AQS线程挂起辅助类LockSupport

/*
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
 * 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 java.util.concurrent.*;
import sun.misc.Unsafe;


/**
 * Basic thread blocking primitives for creating locks and other
 * synchronization classes.
 *
 LockSupport基于线程最原始阻塞，提供锁的创建，服务于其他同步器
 * 
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.)
 *
 LockSupport与所有用到它的每一个线程相关联，permit在某种意义上，可以理解为
 信号量java.util.concurrent.Semaphore。
如果permit可以用，park函数会立即返回，则消费permit，否则肯能阻塞。
如果permit没有可利用的，则unpark会使permit可以用。与信号量不同的是，
permits不允许累计，最多只能有一个。


 * 
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.
 *
park和unpark提供有效分方式blocking and unblocking线程，并且不会遇到Thread.suspend
和Thread.resume方法引起的问题：一个线程park，另一个线程unpark，有由于permit，
线程可能处于liveness（运行）状态。如果当前线程处于中断状态，park会立即返回，
同时支持超时等待park。由于未知的原因，park方法会在任何时候返回，所以必须
循环检查返回的条件。park方法是busy wait的一种优化，不会浪费太多的时间自旋，
park必须与unpark配合使用。

 * 
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}.
 *
 park方法有三种形式，其中一种带Obejct参数的  
 public static void park(Object blocker) 
 。当线程阻塞时，记录线程，以便监控和诊断工具，确定阻塞的原因。
 我们可以用getBlocker方法获取阻塞线程。强烈建议使用带参数的park方法，
 而不是无参数的park方法。待参数的park，阻塞的线程，内部要提供一个lock的实现。


 * 
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. 

 这些方法是为方便创建高质量的同步器，而设计，不是为大多数的并发应用。
 * The {@code park}
 * method is designed for use only in constructions of the form:
 * 
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.
 *
 这一段就不翻译了，暂时理解的不是很透彻


 * 
Sample Usage. Here is a sketch of a first-in-first-out
 * non-reentrant lock class:
 这是一个基于FIFO队列的非重入锁的实现
 * 
{@code
 * class FIFOMutex {
 *   private final AtomicBoolean locked = new AtomicBoolean(false); //原子锁
 *   private final Queue waiters//线程等待队列
 *     = new ConcurrentLinkedQueue();
 *   //加锁
 *   public void lock() {
 *     boolean wasInterrupted = false;
       //获取当前线程加入到，线程等待队列
 *     Thread current = Thread.currentThread();
 *     waiters.add(current);
 *
 *     // Block while not first in queue or cannot acquire lock
       //当前线程，不是队列的头部，并且获取锁失败，则park当前线程
 *     while (waiters.peek() != current ||
 *            !locked.compareAndSet(false, true)) {
 *        LockSupport.park(this);
          //如果线程处于中断状态，则wasInterrupted为true
 *        if (Thread.interrupted()) // ignore interrupts while waiting
 *          wasInterrupted = true;
 *     }
 *     //如果是队列的头部，且获取锁成功，从队列中移除，当前线程
 *     waiters.remove();
 *     if (wasInterrupted)          // reassert interrupt status on exit
 *        current.interrupt();
 *   }
 *   //解锁
 *   public void unlock() {
 *     locked.set(false);//设置锁为打开状态
 *     LockSupport.unpark(waiters.peek());//unpark队列头部线程
 *   }
 * }}
 */

public class LockSupport {
    //LockSupport不支持，实例化，我们可以通过，调用其方法实现相关功能。
    private LockSupport() {} // Cannot be instantiated.

    // Hotspot implementation via intrinsics API
    //Hotspot VM调用操作系统API的辅助工具
    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.
	//即使是volatile，在这里方法调用，hotspot VM 也不需要一个writer barrier
        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
     */
     //当permit不可用时，unpark方法可以使permit对指定线程可用。
     //如果线程被阻塞时，调用此方法，可以unblock，或者说，下次调用park时，
     //保证线程不会被阻塞。当指定线程没有启动，则unpark没有作用。
    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.
     *使当前线程不能被调度，除非permit可用
     * 
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:
     *如果permit可用，则消费掉，并立刻返回；
     否则使当前线程不能被调度，处于睡眠状态，直到下面3个条件发生。
     * 

     * 
Some other thread invokes {@link #unpark unpark} with the
     * current thread as the target; or
     *其他线程unpark当前线程
     * 
Some other thread {@linkplain Thread#interrupt interrupts}
     * the current thread; or
     *其他线程中断当前线程
     * 
The call spuriously (that is, for no reason) returns.
     * 
     *park方法由于未知原因返回
     * 
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.
     *这个方法不会报告什么原因引起return。调用者应该重新检查线程，在第一次被
     park的条件。调用者也可以根据返回，来判断线程的中断状态。
     * @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.
     *此方法与park(Object blocker)类似，只不过要延迟long nanos，才park线程
     * 
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
     */调用者也可以根据返回，来判断线程的中断状态，或等时间耗完，直接返回。
    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.
     * 与上述方法类似，不同的是有一个deadline
     * 
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.
     *返回最近调用park方法，还没有阻塞的线程。返回值是一个瞬间的快照
     * @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.
     *与上述方法类型
     * 
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.
     *等待一段时间park
     * @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.
     *park到指定的时间deadline，除非permit可用，unpark可使permit可用
     * @param deadline the absolute time, in milliseconds from the Epoch,
     *        to wait until
     */
    public static void parkUntil(long deadline) {
        unsafe.park(true, deadline);
    }
}