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(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 | Description |
---|---|---|
class |
AbstractQueuedSynchronizer.ConditionObject |
Condition implementation for a
AbstractQueuedSynchronizer serving as the basis of a Lock implementation. |
Modifier | Constructor | Description |
---|---|---|
protected |
AbstractQueuedSynchronizer() |
Creates a new
AbstractQueuedSynchronizer instance
with initial synchronization state of zero. |
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.
|
Collection<Thread> |
getExclusiveQueuedThreads() |
Returns a collection containing threads that may be waiting to
acquire in exclusive mode.
|
Thread |
getFirstQueuedThread() |
Returns the first (longest-waiting) thread in the queue, or
null if no threads are currently queued. |
Collection<Thread> |
getQueuedThreads() |
Returns a collection containing threads that may be waiting to
acquire.
|
int |
getQueueLength() |
Returns an estimate of the number of threads waiting to
acquire.
|
Collection<Thread> |
getSharedQueuedThreads() |
Returns a collection containing threads that may be waiting to
acquire in shared mode.
|
protected int |
getState() |
Returns the current value of synchronization state.
|
Collection<Thread> |
getWaitingThreads(AbstractQueuedSynchronizer.ConditionObject condition) |
Returns a collection containing those threads that may be
waiting on the given condition associated with this
synchronizer.
|
int |
getWaitQueueLength(AbstractQueuedSynchronizer.ConditionObject condition) |
Returns an estimate of the number of threads waiting on the
given condition associated with this synchronizer.
|
boolean |
hasContended() |
Queries whether any threads have ever contended to acquire this
synchronizer; that is, if an acquire method has ever blocked.
|
boolean |
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
true if synchronization is held exclusively with
respect to the current (calling) thread. |
boolean |
isQueued(Thread thread) |
Returns true if the given thread is currently queued.
|
boolean |
owns(AbstractQueuedSynchronizer.ConditionObject condition) |
Queries whether the given ConditionObject
uses this synchronizer as its lock.
|
boolean |
release(int arg) |
Releases in exclusive mode.
|
boolean |
releaseShared(int arg) |
Releases in shared mode.
|
protected void |
setState(int newState) |
Sets the value of synchronization state.
|
String |
toString() |
Returns a string identifying this synchronizer, as well as its state.
|
protected boolean |
tryAcquire(int arg) |
Attempts to acquire in exclusive mode.
|
boolean |
tryAcquireNanos(int arg,
long nanosTimeout) |
Attempts to acquire in exclusive mode, aborting if interrupted,
and failing if the given timeout elapses.
|
protected int |
tryAcquireShared(int arg) |
Attempts to acquire in shared mode.
|
boolean |
tryAcquireSharedNanos(int arg,
long nanosTimeout) |
Attempts to acquire in shared mode, aborting if interrupted, and
failing if the given timeout elapses.
|
protected boolean |
tryRelease(int arg) |
Attempts to set the state to reflect a release in exclusive
mode.
|
protected boolean |
tryReleaseShared(int arg) |
Attempts to set the state to reflect a release in shared mode.
|
getExclusiveOwnerThread, setExclusiveOwnerThread
protected 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
For further API reference and developer documentation, see Java SE Documentation. That documentation contains more detailed, developer-targeted descriptions, with conceptual overviews, definitions of terms, workarounds, and working code examples.
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