PS2 | WebSocket 1

import java.io.IOException;

import javax.websocket.OnClose;
import javax.websocket.OnMessage;
import javax.websocket.OnOpen;
import javax.websocket.Session;
import javax.websocket.server.ServerEndpoint;

/**
 * @ServerEndpoint gives the relative name for the end point
 * This will be accessed via ws://localhost:8080/EchoChamber/echo
 * Where "localhost" is the address of the host,
 * "EchoChamber" is the name of the package
 * and "echo" is the address to access this class from the server
 */
@ServerEndpoint("/echo")
public class EchoServer {
    /**
     * @OnOpen allows us to intercept the creation of a new session.
     * The session class allows us to send data to the user.
     * In the method onOpen, we'll let the user know that the handshake was
     * successful.
     */
    @OnOpen
    public void onOpen(Session session){
        System.out.println(session.getId() + " has opened a connection");
        try {
            session.getBasicRemote().sendText("Connection Established");
        } catch (IOException ex) {
            ex.printStackTrace();
        }
    }

    /**
     * When a user sends a message to the server, this method will intercept the message
     * and allow us to react to it. For now the message is read as a String.
     */
    @OnMessage
    public void onMessage(String message, Session session){
        System.out.println("Message from " + session.getId() + ": " + message);
        try {
            session.getBasicRemote().sendText(message);
        } catch (IOException ex) {
            ex.printStackTrace();
        }
    }

    /**
     * The user closes the connection.
     *
     * Note: you can't send messages to the client from this method
     */
    @OnClose
    public void onClose(Session session){
        System.out.println("Session " +session.getId()+" has ended");
    }
}


<!DOCTYPE html>

<html>
    <head>
        <title>Echo Chamber</title>
        <meta charset="UTF-8">
        <meta name="viewport" content="width=device-width">
    </head>
    <body>

        <div>
            <input type="text" id="messageinput"/>
        </div>
        <div>
            <button type="button" onclick="openSocket();" >Open</button>
            <button type="button" onclick="send();" >Send</button>
            <button type="button" onclick="closeSocket();" >Close</button>
        </div>
        <!-- Server responses get written here -->
        <div id="messages"></div>

        <!-- Script to utilise the WebSocket -->
        <script type="text/javascript">

            var webSocket;
            var messages = document.getElementById("messages");


            function openSocket(){
                // Ensures only one connection is open at a time
                if(webSocket !== undefined && webSocket.readyState !== WebSocket.CLOSED){
                   writeResponse("WebSocket is already opened.");
                    return;
                }
                // Create a new instance of the websocket
                webSocket = new WebSocket("ws://localhost:8080/EchoChamber/echo");

                /**
                 * Binds functions to the listeners for the websocket.
                 */
                webSocket.onopen = function(event){
                    // For reasons I can't determine, onopen gets called twice
                    // and the first time event.data is undefined.
                    // Leave a comment if you know the answer.
                    if(event.data === undefined)
                        return;

                    writeResponse(event.data);
                };

                webSocket.onmessage = function(event){
                    writeResponse(event.data);
                };

                webSocket.onclose = function(event){
                    writeResponse("Connection closed");
                };
            }

            /**
             * Sends the value of the text input to the server
             */
            function send(){
                var text = document.getElementById("messageinput").value;
                webSocket.send(text);
            }

            function closeSocket(){
                webSocket.close();
            }

            function writeResponse(text){
                messages.innerHTML += "<br/>" + text;
            }

        </script>

    </body>
</html>