Subclasses should be defined as non-public internal helper * classes that are used to implement the synchronization properties * of their enclosing class. Class * {@code 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 * {@code 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 {@code 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 {@code 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 {@code tryAcquire} and/or {@code tryAcquireShared} to * disable barging by internally invoking one or more of the inspection * methods. In particular, a strict FIFO lock can define * {@code tryAcquire} to immediately return {@code false} if {@link * #getFirstQueuedThread} does not return the current thread. A * normally preferable non-strict fair version can immediately return * {@code false} only if {@link #hasQueuedThreads} returns * {@code true} and {@code getFirstQueuedThread} is not the current * thread; or equivalently, that {@code getFirstQueuedThread} is both * non-null and not the current thread. Further 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 {@code 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 {@code 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: * *
{@code
* class Mutex implements Lock, java.io.Serializable {
*
* // Our internal helper class
* private static class Sync extends AbstractQueuedSynchronizer {
* // Reports whether in locked state
* protected boolean isHeldExclusively() {
* return getState() == 1;
* }
*
* // Acquires 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;
* }
*
* // Releases 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;
* }
*
* // Provides a Condition
* Condition newCondition() { return new ConditionObject(); }
*
* // Deserializes 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));
* }
* }}
*
* Here is a latch class that is like a * {@link java.util.concurrent.CountDownLatch CountDownLatch} * except that it only requires a single {@code signal} to * fire. Because a latch is non-exclusive, it uses the {@code shared} * acquire and release methods. * *
{@code
* 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 {@code AbstractQueuedSynchronizer} instance
* with initial synchronization state of zero.
*/
protected AbstractQueuedSynchronizer() { }
/**
* Wait queue node class.
*
* 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. *
* +------+ prev +-----+ +-----+
* head | | <---- | | <---- | | tail
* +------+ +-----+ +-----+
*
*
* 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, dequeuing * 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 enqueuing, 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 {@code 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 {@code 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 {@code 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 -- signals successor and ensures * 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. */ 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 { if (nanosTimeout <= 0L) return false; final long deadline = System.nanoTime() + nanosTimeout; 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; } nanosTimeout = deadline - System.nanoTime(); if (nanosTimeout <= 0L) return false; if (shouldParkAfterFailedAcquire(p, node) && nanosTimeout > spinForTimeoutThreshold) LockSupport.parkNanos(this, nanosTimeout); 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 { if (nanosTimeout <= 0L) return false; final long deadline = System.nanoTime() + nanosTimeout; 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; } } nanosTimeout = deadline - System.nanoTime(); if (nanosTimeout <= 0L) return false; if (shouldParkAfterFailedAcquire(p, node) && nanosTimeout > spinForTimeoutThreshold) LockSupport.parkNanos(this, nanosTimeout); 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): *
{@code
* getFirstQueuedThread() != Thread.currentThread() &&
* hasQueuedThreads()}
*
* 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: * *
{@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 CollectionMethod 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 * {@code 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 {@code 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. *