Javalin is a lightweight Java and Kotlin web framework for REST APIs, WebSockets, and server-rendered applications. A minimal server needs only a Javalin instance, a route, and start(); the sections below build that foundation into a practical API.
Why Javalin?
- No boilerplate: Start a server in 3 lines of code.
- Flexible routing: Define HTTP endpoints with lambda expressions.
- WebSocket support: Build real-time features effortlessly.
- Async-ready: Handle long-running tasks without blocking threads.
- Kotlin-friendly: Leverage Kotlin’s concise syntax for even cleaner code.
Getting Started: Hello World in Java & Kotlin
1. Java Example
Add Maven dependency:
<dependency>
<groupId>io.javalin</groupId>
<artifactId>javalin</artifactId>
<version>6.3.0</version> <!-- Check for latest version -->
</dependency>Start the server:
import io.javalin.Javalin;
public class HelloWorld {
public static void main(String[] args) {
var app = Javalin.create().start(7070);
app.get("/", ctx -> ctx.result("Hello from Java!"));
}
}Output:
Visit http://localhost:7070 in your browser. You’ll see:
Hello from Java!2. Kotlin Example
Add Gradle dependency:
implementation("io.javalin:javalin:6.3.0")Start the server:
import io.javalin.Javalin
fun main() {
val app = Javalin.create().start(7070)
app.get("/") { ctx -> ctx.result("Hello from Kotlin!") }
}Output:
Visit http://localhost:7070:
Hello from Kotlin!Key Features with Code Examples
1. Routing & Path Parameters
app.get("/greet/:name", ctx -> {
String name = ctx.pathParam("name");
ctx.result("Hello, " + name + "!");
});Test:
http://localhost:7070/greet/Alice →
Hello, Alice!2. JSON Responses
app.get("/api/users", ctx -> {
Map<String, Object> user = Map.of("id", 1, "name", "Bob");
ctx.json(user); // Auto-converts to JSON
});Output:
{"id":1,"name":"Bob"}3. WebSocket (Real-Time Updates)
app.ws("/chat", ws -> {
ws.onConnect(ctx -> System.out.println("Client connected!"));
ws.onMessage(ctx -> {
String message = ctx.message();
ctx.send("Echo: " + message);
});
});Test:
Use a WebSocket client (e.g., browser dev tools) to connect to ws://localhost:7070/chat and send a message.
4. Async Handling
app.get("/async", ctx -> {
ctx.future(() -> {
Thread.sleep(1000); // Simulate delay
return "Async response!";
}).thenAccept(ctx::result);
});Output (after 1 second):
Async response!Conclusion
Javalin is ideal for developers who want a fast, unopinionated way to build web apps in Java/Kotlin. Its simplicity doesn’t compromise power—you can scale from tiny APIs to complex apps with plugins (e.g., OpenAPI, templating engines).
Next Steps:
- Explore Javalin’s official docs.
- Try the Kotlin CRUD tutorial.
- Join the Discord community for help.
Happy coding! 🚀
