We Keep Coding

How to connect a java client to SignalR Hub using only websockets?

I need to access a websocket server that runs with SignalR Hub, from my Java desktop application.
There is a library that does exactly that, but it's dead. There are lots of issues and pull requests and it hasn't been updated since 2015.
I also found another library but, as far as I can tell, it's only for Android projects.
I found a SO question that says it's possible to talk directly to the SignalR server (standard websocket calls) with some workarounds, but the given link is dead.
When I tried to access the server directly with websocket, the HTTP handshaking fails (I get HTTP 200 instead of 101).
Can anyone help ?

You will not be able to connect to SignalR 2.x with just bare webSocket. The reason for that is that starting the connection requires a few HTTP requests which have to be sent with in a specific order and contain specific content. I described the SignalR protocol in a blog post. After reading this post you will understand why dedicated client for SignalR 2.x existed.
Having said that - the new version of SignalR for ASP.NET Core no longer has this protocol and requirements and it is possible to connect to the server with just bare websockets. I created a demo a while ago showing how to do this - you can find details here. I also started working on an example for the hubs layer but need to finish it.

Low Latency communication using web sockets & Java Server

I'm looking to establish a low latency 2way communication between a javascript interface (client) and a java server.
The client has to request data from the server (can ask for different set's of data, needs to be async, data are small sets of sensor data).
I was thinking of implementing this using websockets because of it's low latency. However I'm stuck at choosing a java websocket server implementation (I found jetty but there are so many, and there is also a case to be made for node.js and socket.io but there are not going to be a lot of clients in this case; just one client sending multiple requests so correct me if i'm wrong but there doesn't seem to be a reason for going the node.js path).
Last but not least; the server is running on a raspberry pi and is recieving it's sensor data over a special protocol; (but i don't think that's important for this question).
Is there anyone with some experience in this field and wants to share his/her toughts? Thx.

I've been using Kaazing (HTML5 edition) to proxy traffic received via a web-socket to a Java process listening on a traditional TCP server socket.
It's working well, latency is low and was consistently handling over 1000 messages/second (though we found our Java code was the limiting factor in that respect).
Kaazing also provides client APIs for Java, JavaScript and Flex, which allowed us to write an acceptance test suite using the familiar APIs (Concordion in my case).
I don't know how well it'd run on a Raspberry Pi, but given it's free to download there's a simple way to find out.

I've solved my problem by using Atmosphere which is a framework that provides compatibility to all major java servers and web browsers. (The Java Official Standard is still in the workings).
https://github.com/Atmosphere/atmosphere
I've got the chat demo up and running.

Using iOS iPad2 to Connect and Logon to Java Server Running HTTP Service

I am writing an iOS 5.1.1 app for the iPad2 using Xcode 4.4.1. I want to be able to locate a local http server running on Java.
Once I am able to connect to the http server, all I need to communicate with it is the url including the port.
My questions:
1) Should I use Bonjour or a DNS Server running on the http server to discover the http server itself?
2) I need to authenticate the iPad2 user with name and password to work with the http server once I discover it.
I need some help understanding how I would go about accomplishing these two steps including source code if available for the iOS 5 and Xcode 4.4.1.

1) Bonjour is pretty easy to tie in with Java apps. If you're particularly masochistic you can write your own Java-based mDNS (Bonjour) responders (I've done it, it's not rocket science), but the quickest way to get going is to use jMDNS in your http server to advertise its existence. I won't copy & paste the code samples but they suffice for most applications.
On the iOS side, NSNetService is your friend. Fundamentally it involves starting a responder in the background to look for services (i.e. your Java app), then calling a delegate when something appears/disappears:
id delegateObject; // Assume this exists.
NSNetServiceBrowser *serviceBrowser;
serviceBrowser = [[NSNetServiceBrowser alloc] init];
[serviceBrowser setDelegate:delegateObject];
[serviceBrowser searchForServicesOfType:#"_http._tcp" inDomain:#""];
There's a guide that explains it all. The protocol hasn't changed for 10+ years and you count on all modern iOS/OS X versions supporting it. The jMDNS library is pretty well battle-tested at this stage, too.
You might consider creating your own service type if you don't want it to be visible to other apps that search for _http._tcp., although this is just a cosmetic thing.
2) The simplest thing that'd work would be HTTP basic auth; you didn't say what kind of authentication your app supports or how you make HTTP requests on the client side, but this is pretty well covered already.

Java game server with Android client

I'm about to start a game project. It will be a network game with client-server architecture with Java server and a Web and Android clients. Impact is on low bandwidth (for use on mobile devices) and fast response. What technologies / libraries are out there for client-server communication? I am somehow experienced in web applications (GWT/Vaadin and servlets) but have no clue what to use when implementing a game server, mainly for the communication.
I am aware that I could use Java sockets with Object serialization or maybe JSON to pass the data from client to server, but I don't know how efficient in terms of bandwidth these approaches are? Or are there any more suited than these? Just pointing me in the right direction will suffice.
Thanks in advance!

As far as Android client libraries go, there's two main options. The first is just the java.net.* package in which you'll mainly use HttpUrlConnection.
The better option is to use the Apache HTTP package that also comes standard in the Android SDK. It gives you a lot more control / flexibility / verbosity in dealing with network connections.
Here is a decent example of how to use the Apache client libraries. I suggest using these, as the java.net packages are really only suited for the most basic of GET requests.
I suggest using the JSON method, because then you're not stuck having to write a Java servlet backend to deserialize the Java objects. The backend can then change independently of the client.

The main problem is, if you use WebTechnologies, than you can't implement an efficient way to realize the communication from Server to Client ...
Client to Server over HTTP GET works fine, but you must use some kind of Comet to realise the communication from server to client.
Thats why plain old tcp connection over socket would may be the better to communicate with a Client, especially with Android Client.
But implementing it with tcp socket work great with android, but not with a Browser.
My solution: implement the communication via WebSocket.
WebSockets (part of HTML5) is a HTTP extension to enable full duplex communication between client and server.
The most major web browser support WebSocket, like Firefox 4, Chrome 9, Opera 10.7
BUT NOT the Internet Explorer (support planned in IE 10, comming with Windows 8)
And for android, there exists also java libaries to implement the communication and i would excpect, that they work also well with android.
Expample:
http://code.google.com/p/weberknecht/
For the server side: Servlet API 3.0 support that, like Jetty 8
In my opinion WebSocket would save the problem, so you can implement a single server which supports WebSocket and communicate via WebSocket to an Android client as well as with a Browser Client (except Internet Explorer)

It depends on timeframe of your project. If time is enough – try in your hands and compare the technologies related to your purposes.

Async data fetching from Java WebService

I'm implementing a client-server GPS application. Client side is a J2ME midlet that sends GPS location via HTTP/XML to a Java Webservice (Tomcat servlet). The servlet stores positions in SQL database. The other client app is a web browser that can login and see the actual position of midlet using Google Maps. This client is written using GWT. All is deployed on the same Tomcat container. Now I'm wondering how to dynamically update current position in webrowser Google Maps of the mobile so that the client can see how the mobile moves. How to do it in GWT - should I create a timer object in GWT client and asynchronously send HTTP request to server for getting the actual positions? Or is there any mechanism in AJAX/GWT to notify client (web browser) about the data update?
Thanks
Dominik

Regardless of all the talk about "push", the standard HTTP model is still one where the client has to ask the server for updates.
In a GWT app, you should use the well-supported Ajax functionality to request small-granular updates (polling, I guess) from the server at regular intervals (5 seconds, maybe?) and use the newly obtained information to update your map info.

There is a detailed article on the GWT incubator web site about Server Push and how to achieve it with GWT. From the article:
Explains Server Push, sometimes known as 'comet', and how you can achieve this with GWT.
Then, if you take a look at the comments, you can find interested related information and open implementations as gwt-comet, GWTEventService ...

I agree with rok.
If your deployment is going to be small enough to be able to support one permanent connection per web browser client go with Server Push/ Hanging RPC/ Long Polling or whatever you want to call it.