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;
}
@Override
public 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1
final 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state
if (!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1
if (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enq
Node 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 GC
failed = 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;
}