JavaTM 2 Platform Std. Ed. v1.5.0
java.util.concurrent.locks
Class LockSupport
java.lang.Object
java.util.concurrent.locks.LockSupport
public class LockSupport - extends Object
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.
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.
Sample Usage. Here is a sketch of a First-in-first-out
non-reentrant lock class.
class FIFOMutex {
private AtomicBoolean locked = new AtomicBoolean(false);
private 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();
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 void |
park()
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 |
parkUntil(long deadline)
Disables the current thread for thread scheduling purposes, until
the specified deadline, unless the permit is available. |
static void |
unpark(Thread thread)
Make available the permit for the given thread, if it
was not already available. |
Methods inherited from class java.lang.Object |
clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait |
unpark
public static void unpark(Thread thread)
- Make 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.
- Parameters:
thread - the thread to unpark, or null, in which case
this operation has no effect.
park
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:
- 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.
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.
parkNanos
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:
- 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.
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.
- Parameters:
nanos - the maximum number of nanoseconds to wait
parkUntil
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:
- 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.
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.
- Parameters:
deadline - the absolute time, in milliseconds from the Epoch, to
wait until
Copyright 2003 Sun Microsystems, Inc. All rights reserved
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