1. 背景
2. 介绍
可重入锁ReentrantLock，支持可重入的锁。表示同一个线程可对资源重复加锁。
2.1 特性
- 支持公平锁和非公平锁，默认是非公平锁。公平锁的存在是减少“饥饿”发生的概率，等待越久的请求优先满足；
3. 源码
3.1 线程如何重复获取锁及最终的释放（以非公平锁为例）
final boolean nonfairTryAcquire(int acquires) {final Thread current = Thread.currentThread();int c = getState();if (c == 0) {if (compareAndSetState(0, acquires)) {// 如果当前线程获取到锁，则更新setExclusiveOwnerThread(current);return true;}}else if (current == getExclusiveOwnerThread()) {// 判断是否当前线程int nextc = c + acquires;if (nextc < 0) // overflowthrow new Error("Maximum lock count exceeded");setState(nextc);return true;}return false;
}3.2 线程如何保证锁的最终释放（以非公平锁为例）
protected final boolean tryRelease(int releases) {int c = getState() - releases;// 如果不是当前线程，没有释放锁的权利if (Thread.currentThread() != getExclusiveOwnerThread())throw new IllegalMonitorStateException();boolean free = false;if (c == 0) {free = true;// 最终释放锁setExclusiveOwnerThread(null);}setState(c);return free;
}3.3 公平锁如何实现
（1）
protected final boolean tryAcquire(int acquires) {final Thread current = Thread.currentThread();int c = getState();if (c == 0) {// 加入了同步队列中，当前节点是否有前驱节点，有就返回true，代表有线程比当前线程更早地请求获取锁if (!hasQueuedPredecessors() &&compareAndSetState(0, acquires)) {setExclusiveOwnerThread(current);return true;}}else if (current == getExclusiveOwnerThread()) {int nextc = c + acquires;if (nextc < 0)throw new Error("Maximum lock count exceeded");setState(nextc);return true;}return false;
}（2）
- 当队列只有一个节点或者无节点时，无需排队，返回false （第一个节点是不参与排队的，它本身是持有同步状态的）
- 第二个节点是当前线程本身，返回false
public final boolean hasQueuedPredecessors() {// The correctness of this depends on head being initialized// before tail and on head.next being accurate if the current// thread is first in queue.Node t = tail; // Read fields in reverse initialization orderNode h = head;Node s;return h != t &&((s = h.next) == null || s.thread != Thread.currentThread());
}
4. 实战
5. FAQ
5.1 同步队列中不是每个节点都有机会争抢到CPU资源吗？