ForkJoin

package example.test;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
import java.util.Random;
import java.util.concurrent.ForkJoinPool;
import java.util.concurrent.ForkJoinTask;
import java.util.concurrent.RecursiveTask;

public class TSKK {

	public static void main(String[] args) throws Exception {
		// 创建随机数组成的数组:
		Long[] array = new Long[40000000];
		fillRandom(array);

		test01(array);
		test02(array);
	}

	public static void test01(Long[] array) {
		// fork/join task:
		ForkJoinPool fjp = new ForkJoinPool(4); // 最大并发数4
		ForkJoinTask<Long> task = new SumTask(array, 0, array.length);
		long startTime = System.currentTimeMillis();
		Long result = fjp.invoke(task);
		long endTime = System.currentTimeMillis();
		System.out.println("Fork/join sum: " + result + " in " + (endTime - startTime) + " ms.");

	}

	public static void test02(Long[] array) {
		List<Long> arrays = new ArrayList<>(Arrays.asList(array));
		long startTime = System.currentTimeMillis();
		Long result = arrays.stream().parallel().reduce(0L, Long::sum);
		long endTime = System.currentTimeMillis();
		System.out.println("stream/join sum: " + result + " in " + (endTime - startTime) + " ms.");
	}

	public static void fillRandom(Long[] array) {
		Random random = new Random();
		for (int i = 0; i < array.length; i++) {
			array[i] = random.nextLong();
		}
	}
}

@SuppressWarnings("serial")
class SumTask extends RecursiveTask<Long> {

	static final int THRESHOLD = 100000;
	Long[] array;
	int start;
	int end;

	SumTask(Long[] array, int start, int end) {
		this.array = array;
		this.start = start;
		this.end = end;
	}

	@Override
	protected Long compute() {
		if (end - start <= THRESHOLD) {
			// 如果任务足够小,直接计算:
			long sum = 0;
			for (int i = start; i < end; i++) {
				sum += array[i];
			}
			// System.out.println(String.format("compute %d~%d = %d", start, end, sum));
			return sum;
		}
		// 任务太大,一分为二:
		int middle = (end + start) / 2;
		// System.out.println(String.format("split %d~%d ==> %d~%d, %d~%d", start, end,
		// start, middle, middle, end));
		SumTask subtask1 = new SumTask(this.array, start, middle);
		SumTask subtask2 = new SumTask(this.array, middle, end);
		invokeAll(subtask1, subtask2);
		Long subresult1 = subtask1.join();
		Long subresult2 = subtask2.join();
		Long result = subresult1 + subresult2;
		// System.out.println("result = " + subresult1 + " + " + subresult2 + " ==> " +
		// result);
		return result;
	}
}