Semaphore is also an auxiliary class for thread synchronization, which can maintain the number of threads currently accessing itself and provide a synchronization mechanism. Use Semaphore to control the number of threads that access resources at the same time, for example, to achieve the number of concurrent accesses allowed for a file.

Summary of Semaphore's main methods:

void acquire(): Acquire a permit from this semaphore, and block the thread until a permit is provided, otherwise the thread will be interrupted.

void release(): Release a permit and return it to the semaphore.

Int availablePermits(): Returns the number of permits currently available in this semaphore.

boolean hasQueuedThreads(): Query whether there are threads waiting to be acquired.

 1 package com.thread;

 2 

 3 import java.util.concurrent.ExecutorService;

 4 import java.util.concurrent.Executors;

 5 import java.util.concurrent.Semaphore;

 6 

 7 public class SemaphoreTest {

 8     public static void main(String[] args) {

 9         ExecutorService service = Executors.newCachedThreadPool();

10         final  Semaphore sp = new Semaphore(3);//创建Semaphore信号量，初始化许可大小为3

11         for(int i=0;i<10;i++){

12             try {

13                 Thread.sleep(100);

14             } catch (InterruptedException e2) {

15                 e2.printStackTrace();

16             }

17             Runnable runnable = new Runnable(){

18                     public void run(){

19                     try {

20                         sp.acquire();//请求获得许可，如果有可获得的许可则继续往下执行，许可数减1。否则进入阻塞状态

21                     } catch (InterruptedException e1) {

22                         e1.printStackTrace();

23                     }

24                     System.out.println("线程" + Thread.currentThread().getName() + 

25                             "进入，当前已有" + (3-sp.availablePermits()) + "个并发");

26                     try {

27                         Thread.sleep((long)(Math.random()*10000));

28                     } catch (InterruptedException e) {

29                         e.printStackTrace();

30                     }

31                     System.out.println("线程" + Thread.currentThread().getName() + 

32                             "即将离开");                    

33                     sp.release();//释放许可，许可数加1

34                     //下面代码有时候执行不准确，因为其没有和上面的代码合成原子单元

35                     System.out.println("线程" + Thread.currentThread().getName() + 

36                             "已离开，当前已有" + (3-sp.availablePermits()) + "个并发");                    

37                 }

38             };

39             service.execute(runnable);            

40         }

41     }

42 

43 }