public class LockSupport extends Object
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()); } }
| Modifier and Type | Method and Description |
|---|---|
static Object |
getBlocker(Thread
Returns the blocker object supplied to the most recent invocation of a park method that has not yet unblocked, or null if not blocked.
|
static void |
park()
Disables the current thread for thread scheduling purposes unless the permit is available.
|
static void |
park(Object
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 |
parkNanos(Object
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 |
parkUntil(Object
Disables the current thread for thread scheduling purposes, until the specified deadline, unless the permit is available.
|
static void |
unpark(Thread
Makes available the permit for the given thread, if it was not already available.
|
public static void unpark(Threadthread)
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.
thread - the thread to unpark, or
null, in which case this operation has no effect
public static void park(Objectblocker)
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:
unpark with the current thread as the target; or 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.
blocker - the synchronization object responsible for this thread parking
public static void parkNanos(Objectblocker, long nanos)
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:
unpark with the current thread as the target; or 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.
blocker - the synchronization object responsible for this thread parking
nanos - the maximum number of nanoseconds to wait
public static void parkUntil(Objectblocker, long deadline)
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:
unpark with the current thread as the target; or 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.
blocker - the synchronization object responsible for this thread parking
deadline - the absolute time, in milliseconds from the Epoch, to wait until
public static ObjectgetBlocker(Thread t)
t - the thread
NullPointerException - if argument is null
public static void park()
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:
unpark with the current thread as the target; or 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.
public static void parkNanos(long nanos)
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:
unpark with the current thread as the target; or 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.
nanos - the maximum number of nanoseconds to wait
public static void parkUntil(long deadline)
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:
unpark with the current thread as the target; or 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.
deadline - the absolute time, in milliseconds from the Epoch, to wait until