a WebSocket is a protocol that allows bidirectional communication between a client(running on a browser) and a server. It can have interesting applications such as push notifications and chat. As of HTML 5, WebSocket has been introduced as part of the standard javascript. WebSocket operation depends on the browser type and its version, but normally it is supported by all the modern browsers. In this example, we are going to wrap the WebSocket API using JsInterop, and then use it from a GWT app to send and receive messages from a Spring Boot based server.

The WebSocket wrapper

@JsType(isNative=true, namespace=JsPackage.GLOBAL)
public class WebSocket extends EventTarget {
	public Function onclose;
	public Function onerror;
	public Function onmessage;
	public Function onopen;
	public String url;
	public WebSocket(String url){
	public native void send(String data);
	public native void close();

The Server

Then we can initialize a WebSocket from our application:

    WebSocket socket = new WebSocket("ws://localhost:8082/gwidgets-ws");
		socket.onmessage = (evt) -> {
			   MessageEvent event = evt.cast();
			   DivElement div = DOM.createDiv().cast();
			return evt;
		socket.onopen = (evt) -> {  
			   GWT.log("socket open");
			return evt;
		socket.onclose = (evt) -> {
			   GWT.log("socket closed");
			return evt;

We have used lambdas for event handlers to take advantage of GWT 2.8.0 Java 8 capabilities. Finally, we have used Spring Boot, and Spring’s support for WebSockets to implement a server that sends a notification each 2 mins to our app, and also can receive messages and display them on the console.

Our WebSocket configuration looks like:

public class SocketConfig implements WebSocketConfigurer{	
	public void registerWebSocketHandlers(WebSocketHandlerRegistry registry) {
		registry.addHandler(handler(), "/gwidgets-ws").setAllowedOrigins("*");
	public SocketHandler handler(){
		return new SocketHandler();

The SocketHandler implementation can be found here.

and finally, we need to implement our scheduling module that sends a message every two minutes:

public class ScheduledTemplate {
   SocketHandler socketHandler;

   @Scheduled(cron="0 0/2 * * * ?")
    public void publishUpdates(){
	   socketHandler.brodcastMessage("server notification " + LocalDateTime.now().toString());

Full source code can be found here: https://github.com/zak905/gwt-websocket-spring