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 may 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. * *
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. * *
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}. * *
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: * *
{@code
* while (!canProceed()) {
* // ensure request to unpark is visible to other threads
* ...
* LockSupport.park(this);
* }}
*
* 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").
*
* Sample Usage. Here is a sketch of a first-in-first-out * non-reentrant lock class: *
{@code
* class FIFOMutex {
* private final AtomicBoolean locked = new AtomicBoolean(false);
* private final Queue waiters
* = 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
* Class ensureLoaded = LockSupport.class;
* }
* }}
*/
public class LockSupport {
private LockSupport() {} // Cannot be instantiated.
private static void setBlocker(Thread t, Object arg) {
// Even though volatile, hotspot doesn't need a write barrier here.
U.putObject(t, PARKBLOCKER, arg);
}
/**
* Makes available the permit for the given thread, if it
* was not already available. If the thread was blocked on
* {@code park} then it will unblock. Otherwise, its next call
* to {@code 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.
*
* @param thread the thread to unpark, or {@code null}, in which case
* this operation has no effect
*/
public static void unpark(Thread thread) {
if (thread != null)
U.unpark(thread);
}
/**
* 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: * *
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. * * @param blocker the synchronization object responsible for this * thread parking * @since 1.6 */ public static void park(Object blocker) { Thread t = Thread.currentThread(); setBlocker(t, blocker); U.park(false, 0L); setBlocker(t, null); } /** * Disables the current thread for thread scheduling purposes, for up to * the specified waiting time, 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 four * things happens: * *
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. * * @param blocker the synchronization object responsible for this * thread parking * @param nanos the maximum number of nanoseconds to wait * @since 1.6 */ public static void parkNanos(Object blocker, long nanos) { if (nanos > 0) { Thread t = Thread.currentThread(); setBlocker(t, blocker); U.park(false, nanos); setBlocker(t, null); } } /** * Disables the current thread for thread scheduling purposes, until * the specified deadline, 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 four * things happens: * *
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. * * @param blocker the synchronization object responsible for this * thread parking * @param deadline the absolute time, in milliseconds from the Epoch, * to wait until * @since 1.6 */ public static void parkUntil(Object blocker, long deadline) { Thread t = Thread.currentThread(); setBlocker(t, blocker); U.park(true, deadline); setBlocker(t, null); } /** * 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. * * @param t the thread * @return the blocker * @throws NullPointerException if argument is null * @since 1.6 */ public static Object getBlocker(Thread t) { if (t == null) throw new NullPointerException(); return U.getObjectVolatile(t, PARKBLOCKER); } /** * 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: * *
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 park() { U.park(false, 0L); } /** * Disables the current thread for thread scheduling purposes, for up to * the specified waiting time, 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 four * things happens: * *
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. * * @param nanos the maximum number of nanoseconds to wait */ public static void parkNanos(long nanos) { if (nanos > 0) U.park(false, nanos); } /** * Disables the current thread for thread scheduling purposes, until * the specified deadline, 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 four * things happens: * *
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. * * @param deadline the absolute time, in milliseconds from the Epoch, * to wait until */ public static void parkUntil(long deadline) { U.park(true, deadline); } /** * Returns the pseudo-randomly initialized or updated secondary seed. * Copied from ThreadLocalRandom due to package access restrictions. */ static final int nextSecondarySeed() { int r; Thread t = Thread.currentThread(); if ((r = U.getInt(t, SECONDARY)) != 0) { r ^= r << 13; // xorshift r ^= r >>> 17; r ^= r << 5; } else if ((r = java.util.concurrent.ThreadLocalRandom.current().nextInt()) == 0) r = 1; // avoid zero U.putInt(t, SECONDARY, r); return r; } // Hotspot implementation via intrinsics API private static final sun.misc.Unsafe U = sun.misc.Unsafe.getUnsafe(); private static final long PARKBLOCKER; private static final long SECONDARY; static { try { PARKBLOCKER = U.objectFieldOffset (Thread.class.getDeclaredField("parkBlocker")); SECONDARY = U.objectFieldOffset (Thread.class.getDeclaredField("threadLocalRandomSecondarySeed")); } catch (ReflectiveOperationException e) { throw new Error(e); } } }