For Java WebSocket Server Multiple Java WebSocket Clients and JavaScript WebSocket Clients connect and We assume that messages from clients are broadcast to each client.

Operation confirmed Runtime

Local Server : Tomcat 8.5
Azure WebApp Service (Tomcat 9.0)

Server : Java

import java.io.IOException;
import java.util.Collections;
import java.util.HashSet;
import java.util.Iterator;
import java.util.Set;

import javax.websocket.EndpointConfig;
import javax.websocket.OnClose;
import javax.websocket.OnError;
import javax.websocket.OnOpen;
import javax.websocket.Session;
import javax.websocket.server.ServerEndpoint;

@ServerEndpoint("/hellowebsocket")
public class ChatWebSocket {
	//All clients
	private static Set<Session> sessions = Collections.synchronizedSet(new HashSet<Session>());
	//The client that sent the message
	private Session currentSession = null;
	/*
	 *When the connection opens
	 */
	@OnOpen
	public void onOpen(Session session, EndpointConfig ec) {
		sessions.add(session);
		currentSession = session;
	}
	@OnMessage
	public String onMessage(String message) throws IOException {
		Iterator<Session> it = sessions.iterator();
		while (it.hasNext()) {
			Session s = it.next();
			if (s != currentSession) {
				s.getBasicRemote().sendText(message);
			}
		}
		return message;
	}
	@OnClose
	public void onClose(Session session) {
		sessions.remove(session);
	}
}

Client 1 : Java

import java.net.URI;
import java.util.Scanner;
import javax.websocket.ContainerProvider;
import javax.websocket.Session;
import javax.websocket.WebSocketContainer;
public class Client {
	public static void main(String[] args) throws Exception {
		String msg;
		Scanner scanIn = new Scanner(System.in);
		//Get the object of WebSocket container for initialization
		WebSocketContainer container = ContainerProvider.getWebSocketContainer();
		//Server endpoint URI
		URI uri = URI.create("ws://localhost:8080/hellowebsocket");
		//Establish a session with the server endpoint
		Session session = container.connectToServer(new WebSocketClientMain(), uri);

		while (true) {
			msg = scanIn.nextLine();
			session.getBasicRemote().sendText(msg);
			if (msg.equals("")) {
				break;
			}
		}
		scanIn.close();
		session.close();
	}
}

Client 1 : maven

<dependencies>
	<!-- https://mvnrepository.com/artifact/org.glassfish.tyrus.bundles/tyrus-standalone-client -->
	<dependency>
		<groupId>org.glassfish.tyrus.bundles</groupId>
		<artifactId>tyrus-standalone-client</artifactId>
		<version>1.16</version>
	</dependency>
</dependencies>

Client 2 : JavaScript

<html>
<head>
<title>Chat</title>
<script type="text/javascript" src="./jquery.js"></script>
<script>
	var fullpath = document.location.pathname;
	var path = fullpath.substring(0, fullpath.lastIndexOf("/"));
	var url = "ws://" + document.location.host + path + "/hellowebsocket";
	var ws = new WebSocket(url); //important
	$(function() {
		$("#button-search").click(postChat);
		ws.onmessage = function(receive) {
			$("#message").text(receive.data);
		};
	});
	function postChat() {
		var text = $("#q").val();
		var state = ws.readyState;
		if (state == ws.OPEN) {
			ws.send(text);
		} else {
			alert("connection state is not OPEN: " + state);
		}
	}
</script>
</head>
<body>
	<div>
		<div id="inputform">
			<textarea id="q" cols="80" rows="2"></textarea>
			<input type="button" id="button-search" value="POST" />
		</div>
	</div>
	<div id="message"></div>
</body>
</html>

This is all you need WebSocket: Server = Java, Client = Java, JavaScript

Operation confirmed Runtime