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, thereby providing a fair FIFO acquisition order. * In particular, most fair synchronizers can define {@code tryAcquire} * to return {@code false} if {@link #hasQueuedPredecessors} (a method * specifically designed to be used by fair synchronizers) returns * {@code 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 {@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 some instrumentation methods: * *
{@code
* class Mutex implements Lock, java.io.Serializable {
*
* // Our internal helper class
* private static class Sync extends AbstractQueuedSynchronizer {
* // 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 (!isHeldExclusively())
* throw new IllegalMonitorStateException();
* setExclusiveOwnerThread(null);
* setState(0);
* return true;
* }
*
* // Reports whether in locked state
* public boolean isLocked() {
* return getState() != 0;
* }
*
* public boolean isHeldExclusively() {
* // a data race, but safe due to out-of-thin-air guarantees
* return getExclusiveOwnerThread() == Thread.currentThread();
* }
*
* // Provides a Condition
* public 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.isLocked(); }
* public boolean isHeldByCurrentThread() {
* 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() { }
/*
* Overview.
*
* 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 by
* including explicit ("prev" and "next") links plus a "status"
* field that allow nodes to signal successors when releasing
* locks, and handle cancellation due to interrupts and timeouts.
* The status field includes bits that track whether a thread
* needs a signal (using LockSupport.unpark). Despite these
* additions, we maintain most CLH locality properties.
*
* To enqueue into a CLH lock, you atomically splice it in as new
* tail. To dequeue, you set the head field, so the next eligible
* waiter becomes first.
*
* +------+ prev +-------+ +------+
* | head | <---- | first | <---- | tail |
* +------+ +-------+ +------+
*
* Insertion into a CLH queue requires only a single atomic
* operation on "tail", so there is a simple point of demarcation
* from unqueued to queued. The "next" link of the predecessor is
* set by the enqueuing thread after successful CAS. Even though
* non-atomic, this suffices to ensure that any blocked thread is
* signalled by a predecessor when eligible (although in the case
* of cancellation, possibly with the assistance of a signal in
* method cleanQueue). Signalling is based in part on a
* Dekker-like scheme in which the to-be waiting thread indicates
* WAITING status, then retries acquiring, and then rechecks
* status before blocking. The signaller atomically clears WAITING
* status when unparking.
*
* Dequeuing on acquire involves detaching (nulling) a node's
* "prev" node and then updating the "head". Other threads check
* if a node is or was dequeued by checking "prev" rather than
* head. We enforce the nulling then setting order by spin-waiting
* if necessary. Because of this, the lock algorithm is not itself
* strictly "lock-free" because an acquiring thread may need to
* wait for a previous acquire to make progress. When used with
* exclusive locks, such progress is required anyway. However
* Shared mode may (uncommonly) require a spin-wait before
* setting head field to ensure proper propagation. (Historical
* note: This allows some simplifications and efficiencies
* compared to previous versions of this class.)
*
* A node's predecessor can change due to cancellation while it is
* waiting, until the node is first in queue, at which point it
* cannot change. The acquire methods cope with this by rechecking
* "prev" before waiting. The prev and next fields are modified
* only via CAS by cancelled nodes in method cleanQueue. The
* unsplice strategy is reminiscent of Michael-Scott queues in
* that after a successful CAS to prev field, other threads help
* fix next fields. Because cancellation often occurs in bunches
* that complicate decisions about necessary signals, each call to
* cleanQueue traverses the queue until a clean sweep. Nodes that
* become relinked as first are unconditionally unparked
* (sometimes unnecessarily, but those cases are not worth
* avoiding).
*
* A thread may try to acquire if it is first (frontmost) in the
* queue, and sometimes before. Being first does not guarantee
* success; it only gives the right to contend. We balance
* throughput, overhead, and fairness by allowing incoming threads
* to "barge" and acquire the synchronizer while in the process of
* enqueuing, in which case an awakened first thread may need to
* rewait. To counteract possible repeated unlucky rewaits, we
* exponentially increase retries (up to 256) to acquire each time
* a thread is unparked. Except in this case, AQS locks do not
* spin; they instead interleave attempts to acquire with
* bookkeeping steps. (Users who want spinlocks can use
* tryAcquire.)
*
* To improve garbage collectibility, fields of nodes not yet on
* list are null. (It is not rare to create and then throw away a
* node without using it.) Fields of nodes coming off the list are
* nulled out as soon as possible. This accentuates the challenge
* of externally determining the first waiting thread (as in
* method getFirstQueuedThread). This sometimes requires the
* fallback of traversing backwards from the atomically updated
* "tail" when fields appear null. (This is never needed in the
* process of signalling though.)
*
* 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.
*
* Shared mode operations differ from Exclusive in that an acquire
* signals the next waiter to try to acquire if it is also
* Shared. The tryAcquireShared API allows users to indicate the
* degree of propagation, but in most applications, it is more
* efficient to ignore this, allowing the successor to try
* acquiring in any case.
*
* Threads waiting on Conditions use nodes with an additional
* link to maintain the (FIFO) list of conditions. 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 enqueued on the main queue. A special status field
* value is used to track and atomically trigger this.
*
* Accesses to fields head, tail, and state use full Volatile
* mode, along with CAS. Node fields status, prev and next also do
* so while threads may be signallable, but sometimes use weaker
* modes otherwise. Accesses to field "waiter" (the thread to be
* signalled) are always sandwiched between other atomic accesses
* so are used in Plain mode. We use jdk.internal Unsafe versions
* of atomic access methods rather than VarHandles to avoid
* potential VM bootstrap issues.
*
* Most of the above is performed by primary internal method
* acquire, that is invoked in some way by all exported acquire
* methods. (It is usually easy for compilers to optimize
* call-site specializations when heavily used.)
*
* Most AQS methods may be called by JDK components that cannot be
* allowed to fail when encountering OutOfMemoryErrors. The main
* acquire method resorts to spin-waits with backoff if nodes
* cannot be allocated. Condition waits release and reacquire
* locks upon OOME at a slow fixed rate (OOME_COND_WAIT_DELAY)
* designed with the hope that eventually enough memory will be
* recovered; if not performance can be very slow. Effectiveness
* is also limited by the possibility of class loading triggered
* by first-time usages, that may encounter unrecoverable
* OOMEs. Also, it is possible for OutOfMemoryErrors to be thrown
* when attempting to create and throw
* IllegalMonitorStateExceptions and InterruptedExceptions.
*
* There are several arbitrary decisions about when and how to
* check interrupts in both acquire and await before and/or after
* blocking. The decisions are less arbitrary in implementation
* updates because some users appear to rely on original behaviors
* in ways that are racy and so (rarely) wrong in general but hard
* to justify changing.
*
* 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.
*/
// Node status bits, also used as argument and return values
static final int WAITING = 1; // must be 1
static final int CANCELLED = 0x80000000; // must be negative
static final int COND = 2; // in a condition wait
/** CLH Nodes */
abstract static class Node {
volatile Node prev; // initially attached via casTail
volatile Node next; // visibly nonnull when signallable
Thread waiter; // visibly nonnull when enqueued
volatile int status; // written by owner, atomic bit ops by others
// methods for atomic operations
final boolean casPrev(Node c, Node v) { // for cleanQueue
return U.weakCompareAndSetReference(this, PREV, c, v);
}
final boolean casNext(Node c, Node v) { // for cleanQueue
return U.weakCompareAndSetReference(this, NEXT, c, v);
}
final int getAndUnsetStatus(int v) { // for signalling
return U.getAndBitwiseAndInt(this, STATUS, ~v);
}
final void setPrevRelaxed(Node p) { // for off-queue assignment
U.putReference(this, PREV, p);
}
final void setStatusRelaxed(int s) { // for off-queue assignment
U.putInt(this, STATUS, s);
}
final void clearStatus() { // for reducing unneeded signals
U.putIntOpaque(this, STATUS, 0);
}
private static final long STATUS
= U.objectFieldOffset(Node.class, "status");
private static final long NEXT
= U.objectFieldOffset(Node.class, "next");
private static final long PREV
= U.objectFieldOffset(Node.class, "prev");
}
// Concrete classes tagged by type
static final class ExclusiveNode extends Node { }
static final class SharedNode extends Node { }
static final class ConditionNode extends Node
implements ForkJoinPool.ManagedBlocker {
ConditionNode nextWaiter; // link to next waiting node
/**
* Allows Conditions to be used in ForkJoinPools without
* risking fixed pool exhaustion. This is usable only for
* untimed Condition waits, not timed versions.
*/
public final boolean isReleasable() {
return status <= 1 || Thread.currentThread().isInterrupted();
}
public final boolean block() {
while (!isReleasable()) LockSupport.park();
return true;
}
}
/**
* Head of the wait queue, lazily initialized.
*/
private transient volatile Node head;
/**
* Tail of the wait queue. After initialization, modified only via casTail.
*/
private transient volatile Node tail;
/**
* @serial 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 {@code 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) {
return U.compareAndSetInt(this, STATE, expect, update);
}
// Queuing utilities
private boolean casTail(Node c, Node v) {
return U.compareAndSetReference(this, TAIL, c, v);
}
/**
* Tries to CAS a new dummy node for head.
* Returns new tail, or null if OutOfMemory
*/
private Node tryInitializeHead() {
for (Node h = null, t;;) {
if ((t = tail) != null)
return t;
else if (head != null)
Thread.onSpinWait();
else {
if (h == null) {
try {
h = new ExclusiveNode();
} catch (OutOfMemoryError oome) {
return null;
}
}
if (U.compareAndSetReference(this, HEAD, null, h))
return tail = h;
}
}
}
/**
* Enqueues the node unless null. (Currently used only for
* ConditionNodes; other cases are interleaved with acquires.)
*/
final void enqueue(ConditionNode node) {
if (node != null) {
boolean unpark = false;
for (Node t;;) {
if ((t = tail) == null && (t = tryInitializeHead()) == null) {
unpark = true; // wake up to spin on OOME
break;
}
node.setPrevRelaxed(t); // avoid unnecessary fence
if (casTail(t, node)) {
t.next = node;
if (t.status < 0) // wake up to clean link
unpark = true;
break;
}
}
if (unpark)
LockSupport.unpark(node.waiter);
}
}
/** Returns true if node is found in traversal from tail */
final boolean isEnqueued(Node node) {
for (Node t = tail; t != null; t = t.prev)
if (t == node)
return true;
return false;
}
/**
* Wakes up the successor of given node, if one exists, and unsets its
* WAITING status to avoid park race. This may fail to wake up an
* eligible thread when one or more have been cancelled, but
* cancelAcquire ensures liveness.
*/
private static void signalNext(Node h) {
Node s;
if (h != null && (s = h.next) != null && s.status != 0) {
s.getAndUnsetStatus(WAITING);
LockSupport.unpark(s.waiter);
}
}
/** Wakes up the given node if in shared mode */
private static void signalNextIfShared(Node h) {
Node s;
if (h != null && (s = h.next) != null &&
(s instanceof SharedNode) && s.status != 0) {
s.getAndUnsetStatus(WAITING);
LockSupport.unpark(s.waiter);
}
}
/**
* Repeatedly invokes acquire, if its execution throws an Error or a Runtime Exception,
* using an Unsafe.park-based backoff
* @param node which to reacquire
* @param arg the acquire argument
*/
private final void reacquire(Node node, int arg) {
try {
acquire(node, arg, false, false, false, 0L);
} catch (Error | RuntimeException firstEx) {
// While we currently do not emit an JFR events in this situation, mainly
// because the conditions under which this happens are such that it
// cannot be presumed to be possible to actually allocate an event, and
// using a preconstructed one would have limited value in serviceability.
// Having said that, the following place would be the more appropriate
// place to put such logic:
// emit JFR event
for (long nanos = 1L;;) {
U.park(false, nanos); // must use Unsafe park to sleep
if (nanos < 1L << 30) // max about 1 second
nanos <<= 1;
try {
acquire(node, arg, false, false, false, 0L);
} catch (Error | RuntimeException ignored) {
continue;
}
throw firstEx;
}
}
}
/**
* Main acquire method, invoked by all exported acquire methods.
*
* @param node null unless a reacquiring Condition
* @param arg the acquire argument
* @param shared true if shared mode else exclusive
* @param interruptible if abort and return negative on interrupt
* @param timed if true use timed waits
* @param time if timed, the System.nanoTime value to timeout
* @return positive if acquired, 0 if timed out, negative if interrupted
*/
final int acquire(Node node, int arg, boolean shared,
boolean interruptible, boolean timed, long time) {
Thread current = Thread.currentThread();
byte spins = 0, postSpins = 0; // retries upon unpark of first thread
boolean interrupted = false, first = false;
Node pred = null; // predecessor of node when enqueued
/*
* Repeatedly:
* Check if node now first
* if so, ensure head stable, else ensure valid predecessor
* if node is first or not yet enqueued, try acquiring
* else if queue is not initialized, do so by attaching new header node
* resort to spinwait on OOME trying to create node
* else if node not yet created, create it
* resort to spinwait on OOME trying to create node
* else if not yet enqueued, try once to enqueue
* else if woken from park, retry (up to postSpins times)
* else if WAITING status not set, set and retry
* else park and clear WAITING status, and check cancellation
*/
for (;;) {
if (!first && (pred = (node == null) ? null : node.prev) != null &&
!(first = (head == pred))) {
if (pred.status < 0) {
cleanQueue(); // predecessor cancelled
continue;
} else if (pred.prev == null) {
Thread.onSpinWait(); // ensure serialization
continue;
}
}
if (first || pred == null) {
boolean acquired;
try {
if (shared)
acquired = (tryAcquireShared(arg) >= 0);
else
acquired = tryAcquire(arg);
} catch (Throwable ex) {
cancelAcquire(node, interrupted, false);
throw ex;
}
if (acquired) {
if (first) {
node.prev = null;
head = node;
pred.next = null;
node.waiter = null;
if (shared)
signalNextIfShared(node);
if (interrupted)
current.interrupt();
}
return 1;
}
}
Node t;
if ((t = tail) == null) { // initialize queue
if (tryInitializeHead() == null)
return acquireOnOOME(shared, arg);
} else if (node == null) { // allocate; retry before enqueue
try {
node = (shared) ? new SharedNode() : new ExclusiveNode();
} catch (OutOfMemoryError oome) {
return acquireOnOOME(shared, arg);
}
} else if (pred == null) { // try to enqueue
node.waiter = current;
node.setPrevRelaxed(t); // avoid unnecessary fence
if (!casTail(t, node))
node.setPrevRelaxed(null); // back out
else
t.next = node;
} else if (first && spins != 0) {
--spins; // reduce unfairness on rewaits
Thread.onSpinWait();
} else if (node.status == 0) {
node.status = WAITING; // enable signal and recheck
} else {
spins = postSpins = (byte)((postSpins << 1) | 1);
try {
long nanos;
if (!timed)
LockSupport.park(this);
else if ((nanos = time - System.nanoTime()) > 0L)
LockSupport.parkNanos(this, nanos);
else
break;
} catch (Error | RuntimeException ex) {
cancelAcquire(node, interrupted, interruptible); // cancel & rethrow
throw ex;
}
node.clearStatus();
if ((interrupted |= Thread.interrupted()) && interruptible)
break;
}
}
return cancelAcquire(node, interrupted, interruptible);
}
/**
* Spin-waits with backoff; used only upon OOME failures during acquire.
*/
private int acquireOnOOME(boolean shared, int arg) {
for (long nanos = 1L;;) {
if (shared ? (tryAcquireShared(arg) >= 0) : tryAcquire(arg))
return 1;
U.park(false, nanos); // must use Unsafe park to sleep
if (nanos < 1L << 30) // max about 1 second
nanos <<= 1;
}
}
/**
* Possibly repeatedly traverses from tail, unsplicing cancelled
* nodes until none are found. Unparks nodes that may have been
* relinked to be next eligible acquirer.
*/
private void cleanQueue() {
for (;;) { // restart point
for (Node q = tail, s = null, p, n;;) { // (p, q, s) triples
if (q == null || (p = q.prev) == null)
return; // end of list
if (s == null ? tail != q : (s.prev != q || s.status < 0))
break; // inconsistent
if (q.status < 0) { // cancelled
if ((s == null ? casTail(q, p) : s.casPrev(q, p)) &&
q.prev == p) {
p.casNext(q, s); // OK if fails
if (p.prev == null)
signalNext(p);
}
break;
}
if ((n = p.next) != q) { // help finish
if (n != null && q.prev == p) {
p.casNext(n, q);
if (p.prev == null)
signalNext(p);
}
break;
}
s = q;
q = q.prev;
}
}
}
/**
* Cancels an ongoing attempt to acquire.
*
* @param node the node (may be null if cancelled before enqueuing)
* @param interrupted true if thread interrupted
* @param interruptible if should report interruption vs reset
*/
private int cancelAcquire(Node node, boolean interrupted,
boolean interruptible) {
if (node != null) {
node.waiter = null;
node.status = CANCELLED;
if (node.prev != null)
cleanQueue();
}
if (interrupted) {
if (interruptible)
return CANCELLED;
else
Thread.currentThread().interrupt();
}
return 0;
}
// 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 {@link ConditionObject} method. * *
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)) acquire(null, arg, false, false, false, 0L); } /** * 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() || (!tryAcquire(arg) && acquire(null, arg, false, true, false, 0L) < 0)) throw new InterruptedException(); } /** * 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()) { if (tryAcquire(arg)) return true; if (nanosTimeout <= 0L) return false; int stat = acquire(null, arg, false, true, true, System.nanoTime() + nanosTimeout); if (stat > 0) return true; if (stat == 0) return false; } throw new InterruptedException(); } /** * 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)) { signalNext(head); 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) acquire(null, arg, true, false, false, 0L); } /** * 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() || (tryAcquireShared(arg) < 0 && acquire(null, arg, true, true, false, 0L) < 0)) throw new InterruptedException(); } /** * 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()) { if (tryAcquireShared(arg) >= 0) return true; if (nanosTimeout <= 0L) return false; int stat = acquire(null, arg, true, true, true, System.nanoTime() + nanosTimeout); if (stat > 0) return true; if (stat == 0) return false; } throw new InterruptedException(); } /** * 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)) { signalNext(head); 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. * * @return {@code true} if there may be other threads waiting to acquire */ public final boolean hasQueuedThreads() { for (Node p = tail, h = head; p != h && p != null; p = p.prev) if (p.status >= 0) return true; return false; } /** * 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() { Thread first = null, w; Node h, s; if ((h = head) != null && ((s = h.next) == null || (first = s.waiter) == null || s.prev == null)) { // traverse from tail on stale reads for (Node p = tail, q; p != null && (q = p.prev) != null; p = q) if ((w = p.waiter) != null) first = w; } return first; } /** * 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.waiter == 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 instanceof SharedNode) && s.waiter != 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() {
Thread first = null; Node h, s;
if ((h = head) != null && ((s = h.next) == null ||
(first = s.waiter) == null ||
s.prev == null))
first = getFirstQueuedThread(); // retry via getFirstQueuedThread
return first != null && first != 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.waiter != 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 ConditionNode firstWaiter; /** Last node of condition queue. */ private transient ConditionNode lastWaiter; /** * Fixed delay in nanoseconds between releasing and reacquiring * lock during Condition waits that encounter OutOfMemoryErrors */ static final long OOME_COND_WAIT_DELAY = 10L * 1000L * 1000L; // 10 ms /** * Creates a new {@code ConditionObject} instance. */ public ConditionObject() { } // Signalling methods /** * Removes and transfers one or all waiters to sync queue. */ private void doSignal(ConditionNode first, boolean all) { while (first != null) { ConditionNode next = first.nextWaiter; if ((firstWaiter = next) == null) lastWaiter = null; else first.nextWaiter = null; // GC assistance if ((first.getAndUnsetStatus(COND) & COND) != 0) { enqueue(first); if (!all) break; } first = next; } } /** * 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() { ConditionNode first = firstWaiter; if (!isHeldExclusively()) throw new IllegalMonitorStateException(); else if (first != null) doSignal(first, false); } /** * 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() { ConditionNode first = firstWaiter; if (!isHeldExclusively()) throw new IllegalMonitorStateException(); else if (first != null) doSignal(first, true); } // Waiting methods /** * Adds node to condition list and releases lock. * * @param node the node * @return savedState to reacquire after wait */ private int enableWait(ConditionNode node) { if (isHeldExclusively()) { node.waiter = Thread.currentThread(); node.setStatusRelaxed(COND | WAITING); ConditionNode last = lastWaiter; if (last == null) firstWaiter = node; else last.nextWaiter = node; lastWaiter = node; int savedState = getState(); if (release(savedState)) return savedState; } node.status = CANCELLED; // lock not held or inconsistent throw new IllegalMonitorStateException(); } /** * Returns true if a node that was initially placed on a condition * queue is now ready to reacquire on sync queue. * @param node the node * @return true if is reacquiring */ private boolean canReacquire(ConditionNode node) { // check links, not status to avoid enqueue race Node p; // traverse unless known to be bidirectionally linked return node != null && (p = node.prev) != null && (p.next == node || isEnqueued(node)); } /** * Unlinks the given node and other non-waiting nodes from * condition queue unless already unlinked. */ private void unlinkCancelledWaiters(ConditionNode node) { if (node == null || node.nextWaiter != null || node == lastWaiter) { ConditionNode w = firstWaiter, trail = null; while (w != null) { ConditionNode next = w.nextWaiter; if ((w.status & COND) == 0) { w.nextWaiter = null; if (trail == null) firstWaiter = next; else trail.nextWaiter = next; if (next == null) lastWaiter = trail; } else trail = w; w = next; } } } /** * Constructs objects needed for condition wait. On OOME, * releases lock, sleeps, reacquires, and returns null. */ private ConditionNode newConditionNode() { int savedState; if (tryInitializeHead() != null) { try { return new ConditionNode(); } catch (OutOfMemoryError oome) { } } // fall through if encountered OutOfMemoryError if (!isHeldExclusively() || !release(savedState = getState())) throw new IllegalMonitorStateException(); U.park(false, OOME_COND_WAIT_DELAY); acquireOnOOME(false, savedState); return null; } /** * Implements uninterruptible condition wait. *