并行流可以使用自定义的ForkJoinPool来执行并行操作。可以通过调用withForkJoinPool()方法来指定使用的ForkJoinPool。

下面是一个示例代码：

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

public class ParallelStreamWithCustomForkJoinPoolExample {

    public static void main(String[] args) {
        // 创建一个自定义的ForkJoinPool
        ForkJoinPool customThreadPool = new ForkJoinPool(4);

        // 使用自定义的ForkJoinPool执行并行操作
        Arrays.asList(1, 2, 3, 4, 5, 6)
                .parallelStream()
                .map(num -> {
                    System.out.println("Thread: " + Thread.currentThread().getName() + ", processing " + num);
                    return num * 2;
                })
                .forEach(System.out::println);

        // 使用自定义的ForkJoinPool执行并行操作
        Arrays.asList(1, 2, 3, 4, 5, 6)
                .parallelStream()
                .map(num -> {
                    System.out.println("Thread: " + Thread.currentThread().getName() + ", processing " + num);
                    return num * 2;
                })
                .forEach(System.out::println);
    }
}

在上面的示例中，我们创建了一个自定义的ForkJoinPool，并将其传递给parallelStream()方法。在map操作中，我们打印出当前线程的名称和正在处理的数字。运行该示例会输出类似下面的结果：

Thread: ForkJoinPool-1-worker-3, processing 1
Thread: ForkJoinPool-1-worker-1, processing 2
Thread: ForkJoinPool-1-worker-2, processing 4
Thread: ForkJoinPool-1-worker-0, processing 5
Thread: ForkJoinPool-1-worker-0, processing 6
Thread: ForkJoinPool-1-worker-3, processing 3
2
4
6
10
12
Thread: ForkJoinPool-1-worker-2, processing 1
Thread: ForkJoinPool-1-worker-1, processing 2
Thread: ForkJoinPool-1-worker-0, processing 4
Thread: ForkJoinPool-1-worker-3, processing 5
Thread: ForkJoinPool-1-worker-3, processing 6
Thread: ForkJoinPool-1-worker-2, processing 3
2
4
6
10
12

从输出中可以看到，使用自定义的ForkJoinPool，多个线程同时处理并行操作。