13. ForkJoin

ForkJoin :: 分支合并 （JDK 1.7）

• 并行执行任务，提高效率。

• 使用场景：大数据量

• 将大任务拆成小任务

ForkJoin 特点：工作窃取

这个里面维护的都是双端队列，如果一方执行完了，获取另外一方的未完成的任务进行执行

ForkJoin 的操作

package pers.vincent.matrix.subject.forkjoin;

import java.util.concurrent.ForkJoinTask;
import java.util.concurrent.RecursiveTask;

public class ForkJoinDemp extends RecursiveTask<Long> {

    private Long start;

    private Long end;

    private Long temp = 10_0000L;

    public ForkJoinDemp(Long start, Long end){
        this.start = start;
        this.end = end;
    }

    @Override
    protected Long compute() {
        if((end-start) <= temp){
            Long result = 0L;
            for (Long i = start; i <= end; i++) {
                result += i;
            }
            return result;
        }else{
            Long middle = (end + start) / 2;

            ForkJoinDemp task1 = new ForkJoinDemp(start, middle);
            task1.fork();
            ForkJoinDemp task2 = new ForkJoinDemp(middle+1, end);
            task2.fork();

            return task1.join() + task2.join();
        }
    }
}

测试代码：（普通 - ForkJoin - Stream 流计算）

package pers.vincent.matrix.subject.forkjoin;

import java.util.concurrent.ExecutionException;
import java.util.concurrent.ForkJoinPool;
import java.util.concurrent.ForkJoinTask;
import java.util.stream.LongStream;

public class Test1 {

    public static void main(String[] args) throws ExecutionException, InterruptedException {
        long start = 0L;
        long end = 10_0000_0000L;
        test3(start, end);
    }

    /**
     * TOTAL TIME = 5699
     * @param start
     * @param end
     */
    public static void test1(Long start, Long end){
        long beginTime = System.currentTimeMillis();

        Long result = start;
        for (Long i = 0L; i <=end; i++) {
            result += i;
        }

        long endTime = System.currentTimeMillis();

        System.out.println("Time===" + (endTime-beginTime) + ", result = " + result);
    }

    /**
     * TOTAL TIME : 4908
     * @param start
     * @param end
     * @throws ExecutionException
     * @throws InterruptedException
     */
    public static void test2(Long start, Long end) throws ExecutionException, InterruptedException {
        long beginTime = System.currentTimeMillis();

        ForkJoinPool forkJoinPool = new ForkJoinPool();
        ForkJoinTask<Long> forkJoinDemp = new ForkJoinDemp(start, end);
        ForkJoinTask<Long> sum = forkJoinPool.submit(forkJoinDemp);

        long result = sum.get();

        long endTime = System.currentTimeMillis();

        System.out.println("Time===" + (endTime-beginTime) + result);
    }

    /**
     * TOTAL TIME = 250
     * @param start
     * @param end
     */
    public static void test3(Long start, Long end){
        long beginTime = System.currentTimeMillis();

        long sum = LongStream.rangeClosed(start, end).parallel().reduce(0, Long::sum);

        long endTime = System.currentTimeMillis();

        System.out.println("Time===" + (endTime-beginTime) + ", result = " + sum);
    }
}