semaphore研究

package cn.everest.thread;

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

public class Thread20130319 {
	public static void main(String[] args) {
		
		ExecutorService executorService = Executors.newCachedThreadPool();
		final Semaphore semaphore = new Semaphore(3, true);
		for (int i = 1; i <= 10; i++) {
			Runnable runnable = new Runnable() {
				@Override
				public void run() {
					try {
						semaphore.acquire();
					} catch (InterruptedException e) {
						e.printStackTrace();
					}
					System.out.println("线程" + Thread.currentThread().getName() + 
							"进入，当前已有" + (3-semaphore.availablePermits()) + "个并发");
					try {
						Thread.sleep((new Random().nextInt(10) + 1) * 1000);
					} catch (InterruptedException e) {
						e.printStackTrace();
					}
					
					/*
					 * 关键点理解：
					 * 	1、sleep几秒后，线程的运行权就不知道是谁的了，因为同时有三个有信号
					 * 	2、当前sleep的线程确实没有释放执行资格，但是具备执行资格的线程同时有三个
					 */
					System.out.println("线程" + Thread.currentThread().getName() + "即将要退出");
					semaphore.release();
					
					System.out.println("线程" + Thread.currentThread().getName() + 
							"退出，当前已有" + (3-semaphore.availablePermits()) + "个并发");
				} // run
			};
			
			/*
			 * 执行每一个命令
			 */
			executorService.execute(runnable);
		}
	}
}