Subclasses should be defined as non-public internal helper + * classes that are used to implement the synchronization properties + * of their enclosing class. Class + * AbstractQueuedSynchronizer does not implement any + * synchronization interface. Instead it defines methods such as + * {@link #acquireInterruptibly} that can be invoked as + * appropriate by concrete locks and related synchronizers to + * implement their public methods. + * + *
This class supports either or both a default exclusive + * mode and a shared mode. When acquired in exclusive mode, + * attempted acquires by other threads cannot succeed. Shared mode + * acquires by multiple threads may (but need not) succeed. This class + * does not "understand" these differences except in the + * mechanical sense that when a shared mode acquire succeeds, the next + * waiting thread (if one exists) must also determine whether it can + * acquire as well. Threads waiting in the different modes share the + * same FIFO queue. Usually, implementation subclasses support only + * one of these modes, but both can come into play for example in a + * {@link ReadWriteLock}. Subclasses that support only exclusive or + * only shared modes need not define the methods supporting the unused mode. + * + *
This class defines a nested {@link ConditionObject} class that + * can be used as a {@link Condition} implementation by subclasses + * supporting exclusive mode for which method {@link + * #isHeldExclusively} reports whether synchronization is exclusively + * held with respect to the current thread, method {@link #release} + * invoked with the current {@link #getState} value fully releases + * this object, and {@link #acquire}, given this saved state value, + * eventually restores this object to its previous acquired state. No + * AbstractQueuedSynchronizer method otherwise creates such a + * condition, so if this constraint cannot be met, do not use it. The + * behavior of {@link ConditionObject} depends of course on the + * semantics of its synchronizer implementation. + * + *
This class provides inspection, instrumentation, and monitoring + * methods for the internal queue, as well as similar methods for + * condition objects. These can be exported as desired into classes + * using an AbstractQueuedSynchronizer for their + * synchronization mechanics. + * + *
Serialization of this class stores only the underlying atomic + * integer maintaining state, so deserialized objects have empty + * thread queues. Typical subclasses requiring serializability will + * define a readObject method that restores this to a known + * initial state upon deserialization. + * + *
To use this class as the basis of a synchronizer, redefine the + * following methods, as applicable, by inspecting and/or modifying + * the synchronization state using {@link #getState}, {@link + * #setState} and/or {@link #compareAndSetState}: + * + *
You may also find the inherited methods from {@link + * AbstractOwnableSynchronizer} useful to keep track of the thread + * owning an exclusive synchronizer. You are encouraged to use them + * -- this enables monitoring and diagnostic tools to assist users in + * determining which threads hold locks. + * + *
Even though this class is based on an internal FIFO queue, it + * does not automatically enforce FIFO acquisition policies. The core + * of exclusive synchronization takes the form: + * + *
+ * Acquire:
+ * while (!tryAcquire(arg)) {
+ * enqueue thread if it is not already queued;
+ * possibly block current thread;
+ * }
+ *
+ * Release:
+ * if (tryRelease(arg))
+ * unblock the first queued thread;
+ *
+ *
+ * (Shared mode is similar but may involve cascading signals.)
+ *
+ * Because checks in acquire are invoked before
+ * enqueuing, a newly acquiring thread may barge ahead of
+ * others that are blocked and queued. However, you can, if desired,
+ * define tryAcquire and/or tryAcquireShared to
+ * disable barging by internally invoking one or more of the inspection
+ * methods, thereby providing a fair FIFO acquisition order.
+ * In particular, most fair synchronizers can define tryAcquire
+ * to return false if {@link #hasQueuedPredecessors} (a method
+ * specifically designed to be used by fair synchronizers) returns
+ * true. Other variations are possible.
+ *
+ * Throughput and scalability are generally highest for the
+ * default barging (also known as greedy,
+ * renouncement, and convoy-avoidance) strategy.
+ * While this is not guaranteed to be fair or starvation-free, earlier
+ * queued threads are allowed to recontend before later queued
+ * threads, and each recontention has an unbiased chance to succeed
+ * against incoming threads. Also, while acquires do not
+ * "spin" in the usual sense, they may perform multiple
+ * invocations of tryAcquire interspersed with other
+ * computations before blocking. This gives most of the benefits of
+ * spins when exclusive synchronization is only briefly held, without
+ * most of the liabilities when it isn't. If so desired, you can
+ * augment this by preceding calls to acquire methods with
+ * "fast-path" checks, possibly prechecking {@link #hasContended}
+ * and/or {@link #hasQueuedThreads} to only do so if the synchronizer
+ * is likely not to be contended.
+ *
+ * This class provides an efficient and scalable basis for
+ * synchronization in part by specializing its range of use to
+ * synchronizers that can rely on int state, acquire, and
+ * release parameters, and an internal FIFO wait queue. When this does
+ * not suffice, you can build synchronizers from a lower level using
+ * {@link java.util.concurrent.atomic atomic} classes, your own custom
+ * {@link java.util.Queue} classes, and {@link LockSupport} blocking
+ * support.
+ *
+ * Here is a non-reentrant mutual exclusion lock class that uses
+ * the value zero to represent the unlocked state, and one to
+ * represent the locked state. While a non-reentrant lock
+ * does not strictly require recording of the current owner
+ * thread, this class does so anyway to make usage easier to monitor.
+ * It also supports conditions and exposes
+ * one of the instrumentation methods:
+ *
+ * Here is a latch class that is like a {@link CountDownLatch}
+ * except that it only requires a single signal to
+ * fire. Because a latch is non-exclusive, it uses the shared
+ * acquire and release methods.
+ *
+ * The wait queue is a variant of a "CLH" (Craig, Landin, and
+ * Hagersten) lock queue. CLH locks are normally used for
+ * spinlocks. We instead use them for blocking synchronizers, but
+ * use the same basic tactic of holding some of the control
+ * information about a thread in the predecessor of its node. A
+ * "status" field in each node keeps track of whether a thread
+ * should block. A node is signalled when its predecessor
+ * releases. Each node of the queue otherwise serves as a
+ * specific-notification-style monitor holding a single waiting
+ * thread. The status field does NOT control whether threads are
+ * granted locks etc though. A thread may try to acquire if it is
+ * first in the queue. But being first does not guarantee success;
+ * it only gives the right to contend. So the currently released
+ * contender thread may need to rewait.
+ *
+ * To enqueue into a CLH lock, you atomically splice it in as new
+ * tail. To dequeue, you just set the head field.
+ * Insertion into a CLH queue requires only a single atomic
+ * operation on "tail", so there is a simple atomic point of
+ * demarcation from unqueued to queued. Similarly, dequeing
+ * involves only updating the "head". However, it takes a bit
+ * more work for nodes to determine who their successors are,
+ * in part to deal with possible cancellation due to timeouts
+ * and interrupts.
+ *
+ * The "prev" links (not used in original CLH locks), are mainly
+ * needed to handle cancellation. If a node is cancelled, its
+ * successor is (normally) relinked to a non-cancelled
+ * predecessor. For explanation of similar mechanics in the case
+ * of spin locks, see the papers by Scott and Scherer at
+ * http://www.cs.rochester.edu/u/scott/synchronization/
+ *
+ * We also use "next" links to implement blocking mechanics.
+ * The thread id for each node is kept in its own node, so a
+ * predecessor signals the next node to wake up by traversing
+ * next link to determine which thread it is. Determination of
+ * successor must avoid races with newly queued nodes to set
+ * the "next" fields of their predecessors. This is solved
+ * when necessary by checking backwards from the atomically
+ * updated "tail" when a node's successor appears to be null.
+ * (Or, said differently, the next-links are an optimization
+ * so that we don't usually need a backward scan.)
+ *
+ * Cancellation introduces some conservatism to the basic
+ * algorithms. Since we must poll for cancellation of other
+ * nodes, we can miss noticing whether a cancelled node is
+ * ahead or behind us. This is dealt with by always unparking
+ * successors upon cancellation, allowing them to stabilize on
+ * a new predecessor, unless we can identify an uncancelled
+ * predecessor who will carry this responsibility.
+ *
+ * CLH queues need a dummy header node to get started. But
+ * we don't create them on construction, because it would be wasted
+ * effort if there is never contention. Instead, the node
+ * is constructed and head and tail pointers are set upon first
+ * contention.
+ *
+ * Threads waiting on Conditions use the same nodes, but
+ * use an additional link. Conditions only need to link nodes
+ * in simple (non-concurrent) linked queues because they are
+ * only accessed when exclusively held. Upon await, a node is
+ * inserted into a condition queue. Upon signal, the node is
+ * transferred to the main queue. A special value of status
+ * field is used to mark which queue a node is on.
+ *
+ * Thanks go to Dave Dice, Mark Moir, Victor Luchangco, Bill
+ * Scherer and Michael Scott, along with members of JSR-166
+ * expert group, for helpful ideas, discussions, and critiques
+ * on the design of this class.
+ */
+ static final class Node {
+ /** Marker to indicate a node is waiting in shared mode */
+ static final Node SHARED = new Node();
+ /** Marker to indicate a node is waiting in exclusive mode */
+ static final Node EXCLUSIVE = null;
+
+ /** waitStatus value to indicate thread has cancelled */
+ static final int CANCELLED = 1;
+ /** waitStatus value to indicate successor's thread needs unparking */
+ static final int SIGNAL = -1;
+ /** waitStatus value to indicate thread is waiting on condition */
+ static final int CONDITION = -2;
+ /**
+ * waitStatus value to indicate the next acquireShared should
+ * unconditionally propagate
+ */
+ static final int PROPAGATE = -3;
+
+ /**
+ * Status field, taking on only the values:
+ * SIGNAL: The successor of this node is (or will soon be)
+ * blocked (via park), so the current node must
+ * unpark its successor when it releases or
+ * cancels. To avoid races, acquire methods must
+ * first indicate they need a signal,
+ * then retry the atomic acquire, and then,
+ * on failure, block.
+ * CANCELLED: This node is cancelled due to timeout or interrupt.
+ * Nodes never leave this state. In particular,
+ * a thread with cancelled node never again blocks.
+ * CONDITION: This node is currently on a condition queue.
+ * It will not be used as a sync queue node
+ * until transferred, at which time the status
+ * will be set to 0. (Use of this value here has
+ * nothing to do with the other uses of the
+ * field, but simplifies mechanics.)
+ * PROPAGATE: A releaseShared should be propagated to other
+ * nodes. This is set (for head node only) in
+ * doReleaseShared to ensure propagation
+ * continues, even if other operations have
+ * since intervened.
+ * 0: None of the above
+ *
+ * The values are arranged numerically to simplify use.
+ * Non-negative values mean that a node doesn't need to
+ * signal. So, most code doesn't need to check for particular
+ * values, just for sign.
+ *
+ * The field is initialized to 0 for normal sync nodes, and
+ * CONDITION for condition nodes. It is modified using CAS
+ * (or when possible, unconditional volatile writes).
+ */
+ volatile int waitStatus;
+
+ /**
+ * Link to predecessor node that current node/thread relies on
+ * for checking waitStatus. Assigned during enqueing, and nulled
+ * out (for sake of GC) only upon dequeuing. Also, upon
+ * cancellation of a predecessor, we short-circuit while
+ * finding a non-cancelled one, which will always exist
+ * because the head node is never cancelled: A node becomes
+ * head only as a result of successful acquire. A
+ * cancelled thread never succeeds in acquiring, and a thread only
+ * cancels itself, not any other node.
+ */
+ volatile Node prev;
+
+ /**
+ * Link to the successor node that the current node/thread
+ * unparks upon release. Assigned during enqueuing, adjusted
+ * when bypassing cancelled predecessors, and nulled out (for
+ * sake of GC) when dequeued. The enq operation does not
+ * assign next field of a predecessor until after attachment,
+ * so seeing a null next field does not necessarily mean that
+ * node is at end of queue. However, if a next field appears
+ * to be null, we can scan prev's from the tail to
+ * double-check. The next field of cancelled nodes is set to
+ * point to the node itself instead of null, to make life
+ * easier for isOnSyncQueue.
+ */
+ volatile Node next;
+
+ /**
+ * The thread that enqueued this node. Initialized on
+ * construction and nulled out after use.
+ */
+ volatile Thread thread;
+
+ /**
+ * Link to next node waiting on condition, or the special
+ * value SHARED. Because condition queues are accessed only
+ * when holding in exclusive mode, we just need a simple
+ * linked queue to hold nodes while they are waiting on
+ * conditions. They are then transferred to the queue to
+ * re-acquire. And because conditions can only be exclusive,
+ * we save a field by using special value to indicate shared
+ * mode.
+ */
+ Node nextWaiter;
+
+ /**
+ * Returns true if node is waiting in shared mode
+ */
+ final boolean isShared() {
+ return nextWaiter == SHARED;
+ }
+
+ /**
+ * Returns previous node, or throws NullPointerException if null.
+ * Use when predecessor cannot be null. The null check could
+ * be elided, but is present to help the VM.
+ *
+ * @return the predecessor of this node
+ */
+ final Node predecessor() throws NullPointerException {
+ Node p = prev;
+ if (p == null)
+ throw new NullPointerException();
+ else
+ return p;
+ }
+
+ Node() { // Used to establish initial head or SHARED marker
+ }
+
+ Node(Thread thread, Node mode) { // Used by addWaiter
+ this.nextWaiter = mode;
+ this.thread = thread;
+ }
+
+ Node(Thread thread, int waitStatus) { // Used by Condition
+ this.waitStatus = waitStatus;
+ this.thread = thread;
+ }
+ }
+
+ /**
+ * Head of the wait queue, lazily initialized. Except for
+ * initialization, it is modified only via method setHead. Note:
+ * If head exists, its waitStatus is guaranteed not to be
+ * CANCELLED.
+ */
+ private transient volatile Node head;
+
+ /**
+ * Tail of the wait queue, lazily initialized. Modified only via
+ * method enq to add new wait node.
+ */
+ private transient volatile Node tail;
+
+ /**
+ * The synchronization state.
+ */
+ private volatile int state;
+
+ /**
+ * Returns the current value of synchronization state.
+ * This operation has memory semantics of a volatile read.
+ * @return current state value
+ */
+ protected final int getState() {
+ return state;
+ }
+
+ /**
+ * Sets the value of synchronization state.
+ * This operation has memory semantics of a volatile write.
+ * @param newState the new state value
+ */
+ protected final void setState(int newState) {
+ state = newState;
+ }
+
+ /**
+ * Atomically sets synchronization state to the given updated
+ * value if the current state value equals the expected value.
+ * This operation has memory semantics of a volatile read
+ * and write.
+ *
+ * @param expect the expected value
+ * @param update the new value
+ * @return true if successful. False return indicates that the actual
+ * value was not equal to the expected value.
+ */
+ protected final boolean compareAndSetState(int expect, int update) {
+ // See below for intrinsics setup to support this
+ return unsafe.compareAndSwapInt(this, stateOffset, expect, update);
+ }
+
+ // Queuing utilities
+
+ /**
+ * The number of nanoseconds for which it is faster to spin
+ * rather than to use timed park. A rough estimate suffices
+ * to improve responsiveness with very short timeouts.
+ */
+ static final long spinForTimeoutThreshold = 1000L;
+
+ /**
+ * Inserts node into queue, initializing if necessary. See picture above.
+ * @param node the node to insert
+ * @return node's predecessor
+ */
+ private Node enq(final Node node) {
+ for (;;) {
+ Node t = tail;
+ if (t == null) { // Must initialize
+ if (compareAndSetHead(new Node()))
+ tail = head;
+ } else {
+ node.prev = t;
+ if (compareAndSetTail(t, node)) {
+ t.next = node;
+ return t;
+ }
+ }
+ }
+ }
+
+ /**
+ * Creates and enqueues node for current thread and given mode.
+ *
+ * @param mode Node.EXCLUSIVE for exclusive, Node.SHARED for shared
+ * @return the new node
+ */
+ private Node addWaiter(Node mode) {
+ Node node = new Node(Thread.currentThread(), mode);
+ // Try the fast path of enq; backup to full enq on failure
+ Node pred = tail;
+ if (pred != null) {
+ node.prev = pred;
+ if (compareAndSetTail(pred, node)) {
+ pred.next = node;
+ return node;
+ }
+ }
+ enq(node);
+ return node;
+ }
+
+ /**
+ * Sets head of queue to be node, thus dequeuing. Called only by
+ * acquire methods. Also nulls out unused fields for sake of GC
+ * and to suppress unnecessary signals and traversals.
+ *
+ * @param node the node
+ */
+ private void setHead(Node node) {
+ head = node;
+ node.thread = null;
+ node.prev = null;
+ }
+
+ /**
+ * Wakes up node's successor, if one exists.
+ *
+ * @param node the node
+ */
+ private void unparkSuccessor(Node node) {
+ /*
+ * If status is negative (i.e., possibly needing signal) try
+ * to clear in anticipation of signalling. It is OK if this
+ * fails or if status is changed by waiting thread.
+ */
+ int ws = node.waitStatus;
+ if (ws < 0)
+ compareAndSetWaitStatus(node, ws, 0);
+
+ /*
+ * Thread to unpark is held in successor, which is normally
+ * just the next node. But if cancelled or apparently null,
+ * traverse backwards from tail to find the actual
+ * non-cancelled successor.
+ */
+ Node s = node.next;
+ if (s == null || s.waitStatus > 0) {
+ s = null;
+ for (Node t = tail; t != null && t != node; t = t.prev)
+ if (t.waitStatus <= 0)
+ s = t;
+ }
+ if (s != null)
+ LockSupport.unpark(s.thread);
+ }
+
+ /**
+ * Release action for shared mode -- signal successor and ensure
+ * propagation. (Note: For exclusive mode, release just amounts
+ * to calling unparkSuccessor of head if it needs signal.)
+ */
+ private void doReleaseShared() {
+ /*
+ * Ensure that a release propagates, even if there are other
+ * in-progress acquires/releases. This proceeds in the usual
+ * way of trying to unparkSuccessor of head if it needs
+ * signal. But if it does not, status is set to PROPAGATE to
+ * ensure that upon release, propagation continues.
+ * Additionally, we must loop in case a new node is added
+ * while we are doing this. Also, unlike other uses of
+ * unparkSuccessor, we need to know if CAS to reset status
+ * fails, if so rechecking.
+ */
+ for (;;) {
+ Node h = head;
+ if (h != null && h != tail) {
+ int ws = h.waitStatus;
+ if (ws == Node.SIGNAL) {
+ if (!compareAndSetWaitStatus(h, Node.SIGNAL, 0))
+ continue; // loop to recheck cases
+ unparkSuccessor(h);
+ }
+ else if (ws == 0 &&
+ !compareAndSetWaitStatus(h, 0, Node.PROPAGATE))
+ continue; // loop on failed CAS
+ }
+ if (h == head) // loop if head changed
+ break;
+ }
+ }
+
+ /**
+ * Sets head of queue, and checks if successor may be waiting
+ * in shared mode, if so propagating if either propagate > 0 or
+ * PROPAGATE status was set.
+ *
+ * @param node the node
+ * @param propagate the return value from a tryAcquireShared
+ */
+ private void setHeadAndPropagate(Node node, int propagate) {
+ Node h = head; // Record old head for check below
+ setHead(node);
+ /*
+ * Try to signal next queued node if:
+ * Propagation was indicated by caller,
+ * or was recorded (as h.waitStatus) by a previous operation
+ * (note: this uses sign-check of waitStatus because
+ * PROPAGATE status may transition to SIGNAL.)
+ * and
+ * The next node is waiting in shared mode,
+ * or we don't know, because it appears null
+ *
+ * The conservatism in both of these checks may cause
+ * unnecessary wake-ups, but only when there are multiple
+ * racing acquires/releases, so most need signals now or soon
+ * anyway.
+ */
+ if (propagate > 0 || h == null || h.waitStatus < 0) {
+ Node s = node.next;
+ if (s == null || s.isShared())
+ doReleaseShared();
+ }
+ }
+
+ // Utilities for various versions of acquire
+
+ /**
+ * Cancels an ongoing attempt to acquire.
+ *
+ * @param node the node
+ */
+ private void cancelAcquire(Node node) {
+ // Ignore if node doesn't exist
+ if (node == null)
+ return;
+
+ node.thread = null;
+
+ // Skip cancelled predecessors
+ Node pred = node.prev;
+ while (pred.waitStatus > 0)
+ node.prev = pred = pred.prev;
+
+ // predNext is the apparent node to unsplice. CASes below will
+ // fail if not, in which case, we lost race vs another cancel
+ // or signal, so no further action is necessary.
+ Node predNext = pred.next;
+
+ // Can use unconditional write instead of CAS here.
+ // After this atomic step, other Nodes can skip past us.
+ // Before, we are free of interference from other threads.
+ node.waitStatus = Node.CANCELLED;
+
+ // If we are the tail, remove ourselves.
+ if (node == tail && compareAndSetTail(node, pred)) {
+ compareAndSetNext(pred, predNext, null);
+ } else {
+ // If successor needs signal, try to set pred's next-link
+ // so it will get one. Otherwise wake it up to propagate.
+ int ws;
+ if (pred != head &&
+ ((ws = pred.waitStatus) == Node.SIGNAL ||
+ (ws <= 0 && compareAndSetWaitStatus(pred, ws, Node.SIGNAL))) &&
+ pred.thread != null) {
+ Node next = node.next;
+ if (next != null && next.waitStatus <= 0)
+ compareAndSetNext(pred, predNext, next);
+ } else {
+ unparkSuccessor(node);
+ }
+
+ node.next = node; // help GC
+ }
+ }
+
+ /**
+ * Checks and updates status for a node that failed to acquire.
+ * Returns true if thread should block. This is the main signal
+ * control in all acquire loops. Requires that pred == node.prev
+ *
+ * @param pred node's predecessor holding status
+ * @param node the node
+ * @return {@code true} if thread should block
+ */
+ private static boolean shouldParkAfterFailedAcquire(Node pred, Node node) {
+ int ws = pred.waitStatus;
+ if (ws == Node.SIGNAL)
+ /*
+ * This node has already set status asking a release
+ * to signal it, so it can safely park.
+ */
+ return true;
+ if (ws > 0) {
+ /*
+ * Predecessor was cancelled. Skip over predecessors and
+ * indicate retry.
+ */
+ do {
+ node.prev = pred = pred.prev;
+ } while (pred.waitStatus > 0);
+ pred.next = node;
+ } else {
+ /*
+ * waitStatus must be 0 or PROPAGATE. Indicate that we
+ * need a signal, but don't park yet. Caller will need to
+ * retry to make sure it cannot acquire before parking.
+ */
+ compareAndSetWaitStatus(pred, ws, Node.SIGNAL);
+ }
+ return false;
+ }
+
+ /**
+ * Convenience method to interrupt current thread.
+ */
+ private static void selfInterrupt() {
+ Thread.currentThread().interrupt();
+ }
+
+ /**
+ * Convenience method to park and then check if interrupted
+ *
+ * @return {@code true} if interrupted
+ */
+ private final boolean parkAndCheckInterrupt() {
+ LockSupport.park(this);
+ return Thread.interrupted();
+ }
+
+ /*
+ * Various flavors of acquire, varying in exclusive/shared and
+ * control modes. Each is mostly the same, but annoyingly
+ * different. Only a little bit of factoring is possible due to
+ * interactions of exception mechanics (including ensuring that we
+ * cancel if tryAcquire throws exception) and other control, at
+ * least not without hurting performance too much.
+ */
+
+ /**
+ * Acquires in exclusive uninterruptible mode for thread already in
+ * queue. Used by condition wait methods as well as acquire.
+ *
+ * @param node the node
+ * @param arg the acquire argument
+ * @return {@code true} if interrupted while waiting
+ */
+ final boolean acquireQueued(final Node node, int arg) {
+ boolean failed = true;
+ try {
+ boolean interrupted = false;
+ for (;;) {
+ final Node p = node.predecessor();
+ if (p == head && tryAcquire(arg)) {
+ setHead(node);
+ p.next = null; // help GC
+ failed = false;
+ return interrupted;
+ }
+ if (shouldParkAfterFailedAcquire(p, node) &&
+ parkAndCheckInterrupt())
+ interrupted = true;
+ }
+ } finally {
+ if (failed)
+ cancelAcquire(node);
+ }
+ }
+
+ /**
+ * Acquires in exclusive interruptible mode.
+ * @param arg the acquire argument
+ */
+ private void doAcquireInterruptibly(int arg)
+ throws InterruptedException {
+ final Node node = addWaiter(Node.EXCLUSIVE);
+ boolean failed = true;
+ try {
+ for (;;) {
+ final Node p = node.predecessor();
+ if (p == head && tryAcquire(arg)) {
+ setHead(node);
+ p.next = null; // help GC
+ failed = false;
+ return;
+ }
+ if (shouldParkAfterFailedAcquire(p, node) &&
+ parkAndCheckInterrupt())
+ throw new InterruptedException();
+ }
+ } finally {
+ if (failed)
+ cancelAcquire(node);
+ }
+ }
+
+ /**
+ * Acquires in exclusive timed mode.
+ *
+ * @param arg the acquire argument
+ * @param nanosTimeout max wait time
+ * @return {@code true} if acquired
+ */
+ private boolean doAcquireNanos(int arg, long nanosTimeout)
+ throws InterruptedException {
+ long lastTime = System.nanoTime();
+ final Node node = addWaiter(Node.EXCLUSIVE);
+ boolean failed = true;
+ try {
+ for (;;) {
+ final Node p = node.predecessor();
+ if (p == head && tryAcquire(arg)) {
+ setHead(node);
+ p.next = null; // help GC
+ failed = false;
+ return true;
+ }
+ if (nanosTimeout <= 0)
+ return false;
+ if (shouldParkAfterFailedAcquire(p, node) &&
+ nanosTimeout > spinForTimeoutThreshold)
+ LockSupport.parkNanos(this, nanosTimeout);
+ long now = System.nanoTime();
+ nanosTimeout -= now - lastTime;
+ lastTime = now;
+ if (Thread.interrupted())
+ throw new InterruptedException();
+ }
+ } finally {
+ if (failed)
+ cancelAcquire(node);
+ }
+ }
+
+ /**
+ * Acquires in shared uninterruptible mode.
+ * @param arg the acquire argument
+ */
+ private void doAcquireShared(int arg) {
+ final Node node = addWaiter(Node.SHARED);
+ boolean failed = true;
+ try {
+ boolean interrupted = false;
+ for (;;) {
+ final Node p = node.predecessor();
+ if (p == head) {
+ int r = tryAcquireShared(arg);
+ if (r >= 0) {
+ setHeadAndPropagate(node, r);
+ p.next = null; // help GC
+ if (interrupted)
+ selfInterrupt();
+ failed = false;
+ return;
+ }
+ }
+ if (shouldParkAfterFailedAcquire(p, node) &&
+ parkAndCheckInterrupt())
+ interrupted = true;
+ }
+ } finally {
+ if (failed)
+ cancelAcquire(node);
+ }
+ }
+
+ /**
+ * Acquires in shared interruptible mode.
+ * @param arg the acquire argument
+ */
+ private void doAcquireSharedInterruptibly(int arg)
+ throws InterruptedException {
+ final Node node = addWaiter(Node.SHARED);
+ boolean failed = true;
+ try {
+ for (;;) {
+ final Node p = node.predecessor();
+ if (p == head) {
+ int r = tryAcquireShared(arg);
+ if (r >= 0) {
+ setHeadAndPropagate(node, r);
+ p.next = null; // help GC
+ failed = false;
+ return;
+ }
+ }
+ if (shouldParkAfterFailedAcquire(p, node) &&
+ parkAndCheckInterrupt())
+ throw new InterruptedException();
+ }
+ } finally {
+ if (failed)
+ cancelAcquire(node);
+ }
+ }
+
+ /**
+ * Acquires in shared timed mode.
+ *
+ * @param arg the acquire argument
+ * @param nanosTimeout max wait time
+ * @return {@code true} if acquired
+ */
+ private boolean doAcquireSharedNanos(int arg, long nanosTimeout)
+ throws InterruptedException {
+
+ long lastTime = System.nanoTime();
+ final Node node = addWaiter(Node.SHARED);
+ boolean failed = true;
+ try {
+ for (;;) {
+ final Node p = node.predecessor();
+ if (p == head) {
+ int r = tryAcquireShared(arg);
+ if (r >= 0) {
+ setHeadAndPropagate(node, r);
+ p.next = null; // help GC
+ failed = false;
+ return true;
+ }
+ }
+ if (nanosTimeout <= 0)
+ return false;
+ if (shouldParkAfterFailedAcquire(p, node) &&
+ nanosTimeout > spinForTimeoutThreshold)
+ LockSupport.parkNanos(this, nanosTimeout);
+ long now = System.nanoTime();
+ nanosTimeout -= now - lastTime;
+ lastTime = now;
+ if (Thread.interrupted())
+ throw new InterruptedException();
+ }
+ } finally {
+ if (failed)
+ cancelAcquire(node);
+ }
+ }
+
+ // Main exported methods
+
+ /**
+ * Attempts to acquire in exclusive mode. This method should query
+ * if the state of the object permits it to be acquired in the
+ * exclusive mode, and if so to acquire it.
+ *
+ * This method is always invoked by the thread performing
+ * acquire. If this method reports failure, the acquire method
+ * may queue the thread, if it is not already queued, until it is
+ * signalled by a release from some other thread. This can be used
+ * to implement method {@link Lock#tryLock()}.
+ *
+ * The default
+ * implementation throws {@link UnsupportedOperationException}.
+ *
+ * @param arg the acquire argument. This value is always the one
+ * passed to an acquire method, or is the value saved on entry
+ * to a condition wait. The value is otherwise uninterpreted
+ * and can represent anything you like.
+ * @return {@code true} if successful. Upon success, this object has
+ * been acquired.
+ * @throws IllegalMonitorStateException if acquiring would place this
+ * synchronizer in an illegal state. This exception must be
+ * thrown in a consistent fashion for synchronization to work
+ * correctly.
+ * @throws UnsupportedOperationException if exclusive mode is not supported
+ */
+ protected boolean tryAcquire(int arg) {
+ throw new UnsupportedOperationException();
+ }
+
+ /**
+ * Attempts to set the state to reflect a release in exclusive
+ * mode.
+ *
+ * This method is always invoked by the thread performing release.
+ *
+ * The default implementation throws
+ * {@link UnsupportedOperationException}.
+ *
+ * @param arg the release argument. This value is always the one
+ * passed to a release method, or the current state value upon
+ * entry to a condition wait. The value is otherwise
+ * uninterpreted and can represent anything you like.
+ * @return {@code true} if this object is now in a fully released
+ * state, so that any waiting threads may attempt to acquire;
+ * and {@code false} otherwise.
+ * @throws IllegalMonitorStateException if releasing would place this
+ * synchronizer in an illegal state. This exception must be
+ * thrown in a consistent fashion for synchronization to work
+ * correctly.
+ * @throws UnsupportedOperationException if exclusive mode is not supported
+ */
+ protected boolean tryRelease(int arg) {
+ throw new UnsupportedOperationException();
+ }
+
+ /**
+ * Attempts to acquire in shared mode. This method should query if
+ * the state of the object permits it to be acquired in the shared
+ * mode, and if so to acquire it.
+ *
+ * This method is always invoked by the thread performing
+ * acquire. If this method reports failure, the acquire method
+ * may queue the thread, if it is not already queued, until it is
+ * signalled by a release from some other thread.
+ *
+ * The default implementation throws {@link
+ * UnsupportedOperationException}.
+ *
+ * @param arg the acquire argument. This value is always the one
+ * passed to an acquire method, or is the value saved on entry
+ * to a condition wait. The value is otherwise uninterpreted
+ * and can represent anything you like.
+ * @return a negative value on failure; zero if acquisition in shared
+ * mode succeeded but no subsequent shared-mode acquire can
+ * succeed; and a positive value if acquisition in shared
+ * mode succeeded and subsequent shared-mode acquires might
+ * also succeed, in which case a subsequent waiting thread
+ * must check availability. (Support for three different
+ * return values enables this method to be used in contexts
+ * where acquires only sometimes act exclusively.) Upon
+ * success, this object has been acquired.
+ * @throws IllegalMonitorStateException if acquiring would place this
+ * synchronizer in an illegal state. This exception must be
+ * thrown in a consistent fashion for synchronization to work
+ * correctly.
+ * @throws UnsupportedOperationException if shared mode is not supported
+ */
+ protected int tryAcquireShared(int arg) {
+ throw new UnsupportedOperationException();
+ }
+
+ /**
+ * Attempts to set the state to reflect a release in shared mode.
+ *
+ * This method is always invoked by the thread performing release.
+ *
+ * The default implementation throws
+ * {@link UnsupportedOperationException}.
+ *
+ * @param arg the release argument. This value is always the one
+ * passed to a release method, or the current state value upon
+ * entry to a condition wait. The value is otherwise
+ * uninterpreted and can represent anything you like.
+ * @return {@code true} if this release of shared mode may permit a
+ * waiting acquire (shared or exclusive) to succeed; and
+ * {@code false} otherwise
+ * @throws IllegalMonitorStateException if releasing would place this
+ * synchronizer in an illegal state. This exception must be
+ * thrown in a consistent fashion for synchronization to work
+ * correctly.
+ * @throws UnsupportedOperationException if shared mode is not supported
+ */
+ protected boolean tryReleaseShared(int arg) {
+ throw new UnsupportedOperationException();
+ }
+
+ /**
+ * Returns {@code true} if synchronization is held exclusively with
+ * respect to the current (calling) thread. This method is invoked
+ * upon each call to a non-waiting {@link ConditionObject} method.
+ * (Waiting methods instead invoke {@link #release}.)
+ *
+ * The default implementation throws {@link
+ * UnsupportedOperationException}. This method is invoked
+ * internally only within {@link ConditionObject} methods, so need
+ * not be defined if conditions are not used.
+ *
+ * @return {@code true} if synchronization is held exclusively;
+ * {@code false} otherwise
+ * @throws UnsupportedOperationException if conditions are not supported
+ */
+ protected boolean isHeldExclusively() {
+ throw new UnsupportedOperationException();
+ }
+
+ /**
+ * Acquires in exclusive mode, ignoring interrupts. Implemented
+ * by invoking at least once {@link #tryAcquire},
+ * returning on success. Otherwise the thread is queued, possibly
+ * repeatedly blocking and unblocking, invoking {@link
+ * #tryAcquire} until success. This method can be used
+ * to implement method {@link Lock#lock}.
+ *
+ * @param arg the acquire argument. This value is conveyed to
+ * {@link #tryAcquire} but is otherwise uninterpreted and
+ * can represent anything you like.
+ */
+ public final void acquire(int arg) {
+ if (!tryAcquire(arg) &&
+ acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
+ selfInterrupt();
+ }
+
+ /**
+ * Acquires in exclusive mode, aborting if interrupted.
+ * Implemented by first checking interrupt status, then invoking
+ * at least once {@link #tryAcquire}, returning on
+ * success. Otherwise the thread is queued, possibly repeatedly
+ * blocking and unblocking, invoking {@link #tryAcquire}
+ * until success or the thread is interrupted. This method can be
+ * used to implement method {@link Lock#lockInterruptibly}.
+ *
+ * @param arg the acquire argument. This value is conveyed to
+ * {@link #tryAcquire} but is otherwise uninterpreted and
+ * can represent anything you like.
+ * @throws InterruptedException if the current thread is interrupted
+ */
+ public final void acquireInterruptibly(int arg)
+ throws InterruptedException {
+ if (Thread.interrupted())
+ throw new InterruptedException();
+ if (!tryAcquire(arg))
+ doAcquireInterruptibly(arg);
+ }
+
+ /**
+ * Attempts to acquire in exclusive mode, aborting if interrupted,
+ * and failing if the given timeout elapses. Implemented by first
+ * checking interrupt status, then invoking at least once {@link
+ * #tryAcquire}, returning on success. Otherwise, the thread is
+ * queued, possibly repeatedly blocking and unblocking, invoking
+ * {@link #tryAcquire} until success or the thread is interrupted
+ * or the timeout elapses. This method can be used to implement
+ * method {@link Lock#tryLock(long, TimeUnit)}.
+ *
+ * @param arg the acquire argument. This value is conveyed to
+ * {@link #tryAcquire} but is otherwise uninterpreted and
+ * can represent anything you like.
+ * @param nanosTimeout the maximum number of nanoseconds to wait
+ * @return {@code true} if acquired; {@code false} if timed out
+ * @throws InterruptedException if the current thread is interrupted
+ */
+ public final boolean tryAcquireNanos(int arg, long nanosTimeout)
+ throws InterruptedException {
+ if (Thread.interrupted())
+ throw new InterruptedException();
+ return tryAcquire(arg) ||
+ doAcquireNanos(arg, nanosTimeout);
+ }
+
+ /**
+ * Releases in exclusive mode. Implemented by unblocking one or
+ * more threads if {@link #tryRelease} returns true.
+ * This method can be used to implement method {@link Lock#unlock}.
+ *
+ * @param arg the release argument. This value is conveyed to
+ * {@link #tryRelease} but is otherwise uninterpreted and
+ * can represent anything you like.
+ * @return the value returned from {@link #tryRelease}
+ */
+ public final boolean release(int arg) {
+ if (tryRelease(arg)) {
+ Node h = head;
+ if (h != null && h.waitStatus != 0)
+ unparkSuccessor(h);
+ return true;
+ }
+ return false;
+ }
+
+ /**
+ * Acquires in shared mode, ignoring interrupts. Implemented by
+ * first invoking at least once {@link #tryAcquireShared},
+ * returning on success. Otherwise the thread is queued, possibly
+ * repeatedly blocking and unblocking, invoking {@link
+ * #tryAcquireShared} until success.
+ *
+ * @param arg the acquire argument. This value is conveyed to
+ * {@link #tryAcquireShared} but is otherwise uninterpreted
+ * and can represent anything you like.
+ */
+ public final void acquireShared(int arg) {
+ if (tryAcquireShared(arg) < 0)
+ doAcquireShared(arg);
+ }
+
+ /**
+ * Acquires in shared mode, aborting if interrupted. Implemented
+ * by first checking interrupt status, then invoking at least once
+ * {@link #tryAcquireShared}, returning on success. Otherwise the
+ * thread is queued, possibly repeatedly blocking and unblocking,
+ * invoking {@link #tryAcquireShared} until success or the thread
+ * is interrupted.
+ * @param arg the acquire argument
+ * This value is conveyed to {@link #tryAcquireShared} but is
+ * otherwise uninterpreted and can represent anything
+ * you like.
+ * @throws InterruptedException if the current thread is interrupted
+ */
+ public final void acquireSharedInterruptibly(int arg)
+ throws InterruptedException {
+ if (Thread.interrupted())
+ throw new InterruptedException();
+ if (tryAcquireShared(arg) < 0)
+ doAcquireSharedInterruptibly(arg);
+ }
+
+ /**
+ * Attempts to acquire in shared mode, aborting if interrupted, and
+ * failing if the given timeout elapses. Implemented by first
+ * checking interrupt status, then invoking at least once {@link
+ * #tryAcquireShared}, returning on success. Otherwise, the
+ * thread is queued, possibly repeatedly blocking and unblocking,
+ * invoking {@link #tryAcquireShared} until success or the thread
+ * is interrupted or the timeout elapses.
+ *
+ * @param arg the acquire argument. This value is conveyed to
+ * {@link #tryAcquireShared} but is otherwise uninterpreted
+ * and can represent anything you like.
+ * @param nanosTimeout the maximum number of nanoseconds to wait
+ * @return {@code true} if acquired; {@code false} if timed out
+ * @throws InterruptedException if the current thread is interrupted
+ */
+ public final boolean tryAcquireSharedNanos(int arg, long nanosTimeout)
+ throws InterruptedException {
+ if (Thread.interrupted())
+ throw new InterruptedException();
+ return tryAcquireShared(arg) >= 0 ||
+ doAcquireSharedNanos(arg, nanosTimeout);
+ }
+
+ /**
+ * Releases in shared mode. Implemented by unblocking one or more
+ * threads if {@link #tryReleaseShared} returns true.
+ *
+ * @param arg the release argument. This value is conveyed to
+ * {@link #tryReleaseShared} but is otherwise uninterpreted
+ * and can represent anything you like.
+ * @return the value returned from {@link #tryReleaseShared}
+ */
+ public final boolean releaseShared(int arg) {
+ if (tryReleaseShared(arg)) {
+ doReleaseShared();
+ return true;
+ }
+ return false;
+ }
+
+ // Queue inspection methods
+
+ /**
+ * Queries whether any threads are waiting to acquire. Note that
+ * because cancellations due to interrupts and timeouts may occur
+ * at any time, a {@code true} return does not guarantee that any
+ * other thread will ever acquire.
+ *
+ * In this implementation, this operation returns in
+ * constant time.
+ *
+ * @return {@code true} if there may be other threads waiting to acquire
+ */
+ public final boolean hasQueuedThreads() {
+ return head != tail;
+ }
+
+ /**
+ * Queries whether any threads have ever contended to acquire this
+ * synchronizer; that is if an acquire method has ever blocked.
+ *
+ * In this implementation, this operation returns in
+ * constant time.
+ *
+ * @return {@code true} if there has ever been contention
+ */
+ public final boolean hasContended() {
+ return head != null;
+ }
+
+ /**
+ * Returns the first (longest-waiting) thread in the queue, or
+ * {@code null} if no threads are currently queued.
+ *
+ * In this implementation, this operation normally returns in
+ * constant time, but may iterate upon contention if other threads are
+ * concurrently modifying the queue.
+ *
+ * @return the first (longest-waiting) thread in the queue, or
+ * {@code null} if no threads are currently queued
+ */
+ public final Thread getFirstQueuedThread() {
+ // handle only fast path, else relay
+ return (head == tail) ? null : fullGetFirstQueuedThread();
+ }
+
+ /**
+ * Version of getFirstQueuedThread called when fastpath fails
+ */
+ private Thread fullGetFirstQueuedThread() {
+ /*
+ * The first node is normally head.next. Try to get its
+ * thread field, ensuring consistent reads: If thread
+ * field is nulled out or s.prev is no longer head, then
+ * some other thread(s) concurrently performed setHead in
+ * between some of our reads. We try this twice before
+ * resorting to traversal.
+ */
+ Node h, s;
+ Thread st;
+ if (((h = head) != null && (s = h.next) != null &&
+ s.prev == head && (st = s.thread) != null) ||
+ ((h = head) != null && (s = h.next) != null &&
+ s.prev == head && (st = s.thread) != null))
+ return st;
+
+ /*
+ * Head's next field might not have been set yet, or may have
+ * been unset after setHead. So we must check to see if tail
+ * is actually first node. If not, we continue on, safely
+ * traversing from tail back to head to find first,
+ * guaranteeing termination.
+ */
+
+ Node t = tail;
+ Thread firstThread = null;
+ while (t != null && t != head) {
+ Thread tt = t.thread;
+ if (tt != null)
+ firstThread = tt;
+ t = t.prev;
+ }
+ return firstThread;
+ }
+
+ /**
+ * Returns true if the given thread is currently queued.
+ *
+ * This implementation traverses the queue to determine
+ * presence of the given thread.
+ *
+ * @param thread the thread
+ * @return {@code true} if the given thread is on the queue
+ * @throws NullPointerException if the thread is null
+ */
+ public final boolean isQueued(Thread thread) {
+ if (thread == null)
+ throw new NullPointerException();
+ for (Node p = tail; p != null; p = p.prev)
+ if (p.thread == thread)
+ return true;
+ return false;
+ }
+
+ /**
+ * Returns {@code true} if the apparent first queued thread, if one
+ * exists, is waiting in exclusive mode. If this method returns
+ * {@code true}, and the current thread is attempting to acquire in
+ * shared mode (that is, this method is invoked from {@link
+ * #tryAcquireShared}) then it is guaranteed that the current thread
+ * is not the first queued thread. Used only as a heuristic in
+ * ReentrantReadWriteLock.
+ */
+ final boolean apparentlyFirstQueuedIsExclusive() {
+ Node h, s;
+ return (h = head) != null &&
+ (s = h.next) != null &&
+ !s.isShared() &&
+ s.thread != null;
+ }
+
+ /**
+ * Queries whether any threads have been waiting to acquire longer
+ * than the current thread.
+ *
+ * An invocation of this method is equivalent to (but may be
+ * more efficient than):
+ * Note that because cancellations due to interrupts and
+ * timeouts may occur at any time, a {@code true} return does not
+ * guarantee that some other thread will acquire before the current
+ * thread. Likewise, it is possible for another thread to win a
+ * race to enqueue after this method has returned {@code false},
+ * due to the queue being empty.
+ *
+ * This method is designed to be used by a fair synchronizer to
+ * avoid barging.
+ * Such a synchronizer's {@link #tryAcquire} method should return
+ * {@code false}, and its {@link #tryAcquireShared} method should
+ * return a negative value, if this method returns {@code true}
+ * (unless this is a reentrant acquire). For example, the {@code
+ * tryAcquire} method for a fair, reentrant, exclusive mode
+ * synchronizer might look like this:
+ *
+ * Method documentation for this class describes mechanics,
+ * not behavioral specifications from the point of view of Lock
+ * and Condition users. Exported versions of this class will in
+ * general need to be accompanied by documentation describing
+ * condition semantics that rely on those of the associated
+ * AbstractQueuedSynchronizer.
+ *
+ * This class is Serializable, but all fields are transient,
+ * so deserialized conditions have no waiters.
+ */
+ public class ConditionObject implements Condition, java.io.Serializable {
+ private static final long serialVersionUID = 1173984872572414699L;
+ /** First node of condition queue. */
+ private transient Node firstWaiter;
+ /** Last node of condition queue. */
+ private transient Node lastWaiter;
+
+ /**
+ * Creates a new ConditionObject instance.
+ */
+ public ConditionObject() { }
+
+ // Internal methods
+
+ /**
+ * Adds a new waiter to wait queue.
+ * @return its new wait node
+ */
+ private Node addConditionWaiter() {
+ Node t = lastWaiter;
+ // If lastWaiter is cancelled, clean out.
+ if (t != null && t.waitStatus != Node.CONDITION) {
+ unlinkCancelledWaiters();
+ t = lastWaiter;
+ }
+ Node node = new Node(Thread.currentThread(), Node.CONDITION);
+ if (t == null)
+ firstWaiter = node;
+ else
+ t.nextWaiter = node;
+ lastWaiter = node;
+ return node;
+ }
+
+ /**
+ * Removes and transfers nodes until hit non-cancelled one or
+ * null. Split out from signal in part to encourage compilers
+ * to inline the case of no waiters.
+ * @param first (non-null) the first node on condition queue
+ */
+ private void doSignal(Node first) {
+ do {
+ if ( (firstWaiter = first.nextWaiter) == null)
+ lastWaiter = null;
+ first.nextWaiter = null;
+ } while (!transferForSignal(first) &&
+ (first = firstWaiter) != null);
+ }
+
+ /**
+ * Removes and transfers all nodes.
+ * @param first (non-null) the first node on condition queue
+ */
+ private void doSignalAll(Node first) {
+ lastWaiter = firstWaiter = null;
+ do {
+ Node next = first.nextWaiter;
+ first.nextWaiter = null;
+ transferForSignal(first);
+ first = next;
+ } while (first != null);
+ }
+
+ /**
+ * Unlinks cancelled waiter nodes from condition queue.
+ * Called only while holding lock. This is called when
+ * cancellation occurred during condition wait, and upon
+ * insertion of a new waiter when lastWaiter is seen to have
+ * been cancelled. This method is needed to avoid garbage
+ * retention in the absence of signals. So even though it may
+ * require a full traversal, it comes into play only when
+ * timeouts or cancellations occur in the absence of
+ * signals. It traverses all nodes rather than stopping at a
+ * particular target to unlink all pointers to garbage nodes
+ * without requiring many re-traversals during cancellation
+ * storms.
+ */
+ private void unlinkCancelledWaiters() {
+ Node t = firstWaiter;
+ Node trail = null;
+ while (t != null) {
+ Node next = t.nextWaiter;
+ if (t.waitStatus != Node.CONDITION) {
+ t.nextWaiter = null;
+ if (trail == null)
+ firstWaiter = next;
+ else
+ trail.nextWaiter = next;
+ if (next == null)
+ lastWaiter = trail;
+ }
+ else
+ trail = t;
+ t = next;
+ }
+ }
+
+ // public methods
+
+ /**
+ * Moves the longest-waiting thread, if one exists, from the
+ * wait queue for this condition to the wait queue for the
+ * owning lock.
+ *
+ * @throws IllegalMonitorStateException if {@link #isHeldExclusively}
+ * returns {@code false}
+ */
+ public final void signal() {
+ if (!isHeldExclusively())
+ throw new IllegalMonitorStateException();
+ Node first = firstWaiter;
+ if (first != null)
+ doSignal(first);
+ }
+
+ /**
+ * Moves all threads from the wait queue for this condition to
+ * the wait queue for the owning lock.
+ *
+ * @throws IllegalMonitorStateException if {@link #isHeldExclusively}
+ * returns {@code false}
+ */
+ public final void signalAll() {
+ if (!isHeldExclusively())
+ throw new IllegalMonitorStateException();
+ Node first = firstWaiter;
+ if (first != null)
+ doSignalAll(first);
+ }
+
+ /**
+ * Implements uninterruptible condition wait.
+ * Usage Examples
+ *
+ *
+ * class Mutex implements Lock, java.io.Serializable {
+ *
+ * // Our internal helper class
+ * private static class Sync extends AbstractQueuedSynchronizer {
+ * // Report whether in locked state
+ * protected boolean isHeldExclusively() {
+ * return getState() == 1;
+ * }
+ *
+ * // Acquire the lock if state is zero
+ * public boolean tryAcquire(int acquires) {
+ * assert acquires == 1; // Otherwise unused
+ * if (compareAndSetState(0, 1)) {
+ * setExclusiveOwnerThread(Thread.currentThread());
+ * return true;
+ * }
+ * return false;
+ * }
+ *
+ * // Release the lock by setting state to zero
+ * protected boolean tryRelease(int releases) {
+ * assert releases == 1; // Otherwise unused
+ * if (getState() == 0) throw new IllegalMonitorStateException();
+ * setExclusiveOwnerThread(null);
+ * setState(0);
+ * return true;
+ * }
+ *
+ * // Provide a Condition
+ * Condition newCondition() { return new ConditionObject(); }
+ *
+ * // Deserialize properly
+ * private void readObject(ObjectInputStream s)
+ * throws IOException, ClassNotFoundException {
+ * s.defaultReadObject();
+ * setState(0); // reset to unlocked state
+ * }
+ * }
+ *
+ * // The sync object does all the hard work. We just forward to it.
+ * private final Sync sync = new Sync();
+ *
+ * public void lock() { sync.acquire(1); }
+ * public boolean tryLock() { return sync.tryAcquire(1); }
+ * public void unlock() { sync.release(1); }
+ * public Condition newCondition() { return sync.newCondition(); }
+ * public boolean isLocked() { return sync.isHeldExclusively(); }
+ * public boolean hasQueuedThreads() { return sync.hasQueuedThreads(); }
+ * public void lockInterruptibly() throws InterruptedException {
+ * sync.acquireInterruptibly(1);
+ * }
+ * public boolean tryLock(long timeout, TimeUnit unit)
+ * throws InterruptedException {
+ * return sync.tryAcquireNanos(1, unit.toNanos(timeout));
+ * }
+ * }
+ *
+ *
+ *
+ * class BooleanLatch {
+ *
+ * private static class Sync extends AbstractQueuedSynchronizer {
+ * boolean isSignalled() { return getState() != 0; }
+ *
+ * protected int tryAcquireShared(int ignore) {
+ * return isSignalled() ? 1 : -1;
+ * }
+ *
+ * protected boolean tryReleaseShared(int ignore) {
+ * setState(1);
+ * return true;
+ * }
+ * }
+ *
+ * private final Sync sync = new Sync();
+ * public boolean isSignalled() { return sync.isSignalled(); }
+ * public void signal() { sync.releaseShared(1); }
+ * public void await() throws InterruptedException {
+ * sync.acquireSharedInterruptibly(1);
+ * }
+ * }
+ *
+ *
+ * @since 1.5
+ * @author Doug Lea
+ */
+public abstract class AbstractQueuedSynchronizer
+ extends AbstractOwnableSynchronizer
+ implements java.io.Serializable {
+
+ private static final long serialVersionUID = 7373984972572414691L;
+
+ /**
+ * Creates a new AbstractQueuedSynchronizer instance
+ * with initial synchronization state of zero.
+ */
+ protected AbstractQueuedSynchronizer() { }
+
+ /**
+ * Wait queue node class.
+ *
+ *
+ * +------+ prev +-----+ +-----+
+ * head | | <---- | | <---- | | tail
+ * +------+ +-----+ +-----+
+ *
+ *
+ * {@code
+ * getFirstQueuedThread() != Thread.currentThread() &&
+ * hasQueuedThreads()}
+ *
+ * {@code
+ * protected boolean tryAcquire(int arg) {
+ * if (isHeldExclusively()) {
+ * // A reentrant acquire; increment hold count
+ * return true;
+ * } else if (hasQueuedPredecessors()) {
+ * return false;
+ * } else {
+ * // try to acquire normally
+ * }
+ * }}
+ *
+ * @return {@code true} if there is a queued thread preceding the
+ * current thread, and {@code false} if the current thread
+ * is at the head of the queue or the queue is empty
+ * @since 1.7
+ */
+ public final boolean hasQueuedPredecessors() {
+ // The correctness of this depends on head being initialized
+ // before tail and on head.next being accurate if the current
+ // thread is first in queue.
+ Node t = tail; // Read fields in reverse initialization order
+ Node h = head;
+ Node s;
+ return h != t &&
+ ((s = h.next) == null || s.thread != Thread.currentThread());
+ }
+
+
+ // Instrumentation and monitoring methods
+
+ /**
+ * Returns an estimate of the number of threads waiting to
+ * acquire. The value is only an estimate because the number of
+ * threads may change dynamically while this method traverses
+ * internal data structures. This method is designed for use in
+ * monitoring system state, not for synchronization
+ * control.
+ *
+ * @return the estimated number of threads waiting to acquire
+ */
+ public final int getQueueLength() {
+ int n = 0;
+ for (Node p = tail; p != null; p = p.prev) {
+ if (p.thread != null)
+ ++n;
+ }
+ return n;
+ }
+
+ /**
+ * Returns a collection containing threads that may be waiting to
+ * acquire. Because the actual set of threads may change
+ * dynamically while constructing this result, the returned
+ * collection is only a best-effort estimate. The elements of the
+ * returned collection are in no particular order. This method is
+ * designed to facilitate construction of subclasses that provide
+ * more extensive monitoring facilities.
+ *
+ * @return the collection of threads
+ */
+ public final Collection
+ *
+ */
+ public final void awaitUninterruptibly() {
+ Node node = addConditionWaiter();
+ int savedState = fullyRelease(node);
+ boolean interrupted = false;
+ while (!isOnSyncQueue(node)) {
+ LockSupport.park(this);
+ if (Thread.interrupted())
+ interrupted = true;
+ }
+ if (acquireQueued(node, savedState) || interrupted)
+ selfInterrupt();
+ }
+
+ /*
+ * For interruptible waits, we need to track whether to throw
+ * InterruptedException, if interrupted while blocked on
+ * condition, versus reinterrupt current thread, if
+ * interrupted while blocked waiting to re-acquire.
+ */
+
+ /** Mode meaning to reinterrupt on exit from wait */
+ private static final int REINTERRUPT = 1;
+ /** Mode meaning to throw InterruptedException on exit from wait */
+ private static final int THROW_IE = -1;
+
+ /**
+ * Checks for interrupt, returning THROW_IE if interrupted
+ * before signalled, REINTERRUPT if after signalled, or
+ * 0 if not interrupted.
+ */
+ private int checkInterruptWhileWaiting(Node node) {
+ return Thread.interrupted() ?
+ (transferAfterCancelledWait(node) ? THROW_IE : REINTERRUPT) :
+ 0;
+ }
+
+ /**
+ * Throws InterruptedException, reinterrupts current thread, or
+ * does nothing, depending on mode.
+ */
+ private void reportInterruptAfterWait(int interruptMode)
+ throws InterruptedException {
+ if (interruptMode == THROW_IE)
+ throw new InterruptedException();
+ else if (interruptMode == REINTERRUPT)
+ selfInterrupt();
+ }
+
+ /**
+ * Implements interruptible condition wait.
+ *
+ *
+ */
+ public final void await() throws InterruptedException {
+ if (Thread.interrupted())
+ throw new InterruptedException();
+ Node node = addConditionWaiter();
+ int savedState = fullyRelease(node);
+ int interruptMode = 0;
+ while (!isOnSyncQueue(node)) {
+ LockSupport.park(this);
+ if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
+ break;
+ }
+ if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
+ interruptMode = REINTERRUPT;
+ if (node.nextWaiter != null) // clean up if cancelled
+ unlinkCancelledWaiters();
+ if (interruptMode != 0)
+ reportInterruptAfterWait(interruptMode);
+ }
+
+ /**
+ * Implements timed condition wait.
+ *
+ *
+ */
+ public final long awaitNanos(long nanosTimeout)
+ throws InterruptedException {
+ if (Thread.interrupted())
+ throw new InterruptedException();
+ Node node = addConditionWaiter();
+ int savedState = fullyRelease(node);
+ long lastTime = System.nanoTime();
+ int interruptMode = 0;
+ while (!isOnSyncQueue(node)) {
+ if (nanosTimeout <= 0L) {
+ transferAfterCancelledWait(node);
+ break;
+ }
+ LockSupport.parkNanos(this, nanosTimeout);
+ if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
+ break;
+
+ long now = System.nanoTime();
+ nanosTimeout -= now - lastTime;
+ lastTime = now;
+ }
+ if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
+ interruptMode = REINTERRUPT;
+ if (node.nextWaiter != null)
+ unlinkCancelledWaiters();
+ if (interruptMode != 0)
+ reportInterruptAfterWait(interruptMode);
+ return nanosTimeout - (System.nanoTime() - lastTime);
+ }
+
+ /**
+ * Implements absolute timed condition wait.
+ *
+ *
+ */
+ public final boolean awaitUntil(Date deadline)
+ throws InterruptedException {
+ if (deadline == null)
+ throw new NullPointerException();
+ long abstime = deadline.getTime();
+ if (Thread.interrupted())
+ throw new InterruptedException();
+ Node node = addConditionWaiter();
+ int savedState = fullyRelease(node);
+ boolean timedout = false;
+ int interruptMode = 0;
+ while (!isOnSyncQueue(node)) {
+ if (System.currentTimeMillis() > abstime) {
+ timedout = transferAfterCancelledWait(node);
+ break;
+ }
+ LockSupport.parkUntil(this, abstime);
+ if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
+ break;
+ }
+ if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
+ interruptMode = REINTERRUPT;
+ if (node.nextWaiter != null)
+ unlinkCancelledWaiters();
+ if (interruptMode != 0)
+ reportInterruptAfterWait(interruptMode);
+ return !timedout;
+ }
+
+ /**
+ * Implements timed condition wait.
+ *
+ *
+ */
+ public final boolean await(long time, TimeUnit unit)
+ throws InterruptedException {
+ if (unit == null)
+ throw new NullPointerException();
+ long nanosTimeout = unit.toNanos(time);
+ if (Thread.interrupted())
+ throw new InterruptedException();
+ Node node = addConditionWaiter();
+ int savedState = fullyRelease(node);
+ long lastTime = System.nanoTime();
+ boolean timedout = false;
+ int interruptMode = 0;
+ while (!isOnSyncQueue(node)) {
+ if (nanosTimeout <= 0L) {
+ timedout = transferAfterCancelledWait(node);
+ break;
+ }
+ if (nanosTimeout >= spinForTimeoutThreshold)
+ LockSupport.parkNanos(this, nanosTimeout);
+ if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
+ break;
+ long now = System.nanoTime();
+ nanosTimeout -= now - lastTime;
+ lastTime = now;
+ }
+ if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
+ interruptMode = REINTERRUPT;
+ if (node.nextWaiter != null)
+ unlinkCancelledWaiters();
+ if (interruptMode != 0)
+ reportInterruptAfterWait(interruptMode);
+ return !timedout;
+ }
+
+ // support for instrumentation
+
+ /**
+ * Returns true if this condition was created by the given
+ * synchronization object.
+ *
+ * @return {@code true} if owned
+ */
+ final boolean isOwnedBy(AbstractQueuedSynchronizer sync) {
+ return sync == AbstractQueuedSynchronizer.this;
+ }
+
+ /**
+ * Queries whether any threads are waiting on this condition.
+ * Implements {@link AbstractQueuedSynchronizer#hasWaiters}.
+ *
+ * @return {@code true} if there are any waiting threads
+ * @throws IllegalMonitorStateException if {@link #isHeldExclusively}
+ * returns {@code false}
+ */
+ protected final boolean hasWaiters() {
+ if (!isHeldExclusively())
+ throw new IllegalMonitorStateException();
+ for (Node w = firstWaiter; w != null; w = w.nextWaiter) {
+ if (w.waitStatus == Node.CONDITION)
+ return true;
+ }
+ return false;
+ }
+
+ /**
+ * Returns an estimate of the number of threads waiting on
+ * this condition.
+ * Implements {@link AbstractQueuedSynchronizer#getWaitQueueLength}.
+ *
+ * @return the estimated number of waiting threads
+ * @throws IllegalMonitorStateException if {@link #isHeldExclusively}
+ * returns {@code false}
+ */
+ protected final int getWaitQueueLength() {
+ if (!isHeldExclusively())
+ throw new IllegalMonitorStateException();
+ int n = 0;
+ for (Node w = firstWaiter; w != null; w = w.nextWaiter) {
+ if (w.waitStatus == Node.CONDITION)
+ ++n;
+ }
+ return n;
+ }
+
+ /**
+ * Returns a collection containing those threads that may be
+ * waiting on this Condition.
+ * Implements {@link AbstractQueuedSynchronizer#getWaitingThreads}.
+ *
+ * @return the collection of threads
+ * @throws IllegalMonitorStateException if {@link #isHeldExclusively}
+ * returns {@code false}
+ */
+ protected final Collection