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public abstract class AbstractQueuedSynchronizer extends AbstractOwnableSynchronizer 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 {@link #getState}, {@link #setState} and {@link #compareAndSetState} 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 {@link #acquireInterruptibly} that can be invoked as appropriate by concrete locks and related synchronizers to implement their public methods.
This class supports either or both a default exclusive mode and a shared mode. When acquired in exclusive mode, attempted acquires by other threads cannot succeed. Shared mode acquires by multiple threads may (but need not) succeed. This class does not "understand" these differences except in the mechanical sense that when a shared mode acquire succeeds, the next waiting thread (if one exists) must also determine whether it can acquire as well. Threads waiting in the different modes share the same FIFO queue. Usually, implementation subclasses support only one of these modes, but both can come into play for example in a {@link ReadWriteLock}. Subclasses that support only exclusive or only shared modes need not define the methods supporting the unused mode.
This class defines a nested {@link ConditionObject} class that can be used as a {@link Condition} implementation by subclasses supporting exclusive mode for which method {@link #isHeldExclusively} reports whether synchronization is exclusively held with respect to the current thread, method {@link #release} invoked with the current {@link #getState} value fully releases this object, and {@link #acquire}, given this saved state value, eventually restores this object to its previous acquired state. No AbstractQueuedSynchronizer method otherwise creates such a condition, so if this constraint cannot be met, do not use it. The behavior of {@link 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 {@link #getState}, {@link #setState} and/or {@link #compareAndSetState}:
You may also find the inherited methods from {@link 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. In particular, a strict FIFO lock can define tryAcquire to immediately return false if {@link #getFirstQueuedThread} does not return the current thread. A normally preferable non-strict fair version can immediately return false only if {@link #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 {@link #hasContended} and/or {@link #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 {@link java.util.concurrent.atomic atomic} classes, your own custom {@link java.util.Queue} classes, and {@link 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 { // Report whether in locked state protected boolean isHeldExclusively() { return getState() == 1; } // Acquire the lock if state is zero public boolean tryAcquire(int acquires) { assert acquires == 1; // Otherwise unused if (compareAndSetState(0, 1)) { setExclusiveOwnerThread(Thread.currentThread()); return true; } return false; } // Release the lock by setting state to zero protected boolean tryRelease(int releases) { assert releases == 1; // Otherwise unused if (getState() == 0) throw new IllegalMonitorStateException(); setExclusiveOwnerThread(null); setState(0); return true; } // Provide a Condition Condition newCondition() { return new ConditionObject(); } // Deserialize properly private void readObject(ObjectInputStream s) throws IOException, ClassNotFoundException { s.defaultReadObject(); setState(0); // reset to unlocked state } } // The sync object does all the hard work. We just forward to it. private final Sync sync = new Sync(); public void lock() { sync.acquire(1); } public boolean tryLock() { return sync.tryAcquire(1); } public void unlock() { sync.release(1); } public Condition newCondition() { return sync.newCondition(); } public boolean isLocked() { return sync.isHeldExclusively(); } public boolean hasQueuedThreads() { return sync.hasQueuedThreads(); } public void lockInterruptibly() throws InterruptedException { sync.acquireInterruptibly(1); } public boolean tryLock(long timeout, TimeUnit unit) throws InterruptedException { return sync.tryAcquireNanos(1, unit.toNanos(timeout)); } }
Here is a latch class that is like a {@link 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); } }
Nested Class Summary | |
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class |
Condition implementation for a java.util.concurrent.locks.AbstractQueuedSynchronizer serving as the basis of a java.util.concurrent.locks.Lock implementation. |
Constructor Summary | |
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protected |
Creates a new AbstractQueuedSynchronizer instance with initial synchronization state of zero. |
Method Summary | |
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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 |
Returns a collection containing threads that may be waiting to acquire in exclusive mode. |
Thread |
Returns the first (longest-waiting) thread in the queue, or if no threads are currently queued. |
Collection |
Returns a collection containing threads that may be waiting to acquire. |
int |
Returns an estimate of the number of threads waiting to acquire. |
Collection |
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 |
Returns a collection containing those threads that may be waiting on the given condition associated with this synchronizer. |
int |
Returns an estimate of the number of threads waiting on the given condition associated with this synchronizer. |
boolean |
Queries whether any threads have ever contended to acquire this synchronizer; that is if an acquire method has ever blocked. |
boolean |
Queries whether any threads are waiting to acquire. |
boolean |
Queries whether any threads are waiting on the given condition associated with this synchronizer. |
protected boolean |
Returns if synchronization is held exclusively with respect to the current (calling) thread. |
boolean |
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.util.concurrent.locks.AbstractOwnableSynchronizer |
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getExclusiveOwnerThread, setExclusiveOwnerThread |
Methods inherited from class java.lang.Object |
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clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait |
Constructor Detail |
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protected AbstractQueuedSynchronizer()
Method Detail |
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public final void acquire(int arg)
arg
- the acquire argument. This value is conveyed to
{@link #tryAcquire} but is otherwise uninterpreted and
can represent anything you like.public final void acquireInterruptibly(int arg) throws InterruptedException
arg
- the acquire argument. This value is conveyed to
{@link #tryAcquire} but is otherwise uninterpreted and
can represent anything you like.InterruptedException
- if the current thread is interruptedpublic final void acquireShared(int arg)
arg
- the acquire argument. This value is conveyed to
{@link #tryAcquireShared} but is otherwise uninterpreted
and can represent anything you like.public final void acquireSharedInterruptibly(int arg) throws InterruptedException
arg
- the acquire argument.
This value is conveyed to {@link #tryAcquireShared} but is
otherwise uninterpreted and can represent anything
you like.InterruptedException
- if the current thread is interruptedprotected final boolean compareAndSetState(int expect, int update)
expect
- the expected valueupdate
- the new valuepublic final Collection getExclusiveQueuedThreads()
public final Thread getFirstQueuedThread()
In this implementation, this operation normally returns in constant time, but may iterate upon contention if other threads are concurrently modifying the queue.
public final Collection getQueuedThreads()
public final int getQueueLength()
public final Collection getSharedQueuedThreads()
protected final int getState()
public final Collection getWaitingThreads(AbstractQueuedSynchronizer.ConditionObject condition)
condition
- the conditionpublic final int getWaitQueueLength(AbstractQueuedSynchronizer.ConditionObject condition)
condition
- the conditionpublic final boolean hasContended()
In this implementation, this operation returns in constant time.
public final boolean hasQueuedThreads()
In this implementation, this operation returns in constant time.
public final boolean hasWaiters(AbstractQueuedSynchronizer.ConditionObject condition)
condition
- the conditionprotected boolean isHeldExclusively()
The default implementation throws {@link UnsupportedOperationException}. This method is invoked internally only within {@link ConditionObject} methods, so need not be defined if conditions are not used.
public final boolean isQueued(Thread thread)
This implementation traverses the queue to determine presence of the given thread.
thread
- the threadpublic final boolean owns(AbstractQueuedSynchronizer.ConditionObject condition)
condition
- the conditionpublic final boolean release(int arg)
arg
- the release argument. This value is conveyed to
{@link #tryRelease} but is otherwise uninterpreted and
can represent anything you like.public final boolean releaseShared(int arg)
arg
- the release argument. This value is conveyed to
{@link #tryReleaseShared} but is otherwise uninterpreted
and can represent anything you like.protected final void setState(int newState)
newState
- the new state valuepublic String toString()
toString
in class Object
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 {@link Lock#tryLock()}.
The default implementation throws {@link 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.public final boolean tryAcquireNanos(int arg, long nanosTimeout) throws InterruptedException
arg
- the acquire argument. This value is conveyed to
{@link #tryAcquire} but is otherwise uninterpreted and
can represent anything you like.nanosTimeout
- the maximum number of nanoseconds to waitInterruptedException
- if the current thread is interruptedprotected 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 {@link 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.public final boolean tryAcquireSharedNanos(int arg, long nanosTimeout) throws InterruptedException
arg
- the acquire argument. This value is conveyed to
{@link #tryAcquireShared} but is otherwise uninterpreted
and can represent anything you like.nanosTimeout
- the maximum number of nanoseconds to waitInterruptedException
- if the current thread is interruptedprotected boolean tryRelease(int arg)
This method is always invoked by the thread performing release.
The default implementation throws {@link 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.protected boolean tryReleaseShared(int arg)
This method is always invoked by the thread performing release.
The default implementation throws {@link 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.
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