AbstractQueuedLongSynchronizer.java

/*
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.  Oracle designates this
 * particular file as subject to the "Classpath" exception as provided
 * by Oracle in the LICENSE file that accompanied this code.
 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
 * or visit www.oracle.com if you need additional information or have any
 * questions.
 */

/*
 * This file is available under and governed by the GNU General Public
 * License version 2 only, as published by the Free Software Foundation.
 * However, the following notice accompanied the original version of this
 * file:
 *
 * 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.io.Serializable;
import java.lang.invoke.MethodHandles;
import java.lang.invoke.VarHandle;
import java.util.ArrayList;
import java.util.Collection;
import java.util.Date;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.locks.AbstractQueuedSynchronizer.Node;

/**
 * A version of {@link AbstractQueuedSynchronizer} in
 * which synchronization state is maintained as a {@code long}.
 * This class has exactly the same structure, properties, and methods
 * as {@code AbstractQueuedSynchronizer} with the exception
 * that all state-related parameters and results are defined
 * as {@code long} rather than {@code int}. This class
 * may be useful when creating synchronizers such as
 * multilevel locks and barriers that require
 * 64 bits of state.
 *
 * <p>See {@link AbstractQueuedSynchronizer} for usage
 * notes and examples.
 *
 * @since 1.6
 * @author Doug Lea
 */
// 同步队列，除了将关键变量（如许可证数量）的类型提升为long之外，其他地方与AbstractQueuedSynchronizer一样
public abstract class AbstractQueuedLongSynchronizer extends AbstractOwnableSynchronizer implements Serializable {
    
    /*
     * To keep sources in sync, the remainder of this source file is exactly cloned from AbstractQueuedSynchronizer,
     * replacing class name and changing ints related with sync state to longs.
     * Please keep it that way.
     */
    
    private static final long serialVersionUID = 7373984972572414692L;
    
    /**
     * The number of nanoseconds for which it is faster to spin
     * rather than to use timed park. A rough estimate suffices
     * to improve responsiveness with very short timeouts.
     */
    static final long SPIN_FOR_TIMEOUT_THRESHOLD = 1000L;
    
    /**
     * Head of the wait queue, lazily initialized.  Except for
     * initialization, it is modified only via method setHead.  Note:
     * If head exists, its waitStatus is guaranteed not to be
     * CANCELLED.
     */
    private transient volatile Node head;   // 【|同步队列|】的头结点
    
    /**
     * Tail of the wait queue, lazily initialized.  Modified only via
     * method enq to add new wait node.
     */
    private transient volatile Node tail;   // 【|同步队列|】的尾结点
    
    /**
     * The synchronization state.
     */
    // 重入锁计数/许可证数量，在不同的锁中，使用方式有所不同
    private volatile long state;
    
    // VarHandle mechanics
    private static final VarHandle STATE;
    private static final VarHandle HEAD;
    private static final VarHandle TAIL;
    
    
    
    static {
        try {
            MethodHandles.Lookup l = MethodHandles.lookup();
            STATE = l.findVarHandle(AbstractQueuedLongSynchronizer.class, "state", long.class);
            HEAD = l.findVarHandle(AbstractQueuedLongSynchronizer.class, "head", Node.class);
            TAIL = l.findVarHandle(AbstractQueuedLongSynchronizer.class, "tail", Node.class);
        } catch (ReflectiveOperationException e) {
            throw new ExceptionInInitializerError(e);
        }
        
        // Reduce the risk of rare disastrous classloading in first call to
        // LockSupport.park: https://bugs.openjdk.java.net/browse/JDK-8074773
        Class<?> ensureLoaded = LockSupport.class;
    }
    
    
    
    /*▼ 构造方法 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Creates a new {@code AbstractQueuedLongSynchronizer} instance
     * with initial synchronization state of zero.
     */
    protected AbstractQueuedLongSynchronizer() {
    }
    
    /*▲ 构造方法 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 独占锁 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /* 申请 ▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼  */
    
    /**
     * Acquires in exclusive mode, ignoring interrupts.  Implemented
     * by invoking at least once {@link #tryAcquire},
     * returning on success.  Otherwise the thread is queued, possibly
     * repeatedly blocking and unblocking, invoking {@link
     * #tryAcquire} until success.  This method can be used
     * to implement method {@link Lock#lock}.
     *
     * @param arg the acquire argument.  This value is conveyed to
     *            {@link #tryAcquire} but is otherwise uninterpreted and
     *            can represent anything you like.
     */
    // 申请独占锁，允许阻塞带有中断标记的线程（会先将其标记清除）
    public final void acquire(long arg) {
        if(!tryAcquire(arg) && acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
            selfInterrupt();
    }
    
    /**
     * Attempts to acquire in exclusive mode. This method should query
     * if the state of the object permits it to be acquired in the
     * exclusive mode, and if so to acquire it.
     *
     * <p>This method is always invoked by the thread performing
     * acquire.  If this method reports failure, the acquire method
     * may queue the thread, if it is not already queued, until it is
     * signalled by a release from some other thread. This can be used
     * to implement method {@link Lock#tryLock()}.
     *
     * <p>The default
     * implementation throws {@link UnsupportedOperationException}.
     *
     * @param arg the acquire argument. This value is always the one
     *            passed to an acquire method, or is the value saved on entry
     *            to a condition wait.  The value is otherwise uninterpreted
     *            and can represent anything you like.
     *
     * @return {@code true} if successful. Upon success, this object has
     * been acquired.
     *
     * @throws IllegalMonitorStateException  if acquiring would place this
     *                                       synchronizer in an illegal state. This exception must be
     *                                       thrown in a consistent fashion for synchronization to work
     *                                       correctly.
     * @throws UnsupportedOperationException if exclusive mode is not supported
     */
    // 申请一次独占锁，具体的行为模式由子类实现
    protected boolean tryAcquire(long arg) {
        throw new UnsupportedOperationException();
    }
    
    /**
     * Acquires in exclusive uninterruptible mode for thread already in
     * queue. Used by condition wait methods as well as acquire.
     *
     * @param node the node
     * @param arg  the acquire argument
     *
     * @return {@code true} if interrupted while waiting
     */
    // 当node进入排队后再次尝试申请锁，如果还是失败，则可能进入阻塞
    final boolean acquireQueued(final Node node, long arg) {
        boolean interrupted = false;
        try {
            for(; ; ) {
                final Node p = node.predecessor();
                if(p == head && tryAcquire(arg)) {
                    setHead(node);
                    p.next = null; // help GC
                    return interrupted;
                }
                if(shouldParkAfterFailedAcquire(p, node))
                    interrupted |= parkAndCheckInterrupt();
            }
        } catch(Throwable t) {
            cancelAcquire(node);
            if(interrupted)
                selfInterrupt();
            throw t;
        }
    }
    
    /**
     * Acquires in exclusive mode, aborting if interrupted.
     * Implemented by first checking interrupt status, then invoking
     * at least once {@link #tryAcquire}, returning on
     * success.  Otherwise the thread is queued, possibly repeatedly
     * blocking and unblocking, invoking {@link #tryAcquire}
     * until success or the thread is interrupted.  This method can be
     * used to implement method {@link Lock#lockInterruptibly}.
     *
     * @param arg the acquire argument.  This value is conveyed to
     *            {@link #tryAcquire} but is otherwise uninterpreted and
     *            can represent anything you like.
     *
     * @throws InterruptedException if the current thread is interrupted
     */
    // 申请独占锁，不允许阻塞带有中断标记的线程
    public final void acquireInterruptibly(long arg) throws InterruptedException {
        if(Thread.interrupted())
            throw new InterruptedException();
        if(!tryAcquire(arg))
            doAcquireInterruptibly(arg);
    }
    
    /**
     * Acquires in exclusive interruptible mode.
     *
     * @param arg the acquire argument
     */
    // 抢锁失败后，尝试将其阻塞
    private void doAcquireInterruptibly(long arg) throws InterruptedException {
        final Node node = addWaiter(Node.EXCLUSIVE);
        try {
            for(; ; ) {
                final Node p = node.predecessor();
                if(p == head && tryAcquire(arg)) {
                    setHead(node);
                    p.next = null; // help GC
                    return;
                }
                if(shouldParkAfterFailedAcquire(p, node) && parkAndCheckInterrupt())
                    throw new InterruptedException();
            }
        } catch(Throwable t) {
            cancelAcquire(node);
            throw t;
        }
    }
    
    /**
     * Attempts to acquire in exclusive mode, aborting if interrupted,
     * and failing if the given timeout elapses.  Implemented by first
     * checking interrupt status, then invoking at least once {@link
     * #tryAcquire}, returning on success.  Otherwise, the thread is
     * queued, possibly repeatedly blocking and unblocking, invoking
     * {@link #tryAcquire} until success or the thread is interrupted
     * or the timeout elapses.  This method can be used to implement
     * method {@link Lock#tryLock(long, TimeUnit)}.
     *
     * @param arg          the acquire argument.  This value is conveyed to
     *                     {@link #tryAcquire} but is otherwise uninterpreted and
     *                     can represent anything you like.
     * @param nanosTimeout the maximum number of nanoseconds to wait
     *
     * @return {@code true} if acquired; {@code false} if timed out
     *
     * @throws InterruptedException if the current thread is interrupted
     */
    // 申请独占锁，不允许阻塞带有中断标记的线程（一次失败后，带着超时标记继续申请）
    public final boolean tryAcquireNanos(long arg, long nanosTimeout) throws InterruptedException {
        if(Thread.interrupted())
            throw new InterruptedException();
        return tryAcquire(arg) || doAcquireNanos(arg, nanosTimeout);
    }
    
    /**
     * Acquires in exclusive timed mode.
     *
     * @param arg          the acquire argument
     * @param nanosTimeout max wait time
     *
     * @return {@code true} if acquired
     */
    // 申请独占锁，带有超时标记
    private boolean doAcquireNanos(long arg, long nanosTimeout) throws InterruptedException {
        if(nanosTimeout<=0L)
            return false;
        final long deadline = System.nanoTime() + nanosTimeout;
        final Node node = addWaiter(Node.EXCLUSIVE);
        try {
            for(; ; ) {
                final Node p = node.predecessor();
                if(p == head && tryAcquire(arg)) {
                    setHead(node);
                    p.next = null; // help GC
                    return true;
                }
                nanosTimeout = deadline - System.nanoTime();
                if(nanosTimeout<=0L) {
                    cancelAcquire(node);
                    return false;
                }
                if(shouldParkAfterFailedAcquire(p, node) && nanosTimeout>SPIN_FOR_TIMEOUT_THRESHOLD)
                    LockSupport.parkNanos(this, nanosTimeout);
                if(Thread.interrupted())
                    throw new InterruptedException();
            }
        } catch(Throwable t) {
            cancelAcquire(node);
            throw t;
        }
    }
    
    /* 申请 ▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲  */
    
    
    /* 释放 ▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼ */
    
    /**
     * Releases in exclusive mode.  Implemented by unblocking one or
     * more threads if {@link #tryRelease} returns true.
     * This method can be used to implement method {@link Lock#unlock}.
     *
     * @param arg the release argument.  This value is conveyed to
     *            {@link #tryRelease} but is otherwise uninterpreted and
     *            can represent anything you like.
     *
     * @return the value returned from {@link #tryRelease}
     */
    // 释放锁，如果锁已被完全释放，则唤醒后续的阻塞线程。返回值表示本次操作后锁是否自由
    public final boolean release(long arg) {
        if(tryRelease(arg)) {
            Node h = head;
            if(h != null && h.waitStatus != 0)
                unparkSuccessor(h);
            return true;
        }
        return false;
    }
    
    /**
     * Attempts to set the state to reflect a release in exclusive
     * mode.
     *
     * <p>This method is always invoked by the thread performing release.
     *
     * <p>The default implementation throws
     * {@link UnsupportedOperationException}.
     *
     * @param arg the release argument. This value is always the one
     *            passed to a release method, or the current state value upon
     *            entry to a condition wait.  The value is otherwise
     *            uninterpreted and can represent anything you like.
     *
     * @return {@code true} if this object is now in a fully released
     * state, so that any waiting threads may attempt to acquire;
     * and {@code false} otherwise.
     *
     * @throws IllegalMonitorStateException  if releasing would place this
     *                                       synchronizer in an illegal state. This exception must be
     *                                       thrown in a consistent fashion for synchronization to work
     *                                       correctly.
     * @throws UnsupportedOperationException if exclusive mode is not supported
     */
    // 释放一次锁，返回值表示同步锁是否处于自由状态（无线程持有）
    protected boolean tryRelease(long arg) {
        throw new UnsupportedOperationException();
    }
    
    /* 释放 ▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲ */
    
    /*▲ 独占锁 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 共享锁 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /* 申请 ▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼  */
    
    /**
     * Acquires in shared mode, ignoring interrupts.  Implemented by
     * first invoking at least once {@link #tryAcquireShared},
     * returning on success.  Otherwise the thread is queued, possibly
     * repeatedly blocking and unblocking, invoking {@link
     * #tryAcquireShared} until success.
     *
     * @param arg the acquire argument.  This value is conveyed to
     *            {@link #tryAcquireShared} but is otherwise uninterpreted
     *            and can represent anything you like.
     */
    // 申请共享锁，允许阻塞带有中断标记的线程（会先将其标记清除）
    public final void acquireShared(long arg) {
        if(tryAcquireShared(arg)<0)
            doAcquireShared(arg);
    }
    
    /**
     * Attempts to acquire in shared mode. This method should query if
     * the state of the object permits it to be acquired in the shared
     * mode, and if so to acquire it.
     *
     * <p>This method is always invoked by the thread performing
     * acquire.  If this method reports failure, the acquire method
     * may queue the thread, if it is not already queued, until it is
     * signalled by a release from some other thread.
     *
     * <p>The default implementation throws {@link
     * UnsupportedOperationException}.
     *
     * @param arg the acquire argument. This value is always the one
     *            passed to an acquire method, or is the value saved on entry
     *            to a condition wait.  The value is otherwise uninterpreted
     *            and can represent anything you like.
     *
     * @return a negative value on failure; zero if acquisition in shared
     * mode succeeded but no subsequent shared-mode acquire can
     * succeed; and a positive value if acquisition in shared
     * mode succeeded and subsequent shared-mode acquires might
     * also succeed, in which case a subsequent waiting thread
     * must check availability. (Support for three different
     * return values enables this method to be used in contexts
     * where acquires only sometimes act exclusively.)  Upon
     * success, this object has been acquired.
     *
     * @throws IllegalMonitorStateException  if acquiring would place this
     *                                       synchronizer in an illegal state. This exception must be
     *                                       thrown in a consistent fashion for synchronization to work
     *                                       correctly.
     * @throws UnsupportedOperationException if shared mode is not supported
     */
    // 申请共享锁，具体的行为模式由子类实现
    protected long tryAcquireShared(long arg) {
        throw new UnsupportedOperationException();
    }
    
    /**
     * Acquires in shared uninterruptible mode.
     *
     * @param arg the acquire argument
     */
    // 当node进入排队后再次尝试申请锁，如果还是失败，则可能进入阻塞
    private void doAcquireShared(long arg) {
        final Node node = addWaiter(Node.SHARED);
        boolean interrupted = false;
        try {
            for(; ; ) {
                final Node p = node.predecessor();
                if(p == head) {
                    long r = tryAcquireShared(arg);
                    if(r >= 0) {
                        setHeadAndPropagate(node, r);
                        p.next = null; // help GC
                        return;
                    }
                }
                if(shouldParkAfterFailedAcquire(p, node))
                    interrupted |= parkAndCheckInterrupt();
            }
        } catch(Throwable t) {
            cancelAcquire(node);
            throw t;
        } finally {
            if(interrupted)
                selfInterrupt();
        }
    }
    
    /**
     * Acquires in shared mode, aborting if interrupted.  Implemented
     * by first checking interrupt status, then invoking at least once
     * {@link #tryAcquireShared}, returning on success.  Otherwise the
     * thread is queued, possibly repeatedly blocking and unblocking,
     * invoking {@link #tryAcquireShared} until success or the thread
     * is interrupted.
     *
     * @param arg the acquire argument.
     *            This value is conveyed to {@link #tryAcquireShared} but is
     *            otherwise uninterpreted and can represent anything
     *            you like.
     *
     * @throws InterruptedException if the current thread is interrupted
     */
    // 申请共享锁，不允许阻塞带有中断标记的线程
    public final void acquireSharedInterruptibly(long arg) throws InterruptedException {
        if(Thread.interrupted())
            throw new InterruptedException();
        if(tryAcquireShared(arg)<0)
            doAcquireSharedInterruptibly(arg);
    }
    
    /**
     * Acquires in shared interruptible mode.
     *
     * @param arg the acquire argument
     */
    // 抢锁失败后，尝试将其阻塞
    private void doAcquireSharedInterruptibly(long arg) throws InterruptedException {
        final Node node = addWaiter(Node.SHARED);
        try {
            for(; ; ) {
                final Node p = node.predecessor();
                if(p == head) {
                    long r = tryAcquireShared(arg);
                    if(r >= 0) {
                        setHeadAndPropagate(node, r);
                        p.next = null; // help GC
                        return;
                    }
                }
                if(shouldParkAfterFailedAcquire(p, node) && parkAndCheckInterrupt())
                    throw new InterruptedException();
            }
        } catch(Throwable t) {
            cancelAcquire(node);
            throw t;
        }
    }
    
    /**
     * Attempts to acquire in shared mode, aborting if interrupted, and
     * failing if the given timeout elapses.  Implemented by first
     * checking interrupt status, then invoking at least once {@link
     * #tryAcquireShared}, returning on success.  Otherwise, the
     * thread is queued, possibly repeatedly blocking and unblocking,
     * invoking {@link #tryAcquireShared} until success or the thread
     * is interrupted or the timeout elapses.
     *
     * @param arg          the acquire argument.  This value is conveyed to
     *                     {@link #tryAcquireShared} but is otherwise uninterpreted
     *                     and can represent anything you like.
     * @param nanosTimeout the maximum number of nanoseconds to wait
     *
     * @return {@code true} if acquired; {@code false} if timed out
     *
     * @throws InterruptedException if the current thread is interrupted
     */
    // 申请共享锁，不允许阻塞带有中断标记的线程（一次失败后，带着超时标记继续申请）
    public final boolean tryAcquireSharedNanos(long arg, long nanosTimeout) throws InterruptedException {
        if(Thread.interrupted())
            throw new InterruptedException();
        return tryAcquireShared(arg) >= 0 || doAcquireSharedNanos(arg, nanosTimeout);
    }
    
    /**
     * Acquires in shared timed mode.
     *
     * @param arg          the acquire argument
     * @param nanosTimeout max wait time
     *
     * @return {@code true} if acquired
     */
    // 申请共享锁，带有超时标记
    private boolean doAcquireSharedNanos(long arg, long nanosTimeout) throws InterruptedException {
        if(nanosTimeout<=0L)
            return false;
        final long deadline = System.nanoTime() + nanosTimeout;
        final Node node = addWaiter(Node.SHARED);
        try {
            for(; ; ) {
                final Node p = node.predecessor();
                if(p == head) {
                    long r = tryAcquireShared(arg);
                    if(r >= 0) {
                        setHeadAndPropagate(node, r);
                        p.next = null; // help GC
                        return true;
                    }
                }
                nanosTimeout = deadline - System.nanoTime();
                if(nanosTimeout<=0L) {
                    cancelAcquire(node);
                    return false;
                }
                if(shouldParkAfterFailedAcquire(p, node) && nanosTimeout>SPIN_FOR_TIMEOUT_THRESHOLD)
                    LockSupport.parkNanos(this, nanosTimeout);
                if(Thread.interrupted())
                    throw new InterruptedException();
            }
        } catch(Throwable t) {
            cancelAcquire(node);
            throw t;
        }
    }
    
    /* 申请 ▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲  */
    
    
    /* 释放 ▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼  */
    
    /**
     * Releases in shared mode.  Implemented by unblocking one or more
     * threads if {@link #tryReleaseShared} returns true.
     *
     * @param arg the release argument.  This value is conveyed to
     *            {@link #tryReleaseShared} but is otherwise uninterpreted
     *            and can represent anything you like.
     *
     * @return the value returned from {@link #tryReleaseShared}
     */
    // 释放锁，并唤醒排队的结点
    public final boolean releaseShared(long arg) {
        if(tryReleaseShared(arg)) {
            doReleaseShared();
            return true;
        }
        return false;
    }
    
    /**
     * Attempts to set the state to reflect a release in shared mode.
     *
     * <p>This method is always invoked by the thread performing release.
     *
     * <p>The default implementation throws
     * {@link UnsupportedOperationException}.
     *
     * @param arg the release argument. This value is always the one
     *            passed to a release method, or the current state value upon
     *            entry to a condition wait.  The value is otherwise
     *            uninterpreted and can represent anything you like.
     *
     * @return {@code true} if this release of shared mode may permit a
     * waiting acquire (shared or exclusive) to succeed; and
     * {@code false} otherwise
     *
     * @throws IllegalMonitorStateException  if releasing would place this
     *                                       synchronizer in an illegal state. This exception must be
     *                                       thrown in a consistent fashion for synchronization to work
     *                                       correctly.
     * @throws UnsupportedOperationException if shared mode is not supported
     */
    // 释放锁，归还许可证
    protected boolean tryReleaseShared(long arg) {
        throw new UnsupportedOperationException();
    }
    
    /**
     * Release action for shared mode -- signals successor and ensures
     * propagation. (Note: For exclusive mode, release just amounts
     * to calling unparkSuccessor of head if it needs signal.)
     */
    // 为共享结点设置Node.PROPAGATE标记，或唤醒其下一个结点
    private void doReleaseShared() {
        /*
         * Ensure that a release propagates, even if there are other
         * in-progress acquires/releases.  This proceeds in the usual
         * way of trying to unparkSuccessor of head if it needs
         * signal. But if it does not, status is set to PROPAGATE to
         * ensure that upon release, propagation continues.
         * Additionally, we must loop in case a new node is added
         * while we are doing this. Also, unlike other uses of
         * unparkSuccessor, we need to know if CAS to reset status
         * fails, if so rechecking.
         */
        for(; ; ) {
            Node h = head;
            if(h != null && h != tail) {
                int ws = h.waitStatus;
                if(ws == Node.SIGNAL) {
                    if(!h.compareAndSetWaitStatus(Node.SIGNAL, 0))
                        continue;            // loop to recheck cases
                    unparkSuccessor(h);
                } else if(ws == 0 && !h.compareAndSetWaitStatus(0, Node.PROPAGATE))
                    continue;                // loop on failed CAS
            }
            if(h == head)                   // loop if head changed
                break;
        }
    }
    
    /* 释放 ▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲  */
    
    /*▲ 共享锁 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 许可证 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Returns the current value of synchronization state.
     * This operation has memory semantics of a {@code volatile} read.
     *
     * @return current state value
     */
    // 获取当前许可证数量
    protected final long getState() {
        return state;
    }
    
    /**
     * Sets the value of synchronization state.
     * This operation has memory semantics of a {@code volatile} write.
     *
     * @param newState the new state value
     */
    // 更新许可证数量
    protected final void setState(long newState) {
        // See JDK-8180620: Clarify VarHandle mixed-access subtleties
        STATE.setVolatile(this, newState);
    }
    
    /**
     * Atomically sets synchronization state to the given updated
     * value if the current state value equals the expected value.
     * This operation has memory semantics of a {@code volatile} read
     * and write.
     *
     * @param expect the expected value
     * @param update the new value
     *
     * @return {@code true} if successful. False return indicates that the actual
     * value was not equal to the expected value.
     */
    // 原子地更新许可证数量为update，返回true代表更新成功
    protected final boolean compareAndSetState(long expect, long update) {
        return STATE.compareAndSet(this, expect, update);
    }
    
    /*▲ 许可证 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 同步队列 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Queries whether any threads are waiting to acquire. Note that
     * because cancellations due to interrupts and timeouts may occur
     * at any time, a {@code true} return does not guarantee that any
     * other thread will ever acquire.
     *
     * @return {@code true} if there may be other threads waiting to acquire
     */
    // 判断【|同步队列|】中是否存在排队的结点（线程）
    public final boolean hasQueuedThreads() {
        for(Node p = tail, h = head; p != h && p != null; p = p.prev)
            if(p.waitStatus<=0)
                return true;
        return false;
    }
    
    /**
     * Queries whether any threads have ever contended to acquire this
     * synchronizer; that is, if an acquire method has ever blocked.
     *
     * <p>In this implementation, this operation returns in
     * constant time.
     *
     * @return {@code true} if there has ever been contention
     */
    // 判断是否存在线程争用
    public final boolean hasContended() {
        return head != null;
    }
    
    /**
     * Returns true if the given thread is currently queued.
     *
     * <p>This implementation traverses the queue to determine
     * presence of the given thread.
     *
     * @param thread the thread
     *
     * @return {@code true} if the given thread is on the queue
     *
     * @throws NullPointerException if the thread is null
     */
    // 判断指定的线程是否在【|同步队列|】中排队
    public final boolean isQueued(Thread thread) {
        if(thread == null)
            throw new NullPointerException();
        for(Node p = tail; p != null; p = p.prev)
            if(p.thread == thread)
                return true;
        return false;
    }
    
    /**
     * Returns {@code true} if the apparent first queued thread, if one
     * exists, is waiting in exclusive mode.  If this method returns
     * {@code true}, and the current thread is attempting to acquire in
     * shared mode (that is, this method is invoked from {@link
     * #tryAcquireShared}) then it is guaranteed that the current thread
     * is not the first queued thread.  Used only as a heuristic in
     * ReentrantReadWriteLock.
     */
    // 判断【|同步队列|】中首个排队者是否为[独占模式Node]
    final boolean apparentlyFirstQueuedIsExclusive() {
        Node h, s;
        return (h = head) != null && (s = h.next) != null && !s.isShared() && s.thread != null;
    }
    
    /**
     * Queries whether any threads have been waiting to acquire longer
     * than the current thread.
     *
     * <p>An invocation of this method is equivalent to (but may be
     * more efficient than):
     * <pre> {@code
     * getFirstQueuedThread() != Thread.currentThread()
     *   && hasQueuedThreads()}</pre>
     *
     * <p>Note that because cancellations due to interrupts and
     * timeouts may occur at any time, a {@code true} return does not
     * guarantee that some other thread will acquire before the current
     * thread.  Likewise, it is possible for another thread to win a
     * race to enqueue after this method has returned {@code false},
     * due to the queue being empty.
     *
     * <p>This method is designed to be used by a fair synchronizer to
     * avoid <a href="AbstractQueuedSynchronizer.html#barging">barging</a>.
     * Such a synchronizer's {@link #tryAcquire} method should return
     * {@code false}, and its {@link #tryAcquireShared} method should
     * return a negative value, if this method returns {@code true}
     * (unless this is a reentrant acquire).  For example, the {@code
     * tryAcquire} method for a fair, reentrant, exclusive mode
     * synchronizer might look like this:
     *
     * <pre> {@code
     * protected boolean tryAcquire(int arg) {
     *   if (isHeldExclusively()) {
     *     // A reentrant acquire; increment hold count
     *     return true;
     *   } else if (hasQueuedPredecessors()) {
     *     return false;
     *   } else {
     *     // try to acquire normally
     *   }
     * }}</pre>
     *
     * @return {@code true} if there is a queued thread preceding the
     * current thread, and {@code false} if the current thread
     * is at the head of the queue or the queue is empty
     *
     * @since 1.7
     */
    // 判断【|同步队列|】的队头是否还有其他（非当前线程）的线程在排队，返回true代表有，反之则没有
    public final boolean hasQueuedPredecessors() {
        Node h, s;
        if((h = head) != null) {
            if((s = h.next) == null || s.waitStatus>0) {
                s = null; // traverse in case of concurrent cancellation
                for(Node p = tail; p != h && p != null; p = p.prev) {
                    if(p.waitStatus<=0)
                        s = p;
                }
            }
            return s != null && s.thread != Thread.currentThread();
        }
        return false;
    }
    
    /**
     * Returns the first (longest-waiting) thread in the queue, or
     * {@code null} if no threads are currently queued.
     *
     * <p>In this implementation, this operation normally returns in
     * constant time, but may iterate upon contention if other threads are
     * concurrently modifying the queue.
     *
     * @return the first (longest-waiting) thread in the queue, or
     * {@code null} if no threads are currently queued
     */
    // 返回【|同步队列|】中首个排队的线程（可能为空）
    public final Thread getFirstQueuedThread() {
        // handle only fast path, else relay
        return (head == tail) ? null : fullGetFirstQueuedThread();
    }
    
    /**
     * Version of getFirstQueuedThread called when fastpath fails.
     */
    // 返回【|同步队列|】中首个排队的线程（存在排队线程的情形下才使用该方法）
    private Thread fullGetFirstQueuedThread() {
        /*
         * The first node is normally head.next. Try to get its
         * thread field, ensuring consistent reads: If thread
         * field is nulled out or s.prev is no longer head, then
         * some other thread(s) concurrently performed setHead in
         * between some of our reads. We try this twice before
         * resorting to traversal.
         */
        Node h, s;
        Thread st;
        if(((h = head) != null && (s = h.next) != null && s.prev == head && (st = s.thread) != null) || ((h = head) != null && (s = h.next) != null && s.prev == head && (st = s.thread) != null))
            return st;
        
        /*
         * Head's next field might not have been set yet, or may have
         * been unset after setHead. So we must check to see if tail
         * is actually first node. If not, we continue on, safely
         * traversing from tail back to head to find first,
         * guaranteeing termination.
         */
        
        Thread firstThread = null;
        for(Node p = tail; p != null && p != head; p = p.prev) {
            Thread t = p.thread;
            if(t != null)
                firstThread = t;
        }
        return firstThread;
    }
    
    /**
     * Returns an estimate of the number of threads waiting to
     * acquire.  The value is only an estimate because the number of
     * threads may change dynamically while this method traverses
     * internal data structures.  This method is designed for use in
     * monitoring system state, not for synchronization control.
     *
     * @return the estimated number of threads waiting to acquire
     */
    // 获取【|同步队列|】中排队的结点数量
    public final int getQueueLength() {
        int n = 0;
        for(Node p = tail; p != null; p = p.prev) {
            if(p.thread != null)
                ++n;
        }
        return n;
    }
    
    /**
     * Returns a collection containing threads that may be waiting to
     * acquire.  Because the actual set of threads may change
     * dynamically while constructing this result, the returned
     * collection is only a best-effort estimate.  The elements of the
     * returned collection are in no particular order.  This method is
     * designed to facilitate construction of subclasses that provide
     * more extensive monitoring facilities.
     *
     * @return the collection of threads
     */
    // 返回一个集合，包含了所有正在【|同步队列|】中排队的Node中的线程
    public final Collection<Thread> getQueuedThreads() {
        ArrayList<Thread> list = new ArrayList<>();
        for(Node p = tail; p != null; p = p.prev) {
            Thread t = p.thread;
            if(t != null)
                list.add(t);
        }
        return list;
    }
    
    /**
     * Returns a collection containing threads that may be waiting to
     * acquire in exclusive mode. This has the same properties
     * as {@link #getQueuedThreads} except that it only returns
     * those threads waiting due to an exclusive acquire.
     *
     * @return the collection of threads
     */
    // 返回一个集合，包含了所有正在【|同步队列|】中排队的[独占模式Node]中的线程
    public final Collection<Thread> getExclusiveQueuedThreads() {
        ArrayList<Thread> list = new ArrayList<>();
        for(Node p = tail; p != null; p = p.prev) {
            if(!p.isShared()) {
                Thread t = p.thread;
                if(t != null)
                    list.add(t);
            }
        }
        return list;
    }
    
    /**
     * Returns a collection containing threads that may be waiting to
     * acquire in shared mode. This has the same properties
     * as {@link #getQueuedThreads} except that it only returns
     * those threads waiting due to a shared acquire.
     *
     * @return the collection of threads
     */
    // 返回一个集合，包含了所有正在【|同步队列|】中排队的[共享模式Node]中的线程
    public final Collection<Thread> getSharedQueuedThreads() {
        ArrayList<Thread> list = new ArrayList<>();
        for(Node p = tail; p != null; p = p.prev) {
            if(p.isShared()) {
                Thread t = p.thread;
                if(t != null)
                    list.add(t);
            }
        }
        return list;
    }
    
    /*▲ 同步队列 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ ConditionObject ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Queries whether the given ConditionObject
     * uses this synchronizer as its lock.
     *
     * @param condition the condition
     *
     * @return {@code true} if owned
     *
     * @throws NullPointerException if the condition is null
     */
    // 判断给定的条件对象是否归当前的同步队列（锁）所有
    public final boolean owns(ConditionObject condition) {
        return condition.isOwnedBy(this);
    }
    
    /**
     * Queries whether any threads are waiting on the given condition
     * associated with this synchronizer. Note that because timeouts
     * and interrupts may occur at any time, a {@code true} return
     * does not guarantee that a future {@code signal} will awaken
     * any threads.  This method is designed primarily for use in
     * monitoring of the system state.
     *
     * @param condition the condition
     *
     * @return {@code true} if there are any waiting threads
     *
     * @throws IllegalMonitorStateException if exclusive synchronization
     *                                      is not held
     * @throws IllegalArgumentException     if the given condition is
     *                                      not associated with this synchronizer
     * @throws NullPointerException         if the condition is null
     */
    // 判断当前【|条件队列|】中是否存在等待者
    public final boolean hasWaiters(ConditionObject condition) {
        if(!owns(condition))
            throw new IllegalArgumentException("Not owner");
        return condition.hasWaiters();
    }
    
    /**
     * Returns an estimate of the number of threads waiting on the
     * given condition associated with this synchronizer. Note that
     * because timeouts and interrupts may occur at any time, the
     * estimate serves only as an upper bound on the actual number of
     * waiters.  This method is designed for use in monitoring system
     * state, not for synchronization control.
     *
     * @param condition the condition
     *
     * @return the estimated number of waiting threads
     *
     * @throws IllegalMonitorStateException if exclusive synchronization
     *                                      is not held
     * @throws IllegalArgumentException     if the given condition is
     *                                      not associated with this synchronizer
     * @throws NullPointerException         if the condition is null
     */
    // 返回condition中【|条件队列|】长度
    public final int getWaitQueueLength(ConditionObject condition) {
        if(!owns(condition))
            throw new IllegalArgumentException("Not owner");
        return condition.getWaitQueueLength();
    }
    
    /**
     * Returns a collection containing those threads that may be
     * waiting on the given condition associated with this
     * synchronizer.  Because the actual set of threads may change
     * dynamically while constructing this result, the returned
     * collection is only a best-effort estimate. The elements of the
     * returned collection are in no particular order.
     *
     * @param condition the condition
     *
     * @return the collection of threads
     *
     * @throws IllegalMonitorStateException if exclusive synchronization
     *                                      is not held
     * @throws IllegalArgumentException     if the given condition is
     *                                      not associated with this synchronizer
     * @throws NullPointerException         if the condition is null
     */
    // 返回一个集合，该集合包含了condition的【|条件队列|】中所有结点内缓存的线程引用
    public final Collection<Thread> getWaitingThreads(ConditionObject condition) {
        if(!owns(condition))
            throw new IllegalArgumentException("Not owner");
        return condition.getWaitingThreads();
    }
    
    /**
     * Returns true if a node, always one that was initially placed on
     * a condition queue, is now waiting to reacquire on sync queue.
     *
     * @param node the node
     *
     * @return true if is reacquiring
     */
    // 如果结点（最初放置在条件队列中）现在正位于同步队列中等待重新获取锁，则返回true
    final boolean isOnSyncQueue(Node node) {
        if(node.waitStatus == Node.CONDITION || node.prev == null)
            return false;
        if(node.next != null) // If has successor, it must be on queue
            return true;
        /*
         * node.prev can be non-null, but not yet on queue because
         * the CAS to place it on queue can fail. So we have to
         * traverse from tail to make sure it actually made it.  It
         * will always be near the tail in calls to this method, and
         * unless the CAS failed (which is unlikely), it will be
         * there, so we hardly ever traverse much.
         */
        return findNodeFromTail(node);
    }
    
    /**
     * Transfers a node from a condition queue onto sync queue.
     * Returns true if successful.
     *
     * @param node the node
     *
     * @return true if successfully transferred (else the node was
     * cancelled before signal)
     */
    // 尝试将node状态码更新为0，并追加到【|同步队列|】，并为其前驱设置Node.SIGNAL标记（很重要的一步）
    final boolean transferForSignal(Node node) {
        /*
         * If cannot change waitStatus, the node has been cancelled.
         */
        if(!node.compareAndSetWaitStatus(Node.CONDITION, 0))
            return false;
        
        /*
         * Splice onto queue and try to set waitStatus of predecessor to
         * indicate that thread is (probably) waiting. If cancelled or
         * attempt to set waitStatus fails, wake up to resync (in which
         * case the waitStatus can be transiently and harmlessly wrong).
         */
        Node p = enq(node);
        int ws = p.waitStatus;
        if(ws>0 || !p.compareAndSetWaitStatus(ws, Node.SIGNAL))
            LockSupport.unpark(node.thread);
        return true;
    }
    
    /**
     * Transfers node, if necessary, to sync queue after a cancelled wait.
     * Returns true if thread was cancelled before being signalled.
     *
     * @param node the node
     *
     * @return true if cancelled before the node was signalled
     */
    // 尝试将【|条件队列|】中的node状态码更新为0，并追加到【|同步队列|】
    final boolean transferAfterCancelledWait(Node node) {
        if(node.compareAndSetWaitStatus(Node.CONDITION, 0)) {
            enq(node);
            return true;
        }
        /*
         * If we lost out to a signal(), then we can't proceed
         * until it finishes its enq().  Cancelling during an
         * incomplete transfer is both rare and transient, so just
         * spin.
         */
        while(!isOnSyncQueue(node))
            Thread.yield();
        return false;
    }
    
    /**
     * Invokes release with current state value; returns saved state.
     * Cancels node and throws exception on failure.
     * @param node the condition node for this wait
     * @return previous sync state
     */
    // 针对独占锁，丢弃node所在线程持有的所有许可证，并唤醒【|同步队列|】后续的阻塞线程
    final long fullyRelease(Node node) {
        try {
            long savedState = getState();
            if (release(savedState))
                return savedState;
            throw new IllegalMonitorStateException();
        } catch (Throwable t) {
            node.waitStatus = Node.CANCELLED;
            throw t;
        }
    }
    
    /**
     * Returns true if node is on sync queue by searching backwards from tail.
     * Called only when needed by isOnSyncQueue.
     *
     * @return true if present
     */
    // 从【|同步队列|】队尾出发，向前查找node
    private boolean findNodeFromTail(Node node) {
        // We check for node first, since it's likely to be at or near tail.
        // tail is known to be non-null, so we could re-order to "save"
        // one null check, but we leave it this way to help the VM.
        for(Node p = tail; ; ) {
            if(p == node)
                return true;
            if(p == null)
                return false;
            p = p.prev;
        }
    }
    
    /*▲ ConditionObject ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    /*▼ 对锁的支持 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Returns {@code true} if synchronization is held exclusively with
     * respect to the current (calling) thread.  This method is invoked
     * upon each call to a {@link ConditionObject} method.
     *
     * <p>The default implementation throws {@link
     * UnsupportedOperationException}. This method is invoked
     * internally only within {@link ConditionObject} methods, so need
     * not be defined if conditions are not used.
     *
     * @return {@code true} if synchronization is held exclusively;
     * {@code false} otherwise
     *
     * @throws UnsupportedOperationException if conditions are not supported
     */
    // 判断当前线程是否为锁的占用者，由子类实现
    protected boolean isHeldExclusively() {
        throw new UnsupportedOperationException();
    }
    
    /**
     * Wakes up node's successor, if one exists.
     *
     * @param node the node
     */
    // 唤醒node后面陷入阻塞的“后继”
    private void unparkSuccessor(Node node) {
        /*
         * If status is negative (i.e., possibly needing signal) try
         * to clear in anticipation of signalling.  It is OK if this
         * fails or if status is changed by waiting thread.
         */
        int ws = node.waitStatus;
        if(ws<0)
            node.compareAndSetWaitStatus(ws, 0);
        
        /*
         * Thread to unpark is held in successor, which is normally
         * just the next node.  But if cancelled or apparently null,
         * traverse backwards from tail to find the actual
         * non-cancelled successor.
         */
        Node s = node.next;
        if(s == null || s.waitStatus>0) {
            s = null;
            for(Node p = tail; p != node && p != null; p = p.prev)
                if(p.waitStatus<=0)
                    s = p;
        }
        if(s != null)
            LockSupport.unpark(s.thread);
    }
    
    /**
     * Inserts node into queue, initializing if necessary. See picture above.
     *
     * @param node the node to insert
     *
     * @return node's predecessor
     */
    // 使用尾插法将node添加到【|同步队列|】，并返回旧的队尾
    private Node enq(Node node) {
        for(; ; ) {
            Node oldTail = tail;
            if(oldTail != null) {
                node.setPrevRelaxed(oldTail);
                if(compareAndSetTail(oldTail, node)) {
                    oldTail.next = node;
                    return oldTail;
                }
            } else {
                initializeSyncQueue();
            }
        }
    }
    
    /**
     * Initializes head and tail fields on first contention.
     */
    // 【|同步队列|】不存在时，需要初始化一个头结点
    private final void initializeSyncQueue() {
        Node h;
        if(HEAD.compareAndSet(this, null, (h = new Node())))
            tail = h;
    }
    
    /**
     * CASes tail field.
     */
    // 更新队尾游标指向update
    private final boolean compareAndSetTail(Node expect, Node update) {
        return TAIL.compareAndSet(this, expect, update);
    }
    
    /**
     * Creates and enqueues node for current thread and given mode.
     *
     * @param mode Node.EXCLUSIVE for exclusive, Node.SHARED for shared
     *
     * @return the new node
     */
    // 向【|同步队列|】添加一个排队者(线程)
    private Node addWaiter(Node mode) {
        Node node = new Node(mode);
        
        for(; ; ) {
            Node oldTail = tail;
            if(oldTail != null) {
                node.setPrevRelaxed(oldTail);
                if(compareAndSetTail(oldTail, node)) {
                    oldTail.next = node;
                    return node;
                }
            } else {
                initializeSyncQueue();
            }
        }
    }
    
    /**
     * Sets head of queue to be node, thus dequeuing. Called only by
     * acquire methods.  Also nulls out unused fields for sake of GC
     * and to suppress unnecessary signals and traversals.
     *
     * @param node the node
     */
    // 设置node为头结点（即丢掉了原来的头结点）
    private void setHead(Node node) {
        head = node;
        node.thread = null;
        node.prev = null;
    }
    
    /**
     * Sets head of queue, and checks if successor may be waiting
     * in shared mode, if so propagating if either propagate > 0 or
     * PROPAGATE status was set.
     *
     * @param node      the node
     * @param propagate the return value from a tryAcquireShared
     */
    // 更新头结点为node，并为其设置Node.PROPAGATE标记，或唤醒其后续结点
    private void setHeadAndPropagate(Node node, long propagate) {
        Node h = head; // Record old head for check below
        setHead(node);
        /*
         * Try to signal next queued node if:
         *   Propagation was indicated by caller,
         *     or was recorded (as h.waitStatus either before
         *     or after setHead) by a previous operation
         *     (note: this uses sign-check of waitStatus because
         *      PROPAGATE status may transition to SIGNAL.)
         * and
         *   The next node is waiting in shared mode,
         *     or we don't know, because it appears null
         *
         * The conservatism in both of these checks may cause
         * unnecessary wake-ups, but only when there are multiple
         * racing acquires/releases, so most need signals now or soon
         * anyway.
         */
        if(propagate>0 || h == null || h.waitStatus<0 || (h = head) == null || h.waitStatus<0) {
            Node s = node.next;
            if(s == null || s.isShared())
                doReleaseShared();
        }
    }
    
    /**
     * Convenience method to interrupt current thread.
     */
    // 为线程设置中断标记
    static void selfInterrupt() {
        Thread.currentThread().interrupt();
    }
    
    /**
     * Checks and updates status for a node that failed to acquire.
     * Returns true if thread should block. This is the main signal
     * control in all acquire loops.  Requires that pred == node.prev.
     *
     * @param pred node's predecessor holding status
     * @param node the node
     *
     * @return {@code true} if thread should block
     */
    // 抢锁失败时，尝试为node的前驱设置阻塞标记
    private static boolean shouldParkAfterFailedAcquire(Node pred, Node node) {
        int ws = pred.waitStatus;
        if(ws == Node.SIGNAL)
            /*
             * This node has already set status asking a release
             * to signal it, so it can safely park.
             */
            return true;
        if(ws>0) {
            /*
             * Predecessor was cancelled. Skip over predecessors and
             * indicate retry.
             */
            do {
                node.prev = pred = pred.prev;
            } while(pred.waitStatus>0);
            pred.next = node;
        } else {
            /*
             * waitStatus must be 0 or PROPAGATE.  Indicate that we
             * need a signal, but don't park yet.  Caller will need to
             * retry to make sure it cannot acquire before parking.
             */
            pred.compareAndSetWaitStatus(ws, Node.SIGNAL);
        }
        return false;
    }
    
    /**
     * Convenience method to park and then check if interrupted.
     *
     * @return {@code true} if interrupted
     */
    // 设置线程进入阻塞状态，并清除线程的中断状态。返回值代表之前线程是否处于阻塞状态
    private final boolean parkAndCheckInterrupt() {
        LockSupport.park(this);
        return Thread.interrupted();
    }
    
    /**
     * Cancels an ongoing attempt to acquire.
     *
     * @param node the node
     */
    // 标记node结点为Node.CANCELLED（取消）状态
    private void cancelAcquire(Node node) {
        // Ignore if node doesn't exist
        if(node == null)
            return;
        
        node.thread = null;
        
        // Skip cancelled predecessors
        Node pred = node.prev;
        while(pred.waitStatus>0)
            node.prev = pred = pred.prev;
        
        // predNext is the apparent node to unsplice. CASes below will
        // fail if not, in which case, we lost race vs another cancel
        // or signal, so no further action is necessary, although with
        // a possibility that a cancelled node may transiently remain
        // reachable.
        Node predNext = pred.next;
        
        // Can use unconditional write instead of CAS here.
        // After this atomic step, other Nodes can skip past us.
        // Before, we are free of interference from other threads.
        node.waitStatus = Node.CANCELLED;
        
        // If we are the tail, remove ourselves.
        if(node == tail && compareAndSetTail(node, pred)) {
            pred.compareAndSetNext(predNext, null);
        } else {
            // If successor needs signal, try to set pred's next-link
            // so it will get one. Otherwise wake it up to propagate.
            int ws;
            if(pred != head && ((ws = pred.waitStatus) == Node.SIGNAL || (ws<=0 && pred.compareAndSetWaitStatus(ws, Node.SIGNAL))) && pred.thread != null) {
                Node next = node.next;
                if(next != null && next.waitStatus<=0)
                    pred.compareAndSetNext(predNext, next);
            } else {
                unparkSuccessor(node);
            }
            
            node.next = node; // help GC
        }
    }
    
    /*▲ 对锁的支持 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /**
     * Returns a string identifying this synchronizer, as well as its state.
     * The state, in brackets, includes the String {@code "State ="}
     * followed by the current value of {@link #getState}, and either
     * {@code "nonempty"} or {@code "empty"} depending on whether the
     * queue is empty.
     *
     * @return a string identifying this synchronizer, as well as its state
     */
    public String toString() {
        return super.toString()
            + "[State = " + getState() + ", "
            + (hasQueuedThreads() ? "non" : "") + "empty queue]";
    }
    
    
    
    /**
     * Condition implementation for a {@link AbstractQueuedLongSynchronizer}
     * serving as the basis of a {@link Lock} implementation.
     *
     * <p>Method documentation for this class describes mechanics,
     * not behavioral specifications from the point of view of Lock
     * and Condition users. Exported versions of this class will in
     * general need to be accompanied by documentation describing
     * condition semantics that rely on those of the associated
     * {@code AbstractQueuedLongSynchronizer}.
     *
     * <p>This class is Serializable, but all fields are transient,
     * so deserialized conditions have no waiters.
     *
     * @since 1.6
     */
    public class ConditionObject implements Condition, Serializable {
        private static final long serialVersionUID = 1173984872572414699L;
        
        /** First node of condition queue. */
        private transient Node firstWaiter;
        
        /** Last node of condition queue. */
        private transient Node lastWaiter;
        
        /*
         * For interruptible waits, we need to track whether to throw
         * InterruptedException, if interrupted while blocked on
         * condition, versus reinterrupt current thread, if
         * interrupted while blocked waiting to re-acquire.
         */
        
        /** Mode meaning to reinterrupt on exit from wait */
        private static final int REINTERRUPT =  1;
        /** Mode meaning to throw InterruptedException on exit from wait */
        private static final int THROW_IE    = -1;
        
        /**
         * Creates a new {@code ConditionObject} instance.
         */
        public ConditionObject() { }
        
        
        
        /*▼ 暂时阻塞 ████████████████████████████████████████████████████████████████████████████████┓ */
        
        /**
         * Implements uninterruptible condition wait.
         * <ol>
         * <li>Save lock state returned by {@link #getState}.
         * <li>Invoke {@link #release} with saved state as argument,
         * throwing IllegalMonitorStateException if it fails.
         * <li>Block until signalled.
         * <li>Reacquire by invoking specialized version of
         * {@link #acquire} with saved state as argument.
         * </ol>
         */
        /*
         * 使当前活跃的线程暂时陷入阻塞，进入【|条件队列|】排队，并唤醒【|同步队列|】中的等待者...
         * 直到收到signal()命令，该线程才恢复运行
         *
         * 允许阻塞带有中断标记的线程
         */
        public final void awaitUninterruptibly() {
            Node node = addConditionWaiter();
            long savedState = fullyRelease(node);
            boolean interrupted = false;
            while(!isOnSyncQueue(node)) {
                LockSupport.park(this);
                if(Thread.interrupted())
                    interrupted = true;
            }
            if(acquireQueued(node, savedState) || interrupted)
                selfInterrupt();
        }
        
        /**
         * Implements interruptible condition wait.
         * <ol>
         * <li>If current thread is interrupted, throw InterruptedException.
         * <li>Save lock state returned by {@link #getState}.
         * <li>Invoke {@link #release} with saved state as argument,
         * throwing IllegalMonitorStateException if it fails.
         * <li>Block until signalled or interrupted.
         * <li>Reacquire by invoking specialized version of
         * {@link #acquire} with saved state as argument.
         * <li>If interrupted while blocked in step 4, throw InterruptedException.
         * </ol>
         */
        /*
         * 使当前活跃的线程暂时陷入阻塞，进入【|条件队列|】排队，并唤醒【|同步队列|】中的等待者...
         * 直到收到signal()命令，该线程才恢复运行
         *
         * 禁止阻塞带有中断标记的线程
         *
         * 操作步骤：
         * 1.将该线程的引用封装到{条件node}中，并进入【|条件队列|】排队
         * 2.释放掉该线程持有的全部许可证，即让锁重新处于可用状态
         * 3.唤醒【|同步队列|】中的等待者重新争锁
         * 4.将自身陷入阻塞，等待signal()唤醒
         * 5.被signal()唤醒后，将排队的{条件node}移入【|同步队列|】，
         *   并恢复其许可证，让其继续执行...
         */
        public final void await() throws InterruptedException {
            if(Thread.interrupted()) {
                throw new InterruptedException();
            }
            Node node = addConditionWaiter();
            long savedState = fullyRelease(node);
            int interruptMode = 0;
            while(!isOnSyncQueue(node)) {
                LockSupport.park(this);
                if((interruptMode = checkInterruptWhileWaiting(node)) != 0) {
                    break;
                }
            }
            if(acquireQueued(node, savedState) && interruptMode != THROW_IE) {
                interruptMode = REINTERRUPT;
            }
            // clean up if cancelled
            if(node.nextWaiter != null) {
                unlinkCancelledWaiters();
            }
            if(interruptMode != 0) {
                reportInterruptAfterWait(interruptMode);
            }
        }
        
        /**
         * Implements timed condition wait.
         * <ol>
         * <li>If current thread is interrupted, throw InterruptedException.
         * <li>Save lock state returned by {@link #getState}.
         * <li>Invoke {@link #release} with saved state as argument,
         * throwing IllegalMonitorStateException if it fails.
         * <li>Block until signalled, interrupted, or timed out.
         * <li>Reacquire by invoking specialized version of
         * {@link #acquire} with saved state as argument.
         * <li>If interrupted while blocked in step 4, throw InterruptedException.
         * </ol>
         */
        /*
         * 使当前活跃的线程暂时陷入阻塞，进入【|条件队列|】排队，并唤醒【|同步队列|】中的等待者...
         * 直到收到signal()的信号，或超时醒来，该线程才恢复运行
         *
         * 禁止阻塞带有中断标记的线程
         *
         * nanosTimeout是相对时间，代表最长阻塞时间，过了这个时间，即使没有收到signal()的信号，也会自己醒来
         */
        public final long awaitNanos(long nanosTimeout) throws InterruptedException {
            if(Thread.interrupted())
                throw new InterruptedException();
            // We don't check for nanosTimeout <= 0L here, to allow
            // awaitNanos(0) as a way to "yield the lock".
            final long deadline = System.nanoTime() + nanosTimeout;
            long initialNanos = nanosTimeout;
            Node node = addConditionWaiter();
            long savedState = fullyRelease(node);
            int interruptMode = 0;
            while(!isOnSyncQueue(node)) {
                if(nanosTimeout<=0L) {
                    transferAfterCancelledWait(node);
                    break;
                }
                if(nanosTimeout>SPIN_FOR_TIMEOUT_THRESHOLD)
                    LockSupport.parkNanos(this, nanosTimeout);
                if((interruptMode = checkInterruptWhileWaiting(node)) != 0)
                    break;
                nanosTimeout = deadline - System.nanoTime();
            }
            if(acquireQueued(node, savedState) && interruptMode != THROW_IE)
                interruptMode = REINTERRUPT;
            if(node.nextWaiter != null)
                unlinkCancelledWaiters();
            if(interruptMode != 0)
                reportInterruptAfterWait(interruptMode);
            long remaining = deadline - System.nanoTime(); // avoid overflow
            return (remaining<=initialNanos) ? remaining : Long.MIN_VALUE;
        }
        
        /**
         * Implements timed condition wait.
         * <ol>
         * <li>If current thread is interrupted, throw InterruptedException.
         * <li>Save lock state returned by {@link #getState}.
         * <li>Invoke {@link #release} with saved state as argument,
         * throwing IllegalMonitorStateException if it fails.
         * <li>Block until signalled, interrupted, or timed out.
         * <li>Reacquire by invoking specialized version of
         * {@link #acquire} with saved state as argument.
         * <li>If interrupted while blocked in step 4, throw InterruptedException.
         * <li>If timed out while blocked in step 4, return false, else true.
         * </ol>
         */
        // 与awaitNanos(long)方法作用一致，只不过纳秒时间由time转换而来
        public final boolean await(long time, TimeUnit unit) throws InterruptedException {
            long nanosTimeout = unit.toNanos(time);
            if(Thread.interrupted())
                throw new InterruptedException();
            // We don't check for nanosTimeout <= 0L here, to allow
            // await(0, unit) as a way to "yield the lock".
            final long deadline = System.nanoTime() + nanosTimeout;
            Node node = addConditionWaiter();
            long savedState = fullyRelease(node);
            boolean timedout = false;
            int interruptMode = 0;
            while(!isOnSyncQueue(node)) {
                if(nanosTimeout<=0L) {
                    timedout = transferAfterCancelledWait(node);
                    break;
                }
                if(nanosTimeout>SPIN_FOR_TIMEOUT_THRESHOLD)
                    LockSupport.parkNanos(this, nanosTimeout);
                if((interruptMode = checkInterruptWhileWaiting(node)) != 0)
                    break;
                nanosTimeout = deadline - System.nanoTime();
            }
            if(acquireQueued(node, savedState) && interruptMode != THROW_IE)
                interruptMode = REINTERRUPT;
            if(node.nextWaiter != null)
                unlinkCancelledWaiters();
            if(interruptMode != 0)
                reportInterruptAfterWait(interruptMode);
            return !timedout;
        }
        
        /**
         * Implements absolute timed condition wait.
         * <ol>
         * <li>If current thread is interrupted, throw InterruptedException.
         * <li>Save lock state returned by {@link #getState}.
         * <li>Invoke {@link #release} with saved state as argument,
         * throwing IllegalMonitorStateException if it fails.
         * <li>Block until signalled, interrupted, or timed out.
         * <li>Reacquire by invoking specialized version of
         * {@link #acquire} with saved state as argument.
         * <li>If interrupted while blocked in step 4, throw InterruptedException.
         * <li>If timed out while blocked in step 4, return false, else true.
         * </ol>
         */
        /*
         * 使当前活跃的线程暂时陷入阻塞，进入【|条件队列|】排队，并唤醒【|同步队列|】中的等待者...
         * 直到收到signal()的信号，或超时醒来，该线程才恢复运行
         *
         * 禁止阻塞带有中断标记的线程
         *
         * deadline是绝对时间，代表阻塞截止时间，过了这个时间，即使没有收到signal()的信号，也会自己醒来
         */
        public final boolean awaitUntil(Date deadline) throws InterruptedException {
            long abstime = deadline.getTime();
            if(Thread.interrupted())
                throw new InterruptedException();
            Node node = addConditionWaiter();
            long savedState = fullyRelease(node);
            boolean timedout = false;
            int interruptMode = 0;
            while(!isOnSyncQueue(node)) {
                if(System.currentTimeMillis() >= abstime) {
                    timedout = transferAfterCancelledWait(node);
                    break;
                }
                LockSupport.parkUntil(this, abstime);
                if((interruptMode = checkInterruptWhileWaiting(node)) != 0)
                    break;
            }
            if(acquireQueued(node, savedState) && interruptMode != THROW_IE)
                interruptMode = REINTERRUPT;
            if(node.nextWaiter != null)
                unlinkCancelledWaiters();
            if(interruptMode != 0)
                reportInterruptAfterWait(interruptMode);
            return !timedout;
        }
        
        /*▲ 暂时阻塞 ████████████████████████████████████████████████████████████████████████████████┛ */
        
        
        
        /*▼ 准备唤醒 ████████████████████████████████████████████████████████████████████████████████┓ */
        
        /**
         * Moves the longest-waiting thread, if one exists, from the
         * wait queue for this condition to the wait queue for the
         * owning lock.
         *
         * @throws IllegalMonitorStateException if {@link #isHeldExclusively}
         *                                      returns {@code false}
         */
        // 将【|条件队列|】中的首结点追加到【|同步队列|】中，并让其处于待唤醒状态
        public final void signal() {
            if(!isHeldExclusively())
                throw new IllegalMonitorStateException();
            Node first = firstWaiter;
            if(first != null)
                doSignal(first);
        }
        
        /**
         * Removes and transfers nodes until hit non-cancelled one or
         * null. Split out from signal in part to encourage compilers
         * to inline the case of no waiters.
         *
         * @param first (non-null) the first node on condition queue
         */
        // 将【|条件队列|】中的首结点追加到【|同步队列|】中，并让其处于待唤醒状态
        private void doSignal(Node first) {
            do {
                if((firstWaiter = first.nextWaiter) == null)
                    lastWaiter = null;
                first.nextWaiter = null;
            } while(!transferForSignal(first) && (first = firstWaiter) != null);
        }
        
        /**
         * Moves all threads from the wait queue for this condition to
         * the wait queue for the owning lock.
         *
         * @throws IllegalMonitorStateException if {@link #isHeldExclusively}
         *                                      returns {@code false}
         */
        // 将【|条件队列|】中所有结点挨个追加到【|同步队列|】中，并让其处于待唤醒状态
        public final void signalAll() {
            if(!isHeldExclusively())
                throw new IllegalMonitorStateException();
            Node first = firstWaiter;
            if(first != null)
                doSignalAll(first);
        }
        
        /**
         * Removes and transfers all nodes.
         *
         * @param first (non-null) the first node on condition queue
         */
        // 将【|条件队列|】中所有结点挨个追加到【|同步队列|】中，并让其处于待唤醒状态
        private void doSignalAll(Node first) {
            lastWaiter = firstWaiter = null;
            do {
                Node next = first.nextWaiter;
                first.nextWaiter = null;
                transferForSignal(first);
                first = next;
            } while(first != null);
        }
        
        /*▲ 准备唤醒 ████████████████████████████████████████████████████████████████████████████████┛ */
        
        
        /**
         * Queries whether any threads are waiting on this condition.
         * Implements {@link AbstractQueuedLongSynchronizer#hasWaiters(ConditionObject)}.
         *
         * @return {@code true} if there are any waiting threads
         *
         * @throws IllegalMonitorStateException if {@link #isHeldExclusively}
         *                                      returns {@code false}
         */
        // 判断当前【|条件队列|】中是否存在等待者
        protected final boolean hasWaiters() {
            if(!isHeldExclusively())
                throw new IllegalMonitorStateException();
            for(Node w = firstWaiter; w != null; w = w.nextWaiter) {
                if(w.waitStatus == Node.CONDITION)
                    return true;
            }
            return false;
        }
        
        /**
         * Returns an estimate of the number of threads waiting on
         * this condition.
         * Implements {@link AbstractQueuedLongSynchronizer#getWaitQueueLength(ConditionObject)}.
         *
         * @return the estimated number of waiting threads
         *
         * @throws IllegalMonitorStateException if {@link #isHeldExclusively}
         *                                      returns {@code false}
         */
        // 返回【|条件队列|】长度
        protected final int getWaitQueueLength() {
            if(!isHeldExclusively())
                throw new IllegalMonitorStateException();
            int n = 0;
            for(Node w = firstWaiter; w != null; w = w.nextWaiter) {
                if(w.waitStatus == Node.CONDITION)
                    ++n;
            }
            return n;
        }
        
        /**
         * Returns a collection containing those threads that may be
         * waiting on this Condition.
         * Implements {@link AbstractQueuedLongSynchronizer#getWaitingThreads(ConditionObject)}.
         *
         * @return the collection of threads
         *
         * @throws IllegalMonitorStateException if {@link #isHeldExclusively}
         *                                      returns {@code false}
         */
        // 返回一个集合，该集合包含了【|条件队列|】中所有结点内缓存的线程引用
        protected final Collection<Thread> getWaitingThreads() {
            if(!isHeldExclusively())
                throw new IllegalMonitorStateException();
            ArrayList<Thread> list = new ArrayList<>();
            for(Node w = firstWaiter; w != null; w = w.nextWaiter) {
                if(w.waitStatus == Node.CONDITION) {
                    Thread t = w.thread;
                    if(t != null)
                        list.add(t);
                }
            }
            return list;
        }
        
        /**
         * Returns true if this condition was created by the given
         * synchronization object.
         *
         * @return {@code true} if owned
         */
        // 判断当前条件对象是否归同步队列（锁）sync所有
        final boolean isOwnedBy(AbstractQueuedLongSynchronizer sync) {
            return sync == AbstractQueuedLongSynchronizer.this;
        }
        
        /**
         * Adds a new waiter to wait queue.
         *
         * @return its new wait node
         */
        /*
         * 添加一个新的[条件Node]到【|条件队列|】，该Node缓存了对当前线程的引用
         * 添加完成后，返回刚添加的node
         */
        private Node addConditionWaiter() {
            if(!isHeldExclusively())
                throw new IllegalMonitorStateException();
            Node t = lastWaiter;
            // If lastWaiter is cancelled, clean out.
            if(t != null && t.waitStatus != Node.CONDITION) {
                unlinkCancelledWaiters();
                t = lastWaiter;
            }
            
            Node node = new Node(Node.CONDITION);
            
            if(t == null)
                firstWaiter = node;
            else
                t.nextWaiter = node;
            lastWaiter = node;
            return node;
        }
        
        /**
         * Unlinks cancelled waiter nodes from condition queue.
         * Called only while holding lock. This is called when
         * cancellation occurred during condition wait, and upon
         * insertion of a new waiter when lastWaiter is seen to have
         * been cancelled. This method is needed to avoid garbage
         * retention in the absence of signals. So even though it may
         * require a full traversal, it comes into play only when
         * timeouts or cancellations occur in the absence of
         * signals. It traverses all nodes rather than stopping at a
         * particular target to unlink all pointers to garbage nodes
         * without requiring many re-traversals during cancellation
         * storms.
         */
        // 从【|条件队列|】中移除已取消的等待者
        private void unlinkCancelledWaiters() {
            Node t = firstWaiter;
            Node trail = null;
            while(t != null) {
                Node next = t.nextWaiter;
                if(t.waitStatus != Node.CONDITION) {
                    t.nextWaiter = null;
                    if(trail == null)
                        firstWaiter = next;
                    else
                        trail.nextWaiter = next;
                    if(next == null)
                        lastWaiter = trail;
                } else
                    trail = t;
                t = next;
            }
        }
        
        /**
         * Checks for interrupt, returning THROW_IE if interrupted
         * before signalled, REINTERRUPT if after signalled, or
         * 0 if not interrupted.
         */
        /*
         * 检查当前线程是否有中断标记，并清除中断标记
         *
         * 返回0：无中断标记
         * 返回THROW_IE(-1)：有中断标记，且node成功加入到了【|同步队列|】末尾
         * 返回REINTERRUPT(1)：有中断标记，node还在【|条件队列|】中
         */
        private int checkInterruptWhileWaiting(Node node) {
            return Thread.interrupted() ? (transferAfterCancelledWait(node) ? THROW_IE : REINTERRUPT) : 0;
        }
        
        /**
         * Throws InterruptedException, reinterrupts current thread, or
         * does nothing, depending on mode.
         */
        // 对刚刚唤醒的带有中断标记的线程进行特殊处理
        private void reportInterruptAfterWait(int interruptMode) throws InterruptedException {
            if(interruptMode == THROW_IE)
                throw new InterruptedException();
            else if(interruptMode == REINTERRUPT)
                selfInterrupt();
        }
    }
    
}