ForkJoinPool 分支合并框架-工作窃取

Fork/Join 是 jdk1.7 之后出现的：在必要的情况下，将一个大任务，拆分（Fork）成多个小任务，再将小人物运算的结果进行汇总（Join）。

 

实现：

     继承 RecursiveTask 类。

     class ForkJoinSumCalculate extends RecursiveTask<Long>{

　　}

　　

class ForkJoinSumCalculate extends RecursiveTask<Long>{

	/**
	 * 
	 */
	private static final long serialVersionUID = -259195479995561737L;
	
	private long start;
	private long end;
	
	private static final long THURSHOLD = 10000L;  //临界值
	
	public ForkJoinSumCalculate(long start, long end) {
		this.start = start;
		this.end = end;
	}

	@Override
	protected Long compute() {
		long length = end - start;
		
		if(length <= THURSHOLD){
			long sum = 0L;
			
			for (long i = start; i <= end; i++) {
				sum += i;
			}
			
			return sum;
		}else{
			long middle = (start + end) / 2;
			
			ForkJoinSumCalculate left = new ForkJoinSumCalculate(start, middle); 
			left.fork(); //进行拆分，同时压入线程队列
			
			ForkJoinSumCalculate right = new ForkJoinSumCalculate(middle+1, end);
			right.fork(); //
			
			return left.join() + right.join();
		}
	}
	
}

　　main：

public static void main(String[] args) {
		Instant start = Instant.now();
		
		ForkJoinPool pool = new ForkJoinPool();
		
		ForkJoinTask<Long> task = new ForkJoinSumCalculate(0L, 50000000000L);
		
		Long sum = pool.invoke(task);
		
		System.out.println(sum);
		
		Instant end = Instant.now();
		
		System.out.println("耗费时间为：" + Duration.between(start, end).toMillis());//166-1996-10590
	}

　　java 8 对Fork/Join 做了底层实现。

//java8 新特性
	@Test
	public void test2(){
		Instant start = Instant.now();
		
		Long sum = LongStream.rangeClosed(0L, 50000000000L)
							 .parallel()
							 .reduce(0L, Long::sum);
		
		System.out.println(sum);
		
		Instant end = Instant.now();
		
		System.out.println("耗费时间为：" + Duration.between(start, end).toMillis());//1536-8118
	}