Semaphore vs Mutex

Semaphore 信号灯，用来控制同时并发的线程数量
Mutex 互斥锁，控制同一时刻只能有一个线程执行

二者最重要的区别在于所有权这个概念，Mutex 具有所有权，它本身是一个互斥锁，在一个时刻只能被一个线程持有，但是Semaphore没有所有权，任何线程都可以调用此对象的方法。

Semaphore 控制3条线程并发执行

final int clientCount = 3;
final int totalRequestCount = 10;
Semaphore semaphore = new Semaphore(clientCount);
ExecutorService executorService = Executors.newCachedThreadPool();
for (int i = 0; i < totalRequestCount; i++) {
    executorService.execute(()->{
        try {
            semaphore.acquire();
            System.out.println("thread start do something :" + Thread.currentThread() + " :" + semaphore.availablePermits() + " ");
            Thread.sleep(1000);
            System.out.println("thread finish do something :" + Thread.currentThread() + " :" + semaphore.availablePermits() + " ");
        } catch (InterruptedException e) {
            e.printStackTrace();
        } finally {
            semaphore.release();
        }
    });
}
executorService.shutdown();

打印：

thread start do something :Thread[pool-1-thread-1,5,main] :0 
thread start do something :Thread[pool-1-thread-3,5,main] :0 
thread start do something :Thread[pool-1-thread-2,5,main] :0 
thread finish do something :Thread[pool-1-thread-1,5,main] :0 
thread finish do something :Thread[pool-1-thread-2,5,main] :0 
thread finish do something :Thread[pool-1-thread-3,5,main] :1 
thread start do something :Thread[pool-1-thread-5,5,main] :1 
thread start do something :Thread[pool-1-thread-4,5,main] :1 
thread start do something :Thread[pool-1-thread-7,5,main] :0 
...

Mutex 控制1条线程并发执行

final int totalRequestCount = 10;
final ReentrantLock mutex = new ReentrantLock();
ExecutorService executorService = Executors.newCachedThreadPool();
for (int i = 0; i < totalRequestCount; i++) {
    executorService.execute(()->{
        try {
            mutex.lock();
            System.out.println("thread start do something :" + Thread.currentThread());
            Thread.sleep(1000);
            System.out.println("thread finish do something :" + Thread.currentThread());
        } catch (InterruptedException e) {
            e.printStackTrace();
        } finally {
            mutex.unlock();
        }
    });
}
executorService.shutdown();

打印：

thread start do something :Thread[pool-1-thread-1,5,main]
thread finish do something :Thread[pool-1-thread-1,5,main]
thread start do something :Thread[pool-1-thread-2,5,main]
thread finish do something :Thread[pool-1-thread-2,5,main]
thread start do something :Thread[pool-1-thread-3,5,main]
thread finish do something :Thread[pool-1-thread-3,5,main]
thread start do something :Thread[pool-1-thread-4,5,main]
thread finish do something :Thread[pool-1-thread-4,5,main]
thread start do something :Thread[pool-1-thread-5,5,main]
thread finish do something :Thread[pool-1-thread-5,5,main]
...