public abstract class AbstractQueuedSynchronizer extends AbstractOwnableSynchronizer implements Serializable
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.
 
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):
 
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.
 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;
 
 (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 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 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.
 
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 {
     // Reports whether in locked state
     protected boolean isHeldExclusively() {
       return getState() == 1;
     }
     // Acquires the lock if state is zero
     public boolean tryAcquire(int acquires) {
       assert acquires == 1; // Otherwise unused
       if (compareAndSetState(0, 1)) {
         setExclusiveOwnerThread(Thread.currentThread());
         return true;
       }
       return false;
     }
     // Releases the lock by setting state to zero
     protected boolean tryRelease(int releases) {
       assert releases == 1; // Otherwise unused
       if (getState() == 0) throw new IllegalMonitorStateException();
       setExclusiveOwnerThread(null);
       setState(0);
       return true;
     }
     // Provides a Condition
     Condition newCondition() { return new ConditionObject(); }
     // Deserializes properly
     private void readObject(ObjectInputStream s)
         throws IOException, ClassNotFoundException {
       s.defaultReadObject();
       setState(0); // reset to unlocked state
     }
   }
   // The sync object does all the hard work. We just forward to it.
   private final Sync sync = new Sync();
   public void lock()                { sync.acquire(1); }
   public boolean tryLock()          { return sync.tryAcquire(1); }
   public void unlock()              { sync.release(1); }
   public Condition newCondition()   { return sync.newCondition(); }
   public boolean isLocked()         { return sync.isHeldExclusively(); }
   public boolean hasQueuedThreads() { return sync.hasQueuedThreads(); }
   public void lockInterruptibly() throws InterruptedException {
     sync.acquireInterruptibly(1);
   }
   public boolean tryLock(long timeout, TimeUnit unit)
       throws InterruptedException {
     return sync.tryAcquireNanos(1, unit.toNanos(timeout));
   }
 }
 Here is a latch class that is like a
 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);
   }
 }| Modifier and Type | Class and Description | 
|---|---|
| class  | AbstractQueuedSynchronizer.ConditionObjectCondition implementation for a  AbstractQueuedSynchronizerserving as the basis of aLockimplementation. | 
| Modifier | Constructor and Description | 
|---|---|
| protected  | AbstractQueuedSynchronizer()Creates a new  AbstractQueuedSynchronizerinstance
 with initial synchronization state of zero. | 
| Modifier and Type | Method and Description | 
|---|---|
| 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
  nullif 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 | hasQueuedPredecessors()Queries whether any threads have been waiting to acquire longer
 than the current thread. | 
| 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  trueif 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. | 
getExclusiveOwnerThread, setExclusiveOwnerThreadprotected AbstractQueuedSynchronizer()
AbstractQueuedSynchronizer instance
 with initial synchronization state of zero.protected final int getState()
volatile read.protected final void setState(int newState)
volatile write.newState - the new state valueprotected final boolean compareAndSetState(int expect,
                                           int update)
volatile read
 and write.expect - the expected valueupdate - the new valuetrue if successful. False return indicates that the actual
         value was not equal to the expected value.protected boolean tryAcquire(int arg)
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.
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.true if successful. Upon success, this object has
         been acquired.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 supportedprotected boolean tryRelease(int arg)
This method is always invoked by the thread performing release.
The default implementation throws
 UnsupportedOperationException.
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.true if this object is now in a fully released
         state, so that any waiting threads may attempt to acquire;
         and false otherwise.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 supportedprotected int tryAcquireShared(int arg)
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.
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.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 supportedprotected boolean tryReleaseShared(int arg)
This method is always invoked by the thread performing release.
The default implementation throws
 UnsupportedOperationException.
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.true if this release of shared mode may permit a
         waiting acquire (shared or exclusive) to succeed; and
         false otherwiseIllegalMonitorStateException - 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 supportedprotected boolean isHeldExclusively()
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.
true if synchronization is held exclusively;
         false otherwiseUnsupportedOperationException - if conditions are not supportedpublic final void acquire(int arg)
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().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)
                                throws InterruptedException
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().arg - the acquire argument.  This value is conveyed to
        tryAcquire(int) but is otherwise uninterpreted and
        can represent anything you like.InterruptedException - if the current thread is interruptedpublic final boolean tryAcquireNanos(int arg,
                                     long nanosTimeout)
                              throws InterruptedException
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).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 waittrue if acquired; false if timed outInterruptedException - if the current thread is interruptedpublic final boolean release(int arg)
tryRelease(int) returns true.
 This method can be used to implement method Lock.unlock().arg - the release argument.  This value is conveyed to
        tryRelease(int) but is otherwise uninterpreted and
        can represent anything you like.tryRelease(int)public final void acquireShared(int arg)
tryAcquireShared(int),
 returning on success.  Otherwise the thread is queued, possibly
 repeatedly blocking and unblocking, invoking tryAcquireShared(int) until success.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)
                                      throws InterruptedException
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.arg - the acquire argument.
 This value is conveyed to tryAcquireShared(int) but is
 otherwise uninterpreted and can represent anything
 you like.InterruptedException - if the current thread is interruptedpublic final boolean tryAcquireSharedNanos(int arg,
                                           long nanosTimeout)
                                    throws InterruptedException
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.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 waittrue if acquired; false if timed outInterruptedException - if the current thread is interruptedpublic final boolean releaseShared(int arg)
tryReleaseShared(int) returns true.arg - the release argument.  This value is conveyed to
        tryReleaseShared(int) but is otherwise uninterpreted
        and can represent anything you like.tryReleaseShared(int)public final boolean hasQueuedThreads()
true return does not guarantee that any
 other thread will ever acquire.
 In this implementation, this operation returns in constant time.
true if there may be other threads waiting to acquirepublic final boolean hasContended()
In this implementation, this operation returns in constant time.
true if there has ever been contentionpublic final Thread getFirstQueuedThread()
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.
null if no threads are currently queuedpublic final boolean isQueued(Thread thread)
This implementation traverses the queue to determine presence of the given thread.
thread - the threadtrue if the given thread is on the queueNullPointerException - if the thread is nullpublic final boolean hasQueuedPredecessors()
An invocation of this method is equivalent to (but may be more efficient than):
 
 getFirstQueuedThread() != Thread.currentThread() &&
 hasQueuedThreads()
 Note that because cancellations due to interrupts and
 timeouts may occur at any time, a 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 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 tryAcquire(int) method should return
 false, and its tryAcquireShared(int) method should
 return a negative value, if this method returns true
 (unless this is a reentrant acquire).  For example, the tryAcquire method for a fair, reentrant, exclusive mode
 synchronizer might look like this:
  
 
 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
   }
 }true if there is a queued thread preceding the
         current thread, and false if the current thread
         is at the head of the queue or the queue is emptypublic final int getQueueLength()
public final Collection<Thread> getQueuedThreads()
public final Collection<Thread> getExclusiveQueuedThreads()
getQueuedThreads() except that it only returns
 those threads waiting due to an exclusive acquire.public final Collection<Thread> getSharedQueuedThreads()
getQueuedThreads() except that it only returns
 those threads waiting due to a shared acquire.public String toString()
"State ="
 followed by the current value of getState(), and either
 "nonempty" or "empty" depending on whether the
 queue is empty.public final boolean owns(AbstractQueuedSynchronizer.ConditionObject condition)
condition - the conditiontrue if ownedNullPointerException - if the condition is nullpublic final boolean hasWaiters(AbstractQueuedSynchronizer.ConditionObject condition)
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.condition - the conditiontrue if there are any waiting threadsIllegalMonitorStateException - if exclusive synchronization
         is not heldIllegalArgumentException - if the given condition is
         not associated with this synchronizerNullPointerException - if the condition is nullpublic final int getWaitQueueLength(AbstractQueuedSynchronizer.ConditionObject condition)
condition - the conditionIllegalMonitorStateException - if exclusive synchronization
         is not heldIllegalArgumentException - if the given condition is
         not associated with this synchronizerNullPointerException - if the condition is nullpublic final Collection<Thread> getWaitingThreads(AbstractQueuedSynchronizer.ConditionObject condition)
condition - the conditionIllegalMonitorStateException - if exclusive synchronization
         is not heldIllegalArgumentException - if the given condition is
         not associated with this synchronizerNullPointerException - if the condition is null Submit a bug or feature 
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