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LockSupport.java API Doc Java SE 5 API 7248 Fri Aug 26 14:57:26 BST 2005 java.util.concurrent.locks

LockSupport

java.lang.Object

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 {@link java.util.concurrent.Semaphore 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());
}
}

Fields Summary
private static final Unsafe
unsafe
Constructors Summary
private LockSupport()
Methods Summary
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 {@link Thread#interrupt 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.
unsafe.park(false, 0L);
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 {@link Thread#interrupt 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.
param
nanos the maximum number of nanoseconds to wait
if (nanos > 0) unsafe.park(false, nanos);
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 {@link Thread#interrupt 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.
param
deadline the absolute time, in milliseconds from the Epoch, to wait until
unsafe.park(true, deadline);
public static void unpark(java.lang.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.
param
thread the thread to unpark, or null, in which case this operation has no effect.
if (thread != null) unsafe.unpark(thread);