Building RESTful APIs with Kotlin and Spring Boot

Introduction

In today’s digital landscape, building robust and scalable RESTful APIs is essential for modern application development. Kotlin, a versatile programming language, and Spring Boot, a powerful Java-based framework, make an excellent combination for creating high-performance APIs. In this blog, we will explore the process of building RESTful APIs using Kotlin and Spring Boot, complete with code examples. By the end, you’ll have a solid foundation for developing your own APIs with ease.

Table of Contents:

1. Overview of RESTful APIs
2. Introduction to Kotlin and Spring Boot
3. Setting up a Kotlin Spring Boot Project
4. Creating RESTful Endpoints with Spring Boot
   • 4.1. Implementing GET Requests
   • 4.2. Handling POST Requests
   • 4.3. Managing PUT and PATCH Requests
   • 4.4. Deleting Resources with DELETE Requests
5. Request and Response Handling
   • 5.1. Request Body Validation
   • 5.2. Handling Errors and Exceptions
   • 5.3. Customizing Response Formats
6. Security and Authentication
   • 6.1. Securing API Endpoints
7. Testing and Documentation
   • 7.1. Writing Unit Tests for APIs
   • 7.2. Generating API Documentation with Swagger
8. Conclusion and Further Resources

1. Overview of RESTful APIs

REST (Representational State Transfer) is an architectural style that defines a set of principles for designing networked applications. RESTful APIs follow these principles and use HTTP methods (GET, POST, PUT, PATCH, DELETE) to perform operations on resources.

2. Introduction to Kotlin and Spring Boot

Kotlin is a modern programming language that runs on the Java Virtual Machine (JVM) and is fully interoperable with Java. Spring Boot is a framework built on top of the Spring framework that simplifies the development of Java and Kotlin applications. It provides an opinionated approach to application configuration, dependency management, and embedded server deployment.

Prerequisites:

To follow along with this tutorial, you should have a basic understanding of Kotlin programming and RESTful API concepts. Additionally, ensure that you have Kotlin and Spring Boot installed on your development environment.

3. Setting up a Kotlin Spring Boot Project

To get started, set up a new Kotlin Spring Boot project by following these steps:

1. Create a new Spring Boot project using the Spring Initializr (https://start.spring.io/) or your preferred IDE.
2. Add the necessary dependencies for building RESTful APIs, such as Spring Web and Spring Data JPA.
3. Configure the project settings, such as application properties and database connectivity.

Step 1: Create a New Spring Boot Project

Now, let’s create RESTful endpoints for performing CRUD operations on resources. We’ll demonstrate the implementation of various HTTP methods using practical examples.

Open your preferred Integrated Development Environment (IDE), such as IntelliJ IDEA or Eclipse, and create a new Spring Boot project. You can also utilize Spring Initializr to quickly generate a new project with the necessary dependencies.

Step 2: Add Required Dependencies

To build RESTful APIs, we need to include the following dependencies in our project:

dependencies {
    implementation("org.springframework.boot:spring-boot-starter-web")
    implementation("org.jetbrains.kotlin:kotlin-stdlib-jdk8")
}

Ensure that you have the appropriate configuration in your build.gradle.kts or pom.xml file, depending on whether you are using Gradle or Maven as your build tool.

4. Create a RESTful API Endpoint

To illustrate the process, let’s create a simple API endpoint to manage tasks. Open your IDE and create a new Kotlin class called BankController.kt. Use the following code as a starting point:

@RestController
@RequestMapping("api/banks")
class BankController {
}

In the above code snippet, we define a BankController class, annotated @RestController to handle incoming HTTP requests. We specify that all API endpoints under this controller will have the base URL /tasks. The class includes various methods for different CRUD operations (Create, Read, Update, Delete) on tasks.

4.1. Implementing GET Requests

Here, we will explore how to implement GET requests in Spring Boot. We will cover retrieving resources from a database or other data source and returning them as HTTP responses.

       @get:GetMapping
       val banks: Collection<Bank>
        get() = service.getBanks()


    @GetMapping("/{accountNum}")
    fun getBank(@PathVariable accountNum: String?): Bank {
        return service.getBank(accountNum!!)
    }

4.2. Handling POST Requests

This section will focus on handling POST requests, which involve creating new resources. We will demonstrate how to receive and process data sent via POST requests and store it in a database.

    @PostMapping
    @ResponseStatus(HttpStatus.CREATED)
    fun addBank(@RequestBody bank: Bank?): Bank {
        return service.addBank(bank!!)
    }

4.3. Managing PUT and PATCH Requests

In this part, we will cover the implementation of PUT and PATCH requests. Readers will learn how to update existing resources, either by replacing the entire resource (PUT) or partially modifying it (PATCH).

    @PatchMapping
    fun updateBank(@RequestBody bank: Bank?): Bank {
        return service.updateBank(bank!!)
    }

4.4. Deleting Resources with DELETE Requests

Here, we will explain how to handle DELETE requests, which are used to delete specific resources. Readers will understand how to delete resources from a database or perform other necessary operations.

5. Request and Response Handling

This section will focus on handling request and response data effectively in RESTful APIs. We will cover various aspects, including request body validation, error, and exception handling, and customizing response formats.

    @DeleteMapping("/{accountNum}")
    @ResponseStatus(HttpStatus.NO_CONTENT)
    fun deleteBank(@PathVariable accountNum: String?) {
        service.deleteBank(accountNum!!)
    }

5.1. Request Body Validation

We will discuss the importance of validating request bodies and demonstrate how to perform validation using tools and annotations provided by Spring Boot. Readers will learn how to ensure that the incoming data meets specific criteria before processing it.

Here’s an example of how the Bank class might be defined with validation annotations:

data class Bank(
    @field:NotNull
    val accountNum: String?,
    
    @field:NotNull
    @field:Size(min = 2, max = 100)
    val name: String?,
    
    // Other fields and validation annotations
)

In this example, we’ve added @NotNull and @Size annotations to the accountNum and name fields, respectively. This ensures that these fields are not null and that the name field should have a minimum length of 2 and a maximum length of 100 characters.

By applying these validation annotations to the fields of the Bank class, the Spring Boot framework automatically performs the validation when the addBank the method is called. If any validation constraint is violated, Spring Boot will throw a MethodArgumentNotValidException with the appropriate error messages.

5.2. Handling Errors and Exceptions

In this part, we will cover error and exception handling in RESTful APIs. Readers will learn how to gracefully handle different types of errors and exceptions, providing meaningful error messages and appropriate HTTP status codes.

    @ExceptionHandler(NoSuchElementException::class)
    fun handleNotFound(e: NoSuchElementException): ResponseEntity<String> {
        return ResponseEntity.status(HttpStatus.NOT_FOUND).body(e.message)
    }

 
    @ExceptionHandler(IllegalArgumentException::class)
    fun handleBadRequest(e: IllegalArgumentException): ResponseEntity<String> {
        return ResponseEntity.status(HttpStatus.NOT_FOUND).body(e.message)
    }

5.3. Customizing Response Formats

Here, we will explain how to customize the format of API responses, such as using different data serialization formats (JSON, XML) or customizing response headers. Readers will understand how to tailor the responses to meet specific requirements.

[
    {
        "account_number": "ACC",
        "trust": 3.14,
        "default_transaction_fee": 500
    },
    {
        "account_number": "ACC2",
        "trust": 5.14,
        "default_transaction_fee": 1000
    },
    {
        "account_number": "ACC3",
        "trust": 6.14,
        "default_transaction_fee": 4000
    }
]

This section will focus on securing RESTful APIs and implementing authentication and authorization mechanisms to protect sensitive data and resources.

6. Security and Authentication

6.1. Securing API Endpoints

We will discuss the importance of securing API endpoints and explore various security measures such as SSL/TLS, rate limiting, and IP whitelisting. Readers will gain insights into best practices for ensuring the security of their APIs.
To secure API endpoints, you can integrate authentication and authorization mechanisms into your Kotlin Spring Boot application. Here’s an example of how to secure the API endpoints using Spring Security:

Configure Security Dependencies: Add the following dependencies to your build.gradle or pom.xml file:

For Gradle:

implementation 'org.springframework.boot:spring-boot-starter-security'

For Maven:

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-security</artifactId>
</dependency>

Secure API Endpoints: Update your controller class to include appropriate security annotations:

import org.springframework.security.access.annotation.Secured
import org.springframework.security.core.context.SecurityContextHolder
import org.springframework.web.bind.annotation.*

@RestController
@RequestMapping("/api/banks")
class BankController(private val service: BankService) {

    @GetMapping("/{accountNum}")
    @Secured("ROLE_USER")
    fun getBank(@PathVariable accountNum: String?): Bank {
        // Access the authenticated user details if needed
        val username = SecurityContextHolder.getContext().authentication.name

        return service.getBank(accountNum!!)
    }

    // Other controller methods
}

Run the Application

Now that we have our API endpoints defined, it’s time to run the application and test the functionality. Either the command-line interface or your IDE can be used to run the main method. Once the application is operational, you can use programs like Postman or Curl to test the API endpoints.

7. Testing and Documentation

This section will cover testing and documenting RESTful APIs, ensuring their quality and ease of use for developers.

7.1. Writing Unit Tests for APIs

We will discuss the importance of unit testing in API development and demonstrate how to write effective unit tests for different components of the RESTful API, including controllers, services, and data access layers.

@SpringBootTest
@AutoConfigureMockMvc
internal class BankControllerTest @Autowired constructor(
    val mockMvc: MockMvc,
    val objectMapper: ObjectMapper
) {

    val baseUrl = "/api/banks"

    @Nested
    @DisplayName("GET /api/banks")
    @TestInstance(Lifecycle.PER_CLASS)
    inner class GetBanks {

        @Test
        fun `should return all banks`() {
            mockMvc.get(baseUrl)
                .andDo { print() }
                .andExpect {
                    status { isOk() }
                    content { contentType(MediaType.APPLICATION_JSON) }
                    jsonPath("$[0].account_number") { value("ACC") }
                }
        }
    }

7.2. Generating API Documentation with Swagger

Here, we will explore the usage of Swagger for generating API documentation. Readers will learn how to annotate API endpoints with Swagger annotations and generate interactive documentation for their APIs, simplifying API consumption and enhancing collaboration.

@Configuration
class SpringFoxConfiguration {
  
    @Bean
    fun api(): Docket? {
        return Docket(DocumentationType.SWAGGER_2)
                .apiInfo(apiInfo())
                .select()
                .apis(Predicate.not(RequestHandlerSelectors.basePackage("org.springframework.boot")))
                .build()
    }
    private fun apiInfo(): ApiInfo? {
        return ApiInfoBuilder()
                .title("BANK'S SWAGGER API")
                .description("My API Is Listed Here ::")
                .version("0.0.12")
                .build()
    }
}

Conclusion:

In this blog post, we explored the process of building RESTful APIs with Kotlin and Spring Boot. We started by setting up a new Kotlin project with Spring Boot and added the necessary dependencies. Next, we created a TaskController class that included API endpoints for managing tasks. By combining the power of Kotlin and Spring Boot, we developed a robust and scalable API in a concise manner.

This blog only scratches the surface of what Kotlin and Spring Boot can offer for API development. You can further explore advanced features such as request validation, authentication, authorization, and database integration to enhance your APIs. Remember to refer to the official documentation and explore other resources to expand your knowledge in this domain.

References:

1. Kotlin Official Documentation: https://kotlinlang.org/docs/home.html
2. Spring Boot Official Documentation: https://spring.io/projects/spring-boot
3. Spring Framework Official Documentation: https://spring.io/projects/spring-framework
4. RESTful API Design Principles: https://restfulapi.net
5. Spring Security Official Documentation: https://spring.io/projects/spring-security