Java WebSockets made simple with Javalin

WebSockets is a technology that enables establishing an interactive connection between the front-end and the service. Being an advanced web technology, it may appear intimidating. In this blog post, I will show you how to easily build a WebSockets enabled service with Java and Javalin.

A quick intro to WebSockets

WebSockets are a relatively new (2011), but a well-supported communication protocol. At the time of writing every major browser supports them.

What is so great about them? If you are looking to build a very interactive application (think Google Docs, chats or games) they are the protocol to choose. You get an open channel of communication, rather than having to rely on the request-response model.

How do you initiate a WebSocket connection in JavaScript? It is simple:

const ws = new WebSocket(`ws://localhost:7070/some-endpoint/session-id`);

And with that connection, you can simply hook to the following events:

ws.onopen event => {}
ws.onmessage = messageEvent => {}
ws.onerror = event => {}
ws.onclose = closeEvent => {}

I will later show you an example of a simple JavaScript frontend that can be built with these events.

Why Javalin… What is Javalin anyway?

Javalin is an amazing micro-framework for writing microservices. I have chosen it here as it makes writing WebSockets as easy as it gets. If you want to learn more about the framework check out the official site and mine Lightweight Kotlin Microservices with Javalin blog post.

WebSockets basics with Javalin

Working with Javalin and WebSockets is nearly identical to working with JavaScript and WebSockets. The API looks as follows:"/websocket/:path", ws -> {
    ws.onConnect(session -> {});
    ws.onMessage((session, message) -> {});
    ws.onClose((session, statusCode, reason) -> {});
    ws.onError((session, throwable) -> {});

With the WsSession object wrapping Jetty’s Session object and adding convenience methods. The most useful one being  session.send(“message”). The full list can be found with the official documentation:

Making a simple service that receives and sorts a message

I wanted to build something very simple but fun to see as an example. I decided for a service that will receive the message from the frontend and then send back progressively more sorted version of the message. You can see the gif below illustrating the idea:

Smoothly sorting a phrase with WebSockets.

Writing this with request-response would be quite unpleasant as there would have to be quite a lot of polling involved. Imagine if we were dealing here with a similar blocking request with a chunked response. Real-time analytics streaming perhaps?

The service code is very simple with the most difficult part being the actual sorting:

package websockets;

import io.javalin.Javalin;
import io.javalin.embeddedserver.jetty.websocket.WsSession;
import java.util.Arrays;
import java.util.Map;
import java.util.concurrent.ConcurrentHashMap;

public class Main {

    private static Map<String, WsSession> sessions = new ConcurrentHashMap<>();
    public static void main(String[] args) {

                .ws("/demo/:session-id", ws -> {
                    ws.onConnect(session -> {
                        session.send("Hello Session: "+session.param("session-id"));
                    ws.onMessage((session, message) -> {
                        String sortedMessage = "";
                        while(message.length() > 0){
                            sortedMessage = sortedMessage 
                                    + message.substring(0,1);
                            message = message.substring(1);

                            char[] chars = sortedMessage.toCharArray();
                            sortedMessage = new String(chars).trim();

                            String response = "message unsorted: " +
                                    ""+message+"\n"+"message sorted: "+sortedMessage;
                    ws.onError(((wsSession, throwable) ->
                            System.out.println("Something went wrong")
                    ws.onClose((session, status, message) -> {

You also need to add the relevant Javalin dependencies:


On the frontend, we have to deal with the WebSocket appropriately:

<!DOCTYPE html>
<html lang="en">
    <meta charset="UTF-8">
    <title>Javalin WebSockets demo</title>

<textarea style="width: 100%; height: 100px" id="query" placeholder="Type something ..."></textarea>
<textarea style="width: 100%; height: 100px" readonly id="webanswer"></textarea>
    window.onload = setupWebSocket;
    function setupWebSocket() {
        const textQuery = document.querySelector("#query");
        const textAnswer = document.querySelector("#webanswer");
        const ws = new WebSocket(`ws://localhost:7070/demo/session1`);
        ws.onopen = event => {
            console.log('connection established');
        ws.onmessage = messageEvent => {
            textAnswer.value =;
        ws.onerror = event => {
            textAnswer.value = 'error';
        ws.onclose = closeEvent => {
            console.log('connection closed');

        //Send message on pressing return
        textQuery.onkeydown = key =>
            if(key.keyCode === 13) {
                textQuery.value = '';

As you can see, you can make use of WebSockets easily and productively when you chose the right tools. I have shared this code on my GitHub account:

More examples

When writing this blog post I was heavily inspired by two great examples available on the Javalin website:

Creating a simple chat-app with WebSockets

Creating a Google Docs clone with WebSockets

Make sure to check them out for more details and inspiration. Also do not forget, that Javalin fully supports Kotlin.

Do I need to use Javalin when working with WebSockets?

You don’t have to use Javalin. You can use Spring, (with this good article by baeldung explaining how to) or multiple other frameworks.

I have used Javalin here as it provides a very good development experience. Once you understand how to work with WebSockets, you can use them in a less trivial frameworks with confidence.


WebSockets are an exciting technology that I think is not used enough. I believe this is partly because many developers are afraid of potential difficulties when developing with WebSockets. I hope this article gave you some more confidence to give WebSockets a try.