什么是ForkJoin

ForkJoin在JDK1.7，并行执行任务！提高效率，大数据量！

大数据：Map Reduce（把大任务拆分为小任务）

ForkJoin特点：工作窃取

这个里面维护的都是双端队列

当第一行的任务执行完后，会去第二行窃取未执行的任务

缺点：可能会竞争同一个任务

ForkJoin操作

ForkJoinPool的方法

例：

import java.util.concurrent.RecursiveTask;

/**
 *  计算求和任务
 *  如何使用ForkJoin
 *  1.通过ForkJoinPool通过它来执行
 *  2.计算任务：ForkJoinPool.execute(ForkJoinTask<?> task)
 *  3. 计算类要继承RecursiveTask
 */
public class ForkJoinDemo extends RecursiveTask<Long> {
    private Long start;
    private Long end;

    //临界值
    private Long temp = 10000L;

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


    //计算方法
    @Override
    protected Long compute() {
        if ((end-start)<temp){
            Long sum = 0L;
            for (Long i = start; i <= end; i++) {
                sum += i;
            }
            return sum;
        }else { //分支合并计算ForkJoin 递归
            //中间值
            long midden = (start + end) / 2;
            ForkJoinDemo task1 = new ForkJoinDemo(start, midden);
            //拆分任务，把任务压入线程队列
            task1.fork();
            ForkJoinDemo task2 = new ForkJoinDemo(midden + 1, end);
            //拆分任务，把任务压入线程队列
            task2.fork();

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


    //基础递归方法
//public class ForkJoinDemo {
//    public static void main(String[] args) {
//        long sum = 0;
//        for (long i = 1; i <= 10_0000_0000; i++) {
//            sum += i;
//        }
//        System.out.println(sum);
//    }
//}

测试：

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

public class Test {
    public static void main(String[] args) throws ExecutionException, InterruptedException {
//        test1();    
//        test2();    
//        test3();    
    }

    public static void test1() {
        Long sum = 0L;
        long start = System.currentTimeMillis();
        for (Long i = 1L; i <= 10_0000_0000; i++) {
            sum += i;
        }
        long end = System.currentTimeMillis();
        System.out.println("总和：" + sum + " 时间：" + (end - start));
    }

    public static void test2() throws ExecutionException, InterruptedException {
        long start = System.currentTimeMillis();

        ForkJoinPool forkJoinPool = new ForkJoinPool();
        ForkJoinTask<Long>   forkJoinTask = new ForkJoinDemo(0L, 10_0000_0000L);
        //提交任务
        ForkJoinTask<Long> submit = forkJoinPool.submit(forkJoinTask);

        Long sum = submit.get();

        long end = System.currentTimeMillis();
        System.out.println("总和：" + sum + " 时间：" + (end - start));
    }

    public static void test3(){
        long start = System.currentTimeMillis();

        //Stream并行流 range -> ()   rangeClosed -> (]

        long sum = LongStream.rangeClosed(0L, 10_0000_0000L).parallel().reduce(0,Long::sum);

        long end = System.currentTimeMillis();
        System.out.println("总和：" + sum + " 时间：" + (end - start));
    }
}

运行结果

test1：