Fork/Join 框架在并发编程中有何作用？请解释其工作原理和适用场景。

import java.util.concurrent.RecursiveTask;
import java.util.concurrent.ForkJoinPool;

public class FibonacciTask extends RecursiveTask<Integer> {
    private final int n;

    public FibonacciTask(int n) {
        this.n = n;
    }

    @Override
    protected Integer compute() {
        if (n <= 1) { // 基础条件
            return n;
        }

        // 拆分任务
        FibonacciTask f1 = new FibonacciTask(n - 1);
        f1.fork(); // 异步执行

        FibonacciTask f2 = new FibonacciTask(n - 2);

        return f2.compute() + f1.join(); // 合并结果
    }

    public static void main(String[] args) {
        ForkJoinPool pool = new ForkJoinPool();
        FibonacciTask task = new FibonacciTask(10); // 计算第10个斐波那契数
        int result = pool.invoke(task);
        System.out.println("Result: " + result);
    }
}

import java.util.concurrent.RecursiveAction;
import java.util.concurrent.ForkJoinPool;

public class ArraySumTask extends RecursiveAction {
    private final int[] array;
    private final int start;
    private final int end;
    private static final int THRESHOLD = 10; // 阈值

    public ArraySumTask(int[] array, int start, int end) {
        this.array = array;
        this.start = start;
        this.end = end;
    }

    @Override
    protected void compute() {
        if (end - start <= THRESHOLD) { // 任务足够小，直接处理
            int sum = 0;
            for (int i = start; i < end; i++) {
                sum += array[i];
            }
            System.out.println(Thread.currentThread().getName() + ": Sum from " + start + " to " + end + " = " + sum);
        } else {
            // 拆分任务
            int mid = (start + end) / 2;
            ArraySumTask leftTask = new ArraySumTask(array, start, mid);
            ArraySumTask rightTask = new ArraySumTask(array, mid, end);

            invokeAll(leftTask, rightTask); // 并行执行子任务
        }
    }

    public static void main(String[] args) {
        int[] array = new int[100];
        for (int i = 0; i < array.length; i++) {
            array[i] = i + 1; // 初始化数组
        }

        ForkJoinPool pool = new ForkJoinPool();
        ArraySumTask task = new ArraySumTask(array, 0, array.length);
        pool.invoke(task);
    }
}