reentrantlock-unlock

unlock是不分公平锁和非公平锁

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

public final boolean release(int arg) {
    if(tryRelease(arg)) {
        Node h = head;
        if(h != null && h.waitStatus != 0) {
            unparkSuccessor(h);
        }
        return true;
    }
    return false;
}
=====
//release(1) 尝试在当前锁的锁定计数state值上减1，成功返回true，否则为false。如果-1之后新的state=0，则表示当前锁已经被线程释放了，同时会唤醒线程等待队列中的下一个线程，当然该锁不一定就一定会把所有权交给下一个线程。
protected final boolean tryRelease(int releases) {
    int c = getState() - release;
    if(Thread.currentThread() != getExclusiveOwnerThread()) {
        throw new IllegalMonitorStateException();
    } 
    boolean free = false;
    if(c == 0) {
        free = true;
        setExclusiveOwnerThread(null);
    }
    setState(c);
    return free;
}
======
private void unparkSuccessor(Node node) {
    int ws = node.waitStatus;
    if(ws < 0) {
        compareAndSetWaitStatus(node, ws, 0);
    }
    Node s = node.next;
    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) {
        LockSupport.unpark(s.thread);
    }
}

unlock最好放在finally中