Java併發編程入門（十四）CountDownLatch應用場景

時間 2019-11-07

標籤 java 併發編程入門十四 countdownlatch 應用場景欄目 Java 简体版

原文原文鏈接

Java極客 | 做者 / 鏗然一葉這是Java極客的第 42 篇原創文章

1、應用場景

現實生活作中有不少這樣的場景：作F前須要等待A,B,C,D,E完成，A,B,C,D,E能夠併發完成，沒有特定順序，例如：週末在家裏吃飯，有3件事情要作，爸爸作飯，媽媽作菜，兒子收拾餐桌，擺放碗筷。能夠看到這個場景的特徵爲：java

1.在吃飯前有N件事情要作，每件事情都作完後才能吃飯，待處理的事情爲N，每作完一件待處理事情就減小1，當待處理事情爲0時，就能夠吃飯了。編程

2.每件事情能夠並行處理，沒有前後順序，於是提升了效率。緩存

圍繞吃飯，整個過程以下：安全

在應用程序中處理並行操做時和上述場景相似，碰到這種場景時能夠經過java併發工具CountDownLatch來實現。

2、Show me code

代碼類結構以下：bash

I、EatingActivity.java

class EatingActivity implements Runnable {

    private CountDownLatch countDownLatch;

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

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

            //全部事情作完後，await被喚醒，開始吃飯
            System.out.println("Start eating...");

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

/** * 作飯 */
class MakeRice implements Runnable {

    private CountDownLatch countDownLatch;

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

    public void run() {
        exec();
    }

    /** * 模擬作飯 */
    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.");

            //待處理事情減1
            this.countDownLatch.countDown();

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

/** * 作菜 */
class MakeDish implements Runnable {

    private CountDownLatch countDownLatch;

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

    public void run() {
        exec();
    }

    /** * 模擬作菜 */
    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.");

            //待處理事情減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();
    }

    /** * 模擬收拾桌子 */
    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.");

            //待處理事情減1
            this.countDownLatch.countDown();

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

/** * @ClassName CountDownLatchTest * @Description 驗證CountDownLatch * @Author 鏗然一葉 * @Date 2019/10/7 22:32 * @Version 1.0 * javashizhan.com **/
public class CountDownLatchTest {

    public static void main(String[] args) {

        //爲了吃飯，有3件事情要作
        CountDownLatch countDownLatch = new CountDownLatch(3);
        //吃飯活動
        Thread eatingActivity = new Thread(new EatingActivity(countDownLatch));
        eatingActivity.start();

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

        //並行開始作事情
        makeRice.start();
        makeDish.start();
        cleanUpTable.start();
    }
}
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輸出日誌：併發

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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