Kotlin for server side
Kotlin is a great fit for developing server-side applications. It allows you to write concise and expressive code while maintaining full compatibility with existing Java-based technology stacks, all with a smooth learning curve:
Expressiveness: Kotlin's innovative language features, such as its support for type-safe builders and delegated properties, help build powerful and easy-to-use abstractions.
Scalability: Kotlin's support for coroutines helps build server-side applications that scale to massive numbers of clients with modest hardware requirements.
Interoperability: Kotlin is fully compatible with all Java-based frameworks, so you can use your familiar technology stack while reaping the benefits of a more modern language.
Migration: Kotlin supports gradual migration of large codebases from Java to Kotlin. You can start writing new code in Kotlin while keeping older parts of your system in Java.
Tooling: In addition to great IDE support in general, Kotlin offers framework-specific tooling (for example, for Spring) in the plugin for IntelliJ IDEA Ultimate.
Learning Curve: For a Java developer, getting started with Kotlin is very easy. The automated Java-to-Kotlin converter included in the Kotlin plugin helps with the first steps. Kotlin Koans can guide you through the key features of the language with a series of interactive exercises.
Frameworks for server-side development with Kotlin
Here are some examples of the server-side frameworks for Kotlin:
Spring makes use of Kotlin's language features to offer more concise APIs, starting with version 5.0. The online project generator allows you to quickly generate a new project in Kotlin.
Ktor is a framework built by JetBrains for creating Web applications in Kotlin, making use of coroutines for high scalability and offering an easy-to-use and idiomatic API.
Quarkus provides first class support for using Kotlin. The framework is open source and maintained by Red Hat. Quarkus was built from the ground up for Kubernetes and provides a cohesive full-stack framework by leveraging a growing list of hundreds of best-of-breed libraries.
Vert.x, a framework for building reactive Web applications on the JVM, offers dedicated support for Kotlin, including full documentation.
kotlinx.html is a DSL that can be used to build HTML in Web applications. It serves as an alternative to traditional templating systems such as JSP and FreeMarker.
Micronaut is a modern JVM-based full-stack framework for building modular, easily testable microservices and serverless applications. It comes with a lot of useful built-in features.
http4k is the functional toolkit with a tiny footprint for Kotlin HTTP applications, written in pure Kotlin. The library is based on the "Your Server as a Function" paper from Twitter and represents modeling both HTTP servers and clients as simple Kotlin functions that can be composed together.
Javalin is a very lightweight web framework for Kotlin and Java which supports WebSockets, HTTP2, and async requests.
The available options for persistence include direct JDBC access, JPA, and using NoSQL databases through their Java drivers. For JPA, the kotlin-jpa compiler plugin adapts Kotlin-compiled classes to the requirements of the framework.
Deploying Kotlin server-side applications
Kotlin applications can be deployed into any host that supports Java Web applications, including Amazon Web Services, Google Cloud Platform, and more.
To deploy Kotlin applications on Heroku, you can follow the official Heroku tutorial.
AWS Labs provides a sample project showing the use of Kotlin for writing AWS Lambda functions.
Google Cloud Platform offers a series of tutorials for deploying Kotlin applications to GCP, both for Ktor and App Engine and Spring and App engine. In addition, there is an interactive code lab for deploying a Kotlin Spring application.
Products that use Kotlin on the server side
Corda is an open-source distributed ledger platform that is supported by major banks and built entirely in Kotlin.
JetBrains Account, the system responsible for the entire license sales and validation process at JetBrains, is written in 100% Kotlin and has been running in production since 2015 with no major issues.
Next steps
For a more in-depth introduction to the language, check out the Kotlin documentation on this site and Kotlin Koans.
Watch a webinar "Micronaut for microservices with Kotlin" and explore a detailed guide showing how you can use Kotlin extension functions in the Micronaut framework.
http4k provides the CLI to generate fully formed projects, and a starter repo to generate an entire CD pipeline using GitHub, Travis, and Heroku with a single bash command.
Want to migrate from Java to Kotlin? Learn how to perform typical tasks with strings in Java and Kotlin.