A very small taste of WebSockets with WebFlux, Reactive MongoDB data on Spring 5


Today post is about a small experiment using Spring 5 to play with WebFux, I've used it to create a small WebSocket controller to "simulate" an e-mail inbox. The idea was to send some dummy text to the backend, persist it on MongoDB and from time to time check for new messages and send it to the clients connected to the WebSocket endpoint.

The Cast

Let's see an overview of all actors involved in this small experiment. This text probably will be much longer than the code necessary to have everything working with Spring 5.

I'll assume that Spring doesn't need any introduction even because is hard to define it nowadays, if we were back to 2002 ish I would say it is an Ioc container blah blah blah but today...


MongoDB is a free and open-source cross-platform document-oriented database program. Classified as a NoSQL database program, MongoDB uses JSON-like documents with schemata.

MongoDB Reactive Streams Java Driver

MongoDB Reactive Streams Java Driver, providing asynchronous stream processing with non-blocking back pressure for MongoDB.

Project Reactor

Non-Blocking Reactive Streams Foundation for the JVM both implementing a Reactive Extensions inspired API and efficient event streaming support.

The reactive design pattern is an event-based architecture for asynchronous handling of a large volume of concurrent service requests coming from single or multiple service handlers.

And the Spring Reactor project is based on this pattern and has the clear and ambitious goal of building asynchronous, reactive applications on the JVM.


Spring WebFlux is an asynchronous framework from the bottom up. It can run on Servlet Containers using the Servlet 3.1 non-blocking IO API as well as other async runtime environments such as netty or undertow.


WebSocket is a computer communications protocol, providing full-duplex communication channels over a single TCP connection.

The WebSocket protocol enables interaction between a web client (such as a browser) and a web server with lower overheads, facilitating real-time data transfer from and to the server. This is made possible by providing a standardized way for the server to send content to the client without being first requested by the client, and allowing messages to be passed back and forth while keeping the connection open. In this way, an ongoing two-way conversation can take place between the client and the server.

The actual code


Let's start with the pom.xml file, this way you'll have what it takes to play by yourself with your WebSockets. Let's check the most important parts.

The parent pom or the BOM file used to predefine the dependencies versions, so you can add your dependencies without having to figure out about version compatibility between the components.

BOM stands for Bill Of Materials. A BOM is a particular kind of POM that is used to control the versions of a project’s dependencies and provide a central place to define and update those versions.

BOM provides the flexibility to add a dependency to our module without worrying about the version that we should depend on.


The next part, the dependencies I've used, observe that except for the most specific dependencies I didn't configure any version, it's all done by the parent pom.

    <!-- Boot -->

    <!-- Tooling -->

    <!-- Websocket -->

    <!-- Test -->

I won't add the whole pom here. If you want, you can generate your one, indeed more updated from the SPRING INITIALIZR page. Pick any functionality you need for your project, and it will just generate a skeleton for you.


This one is a nice to have, especially if you're going to use database connections, you can place your configurations here instead of having them hardcoded and spread through your code. In case you need extra configuration, and you don't know for sure what property you need, you can check the Common application properties.

For the today's experiment, what I needed was just some port definition, logging, and MongoDB URI connection.




WebSocket configurations

The code below is all you need to have your initial WebSocket configuration, I saw some more complex code essentially doing the same thing, but I suppose they were from previous versions of Spring, with Spring 5, that's all you need to start.

public class WebSocketConfig implements WebSocketMessageBrokerConfigurer {

   public void configureMessageBroker(MessageBrokerRegistry config) {

   public void registerStompEndpoints(StompEndpointRegistry registry) {

The interesting points here are first the @EnableWebSocketMessageBroker annotation, here we're telling Spring that you want to provide WebSockets capabilities on your application, config.enableSimpleBroker("/topic") tells spring that anything starting with /topic is a WebSocket endpoint where clients can subscribe to, config.setApplicationDestinationPrefixes("/app") again gives Spring some information, now it tells that clients will send data to your application through any endpoint starting with /app, registry.addEndpoint("/email") here we're telling Spring about the endpoints we want to use the STOMP protocol, notice that we're overriding registerStompEndpoints these endpoints will probably match some @MessageMapping on your controller.


Here we're primarily providing the definitions of how the MongoDB collection will look like, notice @Document and @Id annotations, they are basically saying to Spring that this class is about collection entries called email identified by the id field, if we want the collection to have a different name we could define Document like @Document(collection = "another_name")

public class Email {
   private String id;
   private String content;
   private final Date sentAt = new Date();


This part is surprising if there's no need for anything special from the repository, the only thing needed is an interface extending ReactiveMongoRepository, Spring will provide basic operations out of the box.

To get just the newest emails since the last verification all we need is to provide an abstract method with a @Query annotation, and anything else is handled by Spring, what a wonderful world we're living these days :)

public interface EmailRepository extends ReactiveMongoRepository {
   @Query("{ 'sentAt' : { $gte : ?0 }}")
   Flux findLastOnes(Date lastExecution);

The controller

Please ignore the private Date lastExecution = new Date() over there, by default Spring creates singleton beans so we're safe here and please, stop thinking about a service layer if you're doing so, we don't need it here right now.

public class EmailController {

   private EmailRepository repository;

   private SimpMessagingTemplate template;

   private Date lastExecution = new Date();

   public void email(final Email message) {

   public void emails() {
      repository.findAll().subscribe(new EmailSubscriber<>());

   @Scheduled(fixedRate = 10000)
   public void updateClients() {
      repository.findLastOnes(lastExecution).subscribe(new EmailSubscriber<>());

   private class EmailSubscriber extends BaseSubscriber {
      protected void hookOnComplete() {
         lastExecution = new Date();

      protected void hookOnError(Throwable throwable) {
         template.convertAndSend("/topic/email/errors", throwable.getMessage());

      protected void hookOnNext(T value) {
         final Email email = (Email) value;
         template.convertAndSend("/topic/email/updates", email);

Now, the interesting parts here, the @MessageMapping annotated methods are the ones that clients will send requests too, just remember that from the client side, they need to hit for example /app/email instead of just /email, do you remember WebSocketConfig, config.setApplicationDestinationPrefixes("/app")?

To simulate that small delay from the point when someone sends us an email and the moment we receive it, I've configured an schedule that runs every 10 seconds, take a look at updateClients. Here some interesting things are going on. First, we're asking the repository to give us the new messages since the last verification and, instead of wait for the answer, we've subscribed, furthermore when something comes back we'll be notified so we can push content to the connected clients when and just when we have something to do so.

subsctibe is a method provided by Flux that in the Reactor world represents a Reactive stream of size from 0 to N, for 0 or single result Mono is the right representation. Ok, we have some results, what happens now? The Subscriber hookOnNext method will be executed once for each of the found results. Here we can do some work like escape characters. Because we need at least one method with more than one line right? From this point just sent it to the proper topic and the subscribed clients will be updated.


Testing it was simple too since with a simple @SpringBootTest annotation the whole stack was made available, we only have to do some initial setup to have an integration test up and running.

@SpringBootTest(webEnvironment = SpringBootTest.WebEnvironment.RANDOM_PORT)
public class EmailControllerTest {

   private static final String SUBSCRIBE_TOPIC_ENDPOINT = "/topic/email/updates";
   private static final String SEND_EMAIL_ENDPOINT = "/app/email";

   private EmailRepository repository;

   private ReactiveCrudRepository operations;

   private int port;
   private String URL;

   private CompletableFuture completableFuture;
   private WebSocketStompClient stompClient;
   private StompSession stompSession;

   public void setup() throws InterruptedException, ExecutionException, TimeoutException {
      completableFuture = new CompletableFuture<>();
      URL = String.format("ws://localhost:%d/email", port);

      stompClient = new WebSocketStompClient(new SockJsClient(createTransportClient()));
      stompClient.setMessageConverter(new MappingJackson2MessageConverter());
      stompSession = stompClient.connect(URL, new StompSessionHandlerAdapter() {
      }).get(1, TimeUnit.SECONDS);

   public void receiveEmails() throws InterruptedException, ExecutionException, TimeoutException {
      // Adding a dummy email
      final Email email = Email

      // Send the email through webSocket to be persisted
      stompSession.send(SEND_EMAIL_ENDPOINT, email);

      // Subscribing to the notification endpoint as a client
      stompSession.subscribe(SUBSCRIBE_TOPIC_ENDPOINT, new MessageStompFrameHandler());

      // Waiting up to 10s for a notification to match the scheduler
      final Email emailReceived = completableFuture.get(10, TimeUnit.SECONDS);

      // Validating content

      // Removing dummy

      // Verifying if is really gone
      final Email foundEmail = repository.findById(email.getId()).block();

   private List createTransportClient() {
      final List transports = new ArrayList<>(1);
      transports.add(new WebSocketTransport(new StandardWebSocketClient()));
      return transports;

   private class MessageStompFrameHandler implements StompFrameHandler {

      public Type getPayloadType(StompHeaders stompHeaders) {
         return Email.class;

      public void handleFrame(StompHeaders stompHeaders, Object o) {
         completableFuture.complete((Email) o);

The client

To play here I've wrote a html, js and css files, placed them inside the src/main/resources/static folder and started the project, when hitting localhost:8080 the client gets connected automatically, and the fun is complete, I'm not proud of them, but you can have them index.html, index.js and index.css.


Once again Spring did an excellent job to provide ways to write applications with low effort. With few lines of code, we can see a new paradigm acting over every single layer of our application. And in case you're wondering if it could be applied somewhere else but WebSockets related endpoints, you can use mainly the same dependencies and approach to see similar results happening for example for rest endpoints.

Hope it provided you a glance of what's possible to achieve with this new Spring version and how much fun you can have playing around with these new toys :)


Spring Boot

Spring 5




WebSocket Support on Spring 5


Project Reactor

Reactive Streams

Spring Initializr

Common Spring application properties