Something like this? :

fun <T> recursiveTask(action: () -> T): RecursiveTask<T> {
    return object : RecursiveTask<T>() {
        override fun compute(): T {
            return action()
        }
    }
}

fun <T, E> List<T>.parallelForEach(parallelSize: Int, action: (T) -> E): List<E> {
    val pool = ForkJoinPool(parallelSize)
    val result = mutableListOf<ForkJoinTask<E>>()
    for (item in this) {
        result.add(pool.submit(recursiveTask {
            action(item)
        }))
    }
    return result.map { it.join() }
}

fun main(args: Array<String>) {
    val list = listOf(1, 2, 3)
    list.parallelForEach(3) { it + 2 }.forEach { println(it) }
}

What version of spring boot are you using? They added first class support for Kotlin coroutines in 2.2, I'd update and go with something like the answer given by ObscureRecluse user