LockSupport

Basic thread blocking primitives for creating locks and other synchronization classes.

<p>This class associates, with each thread that uses it, a permit (in the sense of the {@link java.util.concurrent.Semaphore Semaphore} class). A call to {@code park} will return immediately if the permit is available, consuming it in the process; otherwise it <em>may</em> block. A call to {@code 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.

<p>Methods {@code park} and {@code unpark} provide efficient means of blocking and unblocking threads that do not encounter the problems that cause the deprecated methods {@code Thread.suspend} and {@code Thread.resume} to be unusable for such purposes: Races between one thread invoking {@code park} and another thread trying to {@code unpark} it will preserve liveness, due to the permit. Additionally, {@code park} will return if the caller's thread was interrupted, and timeout versions are supported. The {@code 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 {@code park} serves as an optimization of a "busy wait" that does not waste as much time spinning, but must be paired with an {@code unpark} to be effective.

<p>The three forms of {@code park} each also support a {@code 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 {@link #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 {@code blocker} within a lock implementation is {@code this}.

<p>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 {@code park} method is designed for use only in constructions of the form:

<pre> {@code while (!canProceed()) { // ensure request to unpark is visible to other threads ... LockSupport.park(this); }}</pre>

where no actions by the thread publishing a request to unpark, prior to the call to {@code park}, entail locking or blocking. Because only one permit is associated with each thread, any intermediary uses of {@code park}, including implicitly via class loading, could lead to an unresponsive thread (a "lost unpark").

<p><b>Sample Usage.</b> Here is a sketch of a first-in-first-out non-reentrant lock class: <pre> {@code class FIFOMutex { private final AtomicBoolean locked = new AtomicBoolean(false); private final Queue!(Thread) waiters = new ConcurrentLinkedQueue<>();

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(); }

void unlock() { locked.set(false); LockSupport.unpark(waiters.peek()); }

static { // Reduce the risk of "lost unpark" due to classloading Class<?> ensureLoaded = LockSupport.class; } }}</pre>