Java parknanos locksupport locks concurrent util
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:
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 Queuewaiters = 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()); >> Method Summary
Returns the blocker object supplied to the most recent invocation of a park method that has not yet unblocked, or null if not blocked.
Disables the current thread for thread scheduling purposes, for up to the specified waiting time, unless the permit is available.
Disables the current thread for thread scheduling purposes, for up to the specified waiting time, unless the permit is available.
Disables the current thread for thread scheduling purposes, until the specified deadline, unless the permit is available.
Disables the current thread for thread scheduling purposes, until the specified deadline, unless the permit is available.
Methods inherited from class java.lang.Object
Method Detail
unpark
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.
park
- 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.
parkNanos
- 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.
parkUntil
- 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.
getBlocker
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.
park
- 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.
parkNanos
public static void parkNanos(long nanos)
- 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.
parkUntil
public static void parkUntil(long deadline)
- 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.
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Class LockSupport
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.) Reliable usage requires the use of volatile (or atomic) variables to control when to park or unpark. Orderings of calls to these methods are maintained with respect to volatile variable accesses, but not necessarily non-volatile variable accesses.
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:
where no actions by the thread publishing a request to unpark, prior to the call to park , entail locking or blocking. Because only one permit is associated with each thread, any intermediary uses of park , including implicitly via class loading, could lead to an unresponsive thread (a «lost unpark»).
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 Queuewaiters = new ConcurrentLinkedQueue<>(); public void lock() < boolean wasInterrupted = false; // publish current thread for unparkers waiters.add(Thread.currentThread()); // Block while not first in queue or cannot acquire lock while (waiters.peek() != Thread.currentThread() || !locked.compareAndSet(false, true)) < LockSupport.park(this); // ignore interrupts while waiting if (Thread.interrupted()) wasInterrupted = true; >waiters.remove(); // ensure correct interrupt status on return if (wasInterrupted) Thread.currentThread().interrupt(); > public void unlock() < locked.set(false); LockSupport.unpark(waiters.peek()); >static < // Reduce the risk of "lost unpark" due to classloading ClassensureLoaded = LockSupport.class; > >