Building RESTful APIs with Ktor

Ktor is a lightweight, asynchronous web framework that makes it easy to create high-performance RESTful APIs. Building RESTful APIs using Ktor is a great choice for Kotlin developers. In this blog, we’ll see how we can build a simple RESTful API using Ktor. Let’s get started:

Prerequisites

1. Basic knowledge of Kotlin programming language
2. JDK (Java Development Kit) installed
3. A code editor (e.g., IntelliJ IDEA, Visual Studio Code)
4. cURL or Postman for testing the API endpoints

Step 1. Go to https://ktor.io/create/ and from there generate a new ktor project and add routing plugin. It will download a sample ktor project for you.

Step 2. Add the following dependency in your project

dependencies{
        implementation("io.ktor:ktor-serialization-gson:$ktor_version")
        implementation("io.ktor:ktor-server-content-negotiation:$ktor_version")
    }

Step 3. Application.kt is the main entry point of our application

Step 4. Define Data Model given as below, for example we’re taking a ToDo app data model.

data class Todo(val id: Int, val title: String, val completed: Boolean)

Step 5: Implement the API Endpoints

define the API endpoints under the routing block, and also define in memory storage list before it.

val todos = mutableListOf<Todo>()
var currentId = 1

routing {
    route("/todos") {
        get {
            call.respond(todos)
        }
        get("/{id}") {
            val id = call.parameters["id"]?.toIntOrNull()
            val todo = todos.find { it.id == id }
            if (todo != null) {
                call.respond(todo)
            } else {
                call.respond(HttpStatusCode.NotFound)
            }
        }
        post {
            val todo = call.receive<Todo>()
            val newTodo = todo.copy(id = currentId++)
            todos.add(newTodo)
            call.respond(HttpStatusCode.Created, newTodo)
        }
        put("/{id}") {
            val id = call.parameters["id"]?.toIntOrNull()
            val todoIndex = todos.indexOfFirst { it.id == id }
            if (todoIndex != -1) {
                val updatedTodo = call.receive<Todo>()
                todos[todoIndex] = updatedTodo.copy(id = id!!)
                call.respond(HttpStatusCode.OK, updatedTodo)
            } else {
                call.respond(HttpStatusCode.NotFound)
            }
        }
        delete("/{id}") {
            val id = call.parameters["id"]?.toIntOrNull()
            if (todos.removeIf { it.id == id }) {
                call.respond(HttpStatusCode.NoContent)
            } else {
                call.respond(HttpStatusCode.NotFound)
            }
        }
    }
}

Step 6. Run the Application

Run the main function in your Application.kt file.

Step 7. Test the Endpoints

Use tools like Postman or cURL to test your API endpoints:

• Create a new Todo item:
  POST http://localhost:8080/todos

similarly, We can test other end points.

Conclusion

In conclusion, building RESTful APIs with Ktor offers a powerful and efficient solution for developing web applications in Kotlin. Throughout this blog post, we explored the fundamental steps involved in creating a RESTful API using Ktor, along with a practical example of building a Todo List API.

If you’re new to kotlin, you can explore about kotlin from here!