【J.U.C】LockSupport源码翻译

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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);
            }
        }

    }

来源:https://www.jianshu.com/p/347ea7f881f8

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