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/*
 * Written by Doug Lea with assistance from members of JCP JSR-166
 * Expert Group and released to the public domain, as explained at
 * http://creativecommons.org/publicdomain/zero/1.0/
 */

package java.util.concurrent.locks;
import sun.misc.Unsafe;

/**
 * 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 {@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.) * *

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}.) 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()) { ... LockSupport.park(this); }}
* * where neither {@code canProceed} nor any other actions 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} could interfere with its intended effects. * *

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;
 *     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());
 *   }
 * }}
*/ public class LockSupport { private LockSupport() {} // Cannot be instantiated. // Hotspot implementation via intrinsics API private static final Unsafe unsafe = Unsafe.getUnsafe(); private static final long parkBlockerOffset; static { try { parkBlockerOffset = unsafe.objectFieldOffset (java.lang.Thread.class.getDeclaredField("parkBlocker")); } catch (Exception ex) { throw new Error(ex); } } private static void setBlocker(Thread t, Object arg) { // Even though volatile, hotspot doesn't need a write barrier here. unsafe.putObject(t, parkBlockerOffset, 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) unsafe.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: * *

    *
  • Some other thread invokes {@link #unpark unpark} with the * current thread as the target; or * *
  • Some other thread {@linkplain Thread#interrupt 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. * * @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); unsafe.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: * *

    *
  • Some other thread invokes {@link #unpark unpark} with the * current thread as the target; or * *
  • Some other thread {@linkplain Thread#interrupt interrupts} * the current thread; or * *
  • The specified waiting time elapses; 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, 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); unsafe.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: * *

    *
  • Some other thread invokes {@link #unpark unpark} with the * current thread as the target; or * *
  • Some other thread {@linkplain Thread#interrupt interrupts} the * current thread; or * *
  • The specified deadline passes; 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, 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); unsafe.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 unsafe.getObjectVolatile(t, parkBlockerOffset); } /** * 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 {@link #unpark unpark} with the * current thread as the target; or * *
  • Some other thread {@linkplain Thread#interrupt 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. */ public static void park() { unsafe.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: * *

    *
  • Some other thread invokes {@link #unpark unpark} with the * current thread as the target; or * *
  • Some other thread {@linkplain Thread#interrupt interrupts} * the current thread; or * *
  • The specified waiting time elapses; 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, or the elapsed time * upon return. * * @param nanos the maximum number of nanoseconds to wait */ public static void parkNanos(long nanos) { if (nanos > 0) unsafe.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: * *

    *
  • Some other thread invokes {@link #unpark unpark} with the * current thread as the target; or * *
  • Some other thread {@linkplain Thread#interrupt interrupts} * the current thread; or * *
  • The specified deadline passes; 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, 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) { unsafe.park(true, deadline); } }





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