How to implement a WebSocket Server in java?

Question

I'm working on setting up my first websocket server for a communications app. I can't seem to figure out how websockets are implemented in Java.

I've tried, unsuccessfully, creating an annotation based Endpoint, but I'm not sure where the client info is going to come through. This is basically the gist of my code, without going into mundane details.

I'm trying to make the class MessageHelper deal with the websocket info transfer, I just can't grasp how to actually get the transfer there.

class MainServer implements Runnable {
// VARIABLES
    ServerSocket serverSocket = null;
    int port;
// CONSTRUCTORS
    MainServer(int p) {
        this.port = p;
    }
// METHODS
    public void run() {
        openServerSocket();
        while(!isStopped()){
            try{
                clientSocket = serverSocket.accept();
            } catch(IOException e) {
                // Do something
            }
            new Thread(new MainThread(clientSocket)).start();
        }
    }
}

// Other methods below.

public class MainThread {

    final Socket socket;


    MainThread(Socket s) {
        this.socket = s;
    }

    public void run() {
        try{
            BufferedReader br = new BufferedReader(
                new InputStreamReader(socket.getInputStream()));

            String input = br.readLine(), read = br.readLine();
            while(!input.isEmpty()) {
                read += "\n";
                read += input;
                input = br.readLine();
            }

            /**
            *  Everything works fine, I'm just not sure where to go
            * from here. I tried creating MessageHelper into the java
            * websocket implementation using annotations but it did not 
            * accept input from the client after the handshake was
            * made. My client would send something but it would just 
            * give and EOFException.
            **/
            if(websocketHandshakeRequest(read)) {
                MessageHelper messageHelper = 
                    new MessageHelper(this.socket);
            } else {
                // Do something
            }
        } catch(Exception e) {
            // Do something.
        }
    }
}

xtay2 · Accepted Answer · 2025-01-16 09:48:54Z

20

Don't be confused about the name WebSocket. A TCP socket and a WebSocket are entirely different kind of "sockets".

In Java you use a ServerSocket for TCP sockets. TCP is a transport layer protocol used to implement application layer protocols like POP3 and HTTP.

WebSocket is a HTTP/1.1 protocol upgrade commonly used in web servers and web browsers. You cannot use a ServerSocket for the WebSocket protocol, at least not so straight forward as you might think. First, you have to implement the HTTP/1.1 protocol and then the WebSocket protocol on top of that.

In the Java world you can use web servers like Tomcat or Jetty which provide WebSocket implementations and a high level Java API. This API is part of the Jave Enterprise Edition (JEE). See also the Jave EE 7 Tutorial - Chapter 18 Java API for WebSocket.

E.g. Jetty is a lightweight JEE web server which can be embedded in your application or run as a stand-alone server. See Jetty Development Guide - Chapter 26. WebSocket Introduction.

So in a Java web application running in a WebSocket enabled JEE web server like Jetty you can implement a server side WebSocket as follows:

package com.example.websocket;

import org.apache.log4j.Logger;

import javax.websocket.CloseReason;
import javax.websocket.OnClose;
import javax.websocket.OnError;
import javax.websocket.OnMessage;
import javax.websocket.OnOpen;
import javax.websocket.Session;
import javax.websocket.server.ServerEndpoint;
import java.io.IOException;

@ServerEndpoint("/toUpper")
public class ToUpperWebsocket {

  private static final Logger LOGGER = Logger.getLogger(ToUpperWebsocket.class);

  @OnOpen
  public void onOpen(Session session) {
    LOGGER.debug(String.format("WebSocket opened: %s", session.getId()));
  }

  @OnMessage
  public void onMessage(String txt, Session session) throws IOException {
    LOGGER.debug(String.format("Message received: %s", txt));
    session.getBasicRemote().sendText(txt.toUpperCase());
  }

  @OnClose
  public void onClose(CloseReason reason, Session session) {
    LOGGER.debug(String.format("Closing a WebSocket (%s) due to %s", session.getId(), reason.getReasonPhrase()));
  }

  @OnError
  public void onError(Session session, Throwable t) {
    LOGGER.error(String.format("Error in WebSocket session %s%n", session == null ? "null" : session.getId()), t);
  }
}

You register your class as a WebSocket handler for the specific path with the @ServerEndpoint annotation. Your WebSocket URL is then ws://host:port/context/toUpper or wss://host:port/context/toUpper for HTTPS connections.

Edit: Here is a very simplistic HTML page to demonstrate the client side connection to the above WebSocket. This page is served by the same webserver as the WebSocket. The webapplication containing the WebSocket is deployed at context "websocket" on localhost port 7777.

<html>

<body>
  <h2>WebSocket Test</h2>
  <div>
    <input type="text" id="input" />
  </div>
  <div>
    <input type="button" id="connectBtn" value="CONNECT" onclick="connect()" />
    <input type="button" id="sendBtn" value="SEND" onclick="send()" disable="true" />
  </div>
  <div id="output">
    <h2>Output</h2>
  </div>
</body>
<script type="text/javascript">
  var webSocket;
  var output = document.getElementById("output");
  var connectBtn = document.getElementById("connectBtn");
  var sendBtn = document.getElementById("sendBtn");
  var wsUrl = (location.protocol == "https:" ? "wss://" : "ws://") + location.hostname + (location.port ? ':' + location.port : '') + "/websocket/toUpper";

  function connect() {
    // open the connection if one does not exist
    if (webSocket !== undefined
      && webSocket.readyState !== WebSocket.CLOSED) {
      return;
    }

    updateOutput("Trying to establish a WebSocket connection to <code>" + wsUrl + "</code>");

    // Create a websocket
    webSocket = new WebSocket(wsUrl);

    webSocket.onopen = function (event) {
      updateOutput("Connected!");
      connectBtn.disabled = true;
      sendBtn.disabled = false;
    };

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

    webSocket.onclose = function (event) {
      updateOutput("Connection Closed");
      connectBtn.disabled = false;
      sendBtn.disabled = true;
    };
  }

  function send() {
    var text = document.getElementById("input").value;
    webSocket.send(text);
  }

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

  function updateOutput(text) {
    output.innerHTML += "<br/>" + text;
  }
</script>

</html>

edited Jan 16 at 9:48

xtay2

98810 silver badges28 bronze badges

answered Sep 13, 2019 at 13:08

vanje

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7 Comments

Kyler Over a year ago

Ok, so I have been reading through the links you sent and I am still unsure of how the class ToUpperWebsocket is accessed by the client.

vanje Over a year ago

@Kyler: I updated my answer and included some client side code.

Jason Rich Darmawan Over a year ago

I have build a WebSocket server with ServerSocket to listen to a port, Socket to handle the WebSocket protocol. So far so good, 4 - 12ms to decode a message from a client to the server and to reply it back, Ping/Pong frame and Close frame supported, maybe there are still other parts that I have not convered. I am not a CS student to begin with, so I am lacking in TCP and UDP stuff, do you mean WebSocket does not run on TCP after the Opening Handshake, so ServerSocket is not the right class to use?

mbz Over a year ago

How do I use the ToUpperWebsocket class inside a main method? Where do I define the port and which part of the code tells it to wait for a connection? I'm a bit lost.

vanje Over a year ago

@vincent thorpe: Maybe the websocket JARs are missing in the classpath? It is impossible to help you with your specific errors without any further information. You should consider to create a new question for this.

|

tvicky4j247 · Accepted Answer · 2019-09-13 11:26:29Z

2

If you're willing to use Java Spring - which I think would be great for your use case, it's pretty easy to setup a websocket server and client connection.

There's an example here - https://spring.io/guides/gs/messaging-stomp-websocket/

answered Sep 13, 2019 at 11:26

tvicky4j247

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