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ReentrantLock：lock state 0-> 1成功则获取锁，否则阻塞，获取锁的线程unlock state 1 -> 0，唤醒其他线程

CountDownLatch：awit，获取共享锁，如果state！=0，没获得锁，阻塞等待，countdown释放锁，state-1，如果state-1成功，判断如果state==0，则释放锁，唤醒阻塞线程

    public void await() throws InterruptedException {
        sync.acquireSharedInterruptibly(1);
    }

       protected int tryAcquireShared(int acquires) {
            return (getState() == 0) ? 1 : -1;
        }

protected boolean tryReleaseShared(int releases) {
    // Decrement count; signal when transition to zero
    for (;;) {
        int c = getState();
        if (c == 0)
            return false;
        int nextc = c-1;
        // 减一不成功，则再次循环
        if (compareAndSetState(c, nextc))
            return nextc == 0;
    }
}

Semphore：acquire获取共享锁，state-1，如果<=0，没获得锁，则阻塞

public void acquire(int permits) throws InterruptedException {
    if (permits < 0) throw new IllegalArgumentException();
    sync.acquireSharedInterruptibly(permits);
}

   public boolean tryAcquire(int permits) {
        if (permits < 0) throw new IllegalArgumentException();
        return sync.nonfairTryAcquireShared(permits) >= 0;
    }

       final int nonfairTryAcquireShared(int acquires) {
            for (;;) {
                int available = getState();
                int remaining = available - acquires;
                if (remaining < 0 ||
                    compareAndSetState(available, remaining))
                    return remaining;
            }
        }

				// 释放锁，不需要由获得锁的线程释放
        protected final boolean tryReleaseShared(int releases) {
            for (;;) {
                int current = getState();
                int next = current + releases;
                if (next < current) // overflow
                    throw new Error("Maximum permit count exceeded");
                if (compareAndSetState(current, next))
                    return true;
            }
        }

读锁又称为共享锁，简称S锁，顾名思义，共享锁就是多个事务对于同一数据可以共享一把锁，都能访问到数据，但是只能读不能修改。select … lock in share mode

写锁又称为排他锁，简称X锁，顾名思义，排他锁就是不能与其他所并存，如一个事务获取了一个数据行的排他锁，其他事务就不能再获取该行的其他锁，包括共享锁和排他锁，但是获取排他锁的事务是可以对数据就行读取和修改。

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