【J.U.C】LockSupport源码翻译

JDK LockSuppoprt源码翻译

import sun.misc.Unsafe;

/**
 * Basic thread blocking primitives for creating locks and other
 * synchronization classes.
 * 用于创建锁和其他同步类的基本线程阻塞原语。
 * <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.)
 * 这个类与使用它的每个线程关联一个许可证(和Semaphore类差不多)。
 * 如果许可证可用，对{@code park}的调用将立即返回，并在此过程中使用它;否则它可能会阻塞。
 * 如果许可证尚未可用，则调用{@code unpark}将使许可证可用。
 * (与信号量不同，许可证不会累积。最多只有一个)。
 *
 * <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.
 * 方法{@code park}和{@code unpark}提供了有效的方法来阻塞和解除阻塞线程，
 * 这些线程不会遇到导致废弃方法{@code Thread.suspend}和{@code Thread.resume}不能用于这些目的的问题:
 * 调用{@code park}的线程与试图{@code unpark}的线程之间的竞争将保持活跃，这是由于许可证的原因。
 * 此外，如果调用者的线程被中断，并且支持超时版本，则{@code park}将interrupted。
 * {@code park}方法也可以在任何其他时间返回，原因是“没有任何原因”，所以通常必须在返回时在循环中调用，循环会重新检查条件。
 * 从这个意义上说，{@code park}是一个“繁忙等待”的优化，它不会浪费太多的时间旋转，但必须与{@code unpark}匹配才能有效。
 *
 * <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}.
 * {@code park}的三种形式都支持{@code blocker}对象参数。
 * 在线程被阻塞时记录此对象，以便允许监视和诊断工具识别线程被阻塞的原因。
 * (这些工具可以使用方法{@link #getBlocker(Thread)}访问阻塞程序。)
 * 强烈建议使用这些形式而不是没有此参数的原始形式。
 * 在锁实现中作为{@code blocker}提供的正常参数是{@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:
 * 这些方法被设计为用于创建更高级别的同步实用程序的工具，但它们本身并不适用于大多数并发控制应用程序。
 * {@code park}方法只适用于下列结构:
 * <pre> {@code
 * while (!canProceed()) { ... LockSupport.park(this); }}</pre>
 * <p>
 * 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.
 * 在调用{@code park}之前，{@code canProceed}或任何其他操作都不需要锁定或阻塞。
 * 因为每个线程只与一个许可证相关联，所以任何中介使用{@code park}都可能干扰它的预期效果。
 *
 * <p><b>Sample Usage.</b> Here is a sketch of a first-in-first-out
 * non-reentrant lock class:
 * 下面是一个先进先出非重入锁类的例子：
 * <pre> {@code
 * class FIFOMutex {
 *   //有两种状态 true或者false
 *   private final AtomicBoolean locked = new AtomicBoolean(false);
 *   //线程队列
 *   private final Queue<Thread> waiters
 *     = new ConcurrentLinkedQueue<Thread>();
 *
 *   //加锁
 *   public void lock() {
 *     //中断标志
 *     boolean wasInterrupted = false;
 *     //当前线程
 *     Thread current = Thread.currentThread();
 *     //当前线程入队列
 *     waiters.add(current);
 *
 *     // Block while not first in queue or cannot acquire lock
 *     //在队列中不是第一个或无法获取锁时阻塞
 *     while (waiters.peek() != current ||
 *            !locked.compareAndSet(false, true)) {
 *        //阻塞
 *       LockSupport.park(this);
 *       //在等待时忽略中断
 *       if (Thread.interrupted()) // ignore interrupts while waiting
 *         wasInterrupted = true;
 *     }
 *     //移除当前线程
 *     waiters.remove();
 *     //退出时重申中断状态
 *     if (wasInterrupted)          // reassert interrupt status on exit
 *       current.interrupt();
 *   }
 *   解锁
 *   public void unlock() {
 *     //解锁状态设置
 *     locked.set(false);
 *     //当前线程解锁定
 *     LockSupport.unpark(waiters.peek());
 *   }
 * }}</pre>
 */
public class LockSupport {
    /**
     * 不能被实例化。
     */
    private LockSupport() {
    } // Cannot be instantiated.

    /**
     * Sets blocker.
     *
     * @param t   the t
     * @param arg the arg
     */
    private static void setBlocker(Thread t, Object arg) {
        // Even though volatile, hotspot doesn't need a write barrier here.
        // 尽管hotspot是易失性的，但它并不需要写屏障。
        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.
     * 使给定线程的许可证可用(如果它还没有可用)。如果线程在{@code park}上被阻塞，那么它将解阻塞。
     * 否则，它对{@code park}的下一次调用将保证不会阻塞。
     * 如果没有启动给定的线程，则不能保证此操作具有任何效果。
     *
     * @param thread the thread to unpark, or {@code null}, in which case
     *               this operation has no effect 取消停靠的线程，或{@code null}，在这种情况下，此操作没有效果
     */
    public static void unpark(Thread thread) {
        //如果线程不为空，则处理，解阻塞
        if (thread != null)
            UNSAFE.unpark(thread);
    }

    /**
     * Disables the current thread for thread scheduling purposes unless the
     * permit is available.
     * 除非有许可证，否则为线程调度目的禁用当前线程。
     *
     * <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:
     * 如果许可证是可用的，那么它将被使用，调用立即返回;
     * 否则，当前线程将出于线程调度的目的被禁用，并处于休眠状态，直到以下三种情况之一发生:
     *
     * <ul>
     * <li>Some other thread invokes {@link #unpark unpark} with the
     * current thread as the target; or
     * 其他一些线程以当前线程为目标调用{@link #unpark unpark}
     *
     * <li>Some other thread {@linkplain Thread#interrupt interrupts}
     * the current thread; or
     * 其他线程中断当前线程
     *
     * <li>The call spuriously (that is, for no reason) returns.
     * 虚假的调用(也就是说，没有理由)返回。
     * </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.
     * 此方法不报告是哪些原因导致该方法返回。调用者应该重新检查导致线程首先停车的条件。
     * 例如，调用者还可以在返回时确定线程的中断状态。
     *
     * @param blocker the synchronization object responsible for this
     *                thread parking 负责此操作的同步对象
     * @since 1.6
     */
    public static void park(Object blocker) {
        //获取当前线程
        Thread t = Thread.currentThread();
        //设置线程parkBlocker
        setBlocker(t, blocker);
        //阻塞线程
        UNSAFE.park(false, 0L);
        //清除parkBlocker
        setBlocker(t, null);
    }

    /**
     * Disables the current thread for thread scheduling purposes, for up to
     * the specified waiting time, unless the permit is available.
     * 除非有许可证，否则在指定的等待时间内，为线程调度目的禁用当前线程。
     *
     * <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:
     * 如果许可证是可用的，那么它将被使用，调用立即返回;
     * 否则，当前线程将出于线程调度的目的被禁用，并处于休眠状态，直到以下四种情况之一发生:
     * <ul>
     * <li>Some other thread invokes {@link #unpark unpark} with the
     * current thread as the target; or
     * 其他一些线程以当前线程为目标调用{@link #unpark unpark}
     * <li>Some other thread {@linkplain Thread#interrupt interrupts}
     * the current thread; or
     * 其他线程中断当前线程
     *
     * <li>The specified waiting time elapses; or
     * 指定的等待时间已经过了
     * <li>The call spuriously (that is, for no reason) returns.
     * 虚假的调用(也就是说，没有理由)返回
     * </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.
     * 此方法不报告是哪些原因导致该方法返回。调用者应该重新检查导致线程首先停车的条件。
     * 例如，调用者还可以确定线程的中断状态，或者返回时的运行时间。
     *
     * @param blocker the synchronization object responsible for this
     *                thread parking 负责此操作的同步对象
     * @param nanos   the maximum number of nanoseconds to wait 等待的最大纳秒数
     * @since 1.6
     */
    public static void parkNanos(Object blocker, long nanos) {
        //如果等待纳秒数大于0
        if (nanos > 0) {
            //获取当前线程
            Thread t = Thread.currentThread();
            //设置当前线程parkBlocker
            setBlocker(t, blocker);
            //等待nanos后阻塞线程
            UNSAFE.park(false, nanos);
            //清除当前线程parkBlocker
            setBlocker(t, null);
        }
    }

    /**
     * Disables the current thread for thread scheduling purposes, until
     * the specified deadline, unless the permit is available.
     * 为线程调度目的禁用当前线程，直到指定的截止日期，除非许可证可用。
     *
     * <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:
     * 如果许可证是可用的，那么它将被使用，调用立即返回;
     * 否则，当前线程将出于线程调度的目的被禁用，并处于休眠状态，直到以下四种情况之一发生:
     * <ul>
     * <li>Some other thread invokes {@link #unpark unpark} with the
     * current thread as the target; or
     * 其他一些线程以当前线程为目标调用{@link #unpark unpark}
     * <li>Some other thread {@linkplain Thread#interrupt interrupts} the
     * current thread; or
     * 其他线程中断当前线程
     * <li>The specified deadline passes; or
     * 指定的截止日期过了
     * <li>The call spuriously (that is, for no reason) returns.
     * 虚假的调用(也就是说，没有理由)返回
     * </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.
     * 此方法不报告是哪些原因导致该方法返回。调用者应该重新检查导致线程首先停车的条件。
     * 例如，调用者还可以确定线程的中断状态，或者返回时的运行时间。
     *
     * @param blocker  the synchronization object responsible for this
     *                 thread parking 负责此操作的同步对象
     * @param deadline the absolute time, in milliseconds from the Epoch,
     *                 to wait until 等待到的绝对时间(以毫秒为单位)
     * @since 1.6
     */
    public static void parkUntil(Object blocker, long deadline) {
        //获取当前线程
        Thread t = Thread.currentThread();
        //设置当前线程parkBlocker
        setBlocker(t, blocker);
        //达到截止时间后阻塞线程
        UNSAFE.park(true, deadline);
        //清除当前线程parkBlocker
        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.
     * 返回提供给最近一次调用尚未解阻塞的park方法的blocker对象，如果没有阻塞，则返回null。
     * 返回的值只是一个瞬时快照——线程可能已经在另一个blocker对象上解除了阻塞或阻塞。
     *
     * @param t the thread
     * @return the blocker
     * @throws NullPointerException if argument is null
     * @since 1.6
     */
    public static Object getBlocker(Thread t) {
        //判空
        if (t == null)
            throw new NullPointerException();
        //获取线程parkBlocker
        return UNSAFE.getObjectVolatile(t, parkBlockerOffset);
    }

    /**
     * Disables the current thread for thread scheduling purposes unless the
     * permit is available.
     * 除非有许可证，否则为线程调度目的禁用当前线程。
     *
     * <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:
     * 如果许可证是可用的，那么它将被使用，调用立即返回;
     * 否则，当前线程将出于线程调度的目的被禁用，并处于休眠状态，直到以下三种情况之一发生:
     * <ul>
     *
     * <ul>
     * <li>Some other thread invokes {@link #unpark unpark} with the
     * current thread as the target; or
     * 其他一些线程以当前线程为目标调用{@link #unpark unpark}
     *
     * <li>Some other thread {@linkplain Thread#interrupt interrupts}
     * the current thread; or
     * 其他线程中断当前线程
     *
     * <li>The call spuriously (that is, for no reason) returns.
     * 虚假的调用(也就是说，没有理由)返回。
     * </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.
     * 此方法不报告是哪些原因导致该方法返回。调用者应该重新检查导致线程首先停车的条件。
     * 例如，调用者还可以在返回时确定线程的中断状态。
     */
    public static void park() {
        //阻塞
        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)
            //等待nanos后锁定
            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) {
        //到截至日期后锁定
        UNSAFE.park(true, deadline);
    }

    /**
     * Returns the pseudo-randomly initialized or updated secondary seed.
     * Copied from ThreadLocalRandom due to package access restrictions.
     * 返回伪随机初始化或更新的辅助种子。由于包访问限制，从ThreadLocalRandom复制。
     */
    static final int nextSecondarySeed() {
        //threadLocalRandomSecondarySeed
        int r;
        //当前线程
        Thread t = Thread.currentThread();
        if ((r = UNSAFE.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
        UNSAFE.putInt(t, SECONDARY, r);
        return r;
    }

    // Hotspot implementation via intrinsics API
    private static final Unsafe UNSAFE;
    private static final long parkBlockerOffset;
    private static final long SEED;
    private static final long PROBE;
    private static final long SECONDARY;

    static {
        try {
            UNSAFE = Unsafe.getUnsafe();
            Class<?> tk = Thread.class;
            parkBlockerOffset = UNSAFE.objectFieldOffset
                    (tk.getDeclaredField("parkBlocker"));
            SEED = UNSAFE.objectFieldOffset
                    (tk.getDeclaredField("threadLocalRandomSeed"));
            PROBE = UNSAFE.objectFieldOffset
                    (tk.getDeclaredField("threadLocalRandomProbe"));
            SECONDARY = UNSAFE.objectFieldOffset
                    (tk.getDeclaredField("threadLocalRandomSecondarySeed"));
        } catch (Exception ex) {
            throw new Error(ex);
        }
    }

}