ReenTrantLock

一个例子

public class Demo1 {
    private static final Lock lock = new ReentrantLock();// 默认非公平
    public static void main(String[] args) {
        new Thread(()-> {
           test();
        },"test-1").start();

        new Thread(()-> {
            test();
        },"test-2").start();


    }

    static void test() {
        lock.lock();
        try {
            System.out.println(Thread.currentThread().getName() + "获得锁");
            try {
                TimeUnit.SECONDS.sleep(2);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }finally {
            lock.unlock();
            System.out.println(Thread.currentThread().getName() + "释放锁");
        }
    }
}

2. 调用时序图

image-20200518170412300.png

3. lock.lock()代码分析

  • ReentrantLock.java
public void lock() {
  // 这里是获取锁的入口
    sync.lock();
}
  • NonfairSync 是ReentrantLock的内部类
final void lock() {
    // cas尝试能不能获取到锁,获取成功之后设置state为1
    if (compareAndSetState(0, 1))
        // 成功获取之后设置锁的owner是当前线程
        setExclusiveOwnerThread(Thread.currentThread());
    else
        // 没有获取到锁去排队
        acquire(1);
}

  • AbstractQueuedSynchronizer.java
// cas操作尝试获取锁  
protected final boolean compareAndSetState(int expect, int update) {
        // See below for intrinsics setup to support this
        return unsafe.compareAndSwapInt(this, stateOffset, expect, update);
    }
  • AbstractQueuedSynchronizer.java
// 竞争锁失败去排队
public final void acquire(int arg) {
  // 先尝试一下获取锁,获取锁失败之后走&&后面的逻辑;公平锁和非公平锁的tryAcquire实现不同
        if (!tryAcquire(arg) &&
            acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
            selfInterrupt();// 如果线程在被挂起的过程中被外部中断了,被唤醒之后响应中断的操作
    }
  • NonfairSync.java
// 非公平锁中的实现
 protected final boolean tryAcquire(int acquires) {
            return nonfairTryAcquire(acquires);// 走默认实现
        }
  • Sync.java
//非公平尝试获得锁
       final boolean nonfairTryAcquire(int acquires) {
           final Thread current = Thread.currentThread();
           int c = getState();//获取一下当前锁的状态,万一持有锁的线程已经释放了呢
           if (c == 0) {
               if (compareAndSetState(0, acquires)) {//cas一下,尝试获得锁
                   setExclusiveOwnerThread(current);//成功,设置锁的owner是自己
                   return true;
               }
           }
         // 如果锁已经被持有,看一下是不是自己
           else if (current == getExclusiveOwnerThread()) {
               int nextc = c + acquires; //重入次数增加
               if (nextc < 0) // overflow ??为什么会出现负值
                   throw new Error("Maximum lock count exceeded");
               setState(nextc); // 设置state
               return true;
           }
           return false;  //获得锁失败
       }

  • AbstractQueuedSynchronizer.java
// 把每获得锁的线程封装为一个节点    mode此时=null
private Node addWaiter(Node mode) {
            // 生成一个节点,next为null
        Node node = new Node(Thread.currentThread(), mode);
        // Try the fast path of enq; backup to full enq on failure
        Node pred = tail;
        if (pred != null) {
            node.prev = pred;
          // 这里cas操作是为了原子性操作去设置tail
            if (compareAndSetTail(pred, node)) {
                pred.next = node;
                return node;
            }
        }
            // 生成wait队列的实现
        enq(node);
            // node总是被插入到最后,返回自己
        return node;
    }
  • AbstractQueuedSynchronizer.java
// 生成一个链
private Node enq(final Node node) {
  // 自旋生成链表
        for (;;) {
            Node t = tail;
            if (t == null) { // Must initialize
              // 这里会存在并发,当前线程如果设置head失败,说明有其他线程在设置,自旋操作后会走到else分支
                if (compareAndSetHead(new Node()))
                    tail = head;
            } else {
                node.prev = t;
              // 这里会存在并发,设置当前节点为尾节点,如果设置失败,说明其他线程在设置,自旋后重新尝试设置
                if (compareAndSetTail(t, node)) {
                    t.next = node;
                    return t;
                }
            }
        }
    }
  • AbstractQueuedSynchronizer.java
// 把当前线程挂起,设置前一个节点的waitStatus=-1
// node是封装的当前线程的节点
final boolean acquireQueued(final Node node, int arg) {
        boolean failed = true;
        try {
            boolean interrupted = false;
            for (;;) {
                final Node p = node.predecessor();
              // head的下一个节点总是被限唤醒,因此被唤醒的线程继续cas时,p是head节点
                if (p == head && tryAcquire(arg)) { // unlock之后才走这里,lock时不走这
                  //tryAcquire此时还是会去和新线程竞争,如果没获取到,执行下面的逻辑继续等待
                    setHead(node);// 把自己设置成head节点
                    p.next = null; // help GC
                    failed = false;
                    return interrupted;
                }
              // shouldParkAfterFailedAcquire判断是否要挂起当前线程
              // parkAndCheckInterrupt 挂起当前线程
              // 这两个方法执行完之后,线程被挂起,不再继续执行,线程被唤醒之后从这往下继续执行
                if (shouldParkAfterFailedAcquire(p, node) &&
                    parkAndCheckInterrupt())
                    interrupted = true;
              
            }
        } finally {
            if (failed)
                cancelAcquire(node);
        }
    }
  • AbstractQueuedSynchronizer.java
private final boolean parkAndCheckInterrupt() {
        LockSupport.park(this);// 这里执行完之后,线程就会被挂起,不再继续往下执行
  // 复位并返回中当前断状态;当前有可能被外部线程中断,但是自己此时是无法响应的,需要先保存一下中断状态
        return Thread.interrupted();
    }

至此,获取不到锁的线程会形成一条链表。


image-20200518223919049.png

4. lock.unlock()代码分析

  • ReentrantLock
public void unlock() {
    //  释放锁的入口
        sync.release(1);
    }
  • AbstractQueuedSynchronizer
   public final boolean release(int arg) {
        if (tryRelease(arg)) { // 尝试释放锁,释放成功(state=0)才继续往下执行
            Node h = head;
            if (h != null && h.waitStatus != 0)
                unparkSuccessor(h);
            return true;
        }
        return false;
    }
  • Sync
protected final boolean tryRelease(int releases) {
    int c = getState() - releases; // 执行一次unlock就减1
    if (Thread.currentThread() != getExclusiveOwnerThread())
        throw new IllegalMonitorStateException();
    boolean free = false;
    if (c == 0) {
        free = true;
        setExclusiveOwnerThread(null); // 释放成功,设置锁的owner为空
    }
    setState(c);
    return free; //多次获取锁,没有释放完返回false
}
  • AbstractQueuedSynchronizer
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)
            compareAndSetWaitStatus(node, 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; //找到头节点的下一个节点
        if (s == null || s.waitStatus > 0) {
            s = null;
            for (Node t = tail; t != null && t != node; t = t.prev)
                if (t.waitStatus <= 0)
                    s = t;
        }
        if (s != null)
            LockSupport.unpark(s.thread); //把这个线程唤醒去继续执行
    }

5. 类图

image.png

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