多线程-Fork/Join

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();  
  
    }  
  
}