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| java.lang.Object | |
| ↳ | java.util.concurrent.locks.LockSupport |
Basic thread blocking primitives for creating locks and other synchronization classes.
This class associates, with each thread that uses it, a permit
(in the sense of the
Semaphore
class). A call to
park
will return immediately
if the permit is available, consuming it in the process; otherwise
it
may
block. A call to
unpark
makes the permit
available, if it was not already available. (Unlike with Semaphores
though, permits do not accumulate. There is at most one.)
Methods
park
and
unpark
provide efficient
means of blocking and unblocking threads that do not encounter the
problems that cause the deprecated methods
Thread.suspend
and
Thread.resume
to be unusable for such purposes: Races
between one thread invoking
park
and another thread trying
to
unpark
it will preserve liveness, due to the
permit. Additionally,
park
will return if the caller's
thread was interrupted, and timeout versions are supported. The
park
method may also return at any other time, for "no
reason", so in general must be invoked within a loop that rechecks
conditions upon return. In this sense
park
serves as an
optimization of a "busy wait" that does not waste as much time
spinning, but must be paired with an
unpark
to be
effective.
The three forms of
park
each also support a
blocker
object parameter. This object is recorded while
the thread is blocked to permit monitoring and diagnostic tools to
identify the reasons that threads are blocked. (Such tools may
access blockers using method
getBlocker(Thread)
.) The use of these
forms rather than the original forms without this parameter is
strongly encouraged. The normal argument to supply as a
blocker
within a lock implementation is
this
.
These methods are designed to be used as tools for creating
higher-level synchronization utilities, and are not in themselves
useful for most concurrency control applications. The
park
method is designed for use only in constructions of the form:
while (!canProceed()) { ... LockSupport.park(this);
}
where neither
canProceed
nor any other actions prior to the
call to
park
entail locking or blocking. Because only one
permit is associated with each thread, any intermediary uses of
park
could interfere with its intended effects.
Sample Usage. Here is a sketch of a first-in-first-out non-reentrant lock class:
class FIFOMutex {
private final AtomicBoolean locked = new AtomicBoolean(false);
private final Queue
waiters
= new ConcurrentLinkedQueue
();
public void lock() {
boolean wasInterrupted = false;
Thread current = Thread.currentThread();
waiters.add(current);
// Block while not first in queue or cannot acquire lock
while (waiters.peek() != current ||
!locked.compareAndSet(false, true)) {
LockSupport.park(this);
if (Thread.interrupted()) // ignore interrupts while waiting
wasInterrupted = true;
waiters.remove();
if (wasInterrupted) // reassert interrupt status on exit
current.interrupt();
}
public void unlock() {
locked.set(false);
LockSupport.unpark(waiters.peek());
}
}}
| Public Methods | |||||||||||
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Returns the blocker object supplied to the most recent
invocation of a park method that has not yet unblocked, or null
if not blocked.
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|
|
Disables the current thread for thread scheduling purposes unless the
permit is available.
|
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|
|
Disables the current thread for thread scheduling purposes unless the
permit is available.
|
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|
|
Disables the current thread for thread scheduling purposes, for up to
the specified waiting time, unless the permit is available.
|
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|
|
Disables the current thread for thread scheduling purposes, for up to
the specified waiting time, unless the permit is available.
|
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|
|
Disables the current thread for thread scheduling purposes, until
the specified deadline, unless the permit is available.
|
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|
|
Disables the current thread for thread scheduling purposes, until
the specified deadline, unless the permit is available.
|
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|
|
Makes available the permit for the given thread, if it
was not already available.
|
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|
[Expand]
Inherited Methods
|
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|---|---|---|---|---|---|---|---|---|---|---|---|
From class
java.lang.Object
|
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Returns the blocker object supplied to the most recent invocation of a park method that has not yet unblocked, or null if not blocked. The value returned is just a momentary snapshot -- the thread may have since unblocked or blocked on a different blocker object.
| t | the thread |
|---|
| NullPointerException | if argument is null |
|---|
Disables the current thread for thread scheduling purposes unless the permit is available.
If the permit is available then it is consumed and the call returns immediately; otherwise the current thread becomes disabled for thread scheduling purposes and lies dormant until one of three things happens:
unpark
with the
current thread as the target; or
interrupts
the current thread; or
This method does not report which of these caused the method to return. Callers should re-check the conditions which caused the thread to park in the first place. Callers may also determine, for example, the interrupt status of the thread upon return.
| blocker | the synchronization object responsible for this thread parking |
|---|
Disables the current thread for thread scheduling purposes unless the permit is available.
If the permit is available then it is consumed and the call returns immediately; otherwise the current thread becomes disabled for thread scheduling purposes and lies dormant until one of three things happens:
unpark
with the
current thread as the target; or
interrupts
the current thread; or
This method does not report which of these caused the method to return. Callers should re-check the conditions which caused the thread to park in the first place. Callers may also determine, for example, the interrupt status of the thread upon return.
Disables the current thread for thread scheduling purposes, for up to the specified waiting time, unless the permit is available.
If the permit is available then it is consumed and the call returns immediately; otherwise the current thread becomes disabled for thread scheduling purposes and lies dormant until one of four things happens:
unpark
with the
current thread as the target; or
interrupts
the current thread; or
This method does not report which of these caused the method to return. Callers should re-check the conditions which caused the thread to park in the first place. Callers may also determine, for example, the interrupt status of the thread, or the elapsed time upon return.
| nanos | the maximum number of nanoseconds to wait |
|---|
Disables the current thread for thread scheduling purposes, for up to the specified waiting time, unless the permit is available.
If the permit is available then it is consumed and the call returns immediately; otherwise the current thread becomes disabled for thread scheduling purposes and lies dormant until one of four things happens:
unpark
with the
current thread as the target; or
interrupts
the current thread; or
This method does not report which of these caused the method to return. Callers should re-check the conditions which caused the thread to park in the first place. Callers may also determine, for example, the interrupt status of the thread, or the elapsed time upon return.
| blocker | the synchronization object responsible for this thread parking |
|---|---|
| nanos | the maximum number of nanoseconds to wait |
Disables the current thread for thread scheduling purposes, until the specified deadline, unless the permit is available.
If the permit is available then it is consumed and the call returns immediately; otherwise the current thread becomes disabled for thread scheduling purposes and lies dormant until one of four things happens:
unpark
with the
current thread as the target; or
interrupts
the
current thread; or
This method does not report which of these caused the method to return. Callers should re-check the conditions which caused the thread to park in the first place. Callers may also determine, for example, the interrupt status of the thread, or the current time upon return.
| blocker | the synchronization object responsible for this thread parking |
|---|---|
| deadline | the absolute time, in milliseconds from the Epoch, to wait until |
Disables the current thread for thread scheduling purposes, until the specified deadline, unless the permit is available.
If the permit is available then it is consumed and the call returns immediately; otherwise the current thread becomes disabled for thread scheduling purposes and lies dormant until one of four things happens:
unpark
with the
current thread as the target; or
interrupts
the current thread; or
This method does not report which of these caused the method to return. Callers should re-check the conditions which caused the thread to park in the first place. Callers may also determine, for example, the interrupt status of the thread, or the current time upon return.
| deadline | the absolute time, in milliseconds from the Epoch, to wait until |
|---|
Makes available the permit for the given thread, if it
was not already available. If the thread was blocked on
park
then it will unblock. Otherwise, its next call
to
park
is guaranteed not to block. This operation
is not guaranteed to have any effect at all if the given
thread has not been started.
| thread |
the thread to unpark, or
null
, in which case
this operation has no effect
|
|---|