When I was working on the first version Javalin, I was hanging out in the Kotlin Slack, where I got talking to the Ktor Developers (Ktor is Jetbrains’ official server/client library for Kotlin).

We were discussing API design, and I mentioned how I didn’t like setting up a Ktor server, simply because of how many imports there where. This statement was met with confusion and bafflement in the Slack channel, so I thought it could make for an interesting blogpost.

What’s wrong with imports?

Well, let’s just use Ktor as example. If you visit, you will see the following snippet:

fun main() {
	embeddedServer(Netty, port = 8000) {
		routing {
			get ("/") {
				call.respondText("Hello, world!")
	}.start(wait = true)

This code doesn’t compile, since the imports have been omitted (maybe they took up too much space?). Let’s find an example, with imports, that can serialize JSON.

import com.ryanharter.ktor.moshi.moshi
import io.ktor.application.install
import io.ktor.features.ContentNegotiation
import io.ktor.response.respond
import io.ktor.routing.get
import io.ktor.routing.routing
import io.ktor.server.engine.embeddedServer
import io.ktor.server.netty.Netty

val someData = setOf<String>()

fun main(args: Array<String>) {
  val server = embeddedServer(Netty, 8080) { // create server
    install(ContentNegotiation) {
    routing { // attach endpoint
      get("/data") { // attach endpoint
        call.respond(someData) // send json
  server.start(wait = true) // start server

So, that’s a lot of imports (nine, to be exact). That in and of itself isn’t necessarily bad, but how is the developer supposed to know about these functions? How does someone find out that there is a call, a Netty, a routing, and an embeddedServer? The only real way is by looking at the docs and examples (where in Ktor’s case, imports are usually omitted - I wasn’t actually able to find a complete example like this in Ktor’s docs, I had to search through GitHub).

What’s the alternative?

Let’s compare the previous snippet to declaring a JSON endpoint in Javalin:

import io.javalin.Javalin

val someData = setOf<String>()

fun main() {
    Javalin.create() // create server
        .get("/") { ctx -> ctx.json(someData) } // attach endpoint that serves json
        .start(7000) // start server

Just one import! Javalin operates with two main objects, Javalin (your app), and Context (your context for each HTTP request). If a Javalin user wants to configure a custom server, (like in the Ktor example above), they can do it through

Javalin.create { config ->
    config.server { ... }

In my opinion, there is no need to require the user to import a specific method for each configuration option. If you hit CTRL+SPACE on a JavalinConfig object, you’ll be able to see all the possible config options Javalin offers. If you’re looking for server, you’ll also notice the other config options, thereby learning about features you aren’t even using yet.

Similarly, if a user wants to send some JSON, they do it through Context#json. While finding this method, they are likely to notice the other options available on Context, like status, queryParam, etc.

So, what’s better?

The point of this post isn’t to say that Javalin is “better” simply because it has few imports, but it does make it easier to get started with writing code. The fewer concepts a library throws at a developer, the faster the developer can become productive. If a library has high discoverability, developers are unlikely to need to consult the docs; they can instead rely on their IDE (which again increases productivity).

From Javalin’s yearly-ish survey, I’ve learned that the library is widely used for teaching programming. There is a freeform section of the survey where respondents can write what they think is the best and worst part of Javalin, and professors who teach programming often highlight the simplicity of Javalin apps, which let’s the them focus on teaching their curriculum, instead of teaching Javalin specific concepts.

Javalin is also often used for interview assignments and getting-started examples, most likely for the same reason. It gets out of your way and let’s you focus on your application logic.

I think this way of thinking also carries over to the enterprise world. When you’re building a service, you really shouldn’t have to marry the web library you are using - it should be easy to swap it out with something else (at least if you’re using a hexagonal architecture).


Hopefully this gave you something to think about. Developer experience and library usability are often overlooked in software development, and I Javalin’s focus on these aspects is the primary reason for it’s success.

I’ve made a GitHub issue if anyone wants to discuss this post.


It was noted by /u/RupertMaddenAbbott in a comment that I was equating discoverability with import statement count.

In your blog post, you seem to go even further and equate discoverability with import statement count. I think this is obviously not. For example, ktor could have put all of their methods/objects in the same package and then a single wildcard import would do just fine. The difference in discoverability between Javalin and ktor would remain but the number of import statements would be equivalent.

Conversely, you could have a central class whose methods all return hundreds of different objects. For example, an SDK that exposed rich model objects for each of a REST API's responses. If a developer stores these results in variables then they will have hundreds of imports but the discoverability remains equivalent to Javalin, in my opinion.

I largely with what is being said (although I think it’s a little unrealistic to put your entire public API in one package). The point of this post was not to equate the two, just to point out one vector that is often overlooked when designing an API.