前言
保证线程安全的方式有很多，比如CAS操作、synchronized、原子类、volatile保证可见性和ReentrantLock等，这篇文章我们主要探讨ReentrantLock的相关内容。本文基于JDK1.8讲述ReentrantLock.
一、可重入锁
所谓可重入锁，即一个线程已经获得了某个锁，当这个线程要再次获取这个锁时，依然可以获取成功，不会发生死锁的情况。synchronized就是一个可重入锁，除此之外，JDK提供的ReentrantLock也是一种可重入锁。
二、ReentrantLock
2.1 ReentrantLock的简单使用
public class TestReentrantLock {
private static int i = 0;
public static void main(String[] args) {
ReentrantLock lock = new ReentrantLock();
try {
lock.lock();
i++;
} finally {
lock.unlock();
}
System.out.println(i);
}
}
上面是ReentrantLock的一个简单使用案列，进入同步代码块之前，需要调用lock()方法进行加锁，执行完同步代码块之后，为了防止异常发生时造成死锁，需要在finally块中调用unlock()方法进行解锁。
2.2 ReentrantLock UML图
2.3 lock()方法调用链
上图描述了ReentrantLock.lock()加锁的方法调用过程。在ReentrantLock中有一个成员变量private final Sync sync，Sync是AQS的一个子类。ReentrantLock的lock()方法中，调用了sync的lock()方法，这个方法为抽象方法，具体调用的是NonfairSync中实现的lock()方法：
/**
* Performs lock. Try immediate barge, backing up to normal
* acquire on failure.
*/
final void lock() {
if (compareAndSetState(0, 1))
setExclusiveOwnerThread(Thread.currentThread());
else
acquire(1);
}
在这个方法中，先尝试通过CAS操作进行加锁。如果加锁失败，会调用AQS的acquire()方法：
/**
* 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.
*/
public final void acquire(int arg) {
if (!tryAcquire(arg) &&
acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
selfInterrupt();
}
在AQS的acquire方法中，先尝试调用tryAcquire方法进行加锁，如果失败，会调用acquireQueued进入等待队列当中。acquireQueued方法将会在第三章中讲解，先来看tryAcquire方法的内容。AQS的tryAcquire方法是一个模板方法，其具体实现在NonfairSync的tryAcquire方法中，里面仅仅是调用了nonfairTryAcquire方法：
/**
* Performs non-fair tryLock. tryAcquire is implemented in
* subclasses, but both need nonfair try for trylock method.
*/
final boolean nonfairTryAcquire(int acquires) {
final T<mark>本文来源gaodaimacom搞#代%码@网-</mark>hread current = Thread.currentThread();
int c = getState();
if (c == 0) {
if (compareAndSetState(0, acquires)) {
setExclusiveOwnerThread(current);
return true;
}
}
else if (current == getExclusiveOwnerThread()) {
int nextc = c + acquires;
if (nextc < 0) // overflow
throw new Error("Maximum lock count exceeded");
setState(nextc);
return true;
}
return false;
}
转载请注明原文链接：Java多线程之深入理解ReentrantLock
