running a javax.websocket endpoint in spring?

I have implemented a websocket server by following this tutorial: https://www.baeldung.com/java-websockets

Now how do I run this app? I am using Spring and my main function looks like this:

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

@SpringBootApplication
public class App {
	public static void main(String[] args) {
		SpringApplication.run(App.class, args);
	}
}

and my websocket endpoint looks like this:

import java.io.IOException;

import javax.websocket.*;
import javax.websocket.server.*;

@ServerEndpoint(
		value = "/chat/{username}",
		decoders=MessageDecoder.class,
		encoders = MessageEncoder.class
)
public class Controller {
	// ...
}

now when I run this, and try to connect to the websocket endpoint like this: wsta ws://localhost:8080/chat/aa -I I get the following error:

> WebSocket upgrade request
>
> ---
>
> Host: localhost:8080
>
> Connection: Upgrade
>
> Upgrade: websocket
>
> Sec-WebSocket-Version: 13
>
> Sec-WebSocket-Key: ...
>
> Origin: http://localhost
>
>
> WebSocket upgrade response
>
> ---
>
> 404 Not Found
>
> Vary: OriginAccess-Control-Request-MethodAccess-Control-Request-Headers
>
> Content-Type: application/json
>
> Transfer-Encoding: chunked
>
> Date: Tue, 05 May 2020 12:12:52 GMT
>
>
> WebSocketError: WebSocket response error

The answer here (https://stackoverflow.com/a/57924245/10551293) says that you access the webscoket endpoint over ws://localhost:8080/context/chat/aa. What is context, and how do I get my server to run and accept incoming connections?

In order to get the Baeldung tutorial mentioned in OP running

1. I had to add a maven dependency for spring-boot-starter-websocket to pom.xml

   <dependency>
       <groupId>org.springframework.boot</groupId>
   	<artifactId>spring-boot-starter-websocket</artifactId>
   </dependency>
   

2. Add a spring @Configuration in which @EnableWebSocket is added and a ServerEndpointExporter bean is instantiated:

   @Configuration
   @EnableWebSocket
   public class WsConfig {
   
       @Bean
       public ServerEndpointExporter serverEndpoint() {
           return new ServerEndpointExporter();
       }
   }
   

3. Additionally the endpoint controller has to be a spring @Component:

   @ServerEndpoint(
           value = "/chat/{username}",
           decoders=MessageDecoder.class,
           encoders = MessageEncoder.class
   )
   @Component
   public class Controller {
    ...
   

<hr>
Eventually the websocket endpoint was reachable under ws://localhost:8080/chat/torvalds