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Javalin Web Framework

Javalin is a lightweight web framework for Kotlin and Java, inspired by Spark and Ktor. It is designed to be simple, easy to use, and to have a minimal learning curve.

Javalin provides a powerful API for creating web applications and APIs, allowing developers to quickly build robust and scalable applications.

History​

Javalin was created by Tatu Lund, a software engineer from Finland, and was first released in 2017. Lund wanted to create a web framework that was lightweight and easy to use, without sacrificing performance or features. Javalin quickly gained popularity among Kotlin and Java developers, and it continues to be actively maintained and improved.

Features​

Easy setup and configuration​

Javalin provides a simple and intuitive API for setting up and configuring web applications. You can create a new Javalin instance with just a few lines of code:

import io.javalin.Javalin

fun main() {
    val app = Javalin.create().start(7000)
    // Your application code here
}

In this example, we create a new Javalin instance and start it on port 7000. You can also configure various options, such as enabling CORS, setting up static file serving, and configuring error handlers.

Routing and request handling​

Javalin makes it easy to define routes and handle HTTP requests. You can define routes using the get, post, put, patch, and delete methods, and handle requests using lambda functions:

app.get("/hello") { ctx ->
    ctx.result("Hello, World!")
}

In this example, we define a GET route for "/hello" and return the string "Hello, World!" as the response. You can access request parameters, headers, and other information through the ctx object.

Path parameters​

Javalin supports path parameters, allowing you to define dynamic routes that match different URL patterns. Path parameters are defined using the pathParam method and can be accessed in the request handler:

app.get("/users/:id") { ctx ->
    val userId = ctx.pathParam("id")
    // Your code here
}

In this example, we define a route with a path parameter id. The value of the id parameter can be accessed using the pathParam method.

Query parameters​

Javalin provides a convenient way to handle query parameters. You can access query parameters using the queryParam method:

app.get("/search") { ctx ->
    val query = ctx.queryParam("q")
    // Your code here
}

In this example, we define a GET route for "/search" and access the q query parameter using the queryParam method.

Middleware​

Javalin supports middleware, allowing you to define common functionality that should be applied to multiple routes. Middleware can be used for tasks such as authentication, logging, and request/response modification. You can define middleware using the before and after methods:

app.before { ctx ->
    // Your code here
}

app.after { ctx ->
    // Your code here
}

In this example, we define before and after middleware functions that will be called before and after each request, respectively.

Error handling​

Javalin provides a convenient way to handle errors and exceptions. You can define error handlers using the exception and error methods:

app.exception(Exception::class.java) { e, ctx ->
    // Your code here
}

app.error(404) { ctx ->
    // Your code here
}

In this example, we define an exception handler for Exception and an error handler for 404 Not Found errors.

Examples​

Example 1: Hello World​

import io.javalin.Javalin

fun main() {
    val app = Javalin.create().start(7000)

    app.get("/") { ctx ->
        ctx.result("Hello, World!")
    }
}

In this example, we create a simple "Hello, World!" application using Javalin. The application listens on port 7000 and responds with "Hello, World!" when a GET request is made to the root ("/") URL.

Example 2: User API​

import io.javalin.Javalin

data class User(val id: Int, val name: String)

fun main() {
    val app = Javalin.create().start(7000)

    val users = mutableListOf<User>()

    app.get("/users") { ctx ->
        ctx.json(users)
    }

    app.post("/users") { ctx ->
        val user = ctx.bodyAsClass(User::class.java)
        users.add(user)
        ctx.status(201)
    }
}

In this example, we create a simple User API using Javalin. The application listens on port 7000 and provides two routes: GET "/users" returns a list of users in JSON format, and POST "/users" adds a new user to the list.

Conclusion​

Javalin is a lightweight and easy-to-use web framework for Kotlin and Java. It provides a powerful API for creating web applications and APIs, with features such as routing, request handling, middleware, and error handling. Javalin is actively maintained and well-documented, making it a great choice for building robust and scalable applications.

For more information, you can visit the official Javalin website: https://javalin.io/