WebSocket in Java: Real-Time Applications from A to Z

Introduction: Why WebSocket Became the Standard for Real-Time Communications

Modern web applications require instant reaction to events. Whether it's a chat, a trading terminal, or a multiplayer game — users expect data to update without reloading the page. Traditional HTTP (REST API) with its "request-response" model is inefficient for such tasks: the client is forced to constantly poll the server, which creates unnecessary load and delays.

The solution to this problem is the WebSocket protocol, which establishes a permanent two-way connection between the client and server. In this article, we will break down WebSocket in Java from the basics to creating a full-fledged real-time application. You will learn how to set up a server, write a client, and handle errors.

WebSocket Basics: How It Works

Difference from HTTP

WebSocket is a protocol that works over TCP. It starts as an HTTP request (handshake), after which the connection "upgrades" to a permanent two-way channel. Unlike HTTP, where each request requires a new connection, WebSocket keeps it open, allowing the server to send data to the client independently (push model).

Main advantages:

• Low latency — data is transmitted instantly without repeated handshakes.
• Less traffic — minimal header (only 2-14 bytes) versus hundreds of bytes in HTTP.
• Bidirectionality — both the client and server can initiate data sending.

Architecture of a Real-Time Application in Java

A typical application consists of three components:

1. Client (browser or mobile application) that initiates the connection.
2. Application server (Java) that manages WebSocket sessions.
3. Backend (database, message queues) that handles business logic.

Java provides powerful tools for working with WebSocket — both on the server side (Jakarta WebSocket, Spring WebSocket) and on the client side (standard Java WebSocket API).

Creating a WebSocket Server in Java

Environment Setup

For the server side, we will use Jakarta WebSocket (formerly Java EE WebSocket). This is a standard API that is part of Jakarta EE. You will need:

• JDK 11 or higher.
• An application server (e.g., Apache Tomcat 10 or GlassFish 7).
• Maven for dependency management.

Add the dependency to pom.xml:

<dependency>    <groupId>jakarta.websocket</groupId>    <artifactId>jakarta.websocket-api</artifactId>    <version>2.1.0</version>    <scope>provided</scope></dependency>

Writing a WebSocket Endpoint

Let's create a simple echo server that sends back the received message:

import jakarta.websocket.*;import jakarta.websocket.server.ServerEndpoint;

@ServerEndpoint("/chat")public class ChatEndpoint {

    @OnOpen    public void onOpen(Session session) {        System.out.println("New connection: " + session.getId());    }

    @OnMessage    public String onMessage(String message, Session session) {        System.out.println("Received from " + session.getId() + ": " + message);        return "Echo: " + message;    }

    @OnClose    public void onClose(Session session, CloseReason reason) {        System.out.println("Connection closed: " + session.getId()             + " due to: " + reason.getReasonPhrase());    }

    @OnError    public void onError(Session session, Throwable error) {        System.err.println("Error in session " + session.getId() + ": " + error.getMessage());    }}

The annotation @ServerEndpoint("/chat")

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