Getting Started with Kotlin Spring: A Step-by-Step Guide

Welcome to Spring with Kotlin, where modern programming meets robust web development. This guide explores how Spring Boot simplifies application development and how Kotlin's conciseness and null safety enhance your code.

In this blog, we'll walk you through setting up your environment, creating a new project with IntelliJ IDEA, and building a simple "Hello World" application. By the end, you'll know how to create, configure, and run a Spring Boot application using Kotlin, laying the groundwork for more advanced topics.

Let's dive in and harness the power of Spring Boot and Kotlin for efficient, clean web development!

Overview of Spring Boot with Kotlin

Spring Boot, a robust framework from the Spring team, simplifies the development of new Spring applications through its convention-over-configuration approach. It's designed to get your Spring applications up and running as quickly as possible. The Spring Framework underlies this, offering a comprehensive programming and configuration model.

Kotlin, a modern language running on the JVM, is known for its conciseness and expressive power. Spring Boot's first-class support for Kotlin makes it a potent combination for building web applications. The Kotlin language integrates seamlessly with all Spring modules, including Spring Boot Starter Web and Spring Data JPA. This synergy leverages Kotlin's features like null safety and data class facilities, enhancing your Spring Boot applications with cleaner, more maintainable code.

Objectives of the Blog

In this blog, you'll discover how to harness the power of Spring Boot with Kotlin to create efficient, robust web applications. Whether you're a seasoned Java developer or new to the world of Spring and Kotlin, this tutorial shows you step-by-step how to set up your environment, create a new project using IntelliJ IDEA, and build a simple "Hello World" application. We'll also delve into the specifics of configuring and running your Spring Boot application, ensuring you have a solid foundation to explore more advanced topics in future articles.

You'll learn to navigate Spring Initializr, understand the importance of dependencies like Spring Boot Starter Test, and appreciate Kotlin’s enhancements such as extension functions and first-class properties support. By the end of this article, you will be able to create, configure, and run a Spring Boot application using Kotlin and understand the core principles that underpin these technologies.

Let's dive in and start by setting up your development environment to leverage the robust capabilities of Spring Boot and Kotlin, boosting your productivity and the performance of your applications.

Setting Up the Development Environment

Installing Java and Kotlin

Before diving into the creation of a Spring Boot application using Kotlin, it is essential to set up your development environment starting with the installation of Java and Kotlin. Since Kotlin runs on the JVM (Java Virtual Machine), having Java installed is a prerequisite.

Java Installation

1. Download and Install Java: You can download the latest version of Java JDK from the official Oracle website or use OpenJDK. Ensure you download a version compatible with Spring Boot (as of this writing, Spring Boot 2.5 supports Java 8 through 16).

   For Windows:

   1    choco install openjdk

   For macOS:

   1    brew install java

2. Set JAVA_HOME: Set the JAVA_HOME environment variable to point to the base directory of your Java installation. Update your system's PATH to include the Java bin directory.

Kotlin Installation

Installing Kotlin is straightforward because it does not require a separate installation if you are using an IDE like IntelliJ IDEA, which comes with Kotlin plugin support. However, if you wish to use Kotlin from the command line, you can install it using SDKMAN or Homebrew:

1sdk install kotlin

or

1brew install kotlin

​

Setting Up IntelliJ IDEA

IntelliJ IDEA by JetBrains is an excellent IDE for developing Kotlin applications, especially with Spring Boot, due to its robust support for Kotlin and Java.

IntelliJ IDEA Installation

1. Download IntelliJ IDEA: Visit the official JetBrains website to download the Community Edition for free or the Ultimate Edition if you need advanced features such as full support for web and enterprise development.

2. Install IntelliJ IDEA: Follow the installation instructions specific to your operating system. On Windows, this typically involves running the downloaded installer and following the on-screen instructions. On macOS, drag the IntelliJ IDEA icon to your Applications folder.

Configuring IntelliJ IDEA

1. Install Kotlin Plugin: Although IntelliJ IDEA comes with the Kotlin plugin installed by default, it's a good practice to check for updates:

   • Open IntelliJ IDEA.

   • Go to File > Settings > Plugins and search for Kotlin. Click on Update if it's available.

2. Create/Import Project: Start a new project or import an existing one:

   • To create a new project, select Create New Project from the Welcome screen, then choose Kotlin on the left panel and configure your project settings, such as the SDK (pointing to the JDK you installed earlier).

3. Configure Project SDK and Kotlin: Ensure your project is set up with the correct SDK and Kotlin version:

   • Right-click on your project in the Project Explorer.

   • Select Open Module Settings.

   • In the Project Settings section, set the Project SDK to your installed JDK and specify the Kotlin version you intend to use.

Creating a New Spring Boot Project

Setting up a new Spring Boot project with Kotlin is streamlined through the use of Spring Initializr, which is integrated into IntelliJ IDEA. This tool helps you quickly bootstrap a new Spring Boot application by generating project structure, dependencies, and basic configuration files. Here’s how you can create and configure your new project.

Using Spring Initializr

Spring Initializr is a web-based utility that makes it easy to create Spring Boot projects from scratch. IntelliJ IDEA incorporates Spring Initializr directly, simplifying the process even further.

Creating a Project via IntelliJ IDEA

1. Open IntelliJ IDEA: Start the application and select File > New > Project from the main menu.

2. Select Spring Initializr: Choose "Spring Initializr" from the left panel. Enter the URL for the Spring Initializr instance you are using, typically https://start.spring.io .

3. Set Project Details: Specify the type of project (Maven or Gradle), language (Kotlin), Spring Boot version, and other metadata like Project Group, Artifact, and Name.

4. Select the JVM Version: Ensure the Java version matches the JDK you installed earlier, supporting your Spring Boot version.

With these steps, IntelliJ IDEA will connect to Spring Initializr and generate a new Spring Boot project with Kotlin configured.

Configuring Project Metadata and Dependencies

Configuring the correct metadata and dependencies at the start of your project is crucial for proper project setup and future maintenance.

Setting Project Metadata

1. Group: Typically your organization or personal domain (e.g., com.example).

2. Artifact: The name of the project or application (e.g., helloworld).

3. Name: The name displayed in IntelliJ IDEA and other tools (e.g., Hello World Application).

4. Description: A brief description of what your project does.

5. Package Name: Follows your group and artifact to define your base package structure (e.g., com.example.helloworld).

Adding Dependencies

Select dependencies tailored to your project needs. Commonly used dependencies for a web application in Kotlin include:

• Spring Boot Starter Web: Provides all the necessary dependencies to build a web application. This includes Spring MVC and Tomcat as the default embedded server.

• Spring Boot Starter Data JPA: Facilitates implementing repositories using Spring Data JPA.

• Spring Boot Starter Test: Includes testing tools such as JUnit, Hamcrest, and Mockito.

• Jackson Module Kotlin: Adds support for Kotlin classes serialization and deserialization, respecting Kotlin’s nullability and default parameters.

• Kotlin Reflect: Needed for reflection in Kotlin, which is useful for some advanced features and integrations.

Here is how you would select these dependencies in the IntelliJ IDEA using Spring Initializr:

1Select Dependencies > Web > Spring Web
2Select Dependencies > SQL > Spring Data JPA
3Select Dependencies > Developer Tools > Spring Boot DevTools
4Select Dependencies > I/O > Jackson Kotlin Module

Once you have configured the metadata and selected the appropriate dependencies, click on "Finish" to generate the project. IntelliJ IDEA will create a new Spring Boot project directory with all the necessary files and structure, as well as download all the specified dependencies. You can now proceed to explore the generated project structure, ensuring all settings align with your project goals.

Exploring the Project Structure

Once you've created your Spring Boot project using Kotlin with IntelliJ IDEA, it's essential to understand its structure and the components that make up the project. This understanding will help you navigate and manage your application effectively.

Understanding the Maven or Gradle Build File

Spring Boot projects can be managed with either Maven or Gradle as the build system. The choice between Maven and Gradle depends on your preference or project requirements. Both systems manage project dependencies, build processes, and additional tasks needed to build and run your Spring Boot application.

Maven Build File: pom.xml

If you chose Maven when setting up your project, you would have a pom.xml file at the root directory. This XML file includes:

• Project Metadata: Such as group ID, artifact ID, and version.

• Dependencies: Libraries needed for your project (e.g., spring-boot-starter-web, kotlin-reflect).

• Plugins: Configuration for tools like the Kotlin Maven plugin, which compiles your Kotlin code.

• Properties: Defines properties like Kotlin version and Java version compatibility.

Here's a snippet from a typical pom.xml for a Kotlin Spring Boot project:

1<dependencies>
2    <dependency>
3        <groupId>org.springframework.boot</groupId>
4        <artifactId>spring-boot-starter-web</artifactId>
5    </dependency>
6    <dependency>
7        <groupId>org.jetbrains.kotlin</groupId>
8        <artifactId>kotlin-reflect</artifactId>
9        <version>${kotlin.version}</version>
10    </dependency>
11</dependencies>

​

Gradle Build File: build.gradle

If you chose Gradle, your project configuration would be in build.gradle (or build.gradle.kts for Kotlin syntax). This file similarly includes:

• Dependencies: Defined in a more concise syntax compared to XML.

• Plugins: Such as the Kotlin plugin for Gradle.

• Tasks: Custom tasks for building, running, and testing your application.

Example snippet from build.gradle:

1plugins {
2    id 'org.springframework.boot' version '2.5.0'
3    id 'io.spring.dependency-management' version '1.0.11.RELEASE'
4    id 'org.jetbrains.kotlin.jvm' version '1.5.10'
5}
6
7dependencies {
8    implementation 'org.springframework.boot:spring-boot-starter-web'
9    implementation 'org.jetbrains.kotlin:kotlin-reflect'
10}

​

Key Directories and Files

The typical structure of a Spring Boot project consists of several key directories and files:

• src/main/kotlin: Contains your Kotlin source files. This is where you write your controllers, services, repositories, and other components.

• src/main/resources: This directory contains resources like static files, templates, and the application.properties or application.yml file for configuring your Spring Boot application.

• src/test/kotlin: Holds your test files, where you can write tests using frameworks supported by the spring-boot-starter-test dependency.

• application.properties or application.yml: Configuration files where you can specify your application's settings, such as server port, database configurations, and more.

Writing the Hello World Application

Creating a "Hello World" application is a traditional way to start with any programming language or framework, as it helps you understand the basic setup and execution flow. In Spring Boot with Kotlin, you will set up a simple application that demonstrates the ease of starting a web server and serving a basic response.

Creating a Kotlin Class with a Main Method

In Kotlin, the entry point to any application is the main method. For a Spring Boot application, you typically set up a class annotated with @SpringBootApplication and include a main function that runs the application.

Here’s how you can create this setup in your Spring Boot project:

1. Navigate to the src/main/kotlin directory: This is where your Kotlin source files reside.

2. Create a Kotlin Class: Right-click on the package where you want to create your file, choose New > Kotlin Class/File. Name it HelloWorldApplication.

3. Write the Main Method: In Kotlin, the main method is defined as follows:

1import org.springframework.boot.autoconfigure.SpringBootApplication
2import org.springframework.boot.runApplication
3
4@SpringBootApplication
5class HelloWorldApplication
6
7fun main(args: Array<String>) {
8    runApplication<HelloWorldApplication>(*args)
9}

​

In the above code:

• @SpringBootApplication is an annotation that marks the class as a Spring Boot application entry point.

• runApplication<HelloWorldApplication>(*args) is a Kotlin way of bootstrapping the Spring Boot application, using the class as the configuration source.

Using the @SpringBootApplication Annotation

The @SpringBootApplication annotation is a convenience annotation that adds:

• @Configuration: Tags the class as a source of bean definitions for the application context.

• @EnableAutoConfiguration: Tells Spring Boot to start adding beans based on classpath settings, other beans, and various property settings.

• @ComponentScan: Enables @Component scan on the package where the application is located.

These annotations together set up a configuration that allows Spring Boot to automate much of the application setup and configuration processes.

Creating a Simple Web Controller

To complete your "Hello World" application, you will now add a controller that handles web requests. Create a new Kotlin class called HelloController in the same package:

1import org.springframework.web.bind.annotation.GetMapping
2import org.springframework.web.bind.annotation.RestController
3
4@RestController
5class HelloController {
6    @GetMapping("/")
7    fun sayHello(): String {
8        return "Hello, World from Spring Boot and Kotlin!"
9    }
10}

​

In this controller:

• @RestController marks this class as a controller where every method returns a domain object instead of a view.

• @GetMapping("/") specifies that the sayHello method should be called when the root URL (/) is accessed.

Now, when you run your Spring Boot application and navigate to localhost:8080, you will see the greeting "Hello, World from Spring Boot and Kotlin!" displayed in your browser. This demonstrates how quickly you can get a web application up and running with Spring Boot and Kotlin.

Configuring Application Properties

In Spring Boot, the application.properties or application.yml files are crucial for defining the behavior of your application under different environments and conditions. These files allow you to set configuration properties that control various aspects of a Spring Boot application.

Understanding application.properties or application.yml

Spring Boot supports two types of configuration files: application.properties and application.yml. Both serve the same purpose but differ in format. The properties file uses a simple key-value pair format, while the YAML file offers a more readable tree-like structure, which is beneficial for complex configurations.

application.properties

This file is located in the src/main/resources directory. Here’s an example showing how you might configure common properties:

1spring.application.name=HelloWorldApp
2server.port=8080
3spring.datasource.url=jdbc:mysql://localhost/test
4spring.datasource.username=dbuser
5spring.datasource.password=dbpass

​

application.yml

Alternatively, you can use the YAML format for more structured data, which is also placed in the same directory:

1spring:
2  application:
3    name: HelloWorldApp
4  datasource:
5    url: jdbc:mysql://localhost/test
6    username: dbuser
7    password: dbpass
8server:
9  port: 8080

​

These files allow you to specify various settings such as database connection details, server port, and application-specific properties.

Setting Server Port and Context Path

Customizing the server port and the context path are common requirements for any web application.

Server Port

By default, a Spring Boot application runs on port 8080. However, you can change this by setting the server.port property in your configuration file:

application.properties

1server.port=9090

​

application.yml

1server:
2  port: 9090

​

Setting the port to 9090 means your application will now be accessible at http://localhost:9090.

Context Path

The context path is the prefix of a URL path that is used to determine which application the request is forwarded to. By default, Spring Boot serves content from the root path (/). If you need to serve your application at a different path, you can set the server.servlet.context-path property:

application.properties

1server.servlet.context-path=/helloapp

​

application.yml

1server:
2  servlet:
3    context-path: /helloapp

​

With this configuration, all URLs for your application will now start with /helloapp, so the URL to access your application will be http://localhost:9090/helloapp.

Configuring these properties correctly ensures that your Spring Boot application integrates seamlessly with your overall system and meets the operational requirements. It’s a vital step for both development and production environments, helping to tailor the application’s behavior to specific needs.

Running the Spring Boot Application

After setting up and configuring your Spring Boot project, the next step is to run the application to see it in action. IntelliJ IDEA provides a seamless and efficient environment for running Spring Boot applications directly from the IDE. Additionally, verifying the output ensures that the application is running as expected.

Using IntelliJ IDEA to Run the Application

Running a Spring Boot application in IntelliJ IDEA is straightforward. Here’s how you can do it:

1. Open Your Project: Start IntelliJ IDEA and open your project by selecting Open and navigating to your project directory.

2. Navigate to the Main Class: The main class is the one with the @SpringBootApplication annotation and contains the main method. In the Project Explorer, navigate to src/main/kotlin and find your main class, usually named HelloWorldApplication.kt.

3. Run the Application:

   • Right-click on the file in the Project Explorer.

   • Select Run 'HelloWorldApplication.kt' from the context menu.

Alternatively, you can use the green play button in the gutter next to the main method to run the application.

4. View the Run Output: The bottom pane in IntelliJ IDEA will show the output from your application. You should see messages indicating that the Spring Boot application has started successfully, along with the port it is running on (e.g., Started HelloWorldApplication in 2.345 seconds (JVM running for 3.123)).

Verifying Output in the Browser or Console

Once the application is running, you can verify its correct behavior by accessing the provided endpoints or by observing the console output.

Checking the Web Output

If your application includes a web controller, like the HelloController created earlier, you can test it by:

• Opening a Web Browser: Navigate to http://localhost:8080/ or to whatever port and context path you have configured.

• Verifying the Response: You should see the response from your HelloController, which might be something like Hello, World from Spring Boot and Kotlin!.

Reviewing Console Output

The console output is crucial for debugging and understanding what happens when your application runs. In IntelliJ IDEA’s console window, you should:

• Look for any error messages that might indicate problems.

• Ensure that application startup messages appear as expected, confirming that components are correctly configured and initialized.

This real-time feedback loop, from running the application to verifying its output, is essential for effective development. It allows you to quickly identify and address issues, ensuring that your Spring Boot application works correctly before moving on to more complex development tasks or deploying the application to a production environment.

Running and verifying a Spring Boot application with Kotlin in IntelliJ IDEA demonstrates how effectively these tools work together to provide a powerful development experience that is both efficient and developer-friendly.

Conclusion

In this blog, we’ve guided you through setting up and developing a basic Spring Boot application using Kotlin, highlighting the seamless integration and powerful features of both technologies. Here’s a quick recap of what we covered:

Key Takeaways

• Efficient Setup: We used IntelliJ IDEA and Spring Initializr to quickly scaffold a new Spring Boot project, showcasing the efficiency of setting up a development environment.

• Simplified Coding: Kotlin's concise syntax reduces boilerplate, which, when combined with Spring Boot's configuration magic, speeds up development and maintenance.

• Application Structure and Execution: Understanding the project structure and configuration files like application.properties or application.yml is crucial. Running the application directly from IntelliJ IDEA demonstrated the ease of testing and debugging.

To enhance your skills, explore more advanced topics in Spring Boot and Kotlin such as security enhancements, microservices, and cloud deployments. Continuous learning and experimentation will help you leverage these tools to their full potential, enabling the creation of robust, scalable applications.

Stay curious and keep experimenting to stay ahead in the fast-evolving tech landscape. Happy coding!

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