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 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());
   }
 }