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ReentrantLock源码笔记 - 释放锁(JDK 1.8)

ReentrantLock源码学习 - 释放锁(unlock)


上次谈到了利用ReentrantLock的非公平和公平加锁方式,那么接下来看看释放锁的流程

首先调用ReentrantLock的unlock方法

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public void unlock() {
sync.release(1);
}

然后会调用AbstractQueuedSynchronizer(AQS)的release方法,在这个方法中首先会调用ReentrantLock的Sync的tryRelease方法,来进行尝试释放锁,如果返回true,那么获取CLH队列的头结点,判断头结点不为空并且头结点的状态不为0(None),那么就调用AQS的unparkSuccessor方法。

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public final boolean release(int arg) {
if (tryRelease(arg)) {
Node h = head;
if (h != null && h.waitStatus != 0)
unparkSuccessor(h);
return true;
}
return false;
}

在tryRelease方法里,首先让当前的state与传入的值(这里为1)进行相减,然后得到c,判断当前线程是不是获取独占锁的线程,如果不是,直接抛出异常;如果是,那么需要判断c是否为0,因为只有c为0时,才符合释放独占锁的条件,这是设置独占锁线程为null,最后设置下state的值(注意这里c为0不为0都会设置)

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protected final boolean tryRelease(int releases) {
int c = getState() - releases;
if (Thread.currentThread() != getExclusiveOwnerThread())
throw new IllegalMonitorStateException();
boolean free = false;
if (c == 0) {
free = true;
setExclusiveOwnerThread(null);
}
setState(c);
return free;
}

接下来来看方法unparkSuccessor,该方法的作用就是为了释放node节点的后继结点。

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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.
*/
// 获取节点的状态
int ws = node.waitStatus;
if (ws < 0)
compareAndSetWaitStatus(node, ws, 0); // 利用CAS 将状态设置为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.
*/
// 获取节点的后继节点
Node s = node.next;
// 判断后继节点是否为空 或者 后者后继节点的状态为CANCELLED
if (s == null || s.waitStatus > 0) {
s = null; // 将后继节点置为null
// 从尾节点从后向前开始遍历知道节点为空或者当前节点为止
for (Node t = tail; t != null && t != node; t = t.prev)
if (t.waitStatus <= 0) // 如果此时节点的状态小于等于0
s = t; // 将此节点赋给传入节点的后继节点
}
if (s != null) // 节点不为空,释放
LockSupport.unpark(s.thread);
}

参考:

http://blog.csdn.net/luonanqin/article/details/41871909