https://blog.davidvassallo.me/2019/11/04/speeding-up-spring-mvc-with-completablefuture/

completableFutureSpringMVC.kt

package com.example.mvc

import org.springframework.web.bind.annotation.GetMapping
import org.springframework.web.bind.annotation.RestController
import java.lang.Thread.sleep
import java.util.concurrent.CompletableFuture
import java.util.concurrent.ExecutorService
import java.util.concurrent.Executors

@RestController
class Controller {

    val worker = {
        sleep(1000)
        "Done"
    }

    private val pool : ExecutorService = Executors.newFixedThreadPool(500)

    @GetMapping("/asyncTest")
    fun asyncTest() : CompletableFuture<String> {
        return CompletableFuture.supplyAsync(worker, pool::execute)
    }

    @GetMapping("/syncTest")
    fun syncTest() : String {
        return worker()
    }
}