Getting Started with Java concurrent programming (xiv) CountDownLatch scenarios

First, the application scenarios

In real life do have many such scenes: the need to wait for A, B, C, D before making F, E completed, A, B, C, D, E can be complicated to complete, in no particular order, for example: Weekend eat at home, there are three things to do, Dad cooks, mom cooking, son clear the table, put the dishes. We can see this scene is characterized by:

1. there before dinner N thing to do, to eat after everything done, things to be treated is N, each done a thing to be processed is reduced 1, when the matter is pending 0, you can eat.

2. Everything can be processed in parallel, in no particular order, thus improving efficiency.

Around dinner, the whole process is as follows:

And said parallel processing operations similar to the scenes in the application, can be implemented by means of concurrent java CountDownLatch when confronted with this scenario.

二、Show me code

Category code structured as follows:

I、EatingActivity.java

class EatingActivity implements Runnable {

    private CountDownLatch countDownLatch;

    public EatingActivity(CountDownLatch countDownLatch) {
        this.countDownLatch = countDownLatch;
    }

    public void run() {
        try {
            //等待吃饭
            System.out.println("Waiting for dinner...");
            this.countDownLatch.await();

            //所有事情做完后，await被唤醒，开始吃饭
            System.out.println("Start eating...");

        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}
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II MakeRice.java

/**
 * 做饭
 */
class MakeRice implements Runnable {

    private CountDownLatch countDownLatch;

    public MakeRice(CountDownLatch countDownLatch) {
        this.countDownLatch = countDownLatch;
    }

    public void run() {
        exec();
    }

    /**
     * 模拟做饭
     */
    private void exec() {
        try {
            System.out.println("Start making rice...");

            long millis = ((int)(1+Math.random()*(5-1+1))) * 1000;
            Thread.sleep(millis);
            System.out.println("Making rice is finished.");

            //待处理事情减1
            this.countDownLatch.countDown();

        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}
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III、MakeDish.java

/**
 * 做菜
 */
class MakeDish implements Runnable {

    private CountDownLatch countDownLatch;

    public MakeDish(CountDownLatch countDownLatch) {
        this.countDownLatch = countDownLatch;
    }

    public void run() {
        exec();
    }

    /**
     * 模拟做菜
     */
    private void exec() {
        try {
            System.out.println("Start making dish...");

            long millis = ((int)(1+Math.random()*(5-1+1))) * 1000;
            Thread.sleep(millis);
            System.out.println("Making dish is finished.");

            //待处理事情减1
            this.countDownLatch.countDown();

        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}
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IV、CleanUpTable.java

/**
 * 收拾桌子
 */
class CleanUpTable implements Runnable {

    private CountDownLatch countDownLatch;

    public CleanUpTable(CountDownLatch countDownLatch) {
        this.countDownLatch = countDownLatch;
    }

    public void run() {
        exec();
    }

    /**
     * 模拟收拾桌子
     */
    private void exec() {
        try {
            System.out.println("Start making rice...");

            long millis = ((int)(1+Math.random()*(5-1+1))) * 1000;
            Thread.sleep(millis);
            System.out.println("Cleaning up table is finished.");

            //待处理事情减1
            this.countDownLatch.countDown();

        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}
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V、CountDownLatchTest.java

/**
 * @ClassName CountDownLatchTest
 * @Description 验证CountDownLatch
 * @Author 铿然一叶
 * @Date 2019/10/7 22:32
 * @Version 1.0
 * javashizhan.com
 **/
public class CountDownLatchTest {

    public static void main(String[] args) {

        //为了吃饭，有3件事情要做
        CountDownLatch countDownLatch = new CountDownLatch(3);
        //吃饭活动
        Thread eatingActivity = new Thread(new EatingActivity(countDownLatch));
        eatingActivity.start();

        //做饭
        Thread makeRice = new Thread(new MakeRice(countDownLatch));
        //做菜
        Thread makeDish = new Thread(new MakeDish(countDownLatch));
        //收拾桌子
        Thread cleanUpTable = new Thread(new CleanUpTable(countDownLatch));

        //并行开始做事情
        makeRice.start();
        makeDish.start();
        cleanUpTable.start();
    }
}
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Output log:

Waiting for dinner...
Start making rice...
Start making rice...
Start making dish...
Cleaning up table is finished.
Making rice is finished.
Making dish is finished.
Start eating...
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Third, other scenarios - fight Mission

Mission to fight scenes, how many people can come after the full group, used the counter, looks can be achieved with CountDownLatch, in fact, not necessary, because you can not fight groups in parallel, as long as there are counters can be achieved.

IV Summary

CountDownLatch of application scenarios:

1. few things to begin after the completion of another thing.

2. The need to do several things can be done independently, and can be processed in parallel.

end.

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