Kotlin and Spring Boot: Getting Started

Learn how to use Spring Boot to create a web application with the help of Spring Initializr, build a REST API and test it. By arjuna sky kok.

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Spring is one of the most famous web frameworks for web developers. Many developers choose Spring because it’s powerful and supports Dependency Injection and Aspect-Oriented Programming. However, some developers dislike the Spring framework because it’s a Java web framework bloated with many XML configuration files.

Pivotal created Spring Boot, a project created on top of the Spring framework to address those criticisms. It’s opinionated and assumes how you develop web applications. Spring Boot also uses auto-configuration to set up your web application.

In other words, you no longer have to write a lot of XML configuration files. Fortunately, Spring Boot also supports Kotlin and Gradle, which means you don’t have to use Java and write XML files. Yeah! :]

In this tutorial, you’ll build an NFT marketplace because everyone is excited about NFTs. Throughout this process, you’ll:

  • Initialize a Spring Boot project.
  • Run the web application.
  • Write controller, model, configuration, exception and middleware.
  • Write and run tests.

Getting Started

You got a new job as a web developer at a startup. Your CEO asks you to build an NFT marketplace. She wants to go mobile-first, so you need to build API endpoints.

Your colleagues need the REST API to build a mobile app to trade NFT. You choose Spring Boot because, well, you read this tutorial. :]

To create a new Spring Boot project, head to Spring Initializr, a website for initializing your Spring Boot project:

Spring Initializr website

Choose Gradle Project in Project. Then select Kotlin as the Language. The default version in the Spring Boot will work just fine.

In the Project Metadata section, set com.raywenderlich in Group. Then fill nftmarketplace in Artifact and Name. Type The NFTs Marketplace in Description.

Use com.raywenderlich.nftmarketplace for Package name. Leave the default values for Packaging and Java.

Then, click Add in the Dependencies section:

Add button

The website will present a modal dialog where you can search for any Spring dependencies. For example, if you need a database, this is where you’d select the related component.

Select Spring Web since it comes with a web server where you’ll deploy the web application:

Spring dependencies modal dialog

The final screen looks like this:

Spring Initializr with configurated project

Click Generate at the bottom of the page. Your browser will download the project in a zip file:

Save project dialog

Download the file and extract it. That’s your starter project directory.

To work on this project, you need IntelliJ IDEA. You can learn how to install and set it up on the official IntelliJ IDEA website. Once you’re setup, move onto the next section.

Running the Project

Launch IntelliJ IDEA and open the starter project. Gradle will start syncing the project, which could take a couple of minutes.

In your Spring Boot project, you’ll find the web application code in the src folder. You’ll write the Kotlin code inside the main/kotlin folder.

Project structure

Before you start writing the code for your web application, open build.gradle.kts. This is your Spring Boot Gradle build file where you’ll install the web application’s dependencies and change the project’s configuration:

build.gradle.kts content

In the file, you’ll find the Spring Boot plugin inside the plugins block and further down the Java version for this project. The org.springframework.boot:spring-boot-starter-web dependency is inside the dependencies block. Remember you added the Spring Web component in Spring Initializr.

Time to run the web application and see what it looks like. Open NftmarketplaceApplication.kt:

Running NftmarketplaceApplication

Run the web application by clicking the green play button on the left side of the main function.

A pop-up dialog will appear. Click Run ‘NftmarketplaceApplic…’:

Running NftmarketplaceApplication pop-up dialog

As you can see in the output on the run tool window, IntelliJ IDEA runs the code:

Running NftmarketplaceApplication log output

Your Spring project comes with an embedded Tomcat web server, which runs on port 8080. So what are you waiting for? Open the URL http://localhost:8080 in a web browser and you’ll see:

Whitelabel error page

Unlike Django or Rails, Spring Boot doesn’t come with a default index page. It just gives you a 404 NOT FOUND page.

So, it’s time to work on your web application.

Creating a Controller

First, you’ll build a controller, specifically a REST controller. Under com.raywenderlich.nftmarketplace, create a package named controller by right-clicking it and then choosing New ▸ Package:

Creating a new package

A pop-up dialog will appear. Add controller to the filled-in string:

Creating a new package pop-up dialog

Then, under the controller directory, create a new file named NftmarketplaceController.kt by right-clicking controller and choosing New ▸ Kotlin Class/File:

Creating a new class

In the pop-up dialog, type NftmarketplaceController. Then make sure Class is selected before pressing Enter:

Naming the new class

You’ll get an empty class:

package com.raywenderlich.nftmarketplace.controller

class NftmarketplaceController {

Replace this empty class with:

package com.raywenderlich.nftmarketplace.controller

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

@RestController // 1
class NftmarketplaceController {

  @GetMapping("/homepage") // 2
  fun getHomePage() = "NFTs Marketplace"

Here’s a code breakdown:

  1. First, you annotate the class with the @RestController annotation. Spring Boot will scan it and recognize it as a controller class. Auto-configuration in Spring Boot means no need to write an XML file manually to register your controller.
  2. To map a GET request to a method, you annotate the method with @GetMapping passing the "/homepage" path as parameter. In this example, the method returns a string.

Rerun the project by clicking Run:

Rerunning the app

A pop-up dialog will appear asking if you’re sure you want to rebuild the project. Click Stop and Rerun:

Confirming to rerun app

Open http://localhost:8080/homepage. This time you get a text displayed on the web page:


In addition to getting data from the browser, you can also use curl:

curl http://localhost:8080/homepage

You’ll get:

NFTs Marketplace%

Instead of just simple strings, a REST API needs to return data in JSON format. But before that, you’ll create a model to represent the NFT.

Creating a Model

Create a new package under com.raywenderlich.nftmarketplace and name it model. Then create a new class under model and name it NFT. Replace the contents of the file with:

package com.raywenderlich.nftmarketplace.model

data class NFT(val id: Int, var name: String, var floor_price: Double)

The id and name are self-explanatory. The floor_price is an NFT’s cheapest price in the particular NFT’s collection.

Supporting a JSON GET Request

Go back to the controller code, NftmarketplaceController.kt. Create sample data of NFTs. Then, add the following code inside the class:

private var NFTs = mutableListOf(
  NFT(1, "CryptoPunks", 100.0),
  NFT(2, "Sneaky Vampire Syndicate", 36.9),
  NFT(3, "The Sevens (Official)", 0.6),
  NFT(4, "Art Blocks Curated", 1.1),
  NFT(5, "Pudgy Penguins", 2.5),

Then import:

import com.raywenderlich.nftmarketplace.model.NFT

You don’t need to do anything special to return this data in JSON format. Simply return the objects. Add the following method inside the class:

fun getNFTs() = NFTs

Then import:

import org.springframework.web.bind.annotation.GetMapping

Spring Boot will serialize your mutable list of data class instances to a JSON string behind the scenes. Look at build.gradle.kts and you’ll notice a JSON library, jackson-module-kotlin:

Jackson dependency

You didn’t install it explicitly, but it’s included when you install the Spring Web component.

Rerun the project and open http://localhost:8080:

NFTs in JSON format

The web application displays NFTs in JSON format.

Executing a curl command:

curl http://localhost:8080

You’ll get:

[{"id":1,"name":"CryptoPunks","floor_price":100.0},{"id":2,"name":"Sneaky Vampire Syndicate","floor_price":36.9},{"id":3,"name":"The Sevens (Official)","floor_price":0.6},{"id":4,"name":"Art Blocks Curated","floor_price":1.1},{"id":5,"name":"Pudgy Penguins","floor_price":2.5}]

You need a method to add a new NFT to your mutable list. To do that, you’ll need to support a JSON post request.

Supporting a JSON POST Request

To support this type of request, you’ll need the @PostMapping annotation. Create a method inside the controller class:

@PostMapping("") // 1
@ResponseStatus(HttpStatus.CREATED) // 2
fun postNFT(@RequestBody nft: NFT): NFT { // 3
  val maxId = NFTs.map { it.id }.maxOrNull() ?: 0 // 4
  val nextId = maxId + 1 // 5
  val newNft = NFT(id = nextId, name = nft.name, floor_price = nft.floor_price) // 6
  NFTs.add(newNft) // 7
  return newNft

You’ll need these imports:

import org.springframework.http.HttpStatus
import org.springframework.web.bind.annotation.PostMapping
import org.springframework.web.bind.annotation.RequestBody
import org.springframework.web.bind.annotation.ResponseStatus

Here’s what you did:

  1. First, you annotate the method with @PostMapping to indicate this method handles a POST request. You use the same path, "", as the previous GET request. That’s not a problem because one path can serve two different requests.
  2. Then you add @ResponseStatus with the HttpStatus.CREATED argument because, typically, after creating a resource like a new NFT, you emit the 201 CREATED response status. If you omit this annotation, you’ll get a default 200 OK response status.

  3. The method argument with the @RequestBody is the JSON object you’ll send to this path.
  4. Then you get the greatest id of the existing NFTs.
  5. You calculate the id for the new NFT.
  6. Then you create a new NFT with the calculated id, the name and floor_price, both from the request body.
  7. Finally, you add the new NFT to the list and return it as the response.

Rerun the project and create a new NFT executing this curl request:

curl -H "Content-Type: application/json" --request POST --data '{"name": "Cryptoadz", "floor_price": 26}' http://localhost:8080

And you’ll get:


Check the full list of NFTs by sending a GET request to http://localhost:8080:

curl http://localhost:8080

This is the result:

[{"id":1,"name":"CryptoPunks","floor_price":100.0},{"id":2,"name":"Sneaky Vampire Syndicate","floor_price":36.9},{"id":3,"name":"The Sevens (Official)","floor_price":0.6},{"id":4,"name":"Art Blocks Curated","floor_price":1.1},{"id":5,"name":"Pudgy Penguins","floor_price":2.5},{"id":6,"name":"Cryptoadz","floor_price":26.0}]

Mapping Path Variables

Checking one NFT from a list of a couple of NFTs isn’t a problem. But sometimes, you want to get one NFT’s information.

Create a new GET path by adding the following method to the class:

@GetMapping("/{id}") // 1
fun getNFTById(@PathVariable id: Int) : NFT? { // 2
  return NFTs.firstOrNull { it.id == id } // 3

Add the following import:

import org.springframework.web.bind.annotation.PathVariable

Here’s a code breakdown:

  1. This time, inside the @GetMapping annotation, you have an id parameter between curly braces.
  2. Because you also have a method parameter id: Int annotated with the @PathVariable annotation, your method receives that parameter whenever the server receives a GET request to /1, /2 or any number. Notice the return type is an optional NFT.
  3. You try to find the NFT with the desired id and return it. If it doesn’t exist, you’ll return null.

Rerun the project and execute the following curl requests:

curl http://localhost:8080/1

You’ll get:


Now, execute:

curl http://localhost:8080/100

Notice you don’t get any response since the NFT with id=100 doesn’t exist.

Now, try executing with the verbose option:

curl http://localhost:8080/100 -v

You’ll get:

*   Trying ::1...
* Connected to localhost (::1) port 8080 (#0)
> GET /100 HTTP/1.1
> Host: localhost:8080
> User-Agent: curl/7.64.1
> Accept: */*
< HTTP/1.1 200 
< Content-Length: 0
< Date: Sat, 30 Oct 2021 22:41:49 GMT
* Connection #0 to host localhost left intact
* Closing connection 0

As you can see, the service responds 200 OK, even if the NFT doesn't exist.

A better way to show the NFT doesn't exist is to return a 404 NOT FOUND result. You'll do that in the next section.

Handling Exceptions

Now, you'll create a custom exception for your missing NFT case.

Under com.raywenderlich.nftmarketplace, create a package named exception. Then inside the package, create the NFTNotFoundException class and replace the file's content with:

package com.raywenderlich.nftmarketplace.exception

class NFTNotFoundException : Exception()

Inside the same package, create the ControllerAdvice class. Name it NFTErrorHandler and replace the file's content with:

package com.raywenderlich.nftmarketplace.exception

import org.springframework.http.HttpStatus
import org.springframework.http.ResponseEntity
import org.springframework.web.bind.annotation.ControllerAdvice
import org.springframework.web.bind.annotation.ExceptionHandler
import javax.servlet.http.HttpServletRequest

@ControllerAdvice // 1
class NFTErrorHandler  {
  @ExceptionHandler(NFTNotFoundException::class) // 2
  fun handleNFTNotFoundException(
    servletRequest: HttpServletRequest,
    exception: Exception
  ): ResponseEntity<String> {
    return ResponseEntity("NFT not found", HttpStatus.NOT_FOUND) // 3

Here's what's happening:

  1. When you annotate the class with @ControllerAdvice, Spring Boot scans and registers this class as ControllerAdvice for your controller.
  2. You annotate the method with @ExceptionHandler, which accepts the exception class you created, letting Spring Boot know that this method can handle that exception.
  3. You return a ResponseEntity. The first argument of ResponseEntity is the result you send to the client which can be a simple text string or a JSON string. The second argument is the status type.

Your job isn't finished. Go back to the controller and replace getNFTById with:

import com.raywenderlich.nftmarketplace.exception.NFTNotFoundException


fun getNFTById(@PathVariable id: Int): NFT {
  val nft = NFTs.firstOrNull { it.id == id }
  return nft ?: throw NFTNotFoundException()

Now, instead of returning null, you throw an exception. Therefore, if you try to retrieve an NFT that doesn’t exist, you'll get a 404 NOT FOUND result.

Rerun the project and go to http://localhost:8080/100:

NFT not found

Or executing curl.

curl http://localhost:8080/100 -v

You'll see this result:

*   Trying ::1...
* Connected to localhost (::1) port 8080 (#0)
> GET /100 HTTP/1.1
> Host: localhost:8080
> User-Agent: curl/7.64.1
> Accept: */*
< HTTP/1.1 404 
< Content-Type: text/plain;charset=UTF-8
< Content-Length: 13
< Date: Sun, 31 Oct 2021 05:03:15 GMT
* Connection #0 to host localhost left intact
NFT not found* Closing connection 0


You got a feature request from your CEO. She wants to log the UTM marketing flag, so you can reward people who promote your NFT marketplace.

You could modify all of your controller's methods, but that would be too cumbersome. Instead, use middleware.

Under com.raywenderlich.nftmarketplace, create a new package named middleware. Inside middleware, create a class named RequestLoggingFilter and replace the content of the file with:

package com.raywenderlich.nftmarketplace.middleware

import org.slf4j.LoggerFactory
import org.springframework.stereotype.Component
import javax.servlet.Filter
import javax.servlet.FilterChain
import javax.servlet.ServletRequest
import javax.servlet.ServletResponse

class RequestLoggingFilter : Filter {
  val loggerFactory = LoggerFactory.getLogger("NFT Logger")

  override fun doFilter(
    servletRequest: ServletRequest,
    servletResponse: ServletResponse,
    filterChain: FilterChain
  ) {
    val utmSource = servletRequest.getParameter("utm_source")
    loggerFactory.info("Logging UTM source: $utmSource")
    filterChain.doFilter(servletRequest, servletResponse)

To execute code automatically inside middleware, you create a Filter class and annotate it with @Component so Spring Boot can register your class in middleware.

To add code inside middleware, you create doFilter, which accepts a request, a response and a filter chain. You get the UTM parameter from the request object. In this use case, you don't alter the response, but you could if you wanted to.

Rerun the application and execute:

curl "http://localhost:8080/1?utm_source=raywenderlich"

You'll get this response:


Check the logging output on the run tool window:

Logging utm source

Adding Configuration

Your CEO got another idea. Now she wants to build a white-label NFT marketplace application. You know: In a gold rush, sell shovels.

The idea is simple: You sell your web application to anyone who wants to build an NFT marketplace.

To do that, you need to display a company name flexibly. You can't embed the company code in your code because you want to sell your web application to many companies. That's where the configuration comes in.

You wrote code inside the main folder, specifically inside kotlin. But main has another child folder as well, resources:

Resources folder

resources has static, which contains static files like images, templatethat contains HTML files and application.properties. This file is where you put configuration.

Open application.properties and add:


The variable is on the left, and the value is on the right. They're separated by =.

To use this configuration in the controller, you use the @Value annotation and bind it to a state variable inside the class. Add the state variable inside the controller class:

private lateinit var name: String

Don't forget to import Value:

import org.springframework.beans.factory.annotation.Value

@Value accepts company_name interpolated inside a string. Spring Boot will bind the resulted value to name.

Change getHomePage to use name:

fun getHomePage() = "$name: NFTs Marketplace"

Rerun the project and execute this command:

curl http://localhost:8080/homepage

You'll get this output:

OpenSky: NFTs Marketplace

Here, you put configurations in one place. In addition to the company name, you can include information like the database configuration and the network configuration.

Requesting Mapping

Your CEO got another inspiration: Instead of going mobile-first, now she wants a traditional web application that displays HTML pages as well. So you must prepend the API path with /nfts so http://localhost:8080/2 becomes http://localhost:8080/nfts/2. You reserve the root URL for the traditional web application.

Easy peasy, right? You could do string substitution, but there’s a better way.

Annotate the controller with @RequestMapping. Then add the following code above the controller class declaration:


Then import:

import org.springframework.web.bind.annotation.RequestMapping

Now, Spring Boot prepends all API paths with /nfts. To test it out, rerun the project and execute:

curl http://localhost:8080/nfts/1

This is the response:



You have a working controller. Good!

Do you know what would be nice? Tests!

You'll put your test files in... the tests folder. You'll find it inside src, which is parallel to main.

You find a simple test file there, called NftmarketplaceApplicationTests. It looks like the following:

package com.raywenderlich.nftmarketplace

import org.junit.jupiter.api.Test
import org.springframework.boot.test.context.SpringBootTest

class NftmarketplaceApplicationTests {

 fun contextLoads() {


In this test file, the test class is annotated with @SpringBootTest. The @Test annotation says that the method is a test. In the next few sections, you'll update this code with a few tests.

Writing Your First Test

Replace the placeholder test with the code below:

package com.raywenderlich.nftmarketplace

import com.fasterxml.jackson.databind.ObjectMapper
import org.springframework.beans.factory.annotation.Autowired
import org.springframework.boot.test.autoconfigure.web.servlet.AutoConfigureMockMvc
import org.springframework.boot.test.context.SpringBootTest
import org.springframework.test.web.servlet.MockMvc

class NftMarketplaceApplicationTests(
  @Autowired val mockMvc: MockMvc,
  @Autowired val objectMapper: ObjectMapper
) {

To test the controller, you need MockMvc and ObjectMapper as the arguments of the test class's constructor. In addition, you annotate the class with @AutoConfigureMockMvc and annotate the arguments with @Autowired.

The MockMvc sends requests to the controller while ObjectMapper converts an object in Kotlin to a JSON string.

Create the first test case. Add the following method:

fun `Assert NFTs has CryptoPunks as the first item`() {
  mockMvc.get("/nfts") // 1
    .andExpect { // 2
      status { isOk() } // 3
      content { contentType(MediaType.APPLICATION_JSON) }
      jsonPath("$[0].id") { value(1) } // 4
      jsonPath("$[0].name") { value("CryptoPunks") }
      jsonPath("$[0].floor_price") { value(100) }
      jsonPath("$.length()") { GreaterThan(1) }

Don't forget to add the relevant imports:

import org.junit.jupiter.api.Test
import org.mockito.internal.matchers.GreaterThan
import org.springframework.http.MediaType
import org.springframework.test.web.servlet.get

Here's a code breakdown:

  1. First, you use the get of mockMvc to send a GET request.
  2. Then you chain it with andExpect which accepts a function block.
  3. Inside the block, you test the status of the response with status and the content type with content.
  4. Finally, you validate the JSON result with jsonPath which accepts an argument. As you can see, the $ represents the parsed object from the JSON string. To test the value of the argument of jsonPath, you need to wrap it with value or GreaterThan.

Time to run the test! Click the green play button on the left side of the method or class:

Running tests

A pop-up dialog will appear. Confirm it:

Confirm running tests

You can see the test result on the run tool window:

Test results

Now, you'll add another test.

Writing Your Second Test

This time you'll test the creating NFT method. Add this test method below your existing test method:

fun `Assert that we can create an NFT`() {
         .andExpect {
    status { isNotFound() }
  val newNFT = NFT(0, "Loot", 45.3)
  mockMvc.post("/nfts") {
    contentType = MediaType.APPLICATION_JSON
    content = objectMapper.writeValueAsString(newNFT)
  .andExpect {
     status { isCreated() }
     content { contentType(MediaType.APPLICATION_JSON) }
     jsonPath("$.name") { value("Loot") }
     jsonPath("$.floor_price") { value(45.3) }
     jsonPath("$.id") { value(6) }
         .andExpect {
    status { isOk() }
    content { contentType(MediaType.APPLICATION_JSON) }
    jsonPath("$.name") { value("Loot") }
    jsonPath("$.floor_price") { value(45.3) }
    jsonPath("$.id") { value(6) }

And import:

import com.raywenderlich.nftmarketplace.model.NFT
import org.springframework.test.web.servlet.post

Notice you use post on mockMvc with a function block on which you set the content type to JSON and the content to the JSON string. You use the writeValueAsString of objectMapper to convert a data class instance, newNFT, to the JSON string.

Run the test, and you'll get a successful result.

Where To Go From Here?

You can download the final version of this project by clicking Download Materials at the top or bottom of this tutorial.

This application isn't complete. As a challenge, write a PUT request to update an existing NFT and a DELETE request to remove an NFT.

Be sure to check out the official documentation.

Do you want to write the traditional web application with HTML pages with Spring Boot? Head to the tutorial on building a blog on the Spring website. On the tutorial page, you'll also learn how to use a database in your Spring Boot project instead of the in-memory list that you used in this tutorial.

There's also a tutorial from kotlinlang.org that you should check out.

If you want to learn how to write a REST API using Ktor or compare it against Spring Boot, check out this tutorial.

I hope you enjoyed this tutorial. If you have any questions or comments, please join the forum discussion below!