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Simple bidirectional JavaServer<->JavaScriptBrowser remoting, based on WebSockets and Webbit

tag: v0.1.3

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README.md

Webbit EasyRemote

Webbit EasyRemote is a thin bidirectional RPC layer on top of Webbit's WebSockets. It allows you to invoke Javascript functions in the browwser from server-side Java objects and java methods on the server from Javascript functions.

On the server

Start by creating a Java interface that represents the Javascript functions you want to invoke from the server:

@Remote
interface ChatClient {
  void say(String username, String message);
  void leave(String username);
  void join(String username);
}

The @Remote annotation will cause the interface to be implemented by a dynamic proxy that the server can use to talk to the browser.

Now, implement the object you would like to invoke methods on from the browser Javascript:

public class ChatServer implements Server<ChatClient> {
  @Override
  public void onOpen(WebSocketConnection connection, ChatClient client) throws Exception {
  }

  @Override
  public void onClose(WebSocketConnection connection, ChatClient client) throws Exception {
  }

  @Remote
  public void login(String username) {
  }

  @Remote
  public void say(String message) {
  }
}

The @Remote annotation on the methods exposes methods so they can be invoked by the browser. To make it convenient for the server to know what WebSocket client invoked a particular method, each @Remote method can declare additional arguments of one or more of the following types:

  • org.webbitserver.WebSocketConnection
  • org.webbitserver.HttpRequest
  • The type of the client

For example:

@Remote
public void say(WebSocketConnection connection, ChatClient client, String message) {
}

The only constraint is that the arguments supplied by the client are declared in the same order in the Java method signature.

Finally hook it all up in Webbit:

WebServer webServer = createWebServer(9877)
  .add("/chatsocket", magic(ChatClient.class, new ChatServer()))   // Mount your client proxy and server instance
  .add(new EmbeddedResourceHandler("org/webbitserver/easyremote")) // Serves webbit.easyremote.js
  .start();

On the client

Start by including the Javascript library (2k) in your HTML page:

<script type="text/javascript" src="webbit.easyremote.js"></script>

And hook up to the server:

var chatServer = new WebbitSocket('/chatsocket', {
  onopen: function() {
  },
  onclose: function() {
  },
  say: function(username, message) {
  },
  join: function(username) {
  },
  leave: function(username) {
  }
});

Now you can talk to the server:

chatServer.login("Brian");
chatServer.say("I am NOT the Messiah!");

Using CSV format for server->client communication

By default, Webbit EasyRemote will send a small JSON document to the browser over the WebSocket to invoke a method:

{
  "action": "say",
  "args": ["Brian", "I am NOT the Messiah!"]
}

If the volume of messages going from the server to the client is high, this might clog up the browser since parsing JSON is relatively slow, even for short messages. Parsing this is faster:

say,Brian,I am NOT the Messiah!

To use the CSV format, just construct your WebbitSocket like this:

new WebbitSocket("/websocket", myClient, {serverClientFormat:"csv"});

That's it! See the Chat Room example for a full example. You can run it with:

make chatroom

Error handling

If either side tries to invoke a method/function that doesn't exist on the other side, an error will be logged on the server. If a method/function is invoked with a different number of arguments than the other side accepts an error will also be logged on the server.

If a function on the client called by the server raises an exception, that exception will be caught in the Javascript library and sent down to the server. You will get a better error message on the server if the client has loaded the stacktrace.js script. You can try this out with the chat example and typing the message "fbomb".

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