Package co.paralleluniverse.strands.concurrent

Class AbstractQueuedSynchronizer

  • All Implemented Interfaces:
    java.io.Serializable
    public abstract class AbstractQueuedSynchronizer
extends AbstractOwnableSynchronizer
implements java.io.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 strands cannot succeed. Shared mode acquires by multiple strands 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 strand (if one exists) must also determine whether it can acquire as well. Strands 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 strand, 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 strand 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 strand-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 strand owning an exclusive synchronizer. You are encouraged to use them -- this enables monitoring and diagnostic tools to assist users in determining which strands 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 strand if it is not already queued;
        possibly block current strand;
     }

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

    Because checks in acquire are invoked before enqueuing, a newly acquiring strand 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 strands are allowed to recontend before later queued strands, and each recontention has an unbiased chance to succeed against incoming strands. 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 hasQueuedStrands() 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. While a non-reentrant lock does not strictly require recording of the current owner strand, 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)) {
         setExclusiveOwnerStrand(Strand.currentStrand());
         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();
       setExclusiveOwnerStrand(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 hasQueuedStrands() { return sync.hasQueuedStrands(); }
   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

    • Constructor Summary

      Constructors 
      Modifier Constructor Description
      protected AbstractQueuedSynchronizer()
      Creates a new AbstractQueuedSynchronizer instance with initial synchronization state of zero.

    • Method Summary

      All Methods Instance Methods Concrete Methods 
      Modifier and Type Method 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.
      java.util.Collection<Strand> getExclusiveQueuedStrands()
      Returns a collection containing strands that may be waiting to acquire in exclusive mode.
      Strand getFirstQueuedStrand()
      Returns the first (longest-waiting) strand in the queue, or null if no strands are currently queued.
      java.util.Collection<Strand> getQueuedStrands()
      Returns a collection containing strands that may be waiting to acquire.
      int getQueueLength()
      Returns an estimate of the number of strands waiting to acquire.
      java.util.Collection<Strand> getSharedQueuedStrands()
      Returns a collection containing strands that may be waiting to acquire in shared mode.
      protected int getState()
      Returns the current value of synchronization state.
      java.util.Collection<Strand> getWaitingStrands​(AbstractQueuedSynchronizer.ConditionObject condition)
      Returns a collection containing those strands that may be waiting on the given condition associated with this synchronizer.
      int getWaitQueueLength​(AbstractQueuedSynchronizer.ConditionObject condition)
      Returns an estimate of the number of strands waiting on the given condition associated with this synchronizer.
      boolean hasContended()
      Queries whether any strands have ever contended to acquire this synchronizer; that is if an acquire method has ever blocked.
      boolean hasQueuedPredecessors()
      Queries whether any strands have been waiting to acquire longer than the current strand.
      boolean hasQueuedStrands()
      Queries whether any strands are waiting to acquire.
      boolean hasWaiters​(AbstractQueuedSynchronizer.ConditionObject condition)
      Queries whether any strands 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) strand.
      boolean isQueued​(Strand strand)
      Returns true if the given strand 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.
      java.lang.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 strand performing acquire. If this method reports failure, the acquire method may queue the strand, if it is not already queued, until it is signalled by a release from some other strand. 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:
        java.lang.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.
        java.lang.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 strand 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 strands may attempt to acquire; and false otherwise.
        Throws:
        java.lang.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.
        java.lang.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 strand performing acquire. If this method reports failure, the acquire method may queue the strand, if it is not already queued, until it is signalled by a release from some other strand.

        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 strand 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:
        java.lang.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.
        java.lang.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 strand 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:
        java.lang.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.
        java.lang.UnsupportedOperationException - if shared mode is not supported

      • isHeldExclusively

        protected boolean isHeldExclusively()
        Returns true if synchronization is held exclusively with respect to the current (calling) strand. 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:
        java.lang.UnsupportedOperationException - if conditions are not supported

      • acquire

        @Suspendable
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 strand 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

        @Suspendable
public final void acquireInterruptibly​(int arg)
                                throws java.lang.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 strand is queued, possibly repeatedly blocking and unblocking, invoking tryAcquire(int) until success or the strand 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:
        java.lang.InterruptedException - if the current strand is interrupted

      • tryAcquireNanos

        @Suspendable
public final boolean tryAcquireNanos​(int arg,
                                     long nanosTimeout)
                              throws java.lang.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 strand is queued, possibly repeatedly blocking and unblocking, invoking tryAcquire(int) until success or the strand 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:
        java.lang.InterruptedException - if the current strand is interrupted

      • release

        public final boolean release​(int arg)
        Releases in exclusive mode. Implemented by unblocking one or more strands 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

        @Suspendable
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 strand 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

        @Suspendable
public final void acquireSharedInterruptibly​(int arg)
                                      throws java.lang.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 strand is queued, possibly repeatedly blocking and unblocking, invoking tryAcquireShared(int) until success or the strand 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:
        java.lang.InterruptedException - if the current strand is interrupted

      • tryAcquireSharedNanos

        @Suspendable
public final boolean tryAcquireSharedNanos​(int arg,
                                           long nanosTimeout)
                                    throws java.lang.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 strand is queued, possibly repeatedly blocking and unblocking, invoking tryAcquireShared(int) until success or the strand 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:
        java.lang.InterruptedException - if the current strand is interrupted

      • releaseShared

        public final boolean releaseShared​(int arg)
        Releases in shared mode. Implemented by unblocking one or more strands 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)

      • hasQueuedStrands

        public final boolean hasQueuedStrands()
        Queries whether any strands 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 strand will ever acquire.

        In this implementation, this operation returns in constant time.

        Returns:
        true if there may be other strands waiting to acquire

      • hasContended

        public final boolean hasContended()
        Queries whether any strands 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

      • getFirstQueuedStrand

        public final Strand getFirstQueuedStrand()
        Returns the first (longest-waiting) strand in the queue, or null if no strands are currently queued.

        In this implementation, this operation normally returns in constant time, but may iterate upon contention if other strands are concurrently modifying the queue.

        Returns:
        the first (longest-waiting) strand in the queue, or null if no strands are currently queued

      • isQueued

        public final boolean isQueued​(Strand strand)
        Returns true if the given strand is currently queued.

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

        Parameters:
        strand - the strand
        Returns:
        true if the given strand is on the queue
        Throws:
        java.lang.NullPointerException - if the strand is null

      • hasQueuedPredecessors

        public final boolean hasQueuedPredecessors()
        Queries whether any strands have been waiting to acquire longer than the current strand.

        An invocation of this method is equivalent to (but may be more efficient than): 

         
 getFirstQueuedStrand() != Strand.currentStrand() &&
 hasQueuedStrands()

        Note that because cancellations due to interrupts and timeouts may occur at any time, a true return does not guarantee that some other strand will acquire before the current strand. Likewise, it is possible for another strand 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
   }
 }
        Returns:
        true if there is a queued strand preceding the current strand, and false if the current strand is at the head of the queue or the queue is empty
        Since:
        1.7

      • getQueueLength

        public final int getQueueLength()
        Returns an estimate of the number of strands waiting to acquire. The value is only an estimate because the number of strands 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 strands waiting to acquire

      • getQueuedStrands

        public final java.util.Collection<Strand> getQueuedStrands()
        Returns a collection containing strands that may be waiting to acquire. Because the actual set of strands 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 strands

      • getExclusiveQueuedStrands

        public final java.util.Collection<Strand> getExclusiveQueuedStrands()
        Returns a collection containing strands that may be waiting to acquire in exclusive mode. This has the same properties as getQueuedStrands() except that it only returns those strands waiting due to an exclusive acquire.
        Returns:
        the collection of strands

      • getSharedQueuedStrands

        public final java.util.Collection<Strand> getSharedQueuedStrands()
        Returns a collection containing strands that may be waiting to acquire in shared mode. This has the same properties as getQueuedStrands() except that it only returns those strands waiting due to a shared acquire.
        Returns:
        the collection of strands

      • toString

        public java.lang.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 java.lang.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:
        java.lang.NullPointerException - if the condition is null

      • hasWaiters

        public final boolean hasWaiters​(AbstractQueuedSynchronizer.ConditionObject condition)
        Queries whether any strands 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 strands. This method is designed primarily for use in monitoring of the system state.
        Parameters:
        condition - the condition
        Returns:
        true if there are any waiting strands
        Throws:
        java.lang.IllegalMonitorStateException - if exclusive synchronization is not held
        java.lang.IllegalArgumentException - if the given condition is not associated with this synchronizer
        java.lang.NullPointerException - if the condition is null

      • getWaitQueueLength

        public final int getWaitQueueLength​(AbstractQueuedSynchronizer.ConditionObject condition)
        Returns an estimate of the number of strands 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 strands
        Throws:
        java.lang.IllegalMonitorStateException - if exclusive synchronization is not held
        java.lang.IllegalArgumentException - if the given condition is not associated with this synchronizer
        java.lang.NullPointerException - if the condition is null

      • getWaitingStrands

        public final java.util.Collection<Strand> getWaitingStrands​(AbstractQueuedSynchronizer.ConditionObject condition)
        Returns a collection containing those strands that may be waiting on the given condition associated with this synchronizer. Because the actual set of strands 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 strands
        Throws:
        java.lang.IllegalMonitorStateException - if exclusive synchronization is not held
        java.lang.IllegalArgumentException - if the given condition is not associated with this synchronizer
        java.lang.NullPointerException - if the condition is null