Try Kotlin with spring-boot 2.0.0M3

demo.kt

package com.example.demo

import org.springframework.beans.factory.annotation.Autowired
import org.springframework.boot.SpringApplication
import org.springframework.boot.autoconfigure.EnableAutoConfiguration
import org.springframework.boot.autoconfigure.SpringBootApplication
import org.springframework.stereotype.Service
import org.springframework.web.bind.annotation.*

@SpringBootApplication
@EnableAutoConfiguration
class DemoApplication

fun main(args: Array<String>) {
    SpringApplication.run(DemoApplication::class.java, *args)
}

@RestController
class MyRestCtrl(@Autowired val service:MyService){
    @GetMapping("/users") fun users() = service.get()
    @GetMapping("/user/{name}") fun user(name:String) = service.get().find { it.name==name }
    @DeleteMapping("/users") fun clear(name:String) = service.reset()
    @PostMapping("/user") fun put(@RequestBody u:User):User {
        service.add(u)
        return u
    }
}

data class User(val name:String, val age:Int)

@Service class MyService{
    val lst: MutableList<User> = mutableListOf()
    fun add(u:User) = lst.add(u)
    fun reset() = lst.clear()
    fun get() = lst
}