java7-Fork/Join

Posted 2020-10-19 路迢迢

　　Fork/Join 框架与传统线程池的区别采用“工作窃取”模式（work-stealing）：当执行新的任务时它可以将其拆分分成更小的任务执行，并将小任务加到线程队列中，然后再从一个随机线程的队列中偷一个并把它放在自己的队列中。

　　相对于一般的线程池实现,fork/join框架的优势体现在对其中包含的任务的处理方式上.在一般的线程池中,如果一个线程正在执行的任务由于某些原因无法继续运行,那么该线程会处于等待状态.而在fork/join框架实现中,如果某个子问题由于等待另外一个子问题的完成而无法继续运行.那么处理该子问题的线程会主动寻找其他尚未运行的子问题来执行.这种方式减少了线程的等待时间,提高了性能.

代码体验效果：

import java.util.concurrent.RecursiveTask;

public class ForkJoinCalculate extends RecursiveTask<Long>{

    /**
     * 
     */
    private static final long serialVersionUID = 13475679780L;
    
    private long start;
    private long end;
    
    private static final long THRESHOLD = 10000L; //临界值
    
    public ForkJoinCalculate(long start, long end) {
        this.start = start;
        this.end = end;
    }
    
    @Override
    protected Long compute() {
        long length = end - start;
        
        if(length <= THRESHOLD){
            long sum = 0;
            
            for (long i = start; i <= end; i++) {
                sum += i;
            }
            
            return sum;
        }else{
            long middle = (start + end) / 2;
            
            ForkJoinCalculate left = new ForkJoinCalculate(start, middle);
            left.fork(); //拆分，并将该子任务压入线程队列
            
            ForkJoinCalculate right = new ForkJoinCalculate(middle+1, end);
            right.fork();
            
            return left.join() + right.join();
        }
        
    }

}

测试类：

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

import org.junit.Test;

public class TestForkJoin {
    static long num = 10000000000000L;
    @Test
    public void test1(){
        long start = System.currentTimeMillis();
        
        ForkJoinPool pool = new ForkJoinPool();
        ForkJoinTask<Long> task = new ForkJoinCalculate(0L, num);
        
        long sum = pool.invoke(task);
        System.out.println(sum);
        
        long end = System.currentTimeMillis();
        
        System.out.println("test1耗费的时间为: " + (end - start)); //112-1953-1988-2654-2647-20663-113808
    }
    
    @Test
    public void test2(){
        long start = System.currentTimeMillis();
        
        long sum = 0L;
        
        for (long i = 0L; i <= num; i++) {
            sum += i;
        }
        
        System.out.println(sum);
        
        long end = System.currentTimeMillis();
        
        System.out.println("test2耗费的时间为: " + (end - start)); //34-3174-3132-4227-4223-31583
    }
    
    @Test
    public void test3(){
        long start = System.currentTimeMillis();
        
        Long sum = LongStream.rangeClosed(0L, num)
                             .parallel()
                             .sum();
        
        System.out.println(sum);
        
        long end = System.currentTimeMillis();
        
        System.out.println("test3耗费的时间为: " + (end - start)); //2061-2053-2086-18926
    }

}