Fork/Join

Java7提供了Fork/Join来支持将一个任务拆分成多个“小任务”并行计算，再把多个“小任务”的结果合并成总的计算结果。

类图

Java7提供了ForkJoinPool来支持将一个任务拆分为多个小任务并行计算，再把多个小任务的结果合并成总的计算结果。ForkJoinPool是ExecutorService的实现类，因此是一种特殊的线程池。

ForkJoinPool(int n)创建一个包含n个并行线程的ForkJoinPool

ForkJoinPool()创建一个Runtime.availableProcessors()返回值个数的并行线程。

ForkJoinTask代表一个可以并行、合并的任务，是一个抽象类，它还有两个抽象子类：RecursiveAction和RecursiveTask。其中RecursiveTask代表有返回值的任务，而RecursiveAction代表没有返回值的任务。

RecursiveAction实例：

package org.github.lujiango;  

import java.util.concurrent.ForkJoinPool;
import java.util.concurrent.RecursiveAction;
import java.util.concurrent.TimeUnit;  

class PrintTask extends RecursiveAction {
    private static final long serialVersionUID = 1L;
    private static final int threshold = 50;
    private int start;
    private int end;  

    public PrintTask(int start, int end) {
        this.start = start;
        this.end = end;
    }  

    @Override
    protected void compute() {
        if (end - start < threshold) {
            for (int i = start; i < end; i++) {
                System.out.println(Thread.currentThread().getName() + " i: " + i);
            }
        } else {
            int middle = (start + end) / 2;
            PrintTask left = new PrintTask(start, middle);
            PrintTask right = new PrintTask(middle, end);
            left.fork();
            right.fork();
        }
    }  

}  

public class Test20 {  

    public static void main(String[] args) throws InterruptedException {
        ForkJoinPool pool = new ForkJoinPool();
        pool.submit(new PrintTask(0, 300));
        pool.awaitTermination(2, TimeUnit.SECONDS);
        pool.shutdown();
    }  

}

分解后的任务分别调用fork()方法开始并行执行。

RecursiveTask<T>实例：

如果大任务是有返回值的任务，则可以让任务继承RecursiveTask<T>，其中泛型参数T就代表了该任务的返回值类型。

package org.github.lujiango;  

import java.util.Random;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.ForkJoinPool;
import java.util.concurrent.Future;
import java.util.concurrent.RecursiveTask;  

class CalTask extends RecursiveTask<Integer> {
    private static final long serialVersionUID = 1L;
    private static final int threshold = 20;
    private int[] arr;
    private int start;
    private int end;  

    public CalTask(int[] arr, int start, int end) {
        this.arr = arr;
        this.start = start;
        this.end = end;
    }  

    @Override
    protected Integer compute() {
        int sum = 0;
        if (end - start < threshold) {
            for (int i = start; i < end; i++) {
                sum += arr[i];
            }
            return sum;
        } else {
            int middle = (start + end) / 2;
            CalTask left = new CalTask(arr, start, middle);
            CalTask right = new CalTask(arr, middle, end);
            left.fork();
            right.fork();
            return left.join() + right.join();
        }
    }  

}  

public class Test21 {  

    public static void main(String[] args) throws InterruptedException, ExecutionException {
        int[] arr = new int[100];
        Random random = new Random();
        int total = 0;
        for (int i = 0, len = arr.length; i < len; i++) {
            int tmp = random.nextInt(20);
            total += (arr[i] = tmp);
        }
        System.out.println(total);  

        ForkJoinPool pool = new ForkJoinPool();
        Future<Integer> future = pool.submit(new CalTask(arr, 0, arr.length));
        System.out.println(future.get());
        pool.shutdown();  

    }  

}