ReentrantLock介绍

ReentrantLock 基于AQS实现了公平和非公平的独占锁功能。

ReentrantLock定义AQS的同步状态（synchronization state）如下：

State为0表示锁可用；为1表示被占用；为N表示锁重入的次数，是独占资源。

ReentrantLock实现公平锁原理

案例代码如下：

1、启动文件

public class Main {public static void main(String[] args) throws ParseException {ReentrantLock lock = new ReentrantLock(true);Thread t1 = new Thread(new Task(lock),"Thread-1");Thread t2 = new Thread(new Task(lock),"Thread-2");Thread t3 = new Thread(new Task(lock),"Thread-3");t1.start();t2.start();t3.start();}
}

2、Task

import java.util.concurrent.locks.ReentrantLock;public class Task implements Runnable{private ReentrantLock lock;public Task(ReentrantLock lock) {this.lock = lock;}@Overridepublic void run() {try {lock.lock();System.out.println(Thread.currentThread().getName() + "获取到锁....");Thread.sleep(2000);} catch (InterruptedException e) {e.printStackTrace();} finally {System.out.println(Thread.currentThread().getName() + "释放锁....");lock.unlock();}}
}

3、执行结果：

案例分析

1、Thread-1调用lock方法

ReentrantLock内部继承AQS实现了1个抽象类Sync

继承于Sync实现了2个内部类FairSync（公平的）和NonfairSync（非公平的）

此时调用FairSync的lock方法

acquire方法来自AQS，注意参数是1

tryAcquire是ReentrantLock自己实现的，尝试获取锁

protected final boolean tryAcquire(int acquires) { //参数是1final Thread current = Thread.currentThread(); //当前线程int c = getState(); //获取当前同步状态if (c == 0) { //如果是0，则锁没有被占用//等待队列中，前面没有其他等待的线程，则用CAS的方法更新同步状态stateif (!hasQueuedPredecessors() &&compareAndSetState(0, acquires)) {setExclusiveOwnerThread(current); //成功的话，则设置锁的占有线程为当前线程return true; //返回获取资源成功}}//如果锁已经被占用，则判断是不是自己占用的else if (current == getExclusiveOwnerThread()) {int nextc = c + acquires;//如果是自己占用的，则是重入，增加state值，累加1if (nextc < 0) //重入次数过大，抛出异常throw new Error("Maximum lock count exceeded");setState(nextc); //设置state值return true; //重入返回ture}return false;//没有获取资源返回false}

Thread-1获取了锁资源，没有释放。

2、Thread-2，开始请求资源，调用lock，此时锁资源还被Thread-1占用

addWaiter方法：

private Node addWaiter(Node mode) {//把当前线程包装成节点，准备放入等待队列Node node = new Node(Thread.currentThread(), mode);// Try the fast path of enq; backup to full enq on failure//尝试直接把节点设置成队尾，否则执行enqNode pred = tail;if (pred != null) {node.prev = pred;//当前节点的上一个节点是之前的队尾节点if (compareAndSetTail(pred, node)) {pred.next = node;return node;}}//当前节点插入队尾enq(node);return node;}

enq自旋+初始化等待队列，并返回Thread-2节点

private Node enq(final Node node) {//采用自旋，保证节点插入for (;;) {Node t = tail;if (t == null) { // Must initialize 如果队列为空，则创建一个空的节点，设置为头尾节点if (compareAndSetHead(new Node()))tail = head;} else {node.prev = t;if (compareAndSetTail(t, node)) { //队列不为空，追加到队尾t.next = node;return t;}}}}

然后对Thread-2包装节点执行acquireQueued

final boolean acquireQueued(final Node node, int arg) {boolean failed = true;try {boolean interrupted = false;for (;;) {//判断节点的前任节点是不是头节点，头节点是一个空节点final Node p = node.predecessor();//如果是头节点，则说明当前节点是队列里的第一个节点，首节点。//则尝试获取锁资源，此处因为Thread-1占用着资源，则失败if (p == head && tryAcquire(arg)) {setHead(node);p.next = null; // help GCfailed = false;return interrupted;}//失败之后，则判断当前节点线程Thread-2是不是可以阻塞if (shouldParkAfterFailedAcquire(p, node) &&parkAndCheckInterrupt())interrupted = true;}} finally {if (failed)cancelAcquire(node);}}

是否阻塞shouldParkAfterFailedAcquire

private static boolean shouldParkAfterFailedAcquire(Node pred, Node node) {int ws = pred.waitStatus;//前驱节点的状态if (ws == Node.SIGNAL) //如果是SIGNAL，则说明前驱节点状态可以唤醒后继节点，可以阻塞/** 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; //只有CANCELLED状态大于0，则把取消状态的节点从队列删除} 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.*/compareAndSetWaitStatus(pred, ws, Node.SIGNAL);//设置前驱节点为SIGNAL状态}return false;}

如果可以阻塞，则调用parkAndCheckInterrupt，阻塞线程，至此Thread-2进入队列，并阻塞了，耐心等待

3、Thread-3同Thread-2，略过

4、Thread-1释放锁资源

release方法：

tryRelease

然后通过unparkSuccessor，唤醒首节点，保证公平策略。

至此Thread-1释放完成，Thread-2可以获得资源，依次类推。