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Concurrent - CountDownLatch

分类: 文章 • 2024-06-24 14:56:10

原创转载请注明出处：http://agilestyle.iteye.com/blog/2343903

 

CountDownLatch

Concurrent - CountDownLatch

CountDownLatch所提供的功能是判断count计数不为0时，则当前线程处于wait状态。

await()的作用是实现等待，判断计数是否为0，如果不为0则呈等待状态。

countDown()的作用是继续运行，其他线程可以调用此方法将计数减1，当计数减到为0时，呈等待的线程继续运行。

getCount()的作用是获得当前的计数个数。

 

用CountDownLatch模拟一个田径短跑的例子，10个线程代表10名选手

MyThread.java

package org.fool.java.concurrent.countdownlatch;

import java.util.concurrent.CountDownLatch;

public class MyThread implements Runnable {
    private CountDownLatch comingTag;
    private CountDownLatch waitStartTag;
    private CountDownLatch waitRunTag;
    private CountDownLatch beginTag;
    private CountDownLatch endTag;

    public MyThread(CountDownLatch comingTag,
                    CountDownLatch waitStartTag,
                    CountDownLatch waitRunTag,
                    CountDownLatch beginTag,
                    CountDownLatch endTag) {
        this.comingTag = comingTag;
        this.waitStartTag = waitStartTag;
        this.waitRunTag = waitRunTag;
        this.beginTag = beginTag;
        this.endTag = endTag;
    }

    @Override
    public void run() {
        try {
            System.out.println(Thread.currentThread().getName() + " 走向起跑点...");
            Thread.sleep((int) (Math.random() * 10000));

            System.out.println(Thread.currentThread().getName() + " 到达起跑点...");
            comingTag.countDown();

            waitStartTag.await();

            Thread.sleep((int) (Math.random() * 10000));
            waitRunTag.countDown();

            beginTag.await();
            System.out.println(Thread.currentThread().getName() + " 加速起跑...");
            Thread.sleep((int) (Math.random() * 10000));
            endTag.countDown();

            System.out.println(Thread.currentThread().getName() + " 到达终点...");
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}

CountDownLatchTest4.java

package org.fool.java.concurrent.countdownlatch;

import java.util.concurrent.CountDownLatch;

public class CountDownLatchTest4 {
    public static void main(String[] args) {
        try {
            CountDownLatch comingTag = new CountDownLatch(10);
            CountDownLatch waitStartTag = new CountDownLatch(1);
            CountDownLatch waitRunTag = new CountDownLatch(10);
            CountDownLatch beginTag = new CountDownLatch(1);
            CountDownLatch endTag = new CountDownLatch(10);

            for (int i = 0; i < 10; i++) {
                Thread thread = new Thread(new MyThread(comingTag, waitStartTag, waitRunTag, beginTag, endTag));
                thread.setName("Thread " + (i + 1));
                thread.start();
            }

            System.out.println("裁判在等待选手的到来...");
            comingTag.await();

            System.out.println("裁判确认所有选手就位....");
            Thread.sleep(5000);
            waitStartTag.countDown();

            System.out.println("各就各位!预备！");
            waitRunTag.await();
            Thread.sleep(2000);

            System.out.println("发令枪响起！");
            beginTag.countDown();
            endTag.await();

            System.out.println("所有选手到达终点，统计名次！");
        } catch (InterruptedException e) {
            e.printStackTrace();
        }


    }
}

Run

Concurrent - CountDownLatch
 

await(long timeout, TimeUnit unit)

await(long timeout, TimeUnit unit)的作用是使线程在指定的最大时间单位内进入WAITING状态，如果超过这个时间则自动唤醒，程序继续向下运行。

CountDownLatchTest5.java

package org.fool.java.concurrent.countdownlatch;

import java.util.concurrent.CountDownLatch;
import java.util.concurrent.TimeUnit;

public class CountDownLatchTest5 {
    public static void main(String[] args) {
        Service service = new Service();

        for (int i = 0; i < 3; i++) {
            Thread thread = new Thread(new MyThread(service));

            thread.start();
        }
    }

    public static class Service {
        private CountDownLatch latch = new CountDownLatch(1);

        public void testMethod() {
            try {
                System.out.println(Thread.currentThread().getName() + " start...");

                latch.await(3, TimeUnit.SECONDS);

                System.out.println(Thread.currentThread().getName() + " end...");
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }

    public static class MyThread implements Runnable {

        private Service service;

        public MyThread(Service service) {
            this.service = service;
        }

        @Override
        public void run() {
            service.testMethod();
        }
    }
}

Run

Concurrent - CountDownLatch
 

getCount()

getCount()作用是获取当前计数的值

CountDownLatchTest6.java

package org.fool.java.concurrent.countdownlatch;

import java.util.concurrent.CountDownLatch;

public class CountDownLatchTest6 {
    public static void main(String[] args) {
        CountDownLatch latch = new CountDownLatch(3);

        System.out.println(latch.getCount());
        latch.countDown();
        System.out.println(latch.getCount());
        latch.countDown();
        System.out.println(latch.getCount());
        latch.countDown();
        System.out.println(latch.getCount());
        latch.countDown();
        System.out.println(latch.getCount());
        latch.countDown();
        System.out.println(latch.getCount());
    }
}

Run

Concurrent - CountDownLatch
 

CountDownLatch使用例子

在这个例子中，模拟一个应用程序启动类，它开始时启动了n个线程类，这些线程将检查外部系统并通知闭锁，并且启动类一直在闭锁上等待着。一旦验证和检查了所有外部服务，那么启动类恢复执行。

BaseHealthChecker.java

package org.fool.test.countdownlatch;

import java.util.concurrent.CountDownLatch;

public abstract class BaseHealthChecker implements Runnable {
    private CountDownLatch latch;
    private String serviceName;
    private boolean serviceUp;

    public BaseHealthChecker(String serviceName, CountDownLatch latch) {
        this.serviceName = serviceName;
        this.latch = latch;
        this.serviceUp = false;

        Runtime.getRuntime().addShutdownHook(new Thread(new Runnable() {
            @Override
            public void run() {
                System.out.println(getServiceName() + " down...");
            }
        }));
    }

    @Override
    public void run() {
        try {
            verifyService();
            serviceUp = true;
        } catch (Exception e) {
            e.printStackTrace();
            serviceUp = false;
        } finally {
            if (latch != null) {
                latch.countDown();
            }
        }
    }

    public String getServiceName() {
        return serviceName;
    }

    public boolean isServiceUp() {
        return serviceUp;
    }

    public abstract void verifyService();
}

 NetworkHealthChecker.java

package org.fool.test.countdownlatch;

import java.util.concurrent.CountDownLatch;

public class NetworkHealthChecker extends BaseHealthChecker {

    public NetworkHealthChecker(CountDownLatch latch) {
        super("Network Service", latch);
    }

    @Override
    public void verifyService() {
        System.out.println("Checking " + this.getServiceName());
        try {
            Thread.sleep(5000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println(this.getServiceName() + " is UP");
    }
}

 DatabaseHealthChecker.java

package org.fool.test.countdownlatch;

import java.util.concurrent.CountDownLatch;

public class DatabaseHealthChecker extends BaseHealthChecker {

    public DatabaseHealthChecker(CountDownLatch latch) {
        super("Database Service", latch);
    }

    @Override
    public void verifyService() {
        System.out.println("Checking " + this.getServiceName());
        try {
            Thread.sleep(15000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println(this.getServiceName() + " is UP");
    }
}

 CacheHealthChecker.java

package org.fool.test.countdownlatch;

import java.util.concurrent.CountDownLatch;

public class CacheHealthChecker extends BaseHealthChecker {

    public CacheHealthChecker(CountDownLatch latch) {
        super("Cache Service", latch);
    }

    @Override
    public void verifyService() {
        System.out.println("Checking " + this.getServiceName());
        try {
            Thread.sleep(10000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println(this.getServiceName() + " is UP");
    }
}

 ApplicationStartupUtil.java

package org.fool.test.countdownlatch;

import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

public class ApplicationStartupUtil {
    private static List<BaseHealthChecker> services;

    private static CountDownLatch latch;

    private static ApplicationStartupUtil instance = new ApplicationStartupUtil();

    public static ApplicationStartupUtil getInstance() {
        return instance;
    }

    private ApplicationStartupUtil() {
    }

    public boolean checkExternalServices() throws Exception {
        latch = new CountDownLatch(3);
        services = new ArrayList<>();
        services.add(new NetworkHealthChecker(latch));
        services.add(new CacheHealthChecker(latch));
        services.add(new DatabaseHealthChecker(latch));

        ExecutorService executor = Executors.newFixedThreadPool(services.size());

        for (BaseHealthChecker service : services) {
            executor.execute(service);
        }

        latch.await();

        for (BaseHealthChecker service : services) {
            if(!service.isServiceUp()) {
                return false;
            }
        }

        return true;
    }
}

 MainTest.java

package org.fool.test.countdownlatch;

public class MainTest {
    public static void main(String[] args) throws Exception {
        boolean result = ApplicationStartupUtil.getInstance().checkExternalServices();

        System.out.println("External services validation completed !! Result was :: "+ result);
    }
}

Console Output

Concurrent - CountDownLatch
 

 

Reference

Java并发编程核心方法与框架

https://howtodoinjava.com/core-java/multi-threading/when-to-use-countdownlatch-java-concurrency-example-tutorial/

http://www.importnew.com/15731.html

 

 

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