Originally Kotlin was developed as a language for JVM (Java Virtual Machine), and interoperability with Java is one of its main features. Kotlin libraries provide interfaces that use language features like null safety, default and named arguments, and DSL (Domain Specific Language). Kotlin's ecosystem is growing every year. Here are the top 10 Kotlin libraries that every developer can use in 2023.
Kotest is a testing library. In any testing library, there are three components, the testing framework, the mocking framework, and the assertion framework. You can generally mix and match like JUnit, Mockk, and assertj. Kotest provides you with the testing framework and the assertion framework. Kotest has the advantage of being written in Kotlin. So, it can leverage Kotlin's language features, which allows you to write more idiomatic and concise assertions. It has built-in coroutine support, the ability to use functions such as test lifecycle callbacks, advanced conditional evaluation, powerful data-driven testing, and more.
Exposed is a lightweight SQL library that can be used with a Kotlin web project. It is very easy to set up and use. It follows the important design rule that relies heavily on simplicity and minimalism. So, if you just want a row update, it is pretty straightforward. The library provides you with all the constructs to take control of almost everything from transactions to isolations. It supports DSL syntax, which makes it more flexible and easier to reason the SQL code. It has a good "getting started" guide, and the documentation is getting better. Sometimes you have to dig deeper to find the solution to a particular problem, like transaction isolation or thread pool, which btw makes it interesting to use and work with as well.
Ktor is a framework for building asynchronous servers and clients in connected systems using Kotlin. It's concise and clear. And that is exactly what the framework makes you feel when you use it. It is minimal and easy to set up and use. It has a lot less complexity, but sometimes you need to build more things yourself, which is true with any library that is minimal and simple. It's like having a small toolbox that works extremely well. Spring, on the other hand, is like having the whole hardware store. Everything is in there somewhere, but there is a lot more navigating; it's bulky and magical. With that said, if you need a small application without any IoC or something, Ktor can solve most of your tasks. However, if you need a huge one with potential integration with Kafka, multiple databases, IoC in tests, etc., then you should select Spring.
When you think of a JSON parser, only these names come to mind, Jackson, Gson, or Moshi. And those are the most widely used parsers with Spring and Java applications as well. Kotlin serialization is a Kotlin-oriented, cross-platform, reflectionless serialization library from JetBrains.
Koin is a DI framework for Kotlin developers, completely written in Kotlin. It comes in very handy when you are creating a Kotlin application using the libraries mentioned above. Say you are using Ktor, exposed for a Kotlin web application. You would need a DI framework to manage your controllers, services, repositories, etc. This is where you would need Koin. While working with Koin, there are a few terminologies we need to understand before getting started.
Netflix recently released a library called Netflix DGS to create GraphQL servers in Kotlin and Java. It is open-sourced and is production-ready as Netflix has already been using this internally for quite some time now. DGS framework ties really well with Spring boot and is built on top of graphql-java. There are examples available for both Java and Kotlin.
KMongo is a library that does for MongoDB what Exposed is doing for the SQL databases. It supports both synchronous and asynchronous patterns, and you can use coroutines with it. It is a fairly new library and is not used so much yet. This can be used with Kotlin serialization as well for object mapping. This is what makes it a good choice if your Kotlin application interacts with MongoDB and you are not using Spring Boot. If you don't use KMongo then you will have to use the Java driver for MongoDB. The below example utilizes KMongo and Kotlin serialization to create an entity, save it, and retrieve it. It is really easy to use and simple to set up.
A Javadoc replacement for Kotlin. It is a documentation engine made for Kotlin and has around 2k stars on GitHub. It has loads of configuration options and supports Kotlin multiplatform. The language used to document Kotlin code is called KDoc. Dokka understands standard Javadoc comments in Java files and KDoc comments in Kotlin files and can generate documentation in multiple formats including minimalistic HTML, Javadoc HTML, and Markdown.
Another web framework with support for Kotlin using Kotlin DSL. With the Kotlin DSL for Vaadin, you can start adding your content to the UI class straight away. With less boilerplate, you can focus only on building your app. Vaadin lets you forget about the web and develop user interfaces in much the same way as you would a desktop application with conventional Java toolkits like Swing. Kotlin is a great fit for developing web applications because it allows you to write concise and expressive code.
Mockito is the most common mocking Java library. But you may encounter some issues when testing a Kotlin code with Mockito. For example, notNull method returns null, so we cannot use it for a not-null parameter in Kotlin because NPE is thrown. Also, Mockito does not support other language features of Kotlin. That is why using a library written on Kotlin and especially for Kotlin is the best option. Mockk provides mocking objects, enums, extensions, top-level functions, coroutines, Nothing, chained calls (including in a hierarchical form), private methods, and constructors naturally. Like Mockito, it supports annotations, partial mocks, mocks that implement multiple interfaces, and capturing method arguments.
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