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java.lang.Object
java.util.concurrent.locks.LockSupport
public class LockSupport
extends Object
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());
}
}
Method Summary | |
static Object |
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static void |
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static void | |
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Methods inherited from class java.lang.Object | |
clone , equals , extends Object> getClass , finalize , hashCode , notify , notifyAll , toString , wait , wait , wait |
public 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. The value returned is just a momentary snapshot -- the thread may have since unblocked or blocked on a different blocker object.
- Returns:
- the blocker
- Since:
- 1.6
public static void park()
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: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.
- Some other thread invokes
unpark
with the current thread as the target; or- Some other thread interrupts the current thread; or
- The call spuriously (that is, for no reason) returns.
public static void park(Object blocker)
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: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.
- Some other thread invokes
unpark
with the current thread as the target; or- Some other thread interrupts the current thread; or
- The call spuriously (that is, for no reason) returns.
- Parameters:
blocker
- the synchronization object responsible for this thread parking
- Since:
- 1.6
public 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. 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: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.
- Some other thread invokes
unpark
with the current thread as the target; or- Some other thread interrupts the current thread; or
- The specified waiting time elapses; or
- The call spuriously (that is, for no reason) returns.
- Parameters:
blocker
- the synchronization object responsible for this thread parkingnanos
- the maximum number of nanoseconds to wait
- Since:
- 1.6
public 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. 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: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.
- Some other thread invokes
unpark
with the current thread as the target; or- Some other thread interrupts the current thread; or
- The specified waiting time elapses; or
- The call spuriously (that is, for no reason) returns.
- Parameters:
nanos
- the maximum number of nanoseconds to wait
public static void parkUntil(Object blocker, long deadline)
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: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.
- Some other thread invokes
unpark
with the current thread as the target; or- Some other thread interrupts the current thread; or
- The specified deadline passes; or
- The call spuriously (that is, for no reason) returns.
- Parameters:
blocker
- the synchronization object responsible for this thread parkingdeadline
- the absolute time, in milliseconds from the Epoch, to wait until
- Since:
- 1.6
public static void parkUntil(long deadline)
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: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.
- Some other thread invokes
unpark
with the current thread as the target; or- Some other thread interrupts the current thread; or
- The specified deadline passes; or
- The call spuriously (that is, for no reason) returns.
- Parameters:
deadline
- the absolute time, in milliseconds from the Epoch, to wait until
public static void unpark(Thread thread)
Makes available the permit for the given thread, if it was not already available. If the thread was blocked onpark
then it will unblock. Otherwise, its next call topark
is guaranteed not to block. This operation is not guaranteed to have any effect at all if the given thread has not been started.
- Parameters:
thread
- the thread to unpark, ornull
, in which case this operation has no effect
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