JDK1.5 CountDownLatch

CountDownLatch 实战案例

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本文通过三个实战案例展示了 Java 中 CountDownLatch 的使用方法。案例包括模拟运动会长跑、多个领导审批流程以及主线程等待子线程完成任务的场景。

/*
* 还有一个利用场景: 应用启动的时候 ,检查所有的服务是否正确启动,如果正确启动,应用程序就可以启动成功.
**/



import java.util.Random;
import java.util.concurrent.Callable;
import java.util.concurrent.CompletionService;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.ExecutorCompletionService;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;

/**
 * CountDownLatch 模拟 运动会长跑  ,裁判 发令 --- 运动员开始努力奔跑  ---- 裁判收集结果
 * 1个裁判 和  3个运动员 
 */
public class CountDownLatchTest1 {

	public static void main(String[] args) {
		ExecutorService executor = Executors.newCachedThreadPool();
		final CountDownLatch referee = new CountDownLatch(1); 	//
		final CountDownLatch athletes = new CountDownLatch(3);  //

		CompletionService<String> completionService =new ExecutorCompletionService<String>(executor);  

		final Lock lock=new ReentrantLock();

		//创建要 运行的任务   长跑
		Callable<String> callable=new Callable<String>() {
			@Override
			public String call(){
				try {
					System.out.println("运动员"+Thread.currentThread().getName()+"等待裁判发令");
					referee.await();

					System.out.println("运动员"+Thread.currentThread().getName()+"收到裁判发令,已经出发");
					lock.lock();
					//模拟长跑任务  
					long stime=System.currentTimeMillis();
					String thread_name = Thread.currentThread().getName();
					for (int i = 0; i < thread_name.length(); i++) {
						System.out.print(thread_name.charAt(i));
						Thread.sleep(new Random().nextInt(10)*100);
					}
					System.out.println();
					System.out.println("运动员"+Thread.currentThread().getName()+"已经跑完");

					//跑完了
					athletes.countDown();
					return "运动员"+Thread.currentThread().getName()+"花的时间为:"+(System.currentTimeMillis()-stime);
				} catch (InterruptedException e) {
					e.printStackTrace();
				}finally{
					 //lock.unlock();
				}
				return "运动员比赛出现失误";
			}
		};

		for (int i = 0; i < 3; i++) {
			completionService.submit(callable);
		}

		try {
			System.out.println("裁判"+Thread.currentThread().getName()+"即将发布命令");
			Thread.sleep((long)Math.random()*20000);
			referee.countDown();
			System.out.println("裁判"+Thread.currentThread().getName()+"已经发布命令,等待结果");
			athletes.await();
			System.out.println("裁判"+Thread.currentThread().getName()+"收到了结果如下:-----------------");

			for (int i = 0; i < 3; i++) {
				try {
					System.out.println(completionService.take().get());
				} catch (ExecutionException e) {
					e.printStackTrace();
				}
			}

		} catch (InterruptedException e) {
			e.printStackTrace();
		}
		executor.shutdown();

	}

}



import java.util.concurrent.CountDownLatch;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;

/**
 * @description  三个领导 审批  --一个采购员去采购 
 */
public class CountDownLatchTest2 {

	public static void main(String[] args) {
		ExecutorService service = Executors.newCachedThreadPool();
		final CountDownLatch leader = new CountDownLatch(3); //三个领导
		final CountDownLatch buyer = new CountDownLatch(1);  //只有一个裁判

		final Lock lock=new ReentrantLock();

		Runnable runnable = new Runnable() {
			@Override
			public void run() {
				try{
						System.out.println("领导"+Thread.currentThread().getName()+"开始进行了审批");
						lock.lock();
						String thread_name = Thread.currentThread().getName();
						for (int i = 0; i < thread_name.length(); i++) {
							System.out.print(thread_name.charAt(i));
						}
						System.out.println();
						System.out.println("领导"+Thread.currentThread().getName()+"已经审批完了");
						leader.countDown(); //一个已经结束

						if(leader.getCount()==0){
							buyer.countDown();
						}
				} catch (Exception e) {
					// TODO Auto-generated catch block
					e.printStackTrace();
				}finally{
					lock.unlock();
				}
			}
		};

		for (int i = 0; i < 3; i++) {
			service.execute(runnable);
		}

		try {
			System.out.println("采购员"+Thread.currentThread().getName()+"在等待,领导审批");
			leader.await();
			System.out.println("采购员"+Thread.currentThread().getName()+"的单子,已经通过了所有领导审批");
			System.out.println("采购员去采购了....");
		} catch (InterruptedException e) {
			e.printStackTrace();
		}
		service.shutdown();

	}

}



/**
 * CountDownLatch 是一个倒计数的锁存器，当计数减至0时触发特定的事件。利用这种特性，可以让主线程 等待 子线程的结束运行
 */
public class CountDownLatchTest {

	/**
	 * @param args
	 */
	public static void main(String[] args) {
		ExecutorService service=Executors.newCachedThreadPool();
		final CountDownLatch cdOrder=new CountDownLatch(1);
		final CountDownLatch cdAnswer=new CountDownLatch(3);

		for (int i = 0; i < 3; i++) {
			Runnable runnable =new Runnable() {
				public void run() {
					try {
						System.out.println("线程"+Thread.currentThread().getName()+"正准备接受命令");
						cdOrder.await();
						System.out.println("线程"+Thread.currentThread().getName()+"已接受命令");
						Thread.sleep((long)Math.random()*10000);
						System.out.println("线程"+Thread.currentThread().getName()+"回应命令处理结果");
						cdAnswer.countDown();
					} catch (Exception e) {
						// TODO: handle exception
					}
				}
			};
			service.execute(runnable);
		}
		try {
			Thread.sleep((long)Math.random()*10000);
			System.out.println("线程"+Thread.currentThread().getName()+"即将发布命令");
			cdOrder.countDown();
			System.out.println("线程"+Thread.currentThread().getName()+"已经发布命令,正在等待结果...");
			cdAnswer.await();
			System.out.println("线程"+Thread.currentThread().getName()+"已经收到所有响应结果");

		} catch (Exception e) {
			// TODO: handle exception
		}

	}

}