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WebSockets

Introduction

The WebSocket Protocol enables two-way communication between a client and server. The protocol consists of an opening handshake followed by basic message framing, layered over TCP. The goal of this technology is to provide a mechanism for browser-based applications that need two-way communication with servers that does not rely on opening multiple HTTP connections.

Ninja v6.2.0+ includes comprehensive support for WebSockets. While designed to work with the standard Java WebSocket standard (JSR-356), Ninja includes a number of useful features beyond JSR-356 to make working with web sockets simpler.

Setup

Make sure you’re running Ninja 6.2.0+. Ninja WebSockets will work out-of-the-box in the Jetty standalone. If running your Ninja application as a WAR. Ninja WebSockets will also work in any servlet container that supports JSR-356. Ninja’s implementation has been tested against Jetty 9.3.15+, Tomcat 7.0.81+, and Wildfly 10+.

If running as a WAR in a servlet container, you’ll need to swap your GuiceFilter and use ninja.servlet.NinjaServletFilter. Or you can simply use Ninja’s automatic servlet configuration via its ServletContainerInitializer support and omit even having a web.xml.

Add a WebSocket route

All WebSocket endpoints will be configured in Ninja’s router. A new pseudo HTTP method of WS indicates a route is for a WebSocket. The controller method that handles it will be able to accept/reject the handshake and negotiate the protocol and extensions. If you’ve ever used standard JSR-356 WebSockets you’ll appreciate how much easier Ninja’s implementation is.

public class Routes implements ApplicationRoutes {
    @Override
    public void init(Router router) {

        router.WS().route("/echo").with(EchoWebSocket::handshake);
        router.WS().route("/chats/{id}").with(ChatWebSocket::handshake);

    }
}

Note that you’ll be slightly limited with the URI you can use depending on what’s providing your underlying Ninja WebSocket support. If using standard JSR-356 WebSockets (almost always the case), then you’ll be able to use simple path parameters such as /chat/{id}, but no regexes such as /chat/{id: .*}.

The declaring class of the handshake method you specify in your route will become the WebSocket endpoint. If you’re using JSR-356 WebSockets then that means your controller class will need to implement the javax.websocket.Endpoint interface. See the code below for a full example.

Web socket controller/endpoint

You will want to make sure you do not mark your WebSocket class as a @Singleton. Unless you know what you are doing, its better to create a new instance for each WebSocket session that is created.

Your controller/endpoint will be created using Guice – so the @Inject annotation will be honored when instantiating your endpoint.

WebSocket handshake

One of the issues with Java’s JSR-356 standard is that you have very little control with the initial handshake request and little access to the HTTP headers sent by the client. This design flaw would make using Ninja’s session or context impossible.

Ninja fixes this design flaw with JSR-356 by using a two-step process to complete the handshake. When a client initiates a WebSocket request, Ninja will detect the attempt, instantiate your WebSocket endpoint, and call your handshake controller method (like any standard HTTP GET request). You can then accept/reject the handshake request or save any variables you’ll want to access later once the WebSocket session is established.

public class ChatWebSocket extends Endpoint implements MessageHandler.Whole {
    
    public Result handshake(Context context, WebSocketHandshake handshake) {
        // negotiate the protocol (not always used by clients)
        handshake.selectProtocol("chat");

        // process handshake, save whatever you need, tell ninja to proceed
        // or return a failure result to reject the request
        return Results.webSocketContinue();
    }

    @Override
    public void onOpen(Session session, EndpointConfig config) {
        // jsr-356 stuff here...
    }

If your handshake method returns a status code of 101 then Ninja will let the underlying HTTP container proceed with upgrading the connection to a WebSocket. If the upgrade is successful, Ninja guarantees that the instance created for your handshake will be the same one that will receive the onOpen event.

WebSocket session

Once the handshake completes and a WebSocket session is established, you will then handle processing like any other JSR-356 implementation. There are any number of resources on the Internet for examples.