/kotlin-tooling-java-to-kotlin

Spring Framework Conversion Guide

When This Applies

This guide applies when the Java source contains imports matching org.springframework.*. This covers Spring Boot, Spring MVC, Spring Data, and Spring Security.

Key Rules

1. SpringApplication.run — spread CLI args

In Kotlin, String[] varargs must be spread with the * operator.

• Java: SpringApplication.run(App.class, args);
• Kotlin: SpringApplication.run(App::class.java, *args)

2. Constructor injection over @Autowired

Kotlin's primary constructor makes constructor injection natural. When a class has a single constructor, Spring auto-discovers it — remove @Autowired.

3. Stereotype annotations

@Component, @Service, @RestController, and @Repository target the class. Preserve these annotations exactly. No annotation site target is needed.

4. @Value annotation

Use @Value on constructor parameters. Escape $ in SpEL expressions to prevent Kotlin string template interpretation:

@Value("\${app.name}") val appName: String

5. @ConfigurationProperties

Convert to a data class only if the properties are immutable. For mutable configuration, use a regular class with lateinit var.

6. Spring Data repositories

Interface declarations convert directly. Replace Optional<T> return types with nullable T? in Kotlin for idiomatic usage.

7. @RequestMapping / @GetMapping / @PostMapping etc.

Preserve exactly. Where Java uses array initializer syntax for annotation parameters, use arrayOf() in Kotlin.

8. @Transactional

Preserve exactly. The class must remain open because Spring creates proxies via subclassing. Do not make @Transactional classes final.

9. @Bean methods in @Configuration classes

@Bean methods must be open so that Spring can override them in CGLIB proxies. Alternatively, apply the allopen compiler plugin with a Spring preset, which makes annotated classes and their members open automatically.

Examples

Example 1: Spring Boot Application Main Class

Java:

package com.acme;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

@SpringBootApplication
public class Application {
    public static void main(String[] args) {
        SpringApplication.run(Application.class, args);
    }
}

Kotlin:

package com.acme

import org.springframework.boot.SpringApplication
import org.springframework.boot.autoconfigure.SpringBootApplication
import org.springframework.boot.runApplication

@SpringBootApplication
open class Application

fun main(args: Array<String>) {
    runApplication<Application>(*args)
}

Key changes:

• main becomes a top-level function (no companion object needed).
• runApplication<T> is a Spring Boot Kotlin extension that replaces SpringApplication.run(T::class.java, *args).
• The *args spread operator is required for the varargs parameter.

Example 2: REST Controller with Constructor Injection

Java:

package com.acme.web;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.PathVariable;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RestController;

@RestController
@RequestMapping("/api/users")
public class UserController {

    private final UserService userService;

    @Autowired
    public UserController(UserService userService) {
        this.userService = userService;
    }

    @GetMapping("/{id}")
    public UserDto getUser(@PathVariable Long id) {
        return userService.findById(id);
    }

    @GetMapping
    public List<UserDto> getAllUsers() {
        return userService.findAll();
    }
}

Kotlin:

package com.acme.web

import org.springframework.web.bind.annotation.GetMapping
import org.springframework.web.bind.annotation.PathVariable
import org.springframework.web.bind.annotation.RequestMapping
import org.springframework.web.bind.annotation.RestController

@RestController
@RequestMapping("/api/users")
class UserController(
    private val userService: UserService
) {

    @GetMapping("/{id}")
    fun getUser(@PathVariable id: Long): UserDto? {
        return userService.findById(id)
    }

    @GetMapping
    fun getAllUsers(): List<UserDto> {
        return userService.findAll()
    }
}

Key changes:

• @Autowired is removed — Spring auto-discovers the single constructor.
• The Autowired import is removed because it is no longer referenced.
• Constructor parameter becomes a private val in the primary constructor.
• Return type UserDto becomes UserDto? where the service may return null.

Example 3: @ConfigurationProperties Class

Java:

package com.acme.config;

import org.springframework.boot.context.properties.ConfigurationProperties;
import org.springframework.stereotype.Component;

@Component
@ConfigurationProperties(prefix = "app.mail")
public class MailProperties {

    private String host;
    private int port = 587;
    private String username;
    private String password;

    public String getHost() { return host; }
    public void setHost(String host) { this.host = host; }

    public int getPort() { return port; }
    public void setPort(int port) { this.port = port; }

    public String getUsername() { return username; }
    public void setUsername(String username) { this.username = username; }

    public String getPassword() { return password; }
    public void setPassword(String password) { this.password = password; }
}

Kotlin (mutable config with lateinit var):

package com.acme.config

import org.springframework.boot.context.properties.ConfigurationProperties
import org.springframework.stereotype.Component

@Component
@ConfigurationProperties(prefix = "app.mail")
open class MailProperties {
    lateinit var host: String
    var port: Int = 587
    lateinit var username: String
    lateinit var password: String
}

Key changes:

• Getters and setters are replaced by Kotlin properties.
• lateinit var is used for required String properties that Spring populates after construction.
• port keeps its default value and uses a regular var (lateinit does not support primitive types).
• The class is open so that Spring can create a CGLIB proxy for it.