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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/licenses/publicdomain

 */

package java.util.concurrent.locks;

import java.util.concurrent.*;

import sun.misc.Unsafe;

/**

 * 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.)

 *

 * <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}.) 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>while (!canProceed()) { ... LockSupport.park(this); }</pre>

 * 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.

 *

 * <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<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());

 *   }

 * }}</pre>

 */

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.

     *

     * <p>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:

     *

     * <ul>

     * <li>Some other thread invokes {@link #unpark unpark} with the

     * current thread as the target; or

     *

     * <li>Some other thread {@linkplain Thread#interrupt interrupts}

     * the current thread; or

     *

     * <li>The call spuriously (that is, for no reason) returns.

     * </ul>

     *

     * <p>This method does <em>not</em> 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.

     *

     * <p>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:

     *

     * <ul>

     * <li>Some other thread invokes {@link #unpark unpark} with the

     * current thread as the target; or

     *

     * <li>Some other thread {@linkplain Thread#interrupt interrupts} the current

     * thread; or

     *

     * <li>The specified waiting time elapses; or

     *

     * <li>The call spuriously (that is, for no reason) returns.

     * </ul>

     *

     * <p>This method does <em>not</em> 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.

     *

     * <p>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:

     *

     * <ul>

     * <li>Some other thread invokes {@link #unpark unpark} with the

     * current thread as the target; or

     *

     * <li>Some other thread {@linkplain Thread#interrupt interrupts} the

     * current thread; or

     *

     * <li>The specified deadline passes; or

     *

     * <li>The call spuriously (that is, for no reason) returns.

     * </ul>

     *

     * <p>This method does <em>not</em> 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.

     *

     * @return the blocker

     * @since 1.6

     */

    public static Object getBlocker(Thread t) {

        return unsafe.getObjectVolatile(t, parkBlockerOffset);

    }

    /**

     * Disables the current thread for thread scheduling purposes unless the

     * permit is available.

     *

     * <p>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:

     *

     * <ul>

     *

     * <li>Some other thread invokes {@link #unpark unpark} with the

     * current thread as the target; or

     *

     * <li>Some other thread {@linkplain Thread#interrupt interrupts}

     * the current thread; or

     *

     * <li>The call spuriously (that is, for no reason) returns.

     * </ul>

     *

     * <p>This method does <em>not</em> 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.

     *

     * <p>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:

     *

     * <ul>

     * <li>Some other thread invokes {@link #unpark unpark} with the

     * current thread as the target; or

     *

     * <li>Some other thread {@linkplain Thread#interrupt interrupts}

     * the current thread; or

     *

     * <li>The specified waiting time elapses; or

     *

     * <li>The call spuriously (that is, for no reason) returns.

     * </ul>

     *

     * <p>This method does <em>not</em> 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.

     *

     * <p>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:

     *

     * <ul>

     * <li>Some other thread invokes {@link #unpark unpark} with the

     * current thread as the target; or

     *

     * <li>Some other thread {@linkplain Thread#interrupt interrupts}

     * the current thread; or

     *

     * <li>The specified deadline passes; or

     *

     * <li>The call spuriously (that is, for no reason) returns.

     * </ul>

     *

     * <p>This method does <em>not</em> 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);

    }

}