimeout <= 0L) {
2079 transferAfterCancelledWait(node);
2082 LockSupport.parkNanos(this, nanosTimeout);
2083 if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
2086 long now = System.nanoTime();
2087 nanosTimeout -= now - lastTime;
2090 if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
2091 interruptMode = REINTERRUPT;
2092 if (node.nextWaiter != null)
2093 unlinkCancelledWaiters();
2094 if (interruptMode != 0)
2095 reportInterruptAfterWait(interruptMode);
2096 return nanosTimeout - (System.nanoTime() - lastTime);
2100 * Implements absolute timed condition wait.
2102 * <li> If current thread is interrupted, throw InterruptedException.
2103 * <li> Save lock state returned by {@link #getState}.
2104 * <li> Invoke {@link #release} with
2105 * saved state as argument, throwing
2106 * IllegalMonitorStateException if it fails.
2107 * <li> Block until signalled, interrupted, or timed out.
2108 * <li> Reacquire by invoking specialized version of
2109 * {@link #acquire} with saved state as argument.
2110 * <li> If interrupted while blocked in step 4, throw InterruptedException.
2111 * <li> If timed out while blocked in step 4, return false, else true.
2114 public final boolean awaitUntil(Date deadline)
2115 throws InterruptedException {
2116 if (deadline == null)
2117 throw new NullPointerException();
2118 long abstime = deadline.getTime();
2119 if (Thread.interrupted())
2120 throw new InterruptedException();
2121 Node node = addConditionWaiter();
2122 int savedState = fullyRelease(node);
2123 boolean timedout = false;
2124 int interruptMode = 0;
2125 while (!isOnSyncQueue(node)) {
2126 if (System.currentTimeMillis() > abstime) {
2127 timedout = transferAfterCancelledWait(node);
2130 LockSupport.parkUntil(this, abstime);
2131 if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
2134 if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
2135 interruptMode = REINTERRUPT;
2136 if (node.nextWaiter != null)
2137 unlinkCancelledWaiters();
2138 if (interruptMode != 0)
2139 reportInterruptAfterWait(interruptMode);
2144 * Implements timed condition wait.
2146 * <li> If current thread is interrupted, throw InterruptedException.
2147 * <li> Save lock state returned by {@link #getState}.
2148 * <li> Invoke {@link #release} with
2149 * saved state as argument, throwing
2150 * IllegalMonitorStateException if it fails.
2151 * <li> Block until signalled, interrupted, or timed out.
2152 * <li> Reacquire by invoking specialized version of
2153 * {@link #acquire} with saved state as argument.
2154 * <li> If interrupted while blocked in step 4, throw InterruptedException.
2155 * <li> If timed out while blocked in step 4, return false, else true.
2158 public final boolean await(long time, TimeUnit unit)
2159 throws InterruptedException {
2161 throw new NullPointerException();
2162 long nanosTimeout = unit.toNanos(time);
2163 if (Thread.interrupted())
2164 throw new InterruptedException();
2165 Node node = addConditionWaiter();
2166 int savedState = fullyRelease(node);
2167 long lastTime = System.nanoTime();
2168 boolean timedout = false;
2169 int interruptMode = 0;
2170 while (!isOnSyncQueue(node)) {
2171 if (nanosTimeout <= 0L) {
2172 timedout = transferAfterCancelledWait(node);
2175 if (nanosTimeout >= spinForTimeoutThreshold)
2176 LockSupport.parkNanos(this, nanosTimeout);
2177 if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
2179 long now = System.nanoTime();
2180 nanosTimeout -= now - lastTime;
2183 if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
2184 interruptMode = REINTERRUPT;
2185 if (node.nextWaiter != null)
2186 unlinkCancelledWaiters();
2187 if (interruptMode != 0)
2188 reportInterruptAfterWait(interruptMode);
2192 // support for instrumentation
2195 * Returns true if this condition was created by the given
2196 * synchronization object.
2198 * @return {@code true} if owned
2200 final boolean isOwnedBy(AbstractQueuedSynchronizer sync) {
2201 return sync == AbstractQueuedSynchronizer.this;
2205 * Queries whether any threads are waiting on this condition.
2206 * Implements {@link AbstractQueuedSynchronizer#hasWaiters}.
2208 * @return {@code true} if there are any waiting threads
2209 * @throws IllegalMonitorStateException if {@link #isHeldExclusively}
2210 * returns {@code false}
2212 protected final boolean hasWaiters() {
2213 if (!isHeldExclusively())
2214 throw new IllegalMonitorStateException();
2215 for (Node w = firstWaiter; w != null; w = w.nextWaiter) {
2216 if (w.waitStatus == Node.CONDITION)
2223 * Returns an estimate of the number of threads waiting on
2225 * Implements {@link AbstractQueuedSynchronizer#getWaitQueueLength}.
2227 * @return the estimated number of waiting threads
2228 * @throws IllegalMonitorStateException if {@link #isHeldExclusively}
2229 * returns {@code false}
2231 protected final int getWaitQueueLength() {
2232 if (!isHeldExclusively())
2233 throw new IllegalMonitorStateException();
2235 for (Node w = firstWaiter; w != null; w = w.nextWaiter) {
2236 if (w.waitStatus == Node.CONDITION)
2243 * Returns a collection containing those threads that may be
2244 * waiting on this Condition.
2245 * Implements {@link AbstractQueuedSynchronizer#getWaitingThreads}.
2247 * @return the collection of threads
2248 * @throws IllegalMonitorStateException if {@link #isHeldExclusively}
2249 * returns {@code false}
2251 protected final Collection<Thread> getWaitingThreads() {
2252 if (!isHeldExclusively())
2253 throw new IllegalMonitorStateException();
2254 ArrayList<Thread> list = new ArrayList<Thread>();
2255 for (Node w = firstWaiter; w != null; w = w.nextWaiter) {
2256 if (w.waitStatus == Node.CONDITION) {
2257 Thread t = w.thread;
2267 * Setup to support compareAndSet. We need to natively implement
2268 * this here: For the sake of permitting future enhancements, we
2269 * cannot explicitly subclass AtomicInteger, which would be
2270 * efficient and useful otherwise. So, as the lesser of evils, we
2271 * natively implement using hotspot intrinsics API. And while we
2272 * are at it, we do the same for other CASable fields (which could
2273 * otherwise be done with atomic field updaters).
2275 private static final Unsafe unsafe = Unsafe.getUnsafe();
2276 private static final long stateOffset;
2277 private static final long headOffset;
2278 private static final long tailOffset;
2279 private static final long waitStatusOffset;
2280 private static final long nextOffset;
2284 stateOffset = unsafe.objectFieldOffset
2285 (AbstractQueuedSynchronizer.class.getDeclaredField("state"));
2286 headOffset = unsafe.objectFieldOffset
2287 (AbstractQueuedSynchronizer.class.getDeclaredField("head"));
2288 tailOffset = unsafe.objectFieldOffset
2289 (AbstractQueuedSynchronizer.class.getDeclaredField("tail"));
2290 waitStatusOffset = unsafe.objectFieldOffset
2291 (Node.class.getDeclaredField("waitStatus"));
2292 nextOffset = unsafe.objectFieldOffset
2293 (Node.class.getDeclaredField("next"));
2295 } catch (Exception ex) { throw new Error(ex); }
2299 * CAS head field. Used only by enq.
2301 private final boolean compareAndSetHead(Node update) {
2302 return unsafe.compareAndSwapObject(this, headOffset, null, update);
2306 * CAS tail field. Used only by enq.
2308 private final boolean compareAndSetTail(Node expect, Node update) {
2309 return unsafe.compareAndSwapObject(this, tailOffset, expect, update);
2313 * CAS waitStatus field of a node.
2315 private static final boolean compareAndSetWaitStatus(Node node,
2318 return unsafe.compareAndSwapInt(node, waitStatusOffset,
2323 * CAS next field of a node.
2325 private static final boolean compareAndSetNext(Node node,
2328 return unsafe.compareAndSwapObject(node, nextOffset, expect, update);