- All Implemented Interfaces:
Serializable
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(java.util.concurrent.locks.AbstractQueuedSynchronizer.Node, int, boolean, boolean, boolean, long)
, 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)
:
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.
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 thread, this class does so anyway to make usage easier to monitor. It also supports conditions and exposes some instrumentation methods:
class Mutex implements Lock, java.io.Serializable {
// Our internal helper class
private static class Sync extends AbstractQueuedSynchronizer {
// 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 (!isHeldExclusively())
throw new IllegalMonitorStateException();
setExclusiveOwnerThread(null);
setState(0);
return true;
}
// Reports whether in locked state
public boolean isLocked() {
return getState() != 0;
}
public boolean isHeldExclusively() {
// a data race, but safe due to out-of-thin-air guarantees
return getExclusiveOwnerThread() == Thread.currentThread();
}
// Provides a Condition
public 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.isLocked(); }
public boolean isHeldByCurrentThread() {
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
Modifier and TypeClassDescriptionclass
Condition implementation for aAbstractQueuedSynchronizer
serving as the basis of aLock
implementation. -
Constructor Summary
ModifierConstructorDescriptionprotected
Creates a newAbstractQueuedSynchronizer
instance with initial synchronization state of zero. -
Method Summary
Modifier and TypeMethodDescriptionvoid
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.Returns a collection containing threads that may be waiting to acquire in exclusive mode.Returns the first (longest-waiting) thread in the queue, ornull
if no threads are currently queued.Returns a collection containing threads that may be waiting to acquire.int
Returns an estimate of the number of threads waiting to acquire.Returns a collection containing threads that may be waiting to acquire in shared mode.protected int
getState()
Returns the current value of synchronization state.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 have been waiting to acquire longer than the current thread.boolean
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
Returnstrue
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.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 declared in class java.util.concurrent.locks.AbstractOwnableSynchronizer
getExclusiveOwnerThread, setExclusiveOwnerThread
-
Constructor Details
-
AbstractQueuedSynchronizer
protected AbstractQueuedSynchronizer()Creates a newAbstractQueuedSynchronizer
instance with initial synchronization state of zero.
-
-
Method Details
-
getState
protected final int getState()Returns the current value of synchronization state. This operation has memory semantics of avolatile
read.- Returns:
- current state value
-
setState
protected final void setState(int newState)Sets the value of synchronization state. This operation has memory semantics of avolatile
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 avolatile
read and write.- Parameters:
expect
- the expected valueupdate
- 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; andfalse
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
-
isHeldExclusively
protected boolean isHeldExclusively()Returnstrue
if synchronization is held exclusively with respect to the current (calling) thread. This method is invoked upon each call to aAbstractQueuedSynchronizer.ConditionObject
method.The default implementation throws
UnsupportedOperationException
. This method is invoked internally only withinAbstractQueuedSynchronizer.ConditionObject
methods, so need not be defined if conditions are not used.- Returns:
true
if synchronization is held exclusively;false
otherwise- Throws:
UnsupportedOperationException
- if conditions are not supported
-
acquire
public final void acquire(int arg)Acquires in exclusive mode, ignoring interrupts. Implemented by invoking at least oncetryAcquire(int)
, returning on success. Otherwise the thread is queued, possibly repeatedly blocking and unblocking, invokingtryAcquire(int)
until success. This method can be used to implement methodLock.lock()
.- Parameters:
arg
- the acquire argument. This value is conveyed totryAcquire(int)
but is otherwise uninterpreted and can represent anything you like.
-
acquireInterruptibly
Acquires in exclusive mode, aborting if interrupted. Implemented by first checking interrupt status, then invoking at least oncetryAcquire(int)
, returning on success. Otherwise the thread is queued, possibly repeatedly blocking and unblocking, invokingtryAcquire(int)
until success or the thread is interrupted. This method can be used to implement methodLock.lockInterruptibly()
.- Parameters:
arg
- the acquire argument. This value is conveyed totryAcquire(int)
but is otherwise uninterpreted and can represent anything you like.- Throws:
InterruptedException
- if the current thread is interrupted
-
tryAcquireNanos
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 oncetryAcquire(int)
, returning on success. Otherwise, the thread is queued, possibly repeatedly blocking and unblocking, invokingtryAcquire(int)
until success or the thread is interrupted or the timeout elapses. This method can be used to implement methodLock.tryLock(long, TimeUnit)
.- Parameters:
arg
- the acquire argument. This value is conveyed totryAcquire(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 iftryRelease(int)
returns true. This method can be used to implement methodLock.unlock()
.- Parameters:
arg
- the release argument. This value is conveyed totryRelease(int)
but is otherwise uninterpreted and can represent anything you like.- Returns:
- the value returned from
tryRelease(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, atrue
return does not guarantee that any other thread will ever acquire.- Returns:
true
if there may be other threads waiting to acquire
-
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
Returns the first (longest-waiting) thread in the queue, ornull
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
Returns true if the given thread is currently queued.This implementation traverses the queue to determine presence of the given thread.
- Parameters:
thread
- the thread- Returns:
true
if the given thread is on the queue- Throws:
NullPointerException
- if the thread is null
-
hasQueuedPredecessors
public final boolean hasQueuedPredecessors()Queries whether any threads have been waiting to acquire longer than the current thread.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 returnedfalse
, 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 returnfalse
, and itstryAcquireShared(int)
method should return a negative value, if this method returnstrue
(unless this is a reentrant acquire). For example, thetryAcquire
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 thread preceding the current thread, andfalse
if the current thread 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 threads waiting to acquire. The value is only an estimate because the number of threads may change dynamically while this method traverses internal data structures. This method is designed for use in monitoring system state, not for synchronization control.- Returns:
- the estimated number of threads waiting to acquire
-
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
Returns a collection containing threads that may be waiting to acquire in exclusive mode. This has the same properties asgetQueuedThreads()
except that it only returns those threads waiting due to an exclusive acquire.- Returns:
- the collection of threads
-
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 ofgetState()
, and either"nonempty"
or"empty"
depending on whether the queue is empty. -
owns
Queries whether the given ConditionObject uses this synchronizer as its lock.- Parameters:
condition
- the condition- Returns:
true
if owned- Throws:
NullPointerException
- if the condition is null
-
hasWaiters
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, atrue
return does not guarantee that a futuresignal
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 heldIllegalArgumentException
- if the given condition is not associated with this synchronizerNullPointerException
- if the condition is null
-
getWaitQueueLength
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 system state, not for synchronization control.- Parameters:
condition
- the condition- Returns:
- the estimated number of waiting threads
- Throws:
IllegalMonitorStateException
- if exclusive synchronization is not heldIllegalArgumentException
- if the given condition is not associated with this synchronizerNullPointerException
- if the condition is 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 heldIllegalArgumentException
- if the given condition is not associated with this synchronizerNullPointerException
- if the condition is null
-