共享锁

共享锁和独占锁

• 共享锁：允许多个线程同时获取锁，如Semaphore，CountDownLatch，ReadLock等。
• 独占锁：每次只能一个线程持有锁，如ReentrantLock，synchronized，WriteLock等。

AQS中实现共享锁

AQS中提供了独占锁和共享锁。独占锁使用acquire和release方法实现；共享锁使用acquireShared和releaseShared实现。下面我们看看源码中共享锁具体怎么实现的，以Semaphore和CountDownLatch为例

Semaphore

Semaphore（信号量）：控制同时访问的线程个数。可以设置一个阈值，阈值内的线程可以同时获取锁，超过阈值的线程则需要等待锁释放，有点类似于线程池。

举例：公司有五名员工要去上厕所，但是厕所只有三个坑位，则另外两个人需要等待前面的人上完以后才能进，代码如下

public static void main(String[] args) {
    ThreadFactory threadFactory = new ThreadNameFactory();
    ThreadPoolExecutor threadPoolExecutor = new ThreadPoolExecutor(10, 10, 5, TimeUnit.SECONDS, new ArrayBlockingQueue<>(10),threadFactory);
    //公司里厕所有3个坑位
    Semaphore semaphore = new Semaphore(3);
    //5个人去拉粑粑
    for (int i = 0; i < 5; i++) {
        People people = new People(i);
        threadPoolExecutor.execute(()->{
            try {
                System.out.println(people.getName() + "准备去上厕所");
                semaphore.acquire();
                System.out.println(people.getName() + "找到坑位，开始拉粑粑");
                Thread.sleep(3000);//模拟拉粑粑
                System.out.println(people.getName() + "拉完粑粑，准备出厕所");
                semaphore.release();
                System.out.println(people.getName() + "走出厕所");
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        });
    }
    threadPoolExecutor.shutdown();
}

Semaphore通过new Semaphore(3)设置阈值

public Semaphore(int permits) {
    sync = new NonfairSync(permits);
}

这里可以看出信号量也是支持公平锁的，默认非公平锁。acquire()获取锁，此方法调用的是AQS的tryAcquireShared方法，然后在自己NonfairSync中实现此方法。

static final class NonfairSync extends Sync {
    ......
    protected int tryAcquireShared(int acquires) {
        return nonfairTryAcquireShared(acquires);
    }
}
final int nonfairTryAcquireShared(int acquires) {
    for (;;) {
        int available = getState();
        int remaining = available - acquires;
        if (remaining < 0 ||
            compareAndSetState(available, remaining))
            return remaining;
    }
}

release()释放锁，此方法调用的是AQS的tryReleaseShared方法，然后在自己NonfairSync中实现此方法。

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;
    }
}

如果通过new Semaphore(1)这样来初始化信号量，这就相当于独占锁，和ReentrantLock作用类似。

CountDownLatch

CountDownLatch（线程计数器）：一个线程等待其他线程多个线程执行完再一起执行。使用场景：主线程依赖其他子线程的数据。异步转同步操作。

举例：计算阿里接口返回的数据和腾讯接口返回数据的总和。代码如下

public static void main(String[] args) throws Exception{
    CountDownLatch countDownLatch = new CountDownLatch(2);
    ThreadPoolExecutor threadPoolExecutor = new ThreadPoolExecutor(10, 10, 5, TimeUnit.SECONDS, new ArrayBlockingQueue<>(10), new ThreadNameFactory());
    final Integer[] ints = new Integer[2];
    threadPoolExecutor.execute(()->{
        try {
            System.out.println("请求阿里接口");
            Thread.sleep(2000);
            System.out.println("请求阿里接口结束");
            ints[0] = 10;
        } catch (InterruptedException e) {
            e.printStackTrace();
        } finally {
            countDownLatch.countDown();
        }
    });
    threadPoolExecutor.execute(()->{
        try {
            System.out.println("请求腾讯接口");
            Thread.sleep(3000);
            System.out.println("请求腾讯接口结束");
            ints[1] = 20;
        } catch (InterruptedException e) {
            e.printStackTrace();
        } finally {
            countDownLatch.countDown();
        }
    });
    System.out.println("等待两个接口请求完");
    countDownLatch.await();
    System.out.println("两个接口都已请求完");
    int result = ints[0] + ints[1];
    System.out.println(result);
    threadPoolExecutor.shutdown();
}

new CountDownLatch(int count)当需要几个子线程加入主线程时就传几个，如果传多了，主线程会一直等待。CountDownLatch也是通过AQS来实现的，new CountDownLatch(int count)初始化方法设置AQS中的state的初始值，每调用countDown()方法，state都会减1，当主线程调用await()方法时，会等待state等于0时才会继续往下执行。

总结

共享锁允许多个线程同时获取锁，并发访问共享资源，提高效率。

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