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Java Web App: Code to receive UDP packets

I recently started a dynamic web application using Java/JSP and Tomcat.
I did a bit of research, and there's no specific answer about it. I have my front-end JSPs and their handling is done on the corresponding servlets. This is nice and all, and is sort of local.
But I'd want the application to also run another Java class that loops and waits for incoming UDP packets from other external clients. I have a separate UDP receive class which works fine on its own, but I'd like this code to run in the background of the web application.
Is there any way this could be done?
Thank you.

communication between jruby app and java app that are on different servers

Anyone has expirience on having Jruby project running on Jboss (using torquebox or whatever) with an ability to communicate with another "japps" not on the same jboss where jruby app is, i.e. some java project on another jboss?
I know there is an torque-messanging but dunno if it's possible to communicate with external(out of jruby-app's jboss) app?
Best practices are welcomed.
Thanks in advance.
P.S. placing that other app on the jboss where jruby app is not acceptible solution.

I can recommend you to use Thrift and build communication via them.
Thrift have generator for both your needed languages (Java and JRuby) and provide good and fast communication.
UPDATED:
Thrift is RPC (remote procedure call) framework developed at Facebook. In detail you can read about it in Wiki.
In few word to save you time, what it is and how to use it:
You describe you data structures and service interface in .thrift file(files). And generate from this file all needed source files(with all need serialization) for one or few languages(what you need). Than you can simple create server and client in few lines
Using it inside client will be looks like you just use simple class.
With Thrift you can use what protocol and transport used.
In most cases uses Binary or Compact protocol via Blocked or Not-blocked transport. So network communication will be light and fast + with fast serialization.
SOAP(based on XML on HTTP) packages, its in few times bigger, and inappropriate for sending binary data, but not only this. Also XML-serialization is very slow. So with SOAP you receive big overhead. Also with soap you need to write (or use third-party) lib for calling server(tiny network layer), thrift already made it for you.
SMTP and basically JMS is inappropriate for realtime and question-answer communication.
I mean if you need just to put some message in queue and someone sometime give this message and process it — you can (and should) use JMS or any other MQ services(Thrift can do this to, but MQ architecture is better for this issue).
But if you need realtime query-answer calls, you should use RPC, as protocol it can be HTTP(REST, SOAP), binary(Thrift, ProtoBuf, JDBC, etc) or any other.
Thrift (and ProtoBuf) provide framework for generate client and server, so it incapsulate you from low level issues.
P.S:
I made some example in past https://github.com/imysak/using-thrift (communication via Thrift Java server + Java Client or node.js client), maybe it will be useful for someone . But you can found more simple and better examples.

Torquebox supports JMS. The gem you specified torquebox-messaging allows for publishing and processing of HornetQ messages on the local JBoss AS server/cluster that the JRuby app is running in. I don't think it currently supports connecting to remote servers.
Using this functionality in your JRuby app you could then configure your Java app on another server to communicate with HornetQ running in the JBoss AS that the JRuby app is running on.
Alternatively you could always implement your own communication protocol or use another Java library - you have access to anything Java you want to run from JRuby.

You can use Web Services or JMS for that

Best way to manage asynchronous / push communication with web clients on a Spring based Java server

I need to push events to web clients in a cross-browser manner (iPhone, iPad, Android, IE/FF/Chrome/etc.) from a Spring based Java server. I am using backbone.js on the client side.
To my best knowledge, I can either go with a Web socket only approach, or I can use something like socket.io.
What is the best practice for this issue, and which platform/frameworks should I use?
Thanks

Looks like you're interested in an AJAX Push engine. ICEPush (same group that makes ICEFaces) provides these capabilities, and works with a variety of server- and client-side frameworks. There is also APE.

You can have a look at Lightstreamer.
My company is currently using it to push real time financial data from a web server.

I suppose Grizzly or Netty may fit your needs. Don't have a real experience in that scope, unfortunately.

I'd recommend socket.io as you mentioned in your question, if you're doing browser based eventing from a remote host. Socket.io handles all the connection keep-alives and reconnections directly from javascript and has facilities for channeling messages to specific sessions (users). The real advantage comes from the two-way communication of WebSockets without all the boilerplate code of maintaining the connection.
You will need to do some digging for a java implementation thoughConsider running the server directly from V8.

Server-client Java distributed application

I have to design a distributed application composed by one server (developed in Java) and one or more remote GUI clients (Swing application with windows).
As stated before the clients are Swing GUI application that can connect to the server in order to receive and send data.
The communication is bidirectional (Server <=> Clients).
Data sent over the network is mainly composed by my domain logic objects.
Two brief examples: a client calls the server in order to receive data to populate a table inside a window; the server calls client in order to send data to refresh a specific widget (like a button).
The amount of data transmitted between server and clients and the frequency of the network calls are not particularly high.
Which technology do you suggest me for the server-clients communication?
I've in mind one technology suitable for me but I would like to know your opinions.
Thanks a lot.

The first technology that came to my mind was RMI - suitable if you're communicating between java client and java server. But you may get difficulties if you want do switch the client technology to - say - a webinterface.

I would go with RMI but implement the whole architecture using Spring framework. This way it is independent of technology used and can be switched to other ways of communication (such as HTTP or other ) with almost no coding.
UPDATE: And Spring will allow you to have none of RMI specific code.

I believe sockets should do the trick. They are flexible and not especially hard to code/maintain. Most entry level programmer should also be able to maintain them. They are also fast and adapt to any kind of environment.
Unless, your server is going to be off-site or you expect to have firewall issues. In that case, web services are the way to go since your basic communication happens through port 80.

I would second msparer's suggestion of RMI, except I would just use EJB3 (which uses RMI as the communication protocol). EJB3 are very easy and even if you don't use the other feaures EJB gives you (e.g., security) you can still leverage Container Managed Transactions (CMT). It really does make development easy.
As for the server->client communication, you would probably want to use JMS. Again, using EJB3 this is pretty e3asy to do with annotations. The clients will subscribe to the message service and receive update notifications from the server.
And yes, I am currently working on an application that does this very thing. Unfortunately we are using EJB2.1. Still, it is my opinion that this is where EJBs really shine. Using EJBs in a web app is frequently overkill, but in a distributed client/server app they work very well.

You can try using ICE http://www.zeroc.com for establishing server-client connection.

Communicating between Java and Flash without a Flash-specific server

I have Java and Flash client applications. What is the best way for the two to communicate without special Flash-specific servers such as BlazeDS or Red5? I am looking for a light client-only solution.

Well, you can make http requests from flash to any url... so if your java server has a point where it can listen to incoming requests and process XML or JSON, your flash client can just make the request to that url. BlazeDS and Red5 just aim to make it simpler by handling the translation for you making it possible to call the server-side functions transparently.

Are they running in a browser (applet and SWF), or are they standalone apps?
If they're running in a browser then you can use javascript. Both Flash and Java are can access javascript. It's fragile, but it works.
If they're running as actual applications then you can have Java open a socket connection on some port. Then Flash can connect to that and they can send XML data back and forth.
I've done both of these, so I know they both work. The javascript thing is fragile, but the socket stuff has worked great.

WebORB for Java may be of some help to you. It integrates with your J2EE code.
For more info:
http://www.themidnightcoders.com/weborb/java/
I'm sorry, I reread your question that you are only looking for a client side solution. In this case, WebORB will not help you. Sorry for the misunderstanding.

There's a Flash implementation of Caucho's Hessian web service protocol. This approach would be similar to using JSon or XML, but is more performant, since Hessian is a binary protocol. If you happen to be using Spring on your server, you can use the Spring/Hessian binding to call you Spring services directly from your Flash application with minimal work.

Merapi Bridge API
Merapi allows developers to connect Adobe AIR applications, written in Adobe Flex to Java applications running on the user's local computer.