public abstract class

AbstractQueuedSynchronizer

extends Object
implements Serializable
java.lang.Object
   ↳ java.util.concurrent.locks.AbstractQueuedSynchronizer

Class Overview

Provides a framework for implementing blocking locks and related synchronizers (semaphores, events, etc) that rely on first-in-first-out (FIFO) wait queues. This class is designed to be a useful basis for most kinds of synchronizers that rely on a single atomic int value to represent state. Subclasses must define the protected methods that change this state, and which define what that state means in terms of this object being acquired or released. Given these, the other methods in this class carry out all queuing and blocking mechanics. Subclasses can maintain other state fields, but only the atomically updated int value manipulated using methods getState(), setState(int) and compareAndSetState(int, int) is tracked with respect to synchronization.

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 acquireInterruptibly(int) 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 AbstractQueuedSynchronizer.ConditionObject class that can be used as a Condition implementation by subclasses supporting exclusive mode for which method isHeldExclusively() reports whether synchronization is exclusively held with respect to the current thread, method release(int) invoked with the current getState() value fully releases this object, and acquire(int), 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 AbstractQueuedSynchronizer.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 getState(), setState(int) and/or compareAndSetState(int, int):

Each of these methods by default throws UnsupportedOperationException. Implementations of these methods must be internally thread-safe, and should in general be short and not block. Defining these methods is the only supported means of using this class. All other methods are declared final because they cannot be independently varied.

Even though this class is based on an internal FIFO queue, it does not automatically enforce FIFO acquisition policies. The core of exclusive synchronization takes the form: 

 Acquire:
     while (!tryAcquire(arg)) {
        enqueue thread if it is not already queued;
        possibly block current thread;
     }

 Release:
     if (tryRelease(arg))
        unblock the first queued thread;
(Shared mode is similar but may involve cascading signals.)

Because checks in acquire are invoked before enqueuing, a newly acquiring thread may barge ahead of others that are blocked and queued. However, you can, if desired, define tryAcquire and/or tryAcquireShared to disable barging by internally invoking one or more of the inspection methods, thereby providing a fair FIFO acquisition order. In particular, most fair synchronizers can define tryAcquire to return false if predecessors are queued. 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 hasContended() and/or 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 atomic classes, your own custom 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. 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
       return compareAndSetState(0, 1);
     }

     // Release the lock by setting state to zero
     protected boolean tryRelease(int releases) {
       assert releases == 1; // Otherwise unused
       if (getState() == 0) throw new IllegalMonitorStateException();
       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);
   }
 }

Summary

Nested Classes
class AbstractQueuedSynchronizer.ConditionObject Condition implementation for a AbstractQueuedSynchronizer serving as the basis of a Lock implementation. 
Protected Constructors
AbstractQueuedSynchronizer()
Creates a new AbstractQueuedSynchronizer instance with initial synchronization state of zero.
Public Methods
final void acquire(int arg)
Acquires in exclusive mode, ignoring interrupts.
final void acquireInterruptibly(int arg)
Acquires in exclusive mode, aborting if interrupted.
final void acquireShared(int arg)
Acquires in shared mode, ignoring interrupts.
final void acquireSharedInterruptibly(int arg)
Acquires in shared mode, aborting if interrupted.
final Collection<Thread> getExclusiveQueuedThreads()
Returns a collection containing threads that may be waiting to acquire in exclusive mode.
final Thread getFirstQueuedThread()
Returns the first (longest-waiting) thread in the queue, or null if no threads are currently queued.
final int getQueueLength()
Returns an estimate of the number of threads waiting to acquire.
final Collection<Thread> getQueuedThreads()
Returns a collection containing threads that may be waiting to acquire.
final Collection<Thread> getSharedQueuedThreads()
Returns a collection containing threads that may be waiting to acquire in shared mode.
final int getWaitQueueLength(AbstractQueuedSynchronizer.ConditionObject condition)
Returns an estimate of the number of threads waiting on the given condition associated with this synchronizer.
final Collection<Thread> getWaitingThreads(AbstractQueuedSynchronizer.ConditionObject condition)
Returns a collection containing those threads that may be waiting on the given condition associated with this synchronizer.
final boolean hasContended()
Queries whether any threads have ever contended to acquire this synchronizer; that is if an acquire method has ever blocked.
final boolean hasQueuedThreads()
Queries whether any threads are waiting to acquire.
final boolean hasWaiters(AbstractQueuedSynchronizer.ConditionObject condition)
Queries whether any threads are waiting on the given condition associated with this synchronizer.
final boolean isQueued(Thread thread)
Returns true if the given thread is currently queued.
final boolean owns(AbstractQueuedSynchronizer.ConditionObject condition)
Queries whether the given ConditionObject uses this synchronizer as its lock.
final boolean release(int arg)
Releases in exclusive mode.
final boolean releaseShared(int arg)
Releases in shared mode.
String toString()
Returns a string identifying this synchronizer, as well as its state.
final boolean tryAcquireNanos(int arg, long nanosTimeout)
Attempts to acquire in exclusive mode, aborting if interrupted, and failing if the given timeout elapses.
final boolean tryAcquireSharedNanos(int arg, long nanosTimeout)
Attempts to acquire in shared mode, aborting if interrupted, and failing if the given timeout elapses.
Protected Methods
final boolean compareAndSetState(int expect, int update)
Atomically sets synchronization state to the given updated value if the current state value equals the expected value.
final Thread getExclusiveOwnerThread()
Returns the thread last set by setExclusiveOwnerThread, or null if never set.
final int getState()
Returns the current value of synchronization state.
boolean isHeldExclusively()
Returns true if synchronization is held exclusively with respect to the current (calling) thread.
final void setExclusiveOwnerThread(Thread t)
Sets the thread that currently owns exclusive access.
final void setState(int newState)
Sets the value of synchronization state.
boolean tryAcquire(int arg)
Attempts to acquire in exclusive mode.
int tryAcquireShared(int arg)
Attempts to acquire in shared mode.
boolean tryRelease(int arg)
Attempts to set the state to reflect a release in exclusive mode.
boolean tryReleaseShared(int arg)
Attempts to set the state to reflect a release in shared mode.
[Expand]
Inherited Methods
From class java.lang.Object

Protected Constructors

protected AbstractQueuedSynchronizer ()

Since: API Level 1

Creates a new AbstractQueuedSynchronizer instance with initial synchronization state of zero.

Public Methods

public final void acquire (int arg)

Since: API Level 1

Acquires in exclusive mode, ignoring interrupts. Implemented by invoking at least once tryAcquire(int), returning on success. Otherwise the thread is queued, possibly repeatedly blocking and unblocking, invoking tryAcquire(int) until success. This method can be used to implement method lock().

Parameters
arg the acquire argument. This value is conveyed to tryAcquire(int) but is otherwise uninterpreted and can represent anything you like.

public final void acquireInterruptibly (int arg)

Since: API Level 1

Acquires in exclusive mode, aborting if interrupted. Implemented by first checking interrupt status, then invoking at least once tryAcquire(int), returning on success. Otherwise the thread is queued, possibly repeatedly blocking and unblocking, invoking tryAcquire(int) until success or the thread is interrupted. This method can be used to implement method lockInterruptibly().

Parameters
arg the acquire argument. This value is conveyed to tryAcquire(int) but is otherwise uninterpreted and can represent anything you like.
Throws
InterruptedException if the current thread is interrupted

public final void acquireShared (int arg)

Since: API Level 1

Acquires in shared mode, ignoring interrupts. Implemented by first invoking at least once tryAcquireShared(int), returning on success. Otherwise the thread is queued, possibly repeatedly blocking and unblocking, invoking tryAcquireShared(int) until success.

Parameters
arg the acquire argument. This value is conveyed to tryAcquireShared(int) but is otherwise uninterpreted and can represent anything you like.

public final void acquireSharedInterruptibly (int arg)

Since: API Level 1

Acquires in shared mode, aborting if interrupted. Implemented by first checking interrupt status, then invoking at least once tryAcquireShared(int), returning on success. Otherwise the thread is queued, possibly repeatedly blocking and unblocking, invoking tryAcquireShared(int) until success or the thread is interrupted.

Parameters
arg the acquire argument This value is conveyed to tryAcquireShared(int) but is otherwise uninterpreted and can represent anything you like.
Throws
InterruptedException if the current thread is interrupted

public final Collection<Thread> getExclusiveQueuedThreads ()

Since: API Level 1

Returns a collection containing threads that may be waiting to acquire in exclusive mode. This has the same properties as getQueuedThreads() except that it only returns those threads waiting due to an exclusive acquire.

Returns
  • the collection of threads

public final Thread getFirstQueuedThread ()

Since: API Level 1

Returns the first (longest-waiting) thread in the queue, or 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.

Returns
  • the first (longest-waiting) thread in the queue, or null if no threads are currently queued

public final int getQueueLength ()

Since: API Level 1

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.

Returns
  • the estimated number of threads waiting to acquire

public final Collection<Thread> getQueuedThreads ()

Since: API Level 1

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.

Returns
  • the collection of threads

public final Collection<Thread> getSharedQueuedThreads ()

Since: API Level 1

Returns a collection containing threads that may be waiting to acquire in shared mode. This has the same properties as getQueuedThreads() except that it only returns those threads waiting due to a shared acquire.

Returns
  • the collection of threads

public final int getWaitQueueLength (AbstractQueuedSynchronizer.ConditionObject condition)

Since: API Level 1

Returns an estimate of the number of threads waiting on the given condition associated with this synchronizer. Note that because timeouts and interrupts may occur at any time, the estimate serves only as an upper bound on the actual number of waiters. This method is designed for use in monitoring of the system state, not for synchronization control.

Parameters
condition the condition
Returns
  • the estimated number of waiting threads
Throws
IllegalMonitorStateException if exclusive synchronization is not held
IllegalArgumentException if the given condition is not associated with this synchronizer
NullPointerException if the condition is null

public final Collection<Thread> getWaitingThreads (AbstractQueuedSynchronizer.ConditionObject condition)

Since: API Level 1

Returns a collection containing those threads that may be waiting on the given condition associated with this synchronizer. 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.

Parameters
condition the condition
Returns
  • the collection of threads
Throws
IllegalMonitorStateException if exclusive synchronization is not held
IllegalArgumentException if the given condition is not associated with this synchronizer
NullPointerException if the condition is null

public final boolean hasContended ()

Since: API Level 1

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.

Returns
  • true if there has ever been contention

public final boolean hasQueuedThreads ()

Since: API Level 1

Queries whether any threads are waiting to acquire. Note that because cancellations due to interrupts and timeouts may occur at any time, a true return does not guarantee that any other thread will ever acquire.

In this implementation, this operation returns in constant time.

Returns
  • true if there may be other threads waiting to acquire

public final boolean hasWaiters (AbstractQueuedSynchronizer.ConditionObject condition)

Since: API Level 1

Queries whether any threads are waiting on the given condition associated with this synchronizer. Note that because timeouts and interrupts may occur at any time, a true return does not guarantee that a future signal will awaken any threads. This method is designed primarily for use in monitoring of the system state.

Parameters
condition the condition
Returns
  • true if there are any waiting threads
Throws
IllegalMonitorStateException if exclusive synchronization is not held
IllegalArgumentException if the given condition is not associated with this synchronizer
NullPointerException if the condition is null

public final boolean isQueued (Thread thread)

Since: API Level 1

Returns true if the given thread is currently queued.

This implementation traverses the queue to determine presence of the given thread.

Parameters
thread the thread
Returns
  • true if the given thread is on the queue
Throws
NullPointerException if the thread is null

public final boolean owns (AbstractQueuedSynchronizer.ConditionObject condition)

Since: API Level 1

Queries whether the given ConditionObject uses this synchronizer as its lock.

Parameters
condition the condition
Returns
  • true if owned
Throws
NullPointerException if the condition is null

public final boolean release (int arg)

Since: API Level 1

Releases in exclusive mode. Implemented by unblocking one or more threads if tryRelease(int) returns true. This method can be used to implement method unlock().

Parameters
arg the release argument. This value is conveyed to tryRelease(int) but is otherwise uninterpreted and can represent anything you like.
Returns

public final boolean releaseShared (int arg)

Since: API Level 1

Releases in shared mode. Implemented by unblocking one or more threads if tryReleaseShared(int) returns true.

Parameters
arg the release argument. This value is conveyed to tryReleaseShared(int) but is otherwise uninterpreted and can represent anything you like.
Returns

public String toString ()

Since: API Level 1

Returns a string identifying this synchronizer, as well as its state. The state, in brackets, includes the String "State =" followed by the current value of getState(), and either "nonempty" or "empty" depending on whether the queue is empty.

Returns
  • a string identifying this synchronizer, as well as its state

public final boolean tryAcquireNanos (int arg, long nanosTimeout)

Since: API Level 1

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 tryAcquire(int), returning on success. Otherwise, the thread is queued, possibly repeatedly blocking and unblocking, invoking tryAcquire(int) until success or the thread is interrupted or the timeout elapses. This method can be used to implement method tryLock(long, TimeUnit).

Parameters
arg the acquire argument. This value is conveyed to tryAcquire(int) but is otherwise uninterpreted and can represent anything you like.
nanosTimeout the maximum number of nanoseconds to wait
Returns
  • true if acquired; false if timed out
Throws
InterruptedException if the current thread is interrupted

public final boolean tryAcquireSharedNanos (int arg, long nanosTimeout)

Since: API Level 1

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 tryAcquireShared(int), returning on success. Otherwise, the thread is queued, possibly repeatedly blocking and unblocking, invoking tryAcquireShared(int) until success or the thread is interrupted or the timeout elapses.

Parameters
arg the acquire argument. This value is conveyed to tryAcquireShared(int) but is otherwise uninterpreted and can represent anything you like.
nanosTimeout the maximum number of nanoseconds to wait
Returns
  • true if acquired; false if timed out
Throws
InterruptedException if the current thread is interrupted

Protected Methods

protected final boolean compareAndSetState (int expect, int update)

Since: API Level 1

Atomically sets synchronization state to the given updated value if the current state value equals the expected value. This operation has memory semantics of a volatile read and write.

Parameters
expect the expected value
update the new value
Returns
  • true if successful. False return indicates that the actual value was not equal to the expected value.

protected final Thread getExclusiveOwnerThread ()

Since: API Level 5

Returns the thread last set by setExclusiveOwnerThread, or null if never set. This method does not otherwise impose any synchronization or volatile field accesses.

Returns
  • the owner thread

protected final int getState ()

Since: API Level 1

Returns the current value of synchronization state. This operation has memory semantics of a volatile read.

Returns
  • current state value

protected boolean isHeldExclusively ()

Since: API Level 1

Returns true if synchronization is held exclusively with respect to the current (calling) thread. This method is invoked upon each call to a non-waiting AbstractQueuedSynchronizer.ConditionObject method. (Waiting methods instead invoke release(int).)

The default implementation throws UnsupportedOperationException. This method is invoked internally only within AbstractQueuedSynchronizer.ConditionObject methods, so need not be defined if conditions are not used.

Returns
  • true if synchronization is held exclusively; false otherwise
Throws
UnsupportedOperationException if conditions are not supported

protected final void setExclusiveOwnerThread (Thread t)

Since: API Level 5

Sets the thread that currently owns exclusive access. A null argument indicates that no thread owns access. This method does not otherwise impose any synchronization or volatile field accesses.

protected final void setState (int newState)

Since: API Level 1

Sets the value of synchronization state. This operation has memory semantics of a volatile write.

Parameters
newState the new state value

protected boolean tryAcquire (int arg)

Since: API Level 1

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 tryLock().

The default implementation throws UnsupportedOperationException.

Parameters
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.
Returns
  • 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.
UnsupportedOperationException if exclusive mode is not supported

protected int tryAcquireShared (int arg)

Since: API Level 1

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 UnsupportedOperationException.

Parameters
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.
Returns
  • 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.
UnsupportedOperationException if shared mode is not supported

protected boolean tryRelease (int arg)

Since: API Level 1

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 UnsupportedOperationException.

Parameters
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.
Returns
  • true if this object is now in a fully released state, so that any waiting threads may attempt to acquire; and 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.
UnsupportedOperationException if exclusive mode is not supported

protected boolean tryReleaseShared (int arg)

Since: API Level 1

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 UnsupportedOperationException.

Parameters
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.
Returns
  • true if this release of shared mode may permit a waiting acquire (shared or exclusive) to succeed; and 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.
UnsupportedOperationException if shared mode is not supported