Java fork/join 框架说明示例

fork/join 框架设计为将并行任务递归拆分为较小的任务, 然后将每个子项目的结果合并以产生总体结果。它是 ExecutorService 接口的一个实现, 它将这些子任务分配给线程池中的工作线程, 称为 ForkJoinPool。让我们开始探索如何定义任务和子任务。
需要继承RecursiveTask这个类，并实现唯一的一个抽象方法
protected Long compute() {
return null;
}

此方法定义了将手头的任务拆分为子任务的逻辑和算法, 以便在不再可能或不方便地进一步划分时, 生成一个单独的工作项的结果。为此方法的实施经常类似以下伪代码:

if (task is small enough or no longer divisible(任务足够小或不再可分)) {
compute task sequentially(顺序计算该任务)
} else {
split task in two subtasks(将任务拆分2个子任务)
call this method recursively possibly further splitting each subtask(递归调用此方法可能进一步拆分每个子任务)
wait for the completion of all subtasks(等待所有子任务的完成)
combine the results of each subtask(合并每个子任务的结果)
}

用fork/join框架并行求和示例代码：

import java.util.concurrent.ForkJoinPool;
import java.util.concurrent.ForkJoinTask;
import java.util.concurrent.RecursiveTask;
import java.util.function.Function;
import java.util.stream.LongStream;

@SuppressWarnings("serial")
public class ForkJoinRecursiveTasks extends RecursiveTask{
    public static final ForkJoinPool FORK_JOIN_POOL = new ForkJoinPool();
    public static final long THRESHOLD = 10_000;
    private final long[] numbers;
    private final int start;
    private final int end;

    private ForkJoinRecursiveTasks(long[] numbers, int start, int end) {
        this.numbers = numbers;
        this.start = start;
        this.end = end;
    }

    @Override
    protected Long compute() {
        int length = end - start;
        if (length <= THRESHOLD) {
            return computeSequentially();
        }
        ForkJoinRecursiveTasks leftTask = new ForkJoinRecursiveTasks(numbers, start, start + length/2);
        leftTask.fork();
        ForkJoinRecursiveTasks rightTask = new ForkJoinRecursiveTasks(numbers, start + length/2, end);
        Long rightResult = rightTask.compute();
        Long leftResult = leftTask.join();
        return leftResult + rightResult;
    }

    private Long computeSequentially() {
        long sum = 0;
        for (int i = start; i < end; i++) {
            sum += numbers[i];
        }
        return sum;
    }

    public static long forkJoinSum(long n) {
        long[] numbers = LongStream.rangeClosed(1, n).toArray();
        ForkJoinTask task = new ForkJoinSumCalculator(numbers);
        return FORK_JOIN_POOL.invoke(task);
    }

    public static  long measurePerf(Function f, T input) {
        long fastest = Long.MAX_VALUE;
        for (int i = 0; i < 10; i++) {
            long start = System.nanoTime();
            R result = f.apply(input);
            long duration = (System.nanoTime() - start) / 1_000_000;
            System.out.println("Result: " + result);
            if (duration < fastest) fastest = duration;
        }
        return fastest;
    }

    public static void main(String[] args) {
        System.out.println("Range forkJoinSum done in: " + measurePerf(ForkJoinRecursiveTasks::forkJoinSum, 100_000_000L) + " msecs");
    }
}