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。