Semaphore维护当前访问自身的线程个数

import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Semaphore;

public class SemaphoreTest {
public static void main(String[] args) {
ExecutorService service = Executors.newCachedThreadPool();
final  Semaphore sp = new Semaphore(3);//创建Semaphore信号量，初始化许可大小为3
Runnable runnable = new Runnable(){
public void run(){
try {
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
} catch (InterruptedException e1) {
e1.printStackTrace();
}
System.out.println("线程" + Thread.currentThread().getName() +
"进入，当前已有" + (3-sp.availablePermits()) + "个并发");
try {
Thread.sleep((long)(Math.random()*10000));
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println("线程" + Thread.currentThread().getName() +
"即将离开");
sp.release();//释放许可，许可数加1
//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
System.out.println("线程" + Thread.currentThread().getName() +
"已离开，当前已有" + (3-sp.availablePermits()) + "个并发");
}
};
for(int i=0;i<10;i++){
service.execute(runnable); //提交十个任务
}
}

}