一、概述

synchronized上锁机制是通过对象头来实现的，通过锁升级的过程来完成加锁。（https://blog.csdn.net/zhangting19921121/article/details/106002751）

但是synchronized锁升级的过程犹如一个黑盒，我们无法掌控。因此，在实际的工作中ReentrantLock使用相对比较频繁。ReentrantLock显式地获取锁，释放锁，可中断，同时还支持实现公平锁等。常用的写法如下：

ReentrantLock lock = new ReentrantLock(false);//false为非公平锁， true为公平锁
lock.lock() //加锁
// todo 业务代码
lock.unlock() //解锁

假如说有t0,t1,t2三个线程来进行调用，t0获取锁，开始执行业务代码。此时，t1,t2都应该停在lock.lock()方法中，不能向下执行业务代码。那么怎么停在里面呢？可以借鉴synchronized自旋的实现，

while(true) {
   if (加锁成功) {
      break; // 跳出循环
   }
}

但是，一直让线程进行while循环，其实也是在不断消耗CPU。所以，如何让这些线程让出CPU呢？

while(true) {
   if (加锁成功) {
      break; // 跳出循环
   }
   Thread.yeild(); // 让出CPU的使用权
}

会有这样一个问题，假如说业务代码执行很慢，需要10几秒。此时只有t1,t2两个线程在等待获取锁还好，如果有100多个线程都在等待呢，那这100多个线程只能互相让来让去。如果改为Thread.sleep(睡眠时间);呢，但是这个睡眠时间选多少合适呢，这个时间不好确认，也不是一个很好的方法。那如果我把线程阻塞了，不让它再循环了，等待调度唤醒，减少线程间互相的让来来去或者是找一个合适的睡眠时间。

while(true) {
   if (加锁成功) {
      break; // 跳出循环
   }
   LockSupport.part();//阻塞线程，跳出循环
}

那谁去唤醒这些被阻塞的线程呢？

while(true) {
   if (加锁成功) {
      break; // 跳出循环
   }
   LockSupport.part();//阻塞线程，跳出循环
}
LockSupport.unpart(t);// 唤醒线程t

LockSupport.unpart(t)可以唤醒线程，但是我怎么知道唤醒哪个线程呢？可以使用一个对象来进行保存。

// lock.lock()
while(true) {
   if (加锁成功) {
      break; // 跳出循环
   }
   HashSet.add(t); // LinkedQueue也可以
   LockSupport.part();//阻塞线程，跳出循环
}
// todo 业务逻辑
// lock.unlock()
Thread t = HashSet.get();// LinkedQueue.take()
LockSupport.unpart(t);// 唤醒线程t

以上，便是加锁，解锁的简单实现。但是除了自旋，LockSupport，Queue外还需要什么呢？在if判断的地方，必须要保证只有一个线程能够进入，通过synchronized就可以实现这个功能。此外，java中还有CAS可以实现和synchronized一样的功能,ReentrantLock是一种基于AQS框架的应用实现的。

好啦，接下来就引入了本文的重点啦，继续往下看吧~

二、AQS

AQS（全称AbstractQueuedSynchronized），AQS定义了一套多线程访问共享资源的同步器框架，是一个依赖状态(state)的同步器。

1.AQS基本特性

1-1.阻塞等待队列
1-2.共享/独占
1-3.公平/非公平
1-4.可重入
1-5.允许中断

2.AQS实现逻辑

2-1.AQS内部维护属性volatile int state (32位)：state表示资源的可用状态，为0的时候表示当前锁并未被任何线程所持有。state三种访问方式：getState()、setState()、compareAndSetState()
2-2.AQS定义两种资源共享方式：Exclusive-独占（只有一个线程能执行，如ReentrantLock）、Share-共享（多个线程可以同时执行，如Semaphore/CountDownLatch）
2-3.AQS定义两种队列：同步等待队列、条件等待队列
不同的自定义同步器争用共享资源的方式也不同。自定义同步器在实现时只需要实现共享资源state的获取与释放方式即可，至于具体线程等待队列的维护（如获取资源失败入队/ 唤醒出队等），AQS已经在顶层实现好了。自定义同步器实现时主要实现以下几种方法：
isHeldExclusively()：该线程是否正在独占资源。只有用到condition才需要去实现它。
tryAcquire(int)：独占方式。尝试获取资源，成功则返回true，失败则返回false。
tryRelease(int)：独占方式。尝试释放资源，成功则返回true，失败则返回false。
tryAcquireShared(int)：共享方式。尝试获取资源。负数表示失败；0表示成功，但没有剩余可用资源；正数表示成功，且有剩余资源。
tryReleaseShared(int)：共享方式。尝试释放资源，如果释放后允许唤醒后续等待结点返回true，否则返回false。
在这里插入图片描述

//
// Source code recreated from a .class file by IntelliJ IDEA
// (powered by FernFlower decompiler)
//

package java.util.concurrent.locks;

import java.io.Serializable;
import java.util.ArrayList;
import java.util.Collection;
import java.util.Date;
import java.util.concurrent.TimeUnit;
import sun.misc.Unsafe;

public abstract class AbstractQueuedSynchronizer extends AbstractOwnableSynchronizer implements Serializable {
    private static final long serialVersionUID = 7373984972572414691L;
    // 基于Node(prev、next)来构建的双向链表CLH 
    private transient volatile AbstractQueuedSynchronizer.Node head;
    private transient volatile AbstractQueuedSynchronizer.Node tail;
    // 状态器，为0的时候表示当前锁并未被任何线程所持有
    private volatile int state;
    static final long spinForTimeoutThreshold = 1000L;
    private static final Unsafe unsafe = Unsafe.getUnsafe();
    private static final long stateOffset;
    private static final long headOffset;
    private static final long tailOffset;
    private static final long waitStatusOffset;
    private static final long nextOffset;

    protected AbstractQueuedSynchronizer() {
    }

    protected final int getState() {
        return this.state;
    }

    protected final void setState(int var1) {
        this.state = var1;
    }

    protected final boolean compareAndSetState(int var1, int var2) {
        return unsafe.compareAndSwapInt(this, stateOffset, var1, var2);
    }

    private AbstractQueuedSynchronizer.Node enq(AbstractQueuedSynchronizer.Node var1) {
        while(true) {
            AbstractQueuedSynchronizer.Node var2 = this.tail;
            if (var2 == null) {
                if (this.compareAndSetHead(new AbstractQueuedSynchronizer.Node())) {
                    this.tail = this.head;
                }
            } else {
                var1.prev = var2;
                if (this.compareAndSetTail(var2, var1)) {
                    var2.next = var1;
                    return var2;
                }
            }
        }
    }

    private AbstractQueuedSynchronizer.Node addWaiter(AbstractQueuedSynchronizer.Node var1) {
        AbstractQueuedSynchronizer.Node var2 = new AbstractQueuedSynchronizer.Node(Thread.currentThread(), var1);
        AbstractQueuedSynchronizer.Node var3 = this.tail;
        if (var3 != null) {
            var2.prev = var3;
            if (this.compareAndSetTail(var3, var2)) {
                var3.next = var2;
                return var2;
            }
        }

        this.enq(var2);
        return var2;
    }

    private void setHead(AbstractQueuedSynchronizer.Node var1) {
        this.head = var1;
        var1.thread = null;
        var1.prev = null;
    }

    private void unparkSuccessor(AbstractQueuedSynchronizer.Node var1) {
        int var2 = var1.waitStatus;
        if (var2 < 0) {
            compareAndSetWaitStatus(var1, var2, 0);
        }

        AbstractQueuedSynchronizer.Node var3 = var1.next;
        if (var3 == null || var3.waitStatus > 0) {
            var3 = null;

            for(AbstractQueuedSynchronizer.Node var4 = this.tail; var4 != null && var4 != var1; var4 = var4.prev) {
                if (var4.waitStatus <= 0) {
                    var3 = var4;
                }
            }
        }

        if (var3 != null) {
            LockSupport.unpark(var3.thread);
        }

    }

    private void doReleaseShared() {
        while(true) {
            AbstractQueuedSynchronizer.Node var1 = this.head;
            if (var1 != null && var1 != this.tail) {
                int var2 = var1.waitStatus;
                if (var2 == -1) {
                    if (!compareAndSetWaitStatus(var1, -1, 0)) {
                        continue;
                    }

                    this.unparkSuccessor(var1);
                } else if (var2 == 0 && !compareAndSetWaitStatus(var1, 0, -3)) {
                    continue;
                }
            }

            if (var1 == this.head) {
                return;
            }
        }
    }

    private void setHeadAndPropagate(AbstractQueuedSynchronizer.Node var1, int var2) {
        AbstractQueuedSynchronizer.Node var3 = this.head;
        this.setHead(var1);
        if (var2 > 0 || var3 == null || var3.waitStatus < 0 || (var3 = this.head) == null || var3.waitStatus < 0) {
            AbstractQueuedSynchronizer.Node var4 = var1.next;
            if (var4 == null || var4.isShared()) {
                this.doReleaseShared();
            }
        }

    }

    private void cancelAcquire(AbstractQueuedSynchronizer.Node var1) {
        if (var1 != null) {
            var1.thread = null;

            AbstractQueuedSynchronizer.Node var2;
            for(var2 = var1.prev; var2.waitStatus > 0; var1.prev = var2 = var2.prev) {
            }

            AbstractQueuedSynchronizer.Node var3 = var2.next;
            var1.waitStatus = 1;
            if (var1 == this.tail && this.compareAndSetTail(var1, var2)) {
                compareAndSetNext(var2, var3, (AbstractQueuedSynchronizer.Node)null);
            } else {
                int var4;
                if (var2 != this.head && ((var4 = var2.waitStatus) == -1 || var4 <= 0 && compareAndSetWaitStatus(var2, var4, -1)) && var2.thread != null) {
                    AbstractQueuedSynchronizer.Node var5 = var1.next;
                    if (var5 != null && var5.waitStatus <= 0) {
                        compareAndSetNext(var2, var3, var5);
                    }
                } else {
                    this.unparkSuccessor(var1);
                }

                var1.next = var1;
            }

        }
    }

    private static boolean shouldParkAfterFailedAcquire(AbstractQueuedSynchronizer.Node var0, AbstractQueuedSynchronizer.Node var1) {
        int var2 = var0.waitStatus;
        if (var2 == -1) {
            return true;
        } else {
            if (var2 > 0) {
                do {
                    var1.prev = var0 = var0.prev;
                } while(var0.waitStatus > 0);

                var0.next = var1;
            } else {
                compareAndSetWaitStatus(var0, var2, -1);
            }

            return false;
        }
    }

    static void selfInterrupt() {
        Thread.currentThread().interrupt();
    }

    private final boolean parkAndCheckInterrupt() {
        LockSupport.park(this);
        return Thread.interrupted();
    }

    final boolean acquireQueued(AbstractQueuedSynchronizer.Node var1, int var2) {
        boolean var3 = true;

        try {
            boolean var4 = false;

            while(true) {
                AbstractQueuedSynchronizer.Node var5 = var1.predecessor();
                if (var5 == this.head && this.tryAcquire(var2)) {
                    this.setHead(var1);
                    var5.next = null;
                    var3 = false;
                    boolean var6 = var4;
                    return var6;
                }

                if (shouldParkAfterFailedAcquire(var5, var1) && this.parkAndCheckInterrupt()) {
                    var4 = true;
                }
            }
        } finally {
            if (var3) {
                this.cancelAcquire(var1);
            }

        }
    }

    private void doAcquireInterruptibly(int var1) throws InterruptedException {
        AbstractQueuedSynchronizer.Node var2 = this.addWaiter(AbstractQueuedSynchronizer.Node.EXCLUSIVE);
        boolean var3 = true;

        try {
            AbstractQueuedSynchronizer.Node var4;
            do {
                var4 = var2.predecessor();
                if (var4 == this.head && this.tryAcquire(var1)) {
                    this.setHead(var2);
                    var4.next = null;
                    var3 = false;
                    return;
                }
            } while(!shouldParkAfterFailedAcquire(var4, var2) || !this.parkAndCheckInterrupt());

            throw new InterruptedException();
        } finally {
            if (var3) {
                this.cancelAcquire(var2);
            }

        }
    }

    private boolean doAcquireNanos(int var1, long var2) throws InterruptedException {
        if (var2 <= 0L) {
            return false;
        } else {
            long var4 = System.nanoTime() + var2;
            AbstractQueuedSynchronizer.Node var6 = this.addWaiter(AbstractQueuedSynchronizer.Node.EXCLUSIVE);
            boolean var7 = true;

            try {
                do {
                    AbstractQueuedSynchronizer.Node var8 = var6.predecessor();
                    boolean var9;
                    if (var8 == this.head && this.tryAcquire(var1)) {
                        this.setHead(var6);
                        var8.next = null;
                        var7 = false;
                        var9 = true;
                        return var9;
                    }

                    var2 = var4 - System.nanoTime();
                    if (var2 <= 0L) {
                        var9 = false;
                        return var9;
                    }

                    if (shouldParkAfterFailedAcquire(var8, var6) && var2 > 1000L) {
                        LockSupport.parkNanos(this, var2);
                    }
                } while(!Thread.interrupted());

                throw new InterruptedException();
            } finally {
                if (var7) {
                    this.cancelAcquire(var6);
                }

            }
        }
    }

    private void doAcquireShared(int var1) {
        AbstractQueuedSynchronizer.Node var2 = this.addWaiter(AbstractQueuedSynchronizer.Node.SHARED);
        boolean var3 = true;

        try {
            boolean var4 = false;

            while(true) {
                AbstractQueuedSynchronizer.Node var5 = var2.predecessor();
                if (var5 == this.head) {
                    int var6 = this.tryAcquireShared(var1);
                    if (var6 >= 0) {
                        this.setHeadAndPropagate(var2, var6);
                        var5.next = null;
                        if (var4) {
                            selfInterrupt();
                        }

                        var3 = false;
                        return;
                    }
                }

                if (shouldParkAfterFailedAcquire(var5, var2) && this.parkAndCheckInterrupt()) {
                    var4 = true;
                }
            }
        } finally {
            if (var3) {
                this.cancelAcquire(var2);
            }

        }
    }

    private void doAcquireSharedInterruptibly(int var1) throws InterruptedException {
        AbstractQueuedSynchronizer.Node var2 = this.addWaiter(AbstractQueuedSynchronizer.Node.SHARED);
        boolean var3 = true;

        try {
            AbstractQueuedSynchronizer.Node var4;
            do {
                var4 = var2.predecessor();
                if (var4 == this.head) {
                    int var5 = this.tryAcquireShared(var1);
                    if (var5 >= 0) {
                        this.setHeadAndPropagate(var2, var5);
                        var4.next = null;
                        var3 = false;
                        return;
                    }
                }
            } while(!shouldParkAfterFailedAcquire(var4, var2) || !this.parkAndCheckInterrupt());

            throw new InterruptedException();
        } finally {
            if (var3) {
                this.cancelAcquire(var2);
            }

        }
    }

    private boolean doAcquireSharedNanos(int var1, long var2) throws InterruptedException {
        if (var2 <= 0L) {
            return false;
        } else {
            long var4 = System.nanoTime() + var2;
            AbstractQueuedSynchronizer.Node var6 = this.addWaiter(AbstractQueuedSynchronizer.Node.SHARED);
            boolean var7 = true;

            try {
                do {
                    AbstractQueuedSynchronizer.Node var8 = var6.predecessor();
                    if (var8 == this.head) {
                        int var9 = this.tryAcquireShared(var1);
                        if (var9 >= 0) {
                            this.setHeadAndPropagate(var6, var9);
                            var8.next = null;
                            var7 = false;
                            boolean var10 = true;
                            return var10;
                        }
                    }

                    var2 = var4 - System.nanoTime();
                    if (var2 <= 0L) {
                        boolean var14 = false;
                        return var14;
                    }

                    if (shouldParkAfterFailedAcquire(var8, var6) && var2 > 1000L) {
                        LockSupport.parkNanos(this, var2);
                    }
                } while(!Thread.interrupted());

                throw new InterruptedException();
            } finally {
                if (var7) {
                    this.cancelAcquire(var6);
                }

            }
        }
    }

    // 在AbstractQueuedSynchronizer中并没有实现，具体实现的逻辑都在子类中
    protected boolean tryAcquire(int var1) {
        throw new UnsupportedOperationException();
    }

    protected boolean tryRelease(int var1) {
        throw new UnsupportedOperationException();
    }

    protected int tryAcquireShared(int var1) {
        throw new UnsupportedOperationException();
    }

    protected boolean tryReleaseShared(int var1) {
        throw new UnsupportedOperationException();
    }

    protected boolean isHeldExclusively() {
        throw new UnsupportedOperationException();
    }

    public final void acquire(int var1) {
    //this.acquireQueued(this.addWaiter(AbstractQueuedSynchronizer.Node.EXCLUSIVE), var1)加锁失败强制入队
        if (!this.tryAcquire(var1) && this.acquireQueued(this.addWaiter(AbstractQueuedSynchronizer.Node.EXCLUSIVE), var1)) {
            selfInterrupt();
        }

    }

    public final void acquireInterruptibly(int var1) throws InterruptedException {
        if (Thread.interrupted()) {
            throw new InterruptedException();
        } else {
            if (!this.tryAcquire(var1)) {
                this.doAcquireInterruptibly(var1);
            }

        }
    }

    public final boolean tryAcquireNanos(int var1, long var2) throws InterruptedException {
        if (Thread.interrupted()) {
            throw new InterruptedException();
        } else {
            return this.tryAcquire(var1) || this.doAcquireNanos(var1, var2);
        }
    }

    public final boolean release(int var1) {
        if (this.tryRelease(var1)) {
            AbstractQueuedSynchronizer.Node var2 = this.head;
            if (var2 != null && var2.waitStatus != 0) {
                this.unparkSuccessor(var2);
            }

            return true;
        } else {
            return false;
        }
    }

    public final void acquireShared(int var1) {
        if (this.tryAcquireShared(var1) < 0) {
            this.doAcquireShared(var1);
        }

    }

    public final void acquireSharedInterruptibly(int var1) throws InterruptedException {
        if (Thread.interrupted()) {
            throw new InterruptedException();
        } else {
            if (this.tryAcquireShared(var1) < 0) {
                this.doAcquireSharedInterruptibly(var1);
            }

        }
    }

    public final boolean tryAcquireSharedNanos(int var1, long var2) throws InterruptedException {
        if (Thread.interrupted()) {
            throw new InterruptedException();
        } else {
            return this.tryAcquireShared(var1) >= 0 || this.doAcquireSharedNanos(var1, var2);
        }
    }

    public final boolean releaseShared(int var1) {
        if (this.tryReleaseShared(var1)) {
            this.doReleaseShared();
            return true;
        } else {
            return false;
        }
    }

    public final boolean hasQueuedThreads() {
        return this.head != this.tail;
    }

    public final boolean hasContended() {
        return this.head != null;
    }

    public final Thread getFirstQueuedThread() {
        return this.head == this.tail ? null : this.fullGetFirstQueuedThread();
    }

    private Thread fullGetFirstQueuedThread() {
        AbstractQueuedSynchronizer.Node var1;
        AbstractQueuedSynchronizer.Node var2;
        Thread var3;
        if (((var1 = this.head) == null || (var2 = var1.next) == null || var2.prev != this.head || (var3 = var2.thread) == null) && ((var1 = this.head) == null || (var2 = var1.next) == null || var2.prev != this.head || (var3 = var2.thread) == null)) {
            AbstractQueuedSynchronizer.Node var4 = this.tail;

            Thread var5;
            for(var5 = null; var4 != null && var4 != this.head; var4 = var4.prev) {
                Thread var6 = var4.thread;
                if (var6 != null) {
                    var5 = var6;
                }
            }

            return var5;
        } else {
            return var3;
        }
    }

    public final boolean isQueued(Thread var1) {
        if (var1 == null) {
            throw new NullPointerException();
        } else {
            for(AbstractQueuedSynchronizer.Node var2 = this.tail; var2 != null; var2 = var2.prev) {
                if (var2.thread == var1) {
                    return true;
                }
            }

            return false;
        }
    }

    final boolean apparentlyFirstQueuedIsExclusive() {
        AbstractQueuedSynchronizer.Node var1;
        AbstractQueuedSynchronizer.Node var2;
        return (var1 = this.head) != null && (var2 = var1.next) != null && !var2.isShared() && var2.thread != null;
    }

    public final boolean hasQueuedPredecessors() {
        AbstractQueuedSynchronizer.Node var1 = this.tail;
        AbstractQueuedSynchronizer.Node var2 = this.head;
        AbstractQueuedSynchronizer.Node var3;
        return var2 != var1 && ((var3 = var2.next) == null || var3.thread != Thread.currentThread());
    }

    public final int getQueueLength() {
        int var1 = 0;

        for(AbstractQueuedSynchronizer.Node var2 = this.tail; var2 != null; var2 = var2.prev) {
            if (var2.thread != null) {
                ++var1;
            }
        }

        return var1;
    }

    public final Collection<Thread> getQueuedThreads() {
        ArrayList var1 = new ArrayList();

        for(AbstractQueuedSynchronizer.Node var2 = this.tail; var2 != null; var2 = var2.prev) {
            Thread var3 = var2.thread;
            if (var3 != null) {
                var1.add(var3);
            }
        }

        return var1;
    }

    public final Collection<Thread> getExclusiveQueuedThreads() {
        ArrayList var1 = new ArrayList();

        for(AbstractQueuedSynchronizer.Node var2 = this.tail; var2 != null; var2 = var2.prev) {
            if (!var2.isShared()) {
                Thread var3 = var2.thread;
                if (var3 != null) {
                    var1.add(var3);
                }
            }
        }

        return var1;
    }

    public final Collection<Thread> getSharedQueuedThreads() {
        ArrayList var1 = new ArrayList();

        for(AbstractQueuedSynchronizer.Node var2 = this.tail; var2 != null; var2 = var2.prev) {
            if (var2.isShared()) {
                Thread var3 = var2.thread;
                if (var3 != null) {
                    var1.add(var3);
                }
            }
        }

        return var1;
    }

    public String toString() {
        int var1 = this.getState();
        String var2 = this.hasQueuedThreads() ? "non" : "";
        return super.toString() + "[State = " + var1 + ", " + var2 + "empty queue]";
    }

    final boolean isOnSyncQueue(AbstractQueuedSynchronizer.Node var1) {
        if (var1.waitStatus != -2 && var1.prev != null) {
            return var1.next != null ? true : this.findNodeFromTail(var1);
        } else {
            return false;
        }
    }

    private boolean findNodeFromTail(AbstractQueuedSynchronizer.Node var1) {
        for(AbstractQueuedSynchronizer.Node var2 = this.tail; var2 != var1; var2 = var2.prev) {
            if (var2 == null) {
                return false;
            }
        }

        return true;
    }

    final boolean transferForSignal(AbstractQueuedSynchronizer.Node var1) {
        if (!compareAndSetWaitStatus(var1, -2, 0)) {
            return false;
        } else {
            AbstractQueuedSynchronizer.Node var2 = this.enq(var1);
            int var3 = var2.waitStatus;
            if (var3 > 0 || !compareAndSetWaitStatus(var2, var3, -1)) {
                LockSupport.unpark(var1.thread);
            }

            return true;
        }
    }

    final boolean transferAfterCancelledWait(AbstractQueuedSynchronizer.Node var1) {
        if (compareAndSetWaitStatus(var1, -2, 0)) {
            this.enq(var1);
            return true;
        } else {
            while(!this.isOnSyncQueue(var1)) {
                Thread.yield();
            }

            return false;
        }
    }

    final int fullyRelease(AbstractQueuedSynchronizer.Node var1) {
        boolean var2 = true;

        int var4;
        try {
            int var3 = this.getState();
            if (!this.release(var3)) {
                throw new IllegalMonitorStateException();
            }

            var2 = false;
            var4 = var3;
        } finally {
            if (var2) {
                var1.waitStatus = 1;
            }

        }

        return var4;
    }

    public final boolean owns(AbstractQueuedSynchronizer.ConditionObject var1) {
        return var1.isOwnedBy(this);
    }

    public final boolean hasWaiters(AbstractQueuedSynchronizer.ConditionObject var1) {
        if (!this.owns(var1)) {
            throw new IllegalArgumentException("Not owner");
        } else {
            return var1.hasWaiters();
        }
    }

    public final int getWaitQueueLength(AbstractQueuedSynchronizer.ConditionObject var1) {
        if (!this.owns(var1)) {
            throw new IllegalArgumentException("Not owner");
        } else {
            return var1.getWaitQueueLength();
        }
    }

    public final Collection<Thread> getWaitingThreads(AbstractQueuedSynchronizer.ConditionObject var1) {
        if (!this.owns(var1)) {
            throw new IllegalArgumentException("Not owner");
        } else {
            return var1.getWaitingThreads();
        }
    }

    private final boolean compareAndSetHead(AbstractQueuedSynchronizer.Node var1) {
        return unsafe.compareAndSwapObject(this, headOffset, (Object)null, var1);
    }

    private final boolean compareAndSetTail(AbstractQueuedSynchronizer.Node var1, AbstractQueuedSynchronizer.Node var2) {
        return unsafe.compareAndSwapObject(this, tailOffset, var1, var2);
    }

    private static final boolean compareAndSetWaitStatus(AbstractQueuedSynchronizer.Node var0, int var1, int var2) {
        return unsafe.compareAndSwapInt(var0, waitStatusOffset, var1, var2);
    }

    private static final boolean compareAndSetNext(AbstractQueuedSynchronizer.Node var0, AbstractQueuedSynchronizer.Node var1, AbstractQueuedSynchronizer.Node var2) {
        return unsafe.compareAndSwapObject(var0, nextOffset, var1, var2);
    }

    static {
        try {
            stateOffset = unsafe.objectFieldOffset(AbstractQueuedSynchronizer.class.getDeclaredField("state"));
            headOffset = unsafe.objectFieldOffset(AbstractQueuedSynchronizer.class.getDeclaredField("head"));
            tailOffset = unsafe.objectFieldOffset(AbstractQueuedSynchronizer.class.getDeclaredField("tail"));
            waitStatusOffset = unsafe.objectFieldOffset(AbstractQueuedSynchronizer.Node.class.getDeclaredField("waitStatus"));
            nextOffset = unsafe.objectFieldOffset(AbstractQueuedSynchronizer.Node.class.getDeclaredField("next"));
        } catch (Exception var1) {
            throw new Error(var1);
        }
    }

    public class ConditionObject implements Condition, Serializable {
        private static final long serialVersionUID = 1173984872572414699L;
        private transient AbstractQueuedSynchronizer.Node firstWaiter;
        private transient AbstractQueuedSynchronizer.Node lastWaiter;
        private static final int REINTERRUPT = 1;
        private static final int THROW_IE = -1;

        public ConditionObject() {
        }

        private AbstractQueuedSynchronizer.Node addConditionWaiter() {
            AbstractQueuedSynchronizer.Node var1 = this.lastWaiter;
            if (var1 != null && var1.waitStatus != -2) {
                this.unlinkCancelledWaiters();
                var1 = this.lastWaiter;
            }

            AbstractQueuedSynchronizer.Node var2 = new AbstractQueuedSynchronizer.Node(Thread.currentThread(), -2);
            if (var1 == null) {
                this.firstWaiter = var2;
            } else {
                var1.nextWaiter = var2;
            }

            this.lastWaiter = var2;
            return var2;
        }

        private void doSignal(AbstractQueuedSynchronizer.Node var1) {
            do {
                if ((this.firstWaiter = var1.nextWaiter) == null) {
                    this.lastWaiter = null;
                }

                var1.nextWaiter = null;
            } while(!AbstractQueuedSynchronizer.this.transferForSignal(var1) && (var1 = this.firstWaiter) != null);

        }

        private void doSignalAll(AbstractQueuedSynchronizer.Node var1) {
            this.lastWaiter = this.firstWaiter = null;

            AbstractQueuedSynchronizer.Node var2;
            do {
                var2 = var1.nextWaiter;
                var1.nextWaiter = null;
                AbstractQueuedSynchronizer.this.transferForSignal(var1);
                var1 = var2;
            } while(var2 != null);

        }

        private void unlinkCancelledWaiters() {
            AbstractQueuedSynchronizer.Node var1 = this.firstWaiter;

            AbstractQueuedSynchronizer.Node var3;
            for(AbstractQueuedSynchronizer.Node var2 = null; var1 != null; var1 = var3) {
                var3 = var1.nextWaiter;
                if (var1.waitStatus != -2) {
                    var1.nextWaiter = null;
                    if (var2 == null) {
                        this.firstWaiter = var3;
                    } else {
                        var2.nextWaiter = var3;
                    }

                    if (var3 == null) {
                        this.lastWaiter = var2;
                    }
                } else {
                    var2 = var1;
                }
            }

        }

        public final void signal() {
            if (!AbstractQueuedSynchronizer.this.isHeldExclusively()) {
                throw new IllegalMonitorStateException();
            } else {
                AbstractQueuedSynchronizer.Node var1 = this.firstWaiter;
                if (var1 != null) {
                    this.doSignal(var1);
                }

            }
        }

        public final void signalAll() {
            if (!AbstractQueuedSynchronizer.this.isHeldExclusively()) {
                throw new IllegalMonitorStateException();
            } else {
                AbstractQueuedSynchronizer.Node var1 = this.firstWaiter;
                if (var1 != null) {
                    this.doSignalAll(var1);
                }

            }
        }

        public final void awaitUninterruptibly() {
            AbstractQueuedSynchronizer.Node var1 = this.addConditionWaiter();
            int var2 = AbstractQueuedSynchronizer.this.fullyRelease(var1);
            boolean var3 = false;

            while(!AbstractQueuedSynchronizer.this.isOnSyncQueue(var1)) {
                LockSupport.park(this);
                if (Thread.interrupted()) {
                    var3 = true;
                }
            }

            if (AbstractQueuedSynchronizer.this.acquireQueued(var1, var2) || var3) {
                AbstractQueuedSynchronizer.selfInterrupt();
            }

        }

        private int checkInterruptWhileWaiting(AbstractQueuedSynchronizer.Node var1) {
            return Thread.interrupted() ? (AbstractQueuedSynchronizer.this.transferAfterCancelledWait(var1) ? -1 : 1) : 0;
        }

        private void reportInterruptAfterWait(int var1) throws InterruptedException {
            if (var1 == -1) {
                throw new InterruptedException();
            } else {
                if (var1 == 1) {
                    AbstractQueuedSynchronizer.selfInterrupt();
                }

            }
        }

        public final void await() throws InterruptedException {
            if (Thread.interrupted()) {
                throw new InterruptedException();
            } else {
                AbstractQueuedSynchronizer.Node var1 = this.addConditionWaiter();
                int var2 = AbstractQueuedSynchronizer.this.fullyRelease(var1);
                int var3 = 0;

                while(!AbstractQueuedSynchronizer.this.isOnSyncQueue(var1)) {
                    LockSupport.park(this);
                    if ((var3 = this.checkInterruptWhileWaiting(var1)) != 0) {
                        break;
                    }
                }

                if (AbstractQueuedSynchronizer.this.acquireQueued(var1, var2) && var3 != -1) {
                    var3 = 1;
                }

                if (var1.nextWaiter != null) {
                    this.unlinkCancelledWaiters();
                }

                if (var3 != 0) {
                    this.reportInterruptAfterWait(var3);
                }

            }
        }

        public final long awaitNanos(long var1) throws InterruptedException {
            if (Thread.interrupted()) {
                throw new InterruptedException();
            } else {
                AbstractQueuedSynchronizer.Node var3 = this.addConditionWaiter();
                int var4 = AbstractQueuedSynchronizer.this.fullyRelease(var3);
                long var5 = System.nanoTime() + var1;

                int var7;
                for(var7 = 0; !AbstractQueuedSynchronizer.this.isOnSyncQueue(var3); var1 = var5 - System.nanoTime()) {
                    if (var1 <= 0L) {
                        AbstractQueuedSynchronizer.this.transferAfterCancelledWait(var3);
                        break;
                    }

                    if (var1 >= 1000L) {
                        LockSupport.parkNanos(this, var1);
                    }

                    if ((var7 = this.checkInterruptWhileWaiting(var3)) != 0) {
                        break;
                    }
                }

                if (AbstractQueuedSynchronizer.this.acquireQueued(var3, var4) && var7 != -1) {
                    var7 = 1;
                }

                if (var3.nextWaiter != null) {
                    this.unlinkCancelledWaiters();
                }

                if (var7 != 0) {
                    this.reportInterruptAfterWait(var7);
                }

                return var5 - System.nanoTime();
            }
        }

        public final boolean awaitUntil(Date var1) throws InterruptedException {
            long var2 = var1.getTime();
            if (Thread.interrupted()) {
                throw new InterruptedException();
            } else {
                AbstractQueuedSynchronizer.Node var4 = this.addConditionWaiter();
                int var5 = AbstractQueuedSynchronizer.this.fullyRelease(var4);
                boolean var6 = false;
                int var7 = 0;

                while(!AbstractQueuedSynchronizer.this.isOnSyncQueue(var4)) {
                    if (System.currentTimeMillis() > var2) {
                        var6 = AbstractQueuedSynchronizer.this.transferAfterCancelledWait(var4);
                        break;
                    }

                    LockSupport.parkUntil(this, var2);
                    if ((var7 = this.checkInterruptWhileWaiting(var4)) != 0) {
                        break;
                    }
                }

                if (AbstractQueuedSynchronizer.this.acquireQueued(var4, var5) && var7 != -1) {
                    var7 = 1;
                }

                if (var4.nextWaiter != null) {
                    this.unlinkCancelledWaiters();
                }

                if (var7 != 0) {
                    this.reportInterruptAfterWait(var7);
                }

                return !var6;
            }
        }

        public final boolean await(long var1, TimeUnit var3) throws InterruptedException {
            long var4 = var3.toNanos(var1);
            if (Thread.interrupted()) {
                throw new InterruptedException();
            } else {
                AbstractQueuedSynchronizer.Node var6 = this.addConditionWaiter();
                int var7 = AbstractQueuedSynchronizer.this.fullyRelease(var6);
                long var8 = System.nanoTime() + var4;
                boolean var10 = false;

                int var11;
                for(var11 = 0; !AbstractQueuedSynchronizer.this.isOnSyncQueue(var6); var4 = var8 - System.nanoTime()) {
                    if (var4 <= 0L) {
                        var10 = AbstractQueuedSynchronizer.this.transferAfterCancelledWait(var6);
                        break;
                    }

                    if (var4 >= 1000L) {
                        LockSupport.parkNanos(this, var4);
                    }

                    if ((var11 = this.checkInterruptWhileWaiting(var6)) != 0) {
                        break;
                    }
                }

                if (AbstractQueuedSynchronizer.this.acquireQueued(var6, var7) && var11 != -1) {
                    var11 = 1;
                }

                if (var6.nextWaiter != null) {
                    this.unlinkCancelledWaiters();
                }

                if (var11 != 0) {
                    this.reportInterruptAfterWait(var11);
                }

                return !var10;
            }
        }

        final boolean isOwnedBy(AbstractQueuedSynchronizer var1) {
            return var1 == AbstractQueuedSynchronizer.this;
        }

        protected final boolean hasWaiters() {
            if (!AbstractQueuedSynchronizer.this.isHeldExclusively()) {
                throw new IllegalMonitorStateException();
            } else {
                for(AbstractQueuedSynchronizer.Node var1 = this.firstWaiter; var1 != null; var1 = var1.nextWaiter) {
                    if (var1.waitStatus == -2) {
                        return true;
                    }
                }

                return false;
            }
        }

        protected final int getWaitQueueLength() {
            if (!AbstractQueuedSynchronizer.this.isHeldExclusively()) {
                throw new IllegalMonitorStateException();
            } else {
                int var1 = 0;

                for(AbstractQueuedSynchronizer.Node var2 = this.firstWaiter; var2 != null; var2 = var2.nextWaiter) {
                    if (var2.waitStatus == -2) {
                        ++var1;
                    }
                }

                return var1;
            }
        }

        protected final Collection<Thread> getWaitingThreads() {
            if (!AbstractQueuedSynchronizer.this.isHeldExclusively()) {
                throw new IllegalMonitorStateException();
            } else {
                ArrayList var1 = new ArrayList();

                for(AbstractQueuedSynchronizer.Node var2 = this.firstWaiter; var2 != null; var2 = var2.nextWaiter) {
                    if (var2.waitStatus == -2) {
                        Thread var3 = var2.thread;
                        if (var3 != null) {
                            var1.add(var3);
                        }
                    }
                }

                return var1;
            }
        }
    }

    static final class Node {
        static final AbstractQueuedSynchronizer.Node SHARED = new AbstractQueuedSynchronizer.Node();
        static final AbstractQueuedSynchronizer.Node EXCLUSIVE = null;
        static final int CANCELLED = 1;
        static final int SIGNAL = -1;
        static final int CONDITION = -2;
        static final int PROPAGATE = -3;
        volatile int waitStatus;
        volatile AbstractQueuedSynchronizer.Node prev;
        volatile AbstractQueuedSynchronizer.Node next;
        volatile Thread thread;
        AbstractQueuedSynchronizer.Node nextWaiter;

        final boolean isShared() {
            return this.nextWaiter == SHARED;
        }

        final AbstractQueuedSynchronizer.Node predecessor() throws NullPointerException {
            AbstractQueuedSynchronizer.Node var1 = this.prev;
            if (var1 == null) {
                throw new NullPointerException();
            } else {
                return var1;
            }
        }

        Node() {
        }

        Node(Thread var1, AbstractQueuedSynchronizer.Node var2) {
            this.nextWaiter = var2;
            this.thread = var1;
        }

        Node(Thread var1, int var2) {
            this.waitStatus = var2;
            this.thread = var1;
        }
    }
}

三、ReentrantLock

1.在ReentrantLock内部定义了一个Sync的内部类，该类继承AbstractQueuedSynchronized，对该抽象类的部分方法做了实现；
2.定义了两个子类：
1>FairSync 公平锁的实现
2>NonfairSync 非公平锁的实现
这两个类都继承自Sync，也就是间接继承了AbstractQueuedSynchronized，所以这一个ReentrantLock同时具备公平与非公平特性。

abstract static class Sync extends AbstractQueuedSynchronizer {
        private static final long serialVersionUID = -5179523762034025860L;

        Sync() {
        }

        abstract void lock();

        final boolean nonfairTryAcquire(int var1) {
            Thread var2 = Thread.currentThread();
            int var3 = this.getState();
            if (var3 == 0) {
                if (this.compareAndSetState(0, var1)) {
                    this.setExclusiveOwnerThread(var2);
                    return true;
                }
            } else if (var2 == this.getExclusiveOwnerThread()) {
                int var4 = var3 + var1;
                if (var4 < 0) {
                    throw new Error("Maximum lock count exceeded");
                }

                this.setState(var4);
                return true;
            }

            return false;
        }

        protected final boolean tryRelease(int var1) {
            int var2 = this.getState() - var1;
            if (Thread.currentThread() != this.getExclusiveOwnerThread()) {
                throw new IllegalMonitorStateException();
            } else {
                boolean var3 = false;
                if (var2 == 0) {
                    var3 = true;
                    this.setExclusiveOwnerThread((Thread)null);
                }

                this.setState(var2);
                return var3;
            }
        }

        protected final boolean isHeldExclusively() {
            return this.getExclusiveOwnerThread() == Thread.currentThread();
        }

        final ConditionObject newCondition() {
            return new ConditionObject(this);
        }

        final Thread getOwner() {
            return this.getState() == 0 ? null : this.getExclusiveOwnerThread();
        }

        final int getHoldCount() {
            return this.isHeldExclusively() ? this.getState() : 0;
        }

        final boolean isLocked() {
            return this.getState() != 0;
        }

        private void readObject(ObjectInputStream var1) throws IOException, ClassNotFoundException {
            var1.defaultReadObject();
            this.setState(0);
        }
    }

 static final class FairSync extends ReentrantLock.Sync {
        private static final long serialVersionUID = -3000897897090466540L;

        FairSync() {
        }

        final void lock() {
            this.acquire(1);
        }

        protected final boolean tryAcquire(int var1) {
            Thread var2 = Thread.currentThread();
            int var3 = this.getState();
            if (var3 == 0) {
                if (!this.hasQueuedPredecessors() && this.compareAndSetState(0, var1)) {
                    this.setExclusiveOwnerThread(var2);
                    return true;
                }
            } else if (var2 == this.getExclusiveOwnerThread()) {
                int var4 = var3 + var1;
                if (var4 < 0) {
                    throw new Error("Maximum lock count exceeded");
                }

                this.setState(var4);
                return true;
            }

            return false;
        }
    }

    static final class NonfairSync extends ReentrantLock.Sync {
        private static final long serialVersionUID = 7316153563782823691L;

        NonfairSync() {
        }

        final void lock() {
            if (this.compareAndSetState(0, 1)) {
                this.setExclusiveOwnerThread(Thread.currentThread());
            } else {
                this.acquire(1);
            }

        }

        protected final boolean tryAcquire(int var1) {
            return this.nonfairTryAcquire(var1);
        }
    }