public class LockSupport extends Object
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<Thread> waiters
= new ConcurrentLinkedQueue<Thread>();
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());
}
}| Modifier and Type | Method and Description |
|---|---|
static Object |
getBlocker(Thread t)
Returns the blocker object supplied to the most recent
invocation of a park method that has not yet unblocked, or null
if not blocked.
|
static void |
park()
Disables the current thread for thread scheduling purposes unless the
permit is available.
|
static void |
park(Object blocker)
Disables the current thread for thread scheduling purposes unless the
permit is available.
|
static void |
parkNanos(long nanos)
Disables the current thread for thread scheduling purposes, for up to
the specified waiting time, unless the permit is available.
|
static void |
parkNanos(Object blocker,
long nanos)
Disables the current thread for thread scheduling purposes, for up to
the specified waiting time, unless the permit is available.
|
static void |
parkUntil(long deadline)
Disables the current thread for thread scheduling purposes, until
the specified deadline, unless the permit is available.
|
static void |
parkUntil(Object blocker,
long deadline)
Disables the current thread for thread scheduling purposes, until
the specified deadline, unless the permit is available.
|
static void |
unpark(Thread thread)
Makes available the permit for the given thread, if it
was not already available.
|
public static void unpark(Thread thread)
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 effectpublic static void park(Object blocker)
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
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 parkingpublic static void parkNanos(Object blocker, long nanos)
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
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 parkingnanos - the maximum number of nanoseconds to waitpublic static void parkUntil(Object blocker, long deadline)
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
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 parkingdeadline - the absolute time, in milliseconds from the Epoch,
to wait untilpublic static Object getBlocker(Thread t)
t - the threadNullPointerException - if argument is nullpublic static void park()
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
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.
public static void parkNanos(long nanos)
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
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 waitpublic static void parkUntil(long deadline)
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
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 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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