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Subclasses should be defined as non-public internal helper classes that are used to implement the synchronization properties of their enclosing class. Class AbstractQueuedSynchronizer does not implement any synchronization interface. Instead it defines methods such as AbstractQueuedSynchronizer.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 ReadWriteLock . Subclasses that support only exclusive or only shared modes need not define the methods supporting the unused mode.

This class defines a nested ConditionObject class that can be used as a Condition implementation by subclasses supporting exclusive mode for which method AbstractQueuedSynchronizer.isHeldExclusively reports whether synchronization is exclusively held with respect to the current thread, method AbstractQueuedSynchronizer.release invoked with the current AbstractQueuedSynchronizer.getState value fully releases this object, and AbstractQueuedSynchronizer.acquire , given this saved state value, eventually restores this object to its previous acquired state. No AbstractQueuedSynchronizer method otherwise creates such a condition, so if this constraint cannot be met, do not use it. The behavior of ConditionObject depends of course on the semantics of its synchronizer implementation.

This class provides inspection, instrumentation, and monitoring methods for the internal queue, as well as similar methods for condition objects. These can be exported as desired into classes using an AbstractQueuedSynchronizer for their synchronization mechanics.

Serialization of this class stores only the underlying atomic integer maintaining state, so deserialized objects have empty thread queues. Typical subclasses requiring serializability will define a readObject method that restores this to a known initial state upon deserialization.

Usage

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 AbstractQueuedSynchronizer.getState , AbstractQueuedSynchronizer.setState and/or AbstractQueuedSynchronizer.compareAndSetState :

• AbstractQueuedSynchronizer.tryAcquire
• AbstractQueuedSynchronizer.tryRelease
• AbstractQueuedSynchronizer.tryAcquireShared
• AbstractQueuedSynchronizer.tryReleaseShared
• AbstractQueuedSynchronizer.isHeldExclusively

You may also find the inherited methods from 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;
 

Because checks in acquire are invoked before enqueuing, a newly acquiring thread may barge ahead of others that are blocked and queued. However, you can, if desired, define tryAcquire and/or tryAcquireShared to disable barging by internally invoking one or more of the inspection methods, thereby providing a fair FIFO acquisition order. In particular, most fair synchronizers can define tryAcquire to return false if AbstractQueuedSynchronizer.hasQueuedPredecessors (a method specifically designed to be used by fair synchronizers) returns true. Other variations are possible.

Throughput and scalability are generally highest for the default barging (also known as greedy, renouncement, and convoy-avoidance) strategy. While this is not guaranteed to be fair or starvation-free, earlier queued threads are allowed to recontend before later queued threads, and each recontention has an unbiased chance to succeed against incoming threads. Also, while acquires do not "spin" in the usual sense, they may perform multiple invocations of tryAcquire interspersed with other computations before blocking. This gives most of the benefits of spins when exclusive synchronization is only briefly held, without most of the liabilities when it isn't. If so desired, you can augment this by preceding calls to acquire methods with "fast-path" checks, possibly prechecking AbstractQueuedSynchronizer.hasContended and/or AbstractQueuedSynchronizer.hasQueuedThreads to only do so if the synchronizer is likely not to be contended.

This class provides an efficient and scalable basis for synchronization in part by specializing its range of use to synchronizers that can rely on int state, acquire, and release parameters, and an internal FIFO wait queue. When this does not suffice, you can build synchronizers from a lower level using java.util.concurrent.atomic atomic classes, your own custom java.util.Queue classes, and LockSupport blocking support.

Usage Examples

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:

 class Mutex implements Lock, java.io.Serializable {
 // Our internal helper class
 private static class Sync extends AbstractQueuedSynchronizer {
 // Report whether in locked state
 protected boolean isHeldExclusively() {
 return getState() == 1;
 }
 // Acquire the lock if state is zero
 public boolean tryAcquire(int acquires) {
 assert acquires == 1; // Otherwise unused
 if (compareAndSetState(0, 1)) {
 setExclusiveOwnerThread(Thread.currentThread());
 return true;
 }
 return false;
 }
 // Release the lock by setting state to zero
 protected boolean tryRelease(int releases) {
 assert releases == 1; // Otherwise unused
 if (getState() == 0) throw new IllegalMonitorStateException();
 setExclusiveOwnerThread(null);
 setState(0);
 return true;
 }
 // Provide a Condition
 Condition newCondition() { return new ConditionObject(); }
 // Deserialize properly
 private void readObject(ObjectInputStream s)
 throws IOException, ClassNotFoundException {
 s.defaultReadObject();
 setState(0); // reset to unlocked state
 }
 }
 // The sync object does all the hard work. We just forward to it.
 private final Sync sync = new Sync();
 public void lock()                { sync.acquire(1); }
 public boolean tryLock()          { return sync.tryAcquire(1); }
 public void unlock()              { sync.release(1); }
 public Condition newCondition()   { return sync.newCondition(); }
 public boolean isLocked()         { return sync.isHeldExclusively(); }
 public boolean hasQueuedThreads() { return sync.hasQueuedThreads(); }
 public void lockInterruptibly() throws InterruptedException {
 sync.acquireInterruptibly(1);
 }
 public boolean tryLock(long timeout, TimeUnit unit)
 throws InterruptedException {
 return sync.tryAcquireNanos(1, unit.toNanos(timeout));
 }
 }
 

Here is a latch class that is like a CountDownLatch except that it only requires a single signal to fire. Because a latch is non-exclusive, it uses the shared acquire and release methods.

 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);
 }
 }
 

This method is always invoked by the thread performing release.

The default implementation throws UnsupportedOperationException .
Parameters:
  arg - the release argument.

This method is always invoked by the thread performing release.

The default implementation throws UnsupportedOperationException .
Parameters:
  arg - the release argument.

In this implementation, this operation normally returns in constant time, but may iterate upon contention if other threads are concurrently modifying the queue. the first (longest-waiting) thread in the queue, or null if no threads are currently queued

In this implementation, this operation returns in constant time. true if there has ever been contention

In this implementation, this operation returns in constant time. true if there may be other threads waiting to acquire

The default implementation throws UnsupportedOperationException . This method is invoked internally only within ConditionObject methods, so need not be defined if conditions are not used. true if synchronization is held exclusively; false otherwise
throws:
  UnsupportedOperationException - if conditions are not supported

This implementation traverses the queue to determine presence of the given thread.
Parameters:
  thread - the thread true if the given thread is on the queue
throws:
  NullPointerException - if the thread is null

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 Lock.tryLock .

The default implementation throws UnsupportedOperationException .
Parameters:
  arg - the acquire argument. This value is always the onepassed to an acquire method, or is the value saved on entryto a condition wait. The value is otherwise uninterpretedand can represent anything you like. true if successful. Upon success, this object hasbeen acquired.
throws:
  IllegalMonitorStateException - if acquiring would place thissynchronizer in an illegal state. This exception must bethrown in a consistent fashion for synchronization to workcorrectly.
throws:
  UnsupportedOperationException - if exclusive mode is not supported

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 UnsupportedOperationException .
Parameters:
  arg - the acquire argument. This value is always the onepassed to an acquire method, or is the value saved on entryto a condition wait. The value is otherwise uninterpretedand can represent anything you like. a negative value on failure; zero if acquisition in sharedmode succeeded but no subsequent shared-mode acquire cansucceed; and a positive value if acquisition in sharedmode succeeded and subsequent shared-mode acquires mightalso succeed, in which case a subsequent waiting threadmust check availability. (Support for three differentreturn values enables this method to be used in contextswhere acquires only sometimes act exclusively.) Uponsuccess, this object has been acquired.
throws:
  IllegalMonitorStateException - if acquiring would place thissynchronizer in an illegal state. This exception must bethrown in a consistent fashion for synchronization to workcorrectly.
throws:
  UnsupportedOperationException - if shared mode is not supported

This method is always invoked by the thread performing release.

The default implementation throws UnsupportedOperationException .
Parameters:
  arg - the release argument. This value is always the onepassed to a release method, or the current state value uponentry to a condition wait. The value is otherwiseuninterpreted and can represent anything you like. true if this object is now in a fully releasedstate, so that any waiting threads may attempt to acquire;and false otherwise.
throws:
  IllegalMonitorStateException - if releasing would place thissynchronizer in an illegal state. This exception must bethrown in a consistent fashion for synchronization to workcorrectly.
throws:
  UnsupportedOperationException - if exclusive mode is not supported

This method is always invoked by the thread performing release.

The default implementation throws UnsupportedOperationException .
Parameters:
  arg - the release argument. This value is always the onepassed to a release method, or the current state value uponentry to a condition wait. The value is otherwiseuninterpreted and can represent anything you like. true if this release of shared mode may permit awaiting acquire (shared or exclusive) to succeed; and false otherwise
throws:
  IllegalMonitorStateException - if releasing would place thissynchronizer in an illegal state. This exception must bethrown in a consistent fashion for synchronization to workcorrectly.
throws:
  UnsupportedOperationException - if shared mode is not supported