LockSupport JDK 1.6)

Description

public class 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 LockSupport.getBlocker(java.lang.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());
   }
 }

See also:

Methods

Hide/Show inherited methods

publicstatic 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.

publicstatic void park ()

Disables the current thread for thread scheduling purposes unless the permit is available.

publicstatic void park (Object blocker)

Disables the current thread for thread scheduling purposes unless the permit is available.

publicstatic void parkNanos (long nanos)

Disables the current thread for thread scheduling purposes, for up to the specified waiting time, unless the permit is available.

publicstatic 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.

publicstatic void parkUntil (long deadline)

Disables the current thread for thread scheduling purposes, until the specified deadline, unless the permit is available.

publicstatic void parkUntil (Object blocker, long deadline)

Disables the current thread for thread scheduling purposes, until the specified deadline, unless the permit is available.

publicstatic void unpark (Thread thread)

Makes available the permit for the given thread, if it was not already available.

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.
Parameters:
- thread - the thread to unpark, or {@code null}, in which case this operation has no effect