下面是一个使用并行归并排序进行基准测试的示例代码：

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

public class ParallelMergeSortBenchmark {

    // 并行归并排序的阈值
    private static final int THRESHOLD = 1000;

    public static void main(String[] args) {
        // 创建一个随机数组
        int[] arr = generateRandomArray(1000000);

        // 串行归并排序
        int[] serialResult = serialMergeSort(arr.clone());

        // 并行归并排序
        int[] parallelResult = parallelMergeSort(arr.clone());

        // 验证结果是否正确
        System.out.println("结果是否正确：" + Arrays.equals(serialResult, parallelResult));
    }

    // 串行归并排序
    public static int[] serialMergeSort(int[] arr) {
        if (arr.length <= 1) {
            return arr;
        }

        int mid = arr.length / 2;
        int[] left = Arrays.copyOfRange(arr, 0, mid);
        int[] right = Arrays.copyOfRange(arr, mid, arr.length);

        left = serialMergeSort(left);
        right = serialMergeSort(right);

        return merge(left, right);
    }

    // 并行归并排序
    public static int[] parallelMergeSort(int[] arr) {
        ForkJoinPool pool = new ForkJoinPool();
        MergeSortTask task = new MergeSortTask(arr);
        pool.invoke(task);
        return task.getResult();
    }

    // 归并操作
    public static int[] merge(int[] left, int[] right) {
        int[] merged = new int[left.length + right.length];
        int leftPtr = 0, rightPtr = 0, mergedPtr = 0;

        while (leftPtr < left.length && rightPtr < right.length) {
            if (left[leftPtr] <= right[rightPtr]) {
                merged[mergedPtr++] = left[leftPtr++];
            } else {
                merged[mergedPtr++] = right[rightPtr++];
            }
        }

        while (leftPtr < left.length) {
            merged[mergedPtr++] = left[leftPtr++];
        }

        while (rightPtr < right.length) {
            merged[mergedPtr++] = right[rightPtr++];
        }

        return merged;
    }

    // 生成随机数组
    public static int[] generateRandomArray(int size) {
        int[] arr = new int[size];
        Random random = new Random();
        for (int i = 0; i < size; i++) {
            arr[i] = random.nextInt(1000);
        }
        return arr;
    }

    // 并行归并排序任务
    static class MergeSortTask extends RecursiveAction {
        private final int[] arr;
        private int[] result;

        public MergeSortTask(int[] arr) {
            this.arr = arr;
        }

        public int[] getResult() {
            return result;
        }

        @Override
        protected void compute() {
            if (arr.length <= THRESHOLD) {
                result = serialMergeSort(arr);
                return;
            }

            int mid = arr.length / 2;
            int[] left = Arrays.copyOfRange(arr, 0, mid);
            int[] right = Arrays.copyOfRange(arr, mid, arr.length);

            MergeSortTask leftTask = new MergeSortTask(left);
            MergeSortTask rightTask = new MergeSortTask(right);

            invokeAll(leftTask, rightTask);

            result = merge(leftTask.getResult(), rightTask.getResult());
        }
    }
}

这个示例中，首先生成一个随机数组，并使用串行归并排序和并行归并排序对数组进行排序。然后，验证两种排序方法得到的结果是否相同。