Going Beyond Android: How Kotlin Works on the Backend

Kotlin, a statically typed programming language developed by JetBrains, has seen rapid adoption since its first stable release in 2016. While initially gaining traction as a more modern, expressive alternative to Java for Android mobile development, Kotlin is a versatile language that‘s highly suitable for backend development as well.

Kotlin‘s Rise in Popularity

Kotlin‘s growth can be attributed to a number of factors:

1. Concise and expressive syntax: Kotlin allows developers to accomplish more with fewer lines of code compared to Java. Features like type inference, data classes, and extension functions help reduce boilerplate.

2. Interoperability with Java: Kotlin is 100% interoperable with Java, allowing for easy integration with existing Java codebases and libraries. This enables incremental adoption in established projects.

3. Null safety: Kotlin‘s type system distinguishes between nullable and non-nullable references, preventing many common null pointer exceptions. This results in more reliable code.

4. Official support by Google: In 2017, Google announced first-class support for Kotlin on Android, further boosting its popularity and credibility.

According to the Stack Overflow Developer Survey 2022, Kotlin ranked as the 4th most loved programming language, with 62.9% of respondents expressing interest in continuing to develop with it. The JetBrains State of Developer Ecosystem 2021 report also found that 61% of surveyed developers used Kotlin, a significant increase from 47% in 2020.

Why Kotlin for Backend Development?

Java Interoperability

For companies with existing Java backend infrastructure, Kotlin‘s seamless interoperability makes it a compelling choice. Kotlin code can call Java, and vice versa, without any wrappers or special interfaces. This allows development teams to introduce Kotlin gradually, without embarking on an extensive rewrite.

Enhanced Productivity

Kotlin‘s concise syntax can reduce the amount of boilerplate code needed compared to Java. A 2019 study by Trendyol Tech found that Kotlin code required an average of 25-30% fewer lines compared to equivalent Java code. Less code generally means faster development velocity and easier maintenance.

Null Safety and Immutability

Null pointer exceptions (NPEs) are a common source of bugs in Java applications. Kotlin‘s type system addresses this issue at compile time by incorporating nullability into the type system. Types are non-nullable by default, and nullable types must be explicitly declared. This provides greater null safety guarantees and eliminates a whole category of errors.

Kotlin also encourages the use of immutable data with constructs like val (read-only properties), data classes, and read-only collections. Immutability makes it easier to reason about concurrent code, as there are no risks of sudden value changes by another thread.

Functional Programming

Kotlin includes a number of features from functional programming (FP) that can lead to more concise, expressive code. While it‘s not a purely functional language, Kotlin treats functions as first-class citizens. It supports higher-order functions, lambda expressions, and function types.

The Kotlin standard library provides a rich set of functions for working with collections in a functional manner, such as map, filter, and fold. These allow for more declarative-style data processing pipelines.

val transactions: List<Transaction> = getTransactions()
val totalAmount = transactions
    .filter { it.status == TransactionStatus.COMPLETED }
    .map { it.amount }
    .sum()

Coroutines for Concurrency

Kotlin provides a built-in way to manage concurrency using coroutines. Coroutines allow writing asynchronous code in a sequential manner, without the "callback hell" often associated with traditional approaches like Futures and callbacks.

Under the hood, coroutines are lightweight threads managed by the Kotlin runtime. They provide high throughput and low overhead, making them suitable for I/O intensive applications like web services.

Here‘s an example of making concurrent HTTP calls using the Ktor client:

val client = HttpClient(CIO)

suspend fun fetchItems(): List<Item> = withContext(Dispatchers.IO) {
    val response1 = async { client.get<List<Item>>("https://api.example.com/items/1") }
    val response2 = async { client.get<List<Item>>("https://api.example.com/items/2") }
    response1.await() + response2.await()
}

Kotlin Backend Frameworks

Ktor

Ktor is an asynchronous web framework developed by JetBrains. It‘s built from the ground up in Kotlin, making full use of language features like coroutines.

Ktor‘s core use cases include:

• Web applications and HTTP APIs
• HTTP client applications
• WebSocket client and server applications

Ktor aims to be minimalistic and unopinionated, allowing developers to structure applications as they see fit. It provides essential functionality like routing, content negotiation, and templating, while also supporting a plugin system to augment the core framework.

Example of a Ktor server application:

fun main() {
    embeddedServer(Netty, port = 8080) {
        routing {
            get("/") {
                call.respondText("Hello, world!")
            }
        }
    }.start(wait = true)
}

Spring Framework & Boot

Spring is a popular application framework in the Java ecosystem, and its Kotlin support has steadily improved since the release of Spring Framework 5. Specific Kotlin features in Spring include:

• Null-safety support in the core framework API
• Kotlin-based configuration with lambda expressions
• Utilizing Kotlin reified type parameters to avoid specifying class parameters

Spring Boot makes it easy to bootstrap a Spring application in Kotlin. The Spring Initializr provides a web-based UI to generate a project structure with your choice of dependencies.

Example of a Spring Boot application in Kotlin:

@SpringBootApplication
class DemoApplication

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

@RestController
class MessageController {
    @GetMapping("/")
    fun index(): String {
        return "Hello, world!"
    }
}

kweb

kweb is a Kotlin web framework designed for building rich, reactive web applications. Its main selling points are:

• Allowing developers to write both backend and frontend code in Kotlin
• Real-time communication between web browser and server using WebSockets
• DOM manipulation using a type-safe HTML DSL

With kweb, the server can efficiently update web page state without refreshing, enabling dynamic UIs. Here‘s an example kweb application:

fun main() {
    Kweb(port = 8091) {
        doc.body.h1().text("Hello World!")
        doc.body.p().text("This is an example kweb application.")
    }
}

Full-Stack Development with Kotlin

In addition to its use on the backend, Kotlin can also be transpiled to JavaScript for frontend web development, enabling full-stack development in a single language.

Kotlin/JS has seen significant improvements with the release of Kotlin 1.4, including a new IR (intermediate representation) compiler backend. This results in smaller, more optimized output bundles.

Tools and libraries for Kotlin/JS development include:

• kotlin-wrappers: Kotlin wrappers for popular JavaScript libraries like React, styled-components, and Redux
• Kotlin/JS Gradle Plugin: Automates bundling, compiling, and running Kotlin/JS projects
• KotlinConf App: Full-stack sample application using Kotlin/JS for the frontend, and Ktor on the backend

Kotlin Backend Adoption

A number of companies have successfully deployed Kotlin in their backend production environments:

1. Netflix: Uses Kotlin on the server-side to power Metaflow, a framework for data science projects.

2. Amazon Web Services: Uses Kotlin for AWS Cloud Development Kit, which provides infrastructure-as-code capabilities across various AWS services.

3. Atlassian: Uses Kotlin and Kotlin coroutines extensively in their Jira Cloud Platform.

4. Pivotal (now VMware Tanzu): Officially supports Kotlin for developing Spring applications, considering it "enterprise-ready".

5. Corda: The Corda blockchain platform by R3 is written almost entirely in Kotlin, over 98% of its codebase.

Resources for Learning

For Java developers looking to get started with Kotlin, JetBrains provides a handy Java to Kotlin converter tool to automatically convert existing code.

Useful resources for learning Kotlin backend development include:

• Kotlin Docs: Server-side Development
• Kotlin for Java Developers Coursera Course
• Building web applications with Spring Boot and Kotlin
• Kotlin for Enterprise: Building Scalable Backend Systems (KotlinConf 2019 Workshop)

The Kotlin Slack is also an active community for discussions and getting help from other developers.

Conclusion

Kotlin‘s popularity and usage continue to grow beyond Android mobile development. Its concise syntax, null safety guarantees, and seamless interoperability with Java make it an appealing choice for backend development.

Established Java frameworks like Spring have embraced Kotlin, while newer frameworks like Ktor take full advantage of modern language features. Companies are also finding success deploying Kotlin backends in production.

For full-stack development, Kotlin/JS enables writing frontend and backend code in the same language. Multiplatform projects can even share code between mobile apps, web frontends, and the backend.

As Kotlin continues to mature and expand its ecosystem, it‘s certainly a language worth considering for your next backend project. Its expressive power and safety features can boost productivity, code quality, and developer satisfaction.