一:代码
reentrantLock.unlock();
虽然只有一句,但是源码却比较多:
public final boolean release(int arg) { if (tryRelease(arg)) { Node h = head; if (h != null && h.waitStatus != 0) unparkSuccessor(h); return true; } return false; }
这个方法中各个方法意义:
1、tryRelease(arg),尝试去解锁,通过改变state的值来释放锁,如果state的值变成了0,那么返回true,则锁释放完成,否则返回false;
2、unparkSuccessor,如果继任的线程节点存在,就去唤醒这个继任的节点。
二、首先调用的是sync类下的tryRelease方法
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; }
首先拿到重入的次数,释放一次重入就减少一次,只有重入的次数减少到0时,才返回true。
三、unparkSuccessor源码:
/** * Wakes up node‘s successor, if one exists. * * @param node the node */ 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; // 获取头节点的waitStatus if (ws < 0) // 如果头节点的ws状态与预期的一样,就把头节点的ws设置为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; // 如果头节点的继任节点时空,或者被取消了,那么就不会有节点挂在这个继任节点下面, // 那么就从尾部一直往前追溯,直到t.waitStatus <= 0 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) // 释放锁,令牌permit被下一个线程节点拿到。 LockSupport.unpark(s.thread); }