WebSockets in Dropwizard

_WebSocketsInDropwizard.md

Instructions for adding in websocket support to your Dropwizard service without any more dependencies than Jettys websocket implementation and the websocket api.

The pom.xml below contains the required dependency, update jetty version as needed.

The WebSocketsInDropwizardApplication.java shows what is needed in the Dropwizard java implementation in your app to configure websocket support and register server endpoints.

Finally the WebsocketImplementation.java just contains a basically empty server endpoint.

pom.xml

        <dependency>
            <groupId>org.eclipse.jetty.websocket</groupId>
            <artifactId>javax-websocket-server-impl</artifactId>
            <version>9.4.35.v20201120</version>
        </dependency>

WebSocketImplementation.java

@ServerEndpoint("/socket-endpoint")
public class WebSocketImplementation {
    @OnOpen
    public void onOpen(Session session) throws IOException {
        session.getBasicRemote().sendText("Hello World");
    }

    @OnMessage
    public void onMessage(Session session, String message) throws IOException {

    }

    @OnError
    public void onError(Session session, Throwable throwable) throws IOException {

    }

    @OnClose
    public void onClose(Session session) throws IOException {

    }
}

WebSocketsInDropwizardApplication.java

    @Override
    public void run(ApplicationConfiguration applicationConfiguration, Environment environment) throws IOException {
        WebSocketServerContainerInitializer.configure(environment.getApplicationContext(), (servletContext, serverContainer) -> {
            // All the default values
            serverContainer.getPolicy().setMaxTextMessageSize(64 * 1024);
            serverContainer.getPolicy().setMaxBinaryMessageSize(64 * 1024);
            serverContainer.getPolicy().setAsyncWriteTimeout(Duration.ofMinutes(1).toMillis());
            serverContainer.getPolicy().setIdleTimeout(Duration.ofMinutes(5).toMillis());

            // Registering endpoints, the first way is the "normal" one where you get one instance per connection
            serverContainer.addEndpoint(WebsocketImplementation.class);
            // or this second way where a single instance is reused
            serverContainer.addEndpoint(createServerEndpointConfigFromInstance(new WebsocketImplementation()));
        });
    }

    private ServerEndpointConfig createServerEndpointConfigFromInstance(Object instance) {
        Class<?> instanceClass = instance.getClass();
        ServerEndpoint annotation = instanceClass.getAnnotation(ServerEndpoint.class);
        if (annotation == null) {
            throw new IllegalArgumentException("Instance is not annotated with @ServerEndpoint");
        }
        return ServerEndpointConfig.Builder.create(instanceClass, annotation.value()).configurator(new ServerEndpointConfig.Configurator() {
            @SuppressWarnings("unchecked")
            @Override
            public <T> T getEndpointInstance(Class<T> endpointClass) {
                return (T) instance;
            }
        }).build();
    }