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/OpenJDK 8

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.

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<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());
   }
 }

Methods

unpark

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 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(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:

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.

Parameters:: blocker - the synchronization object responsible for this thread parking
Since:: 1.6

parkNanos

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.

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 parking; nanos - the maximum number of nanoseconds to wait
Since:: 1.6

parkUntil

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.

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 parking; deadline - the absolute time, in milliseconds from the Epoch, to wait until
Since:: 1.6

getBlocker

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.

Parameters:: t - the thread
Returns:: the blocker
Throws:: NullPointerException - if argument is null
Since:: 1.6