Java CountDownLatch

CountDownLatch

CountDownLatch使用在当有多个线程一起执行，有任务需要在一定条件下才能执行,CountDownLatch维护一个数，当数减为0时后面的才开始执行。

使用结构

• 在统筹线程中即最后再执行的线程中新建CountDownLatch

public static void main( String[] args ) {
        List<String> contents = readFile();
        int lineCount = contents.size();

        ParallelCalculate_CountDownLatch pc = new ParallelCalculate_CountDownLatch(lineCount);

        //新建CountDownLatch，来统筹等待
        CountDownLatch latch = new CountDownLatch(lineCount);

        for (int i = 0; i < lineCount; i++) {
            final int j = i;
            new Thread(() -> pc.calc(contents.get(j), j, latch)).start();
        }

        try {
            latch.await();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

        pc.sum();

    }

• 在需要多组线程一起执行的方法中传入CountDownLatch,在方法执行完成后调用CountDownLatch.countdown()

public void calc( String line, int index , CountDownLatch latch ) {
        //将每一行的数字切分出来
        String[] nus = line.split(",");

        int total = 0;
        for (String num : nus) {
            total += Integer.parseInt(num);
        }

        nums[index] = total;
        System.out.println(Thread.currentThread().getName() + " 正在执行计算任务..." + line + "  结果为：" + total);

        //countdown
        latch.countDown();
    }

• 在需要最后等待执行的方法前调用CountDownLatch.await()
  在需要最后等待执行的方法前调用CountDownLatch.await()，当其他线程没有执行完之前他会一直等待，知道countdownLatch的方法执行完成

完成代码示例

package JavaCore.MultiThread.advanced.ThreadCorrespondence.CountDownLatch;

import java.io.BufferedReader;
import java.io.FileNotFoundException;
import java.io.FileReader;
import java.io.IOException;
import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.CountDownLatch;

/*******************************************************************************
 * @Copyright (C), 2018-2019,github:Swagger-Ranger 
 * @FileName: ParallelCalculate
 * @Author: liufei32@outlook.com
 * @Date: 2019/3/12 10:22
 * @Description: 不使用CountDownLatch的并行计数，将文本中每行的数字并行求和然后等每一行都得出结果后再汇总求和
 * @Aha-eureka:
 *******************************************************************************/

public class ParallelCalculate_CountDownLatch {

    //数组用来保存每一行的值
    private int[] nums;

    //构造函数初始化要求和的长度
    public ParallelCalculate_CountDownLatch( int line ) {
        nums = new int[line];
    }

    /**
     * 单独计算每一行的和
     * @param line
     * @param index
     */
    public void calc( String line, int index , CountDownLatch latch ) {
        //将每一行的数字切分出来
        String[] nus = line.split(",");

        int total = 0;
        for (String num : nus) {
            total += Integer.parseInt(num);
        }

        nums[index] = total;
        System.out.println(Thread.currentThread().getName() + " 正在执行计算任务..." + line + "  结果为：" + total);

        latch.countDown();
    }


    public void sum() {
        System.out.println("线程汇总开始执行...");

        int total = 0;
        for (int i = 0; i < nums.length; i++) {
            total += nums[i];
        }

        System.out.println("最终结果为：" + total);
    }

    /**
     * 文件读取工具方法
     * @return
     */
    private static List<String> readFile() {
        List<String> contents = new ArrayList<>();
        String line = null;

        BufferedReader br = null;
        try {
            br = new BufferedReader(new FileReader(System.getProperty("user.dir") + "/src/JavaCore/MultiThread/advanced/ThreadCorrespondence/CountDownLatch/data"));
            while ((line = br.readLine()) != null) {
                contents.add(line);
            }
        } catch (FileNotFoundException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }finally {
            try {
                if (br!=null) {
                    br.close();
                }
            } catch (IOException e) {
                e.printStackTrace();
            }
        }

        return contents;
    }


    public static void main( String[] args ) {
        List<String> contents = readFile();
        int lineCount = contents.size();

        ParallelCalculate_CountDownLatch pc = new ParallelCalculate_CountDownLatch(lineCount);

        //新建CountDownLatch，来统筹等待
        CountDownLatch latch = new CountDownLatch(lineCount);

        for (int i = 0; i < lineCount; i++) {
            final int j = i;
            new Thread(() -> pc.calc(contents.get(j), j, latch)).start();
        }

        try {
            latch.await();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

        pc.sum();

    }
}

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