java.util.concurrent.locks
Class AbstractQueuedSynchronizer

java.lang.Object
  java.util.concurrent.locks.AbstractQueuedSynchronizer

All Implemented Interfaces:

Serializable

public abstract class AbstractQueuedSynchronizer
extends Object
implements Serializable

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):

• tryAcquire(int)
• tryRelease(int)
• tryAcquireShared(int)
• tryReleaseShared(int)
• isHeldExclusively()

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. In particular, a strict FIFO lock can define tryAcquire to immediately return false if getFirstQueuedThread() does not return the current thread. A normally preferable non-strict fair version can immediately return false only if hasQueuedThreads() returns true and getFirstQueuedThread is not the current thread; or equivalently, that getFirstQueuedThread is both non-null and not the current thread. Further variations are possible.

Throughput and scalability are generally highest for the default barging (also known as greedy, renouncement, and convoy-avoidance) strategy. While this is not guaranteed to be fair or starvation-free, earlier queued threads are allowed to recontend before later queued threads, and each recontention has an unbiased chance to succeed against incoming threads. Also, while acquires do not "spin" in the usual sense, they may perform multiple invocations of 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);
    }
 }

 

Since:

1.5

See Also:

Serialized Form

Nested Class Summary

class

AbstractQueuedSynchronizer.ConditionObject Condition implementation for a AbstractQueuedSynchronizer serving as the basis of a Lock implementation.

Constructor Summary

protected

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

Method Summary

void	acquire(int arg) Acquires in exclusive mode, ignoring interrupts.
void	acquireInterruptibly(int arg) Acquires in exclusive mode, aborting if interrupted.
void	acquireShared(int arg) Acquires in shared mode, ignoring interrupts.
void	acquireSharedInterruptibly(int arg) Acquires in shared mode, aborting if interrupted.
protected boolean	compareAndSetState(int expect, int update) Atomically sets synchronization state to the given updated value if the current state value equals the expected value.
Collection<Thread>	getExclusiveQueuedThreads() Returns a collection containing threads that may be waiting to acquire in exclusive mode.
Thread	getFirstQueuedThread() Returns the first (longest-waiting) thread in the queue, or null if no threads are currently queued.
Collection<Thread>	getQueuedThreads() Returns a collection containing threads that may be waiting to acquire.
int	getQueueLength() Returns an estimate of the number of threads waiting to acquire.
Collection<Thread>	getSharedQueuedThreads() Returns a collection containing threads that may be waiting to acquire in shared mode.
protected int	getState() Returns the current value of synchronization state.
Collection<Thread>	getWaitingThreads(AbstractQueuedSynchronizer.ConditionObject condition) Returns a collection containing those threads that may be waiting on the given condition associated with this synchronizer.
int	getWaitQueueLength(AbstractQueuedSynchronizer.ConditionObject condition) Returns an estimate of the number of threads waiting on the given condition associated with this synchronizer.
boolean	hasContended() Queries whether any threads have ever contended to acquire this synchronizer; that is if an acquire method has ever blocked.
boolean	hasQueuedThreads() Queries whether any threads are waiting to acquire.
boolean	hasWaiters(AbstractQueuedSynchronizer.ConditionObject condition) Queries whether any threads are waiting on the given condition associated with this synchronizer.
protected boolean	isHeldExclusively() Returns true if synchronization is held exclusively with respect to the current (calling) thread.
boolean	isQueued(Thread thread) Returns true if the given thread is currently queued.
boolean	owns(AbstractQueuedSynchronizer.ConditionObject condition) Queries whether the given ConditionObject uses this synchronizer as its lock.
boolean	release(int arg) Releases in exclusive mode.
boolean	releaseShared(int arg) Releases in shared mode.
protected void	setState(int newState) Sets the value of synchronization state.
String	toString() Returns a string identifying this synchronizer, as well as its state.
protected boolean	tryAcquire(int arg) Attempts to acquire in exclusive mode.
boolean	tryAcquireNanos(int arg, long nanosTimeout) Attempts to acquire in exclusive mode, aborting if interrupted, and failing if the given timeout elapses.
protected int	tryAcquireShared(int arg) Attempts to acquire in shared mode.
boolean	tryAcquireSharedNanos(int arg, long nanosTimeout) Attempts to acquire in shared mode, aborting if interrupted, and failing if the given timeout elapses.
protected boolean	tryRelease(int arg) Attempts to set the state to reflect a release in exclusive mode.
protected boolean	tryReleaseShared(int arg) Attempts to set the state to reflect a release in shared mode.

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, wait, wait, wait

Constructor Detail

AbstractQueuedSynchronizer

protected AbstractQueuedSynchronizer()

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

Method Detail

getState

protected final int getState()

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

Returns:

current state value

setState

protected final void setState(int newState)

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

Parameters:

newState - the new state value

compareAndSetState

protected 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. 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.

tryAcquire

protected boolean tryAcquire(int arg)

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 Lock.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

tryRelease

protected boolean tryRelease(int arg)

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

tryAcquireShared

protected int tryAcquireShared(int arg)

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 on exclusive success, and a positive value if non-exclusively successful, 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

tryReleaseShared

protected boolean tryReleaseShared(int arg)

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 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 shared mode is not supported

isHeldExclusively

protected boolean isHeldExclusively()

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; else false

Throws:

UnsupportedOperationException - if conditions are not supported

acquire

public final void acquire(int arg)

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.lock()

Parameters:

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

acquireInterruptibly

public final void acquireInterruptibly(int arg)
                                throws InterruptedException

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 Lock.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

tryAcquireNanos

public final boolean tryAcquireNanos(int arg,
                                     long nanosTimeout)
                              throws 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 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 Lock.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

release

public final boolean release(int arg)

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

Parameters:

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

Returns:

the value returned from tryRelease(int)

acquireShared

public final void acquireShared(int arg)

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.

acquireSharedInterruptibly

public final void acquireSharedInterruptibly(int arg)
                                      throws InterruptedException

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

tryAcquireSharedNanos

public final boolean tryAcquireSharedNanos(int arg,
                                           long nanosTimeout)
                                    throws 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 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

releaseShared

public final boolean releaseShared(int arg)

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:

the value returned from tryReleaseShared(int)

hasQueuedThreads

public final boolean hasQueuedThreads()

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 the lock.

hasContended

public final boolean hasContended()

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

getFirstQueuedThread

public final Thread getFirstQueuedThread()

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.

isQueued

public final boolean isQueued(Thread thread)

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 in on the queue

Throws:

NullPointerException - if thread null

getQueueLength

public final int getQueueLength()

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 for this lock

getQueuedThreads

public final Collection<Thread> getQueuedThreads()

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

getExclusiveQueuedThreads

public final Collection<Thread> getExclusiveQueuedThreads()

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

getSharedQueuedThreads

public final Collection<Thread> getSharedQueuedThreads()

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

toString

public String toString()

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.

Overrides:

toString in class Object

Returns:

a string identifying this synchronizer, as well as its state.

owns

public final boolean owns(AbstractQueuedSynchronizer.ConditionObject condition)

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

Parameters:

condition - the condition

Returns:

true if owned

Throws:

NullPointerException - if condition null

hasWaiters

public final boolean hasWaiters(AbstractQueuedSynchronizer.ConditionObject condition)

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 condition null

getWaitQueueLength

public final int getWaitQueueLength(AbstractQueuedSynchronizer.ConditionObject condition)

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 condition null

getWaitingThreads

public 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. 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 condition null