WebSocket + Kotlin + Spring boot simple application example

Hello everyone, Today we will learn how to develop a Simple Kotlin - Spring Boot - WebSocket, chat application.



Why WebSocket?

We already know traditional HTTP requests are unidirectional and heavy. But WebSocket is bi-directional. The initial connection is using HTTP, then this connection gets upgraded to a socket-based connection. This single connection is then used for all future communication. The WebSocket message data exchange is much lighter compared to HTTP.


Project Directory:

Pom.xml[maven]

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0" 
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
   xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 
https://maven.apache.org/xsd/maven-4.0.0.xsd">
   <modelVersion>4.0.0</modelVersion>
   <parent>
      <groupId>org.springframework.boot</groupId>
      <artifactId>spring-boot-starter-parent</artifactId>
      <version>2.3.7.RELEASE</version>
      <relativePath/> <!-- lookup parent from repository -->
   </parent>
   <groupId>com.knf.dev</groupId>
   <artifactId>kotlin_spring_websocket_example</artifactId>
   <version>0.0.1-SNAPSHOT</version>
   <name>kotlin_spring_websocket_example</name>
   <description>Demo project for Spring Boot</description>

   <properties>
      <java.version>1.8</java.version>
      <kotlin.version>1.3.72</kotlin.version>
   </properties>

   <dependencies>
      <dependency>
         <groupId>org.springframework.boot</groupId>
         <artifactId>spring-boot-starter-thymeleaf</artifactId>
      </dependency>
      <dependency>
         <groupId>org.springframework.boot</groupId>
         <artifactId>spring-boot-starter-web</artifactId>
      </dependency>
      <dependency>
         <groupId>org.springframework.boot</groupId>
         <artifactId>spring-boot-starter-websocket</artifactId>
      </dependency>
      <dependency>
         <groupId>com.fasterxml.jackson.module</groupId>
         <artifactId>jackson-module-kotlin</artifactId>
      </dependency>
      <dependency>
         <groupId>org.jetbrains.kotlin</groupId>
         <artifactId>kotlin-reflect</artifactId>
      </dependency>
      <dependency>
         <groupId>org.jetbrains.kotlin</groupId>
         <artifactId>kotlin-stdlib-jdk8</artifactId>
      </dependency>

      <dependency>
         <groupId>org.springframework.boot</groupId>
         <artifactId>spring-boot-starter-test</artifactId>
         <scope>test</scope>
         <exclusions>
            <exclusion>
               <groupId>org.junit.vintage</groupId>
               <artifactId>junit-vintage-engine</artifactId>
            </exclusion>
         </exclusions>
      </dependency>
        <dependency>
            <groupId>com.vaadin.external.google</groupId>
            <artifactId>android-json</artifactId>
            <version>0.0.20131108.vaadin1</version>
            <scope>compile</scope>
        </dependency>
    </dependencies>

   <build>
      <sourceDirectory>${project.basedir}/src/main/kotlin</sourceDirectory>
      <testSourceDirectory>${project.basedir}/src/test/kotlin</testSourceDirectory>
      <plugins>
         <plugin>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-maven-plugin</artifactId>
         </plugin>
         <plugin>
            <groupId>org.jetbrains.kotlin</groupId>
            <artifactId>kotlin-maven-plugin</artifactId>
            <configuration>
               <args>
                  <arg>-Xjsr305=strict</arg>
               </args>
               <compilerPlugins>
                  <plugin>spring</plugin>
               </compilerPlugins>
            </configuration>
            <dependencies>
               <dependency>
                  <groupId>org.jetbrains.kotlin</groupId>
                  <artifactId>kotlin-maven-allopen</artifactId>
                  <version>${kotlin.version}</version>
               </dependency>
            </dependencies>
         </plugin>
      </plugins>
   </build>

</project>

Creating the Socket Configuration

package com.knf.dev.config

import com.knf.dev.handler.TextHandler
import org.springframework.context.annotation.Configuration
import org.springframework.web.socket.config.annotation.WebSocketHandlerRegistry
import org.springframework.web.socket.config.annotation.WebSocketConfigurer
import org.springframework.web.socket.config.annotation.EnableWebSocket

@Configuration
@EnableWebSocket
class SocketConfig : WebSocketConfigurer {
    override fun registerWebSocketHandlers(registry: WebSocketHandlerRegistry) {
        registry.addHandler(TextHandler(), "/getMessage")
    }
}

Creating the Handler

package com.knf.dev.handler

import org.springframework.web.socket.TextMessage
import org.json.JSONException
import org.json.JSONObject
import org.springframework.stereotype.Component
import java.io.IOException
import org.springframework.web.socket.WebSocketSession
import org.springframework.web.socket.handler.TextWebSocketHandler

@Component
class TextHandler : TextWebSocketHandler() {
    @Throws(InterruptedException::class, IOException::class, JSONException::class)
    public override fun handleTextMessage(session: WebSocketSession, message: 
                             TextMessage) {
        val payload = message.payload
        val jsonObject = JSONObject(payload)
        session.sendMessage(TextMessage("Your message:" + jsonObject.get("user")))
    }
}

Creating the Main Driver

package com.knf.dev

import org.springframework.boot.autoconfigure.SpringBootApplication
import org.springframework.boot.runApplication

@SpringBootApplication
class KotlinSpringWebsocketExampleApplication

fun main(args: Array<String>) {
   runApplication<KotlinSpringWebsocketExampleApplication>(*args)
}

Creating the view

<!DOCTYPE html>
<html>

<head>
    <title>WebSocket Chat Application</title>
    <link href="/bootstrap.min.css" rel="stylesheet">
    <script src="/jquery.min.js"></script>
    <script src="/main.js"></script>
    <script src="/bootstrap.min.js"></script>
</head>

<body>
<div class="container">
    <br> <br> <br>
    <div class="row">
        <div class="col-md-2"></div>
        <div class="col-md-8">
            <form class="form-inline">
                <div class="form-group">
                    <label for="connect">Kotlin + Spring Boot socket programming
                        example:</label>
                    <button class="btn btn-success" id="connect" 
                                                type="button">Start
                    </button>
                    <button class="btn btn-danger" id="disconnect" 
                                type="button" disabled="disabled">End</button>
                </div>
            </form>
        </div>
        <div class="col-md-2"></div>
    </div>
    <br> <br> <br>
    <div class="row">
        <div class="col-md-2">
            <table id="chat">
                <thead>
                <tr>
                    <th>Welcome,Please enter your message</th>
                </tr>
                </thead>
            </table>
        </div>
        <div class="col-md-2"></div>
        <div class="col-md-6">
            <form>
                <div class="form-group">
            <textarea class="form-control" rows="5" id="user" 
                               placeholder="Write your message here..."
                      required></textarea>
                </div>
                <button class="btn btn-primary" id="send" type="submit">Send</button>
            </form>
        </div>
        <div class="col-md-2"></div>
        <br> <br>
    </div>
    <div class="raw">
        <div class="col-md-2">
            <table id="response">
                <thead>
                <tr>
                    <th>Your Message</th>
                </tr>
                </thead>
            </table>
        </div>
        <div class="col-md-2"></div>
        <div class="col-md-8">
            <form>
                <div class="form-group">
            <textarea id="helloworldmessage" class="form-control" 
                        rows="15" placeholder="Your Message"
                      required></textarea>
                </div>
            </form>
        </div>
    </div>
</div>
</body>

</html>

main.js

var ws;
function setConnected(connected) {
   $("#connect").prop("disabled", connected);
   $("#disconnect").prop("disabled", !connected);
}

function connect() {
   ws = new WebSocket('ws://localhost:8080/getMessage');
   ws.onmessage = function(data) {
      helloWorld(data.data);
   }
   setConnected(true);
}
function disconnect() {
   if (ws != null) {
      ws.close();
   }
   setConnected(false);
   console.log("Websocket is in disconnected state");
}
function sendData() {
   var data = JSON.stringify({
      'user' : $("#user").val()
   })
   ws.send(data);
}

function helloWorld(message) {
   $("#helloworldmessage").append("\n " + message + "");
}

$(function() {
   $("form").on('submit', function(e) {
      e.preventDefault();
   });
   $("#connect").click(function() {
      connect();
   });
   $("#disconnect").click(function() {
      disconnect();
   });
   $("#send").click(function() {
      sendData();
   });
});

Run 

$ mvn spring-boot:run

Download source code

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