WebSocket

Maven Coordinates

To enable WebSocket, add the following dependency to your project’s pom.xml (see Managing Dependencies).

<dependency>
  <groupId>io.helidon.webserver</groupId>
  <artifactId>helidon-webserver-websocket</artifactId>
</dependency>

Example

This section describes the implementation of a simple application that uses a REST resource to push messages into a shared queue and a programmatic WebSocket endpoint to download messages from the queue, one at a time, over a connection. The example will show how REST and WebSocket connections can be seamlessly combined into a Helidon application.

The complete Helidon SE example is available here. Let us start by looking at MessageQueueService:

record MessageQueueService(Queue<String> messageQueue) implements HttpService {
    @Override
    public void routing(HttpRules routingRules) {
        routingRules.post("/board", (req, res) -> {
            messageQueue.add(req.content().as(String.class));
            res.status(204).send();
        });
    }
}

This class exposes a REST resource where messages can be posted. Upon receiving a message, it simply pushes it into a shared queue and returns 204 (No Content).

Messages pushed into the queue can be obtained by opening a WebSocket connection served by MessageBoardEndpoint:

record MessageBoardEndpoint(Queue<String> messageQueue) implements WsListener {
    @Override
    public void onMessage(WsSession session, String text, boolean last) {
        // Send all messages in the queue
        if (text.equals("send")) {
            while (!messageQueue.isEmpty()) {
                session.send(messageQueue.poll(), last);
            }
        }
    }
}

This is an example of a programmatic endpoint that extends WsListener. The method onMessage will be invoked for every message. In this example, when the special send message is received, it empties the shared queue sending messages one at a time over the WebSocket connection.

In Helidon SE, REST and WebSocket classes need to be manually registered into the web server. This is accomplished via a Routing builder:

StaticContentService staticContent = StaticContentService.builder("/WEB")
        .welcomeFileName("index.html")
        .build();
Queue<String> messageQueue = new ConcurrentLinkedQueue<>();
server.routing(it -> it
                .register("/web", staticContent)
                .register("/rest", new MessageQueueService(messageQueue)))
        .addRouting(WsRouting.builder()
                            .endpoint("/websocket/board", new MessageBoardEndpoint(messageQueue)));

This code snippet registers MessageBoardEndpoint at /websocket/board and associates.

Reference

• Helidon WebSocket Javadoc