I am trying to set up a webserver on an old machine of mine. I have installed ubuntu server edition and aim to use it for the following:
I want to run a java program on the server. I want to be able to retrieve data from the program from another computer/phone using an internet connection. I also want to be able to give the program data, and get a response saying whether or not the data has been received correctly.
So for example:
A .jar program runs on my server and holds a variable x
I want to be able to query the value of x from another device (over the internet).
I want to be able to set the value of x remotely from another device, and get a response saying it was successful in altering the value.
What are my options here? I would like to try and keep things simple. It is perhaps worth mentioning that I will be the only one using the system. The server will be used exclusively for dealing with the two requests outline above.
Is it simply the case of creating a java program that listens out for incoming requests and running that on the server?
As you mentioned, you can start with custom ServerSocket wrapper which will decode incoming requests and do as it's bid. Currently, whole frameworks are done to encapsulate common code of this task -- see my 3rd point.
Old-school java solution: use RMI. See RMI tutorial.
New-school java solution: devise some simple text-based protocol with 2 commands:
Read()
Set(newVal)
Then implement that protocol over some new trendy Java framework, like Apache MINA, which is created specifically to facilitate quick development of network apps in Java.
I, personally, started with RMI for such kind of tasks. Since RMI is considered Core Java technology, it's wise to learn it.
Related
During my studies, I have to make a project connected with programming in Java. I learn Java from a few months and I would like to make something interesting (not an application for bank, library, car renting etc). I'm wondering whether it is possible to create real-time web game/application, where you can type something and your friend on another laptop see this message and can send you response? (using internet/Bluetooth) If yes, what I should look for to find information about this type of applications?
Yes, creating something like this is definitely possible. It really will just depend on exactly how you want to implement this (it sounds like you're still not sure EXACTLY what you want, as your description is vague).
What I mean by that, is what do you want as your medium? Would you like the two users to be on their laptops and communicating through their web browsers? Or would you rather have a standalone application that accomplishes this? If so, what Operating Systems will you support? Will it have a graphical user interface, or will it run on the command line?
Let's assume that you want to develop a standalone Windows application that allows the users to exchange messages. Keep in mind that doing this gracefully would involve users logging into your system with authentication, a fairly sophisticated GUI, and lots of encryption for privacy reasons. That being said, a very basic implementation of this could probably be as follows:
You'd have an app that runs locally on the users machine, and also some sort of database backend that your app communicates with. I'd recommend using a mySQL database hosted by Amazons RDS (Here's a tutorial that got me using Javas JDBC library to work with an Amazon RDS database - https://www.youtube.com/watch?v=2i4t-SL1VsU).
Rather than worry about a GUI, I'd suggest trying to get your prototype working on the command line. Your app could preform the following steps when booting up:
Ask user to input the word send followed by a message to send messages ("SEND %MESSAGE%"), or "RECEIVE" to receive messages.
If "SEND %MESSAGE%" is input, add message to database
if "RECEIVE" is input, query the database for all message entries and output them to the user.
You can see that this would accomplish a very crude version of what you asked for, and the devil is in the details. I'd suggest building something very simple like this, and then adding functionality by tweaking and improving features one at a time.
I am having problems with the interconnection between two modules of my system, I hope you can help me.
My hardware is:
Desktop computer running Windows.
Linux server that runs a Java daemon.
I have a Java app that connects to the daemon and shares information properly, basically it is a TCP connection, but it creates objects to operate with the send, receive methods.
However, it is mandatory for other reasons to send and receive data from Matlab to the server, and viceversa.
I have never used Matlab for connections, so I was wondering if you know how can I connect these modules.
I have thought about including the Java code into Matlab, but I am doubting about its feasibility, as it uses very particular libraries.
I also thought about using tcpip Matlab Function, but I think it won't work.
Thank you for your help!
George.
Well, basically there's only three options:
Use tcpip - if the corresponding toolbox is available.
Use Java: This should be straightforward - you could either use the standard java environment to open sockets and perform the read/write operations or try to include your own java libraries.
Implement the connection in C/C++ and include it via mex - which seems to be the most inconvenient alternative.
Given that you seem to have working Java classes doing your work, option 2 seems to be the best approach, imho.
Including third party libraries in MATLAB usually can work pretty good.
For a start, see e.g.:
http://www.mathworks.de/de/help/matlab/matlab_external/bringing-java-classes-and-methods-into-matlab-workspace.html
I happened to work within alternative 2 using a third party library for object-based communication with some database-interface.
I'm developing a distributed application, and I need to connect a client Java based to a server C++ based. Both of them will need to send information to each other, but I need them to be able to do things while waiting for the information, and they don't know when they are gonna get new information, or send information.
How can I achieve this? Now I'm trying to implement a basic communication with Sockets, but I don't really get to communicate them. I have read that using sockets + threads is usually a good approach for client-server apps.
Could you please recommend me some web or book to read about this, or send me some example code to learn?
Do you think that i should use other approach, better than sockets? maybe a higher level library (i would need it for c++ and java) or a totally different way?
EDIT:
I will add some extra information.
What I would love to achieve is the following:
My C++ program has a main loop, where I would like to have a call like GetUpdatedDataFromRemoteDevice() where I read the new values of some numerical variables that previously got updated from the net (the socket, for example).
Eventually, the C++ program will need to send a message to the remote device, to tell him to send other kind of data, and after that, keep getting the updated values.
From the Java program (remote device) the application running is an interactive touchable screen, that cant get blocked by the network transmissions, because it must keep working for the user, so all the networking should be done in a separated thread.
That thread, should connect to the server, and when a button is pushed, start to send the data (4 changing numerical values) in a loop until another event happens.
It would be nice also to be easily re-connectable to the server.
ICE is a modern and good library for distributed applications:
many languages as C++ and Java
many platforms
GNU GPL
good performance
easy to use
First, you define the messages you want to exchange between server and client.
Then, you implement the C++ and Java source code to handle these messages.
More info at http://zeroc.com/ice.html
Have fun ;-)
EDIT: I have to use ACE in some projects. I can tell ACE is very old, maybe mature, but uses outdated C++ coding rules :-(
Therefore ACE is not as easy to use as STL or BOOST. Moreover, ACE is not really efficient... I prefer ICE ;-)
I don't know what your application is but robust client server socket programming is pretty hairy task to do properly. Hardware byte order, String encoding, Network errors, retries, duplicate messages, acks etc.. require lots of good design and careful programming. You need to get it work well as single-threaded before even thinking using multiple threads.
Unless you need instant notifications from server to client I suggest that you use HTTP as protocol between client and server. Client can poll server occasionally for new messages.
Anyway the problem has been solved multiple times already.
http://activemq.apache.org/
http://www.rabbitmq.com/devtools.html
http://www.cs.wustl.edu/~schmidt/ACE-overview.html
I did something of this sort once. In my case it was easier to connect my C++ app to a local Java app using JNI and then have the two Java apps talk to each other.
Is there a way I can pass data, from Java server code, to a Java program which is already running?
I need to transfer data in real time, and I cannot integrate the two programs as they use a different compiler.
Any ideas?
Thanks
** ADDED INFO **
I am looking to control a robot over the internet. I have the server code which communicates with the client, and a program coded in a unique type of Java code (LeJOS if anyone is familiar with it). From the server side I need to pass data to the PC LeJOS Program in real time. Hence, the LeJOS PC program must be able to receive some data from the server code, whilst it is running.
Is this possible? To clarify, both the server code and the LeJOS PC Program will be running on the same machine.
Another possibility could be to integrate the server code and LeJOS PC program into one, so that the LeJOS PC program is run directly from the server code. Would it be possible to do this?
Thanks!
Hm, that's a pretty broad question, and it really depends on what the restrictions are on your app. Basically you're doing to have to transfer it through some intermediary channel.
Options:
Have the server write the data to a file, and have the Java app read it
Have the Java app connect to the server via a Socket and read the data
...and going to much more complicated methods:
RMI (Remote Method Invocation)
If you need a more specific answer, you'll have to tell us more about:
why are you transferring this data?
what sort of data you're trying to transfer between the two apps?
do you have access to modify the two programs, or is their code not controlled by you?
what have you already tried?
Have the client connect to the server via a socket. It doesn't matter if they're both running on the same machine or not. That is pretty much they way you do 'ipc' in Java, and if you want to extend the control over the network you'll have to go there anyway.
You can send data bidirectionally, so its up to you how you want to use that. The simplest way to implement your protocol is something textual, that way its easy to debug. If you can make it request/response oriented then that makes things simpler too.
Here's a basic tutorial for network IO in Java. http://docs.oracle.com/javase/tutorial/networking/sockets/index.html
Good luck.
I am implementing a website using PHP for the front end and a Java service as the back end. The two parts are as follows:
PHP front end listens to http requests and interacts with the database.
The Java back end run continuously and responds to calls from the front end.
More specifically, the back end is a daemon that connects and maintain the link to several IM services (AOL, MSN, Yahoo, Jabber...).
Both of the layers will be deployed on the same system (a CentOS box, I suppose) and introducing a middle layer (for instance: using XML-RPC) will reduce the performance (the resource is also rather limited).
Question: Is there a way to link the two layers directly? (no more web services in between)
Since this is communication between two separate running processes, a "direct" call (as in JNI) is not possible. The easiest ways to do such interprocess communcation are probably named pipes and network sockets. In both cases, you'll have to define a communication protocol and implement it on both sides. Using a standard protocol such as XML-RPC makes this easier, but is not strictly necessary.
There are generally four patterns for application integration:
via Filesystem, ie. one producers writes data to a directory monitored by the consumer
via Database, ie. two applications share a schema or table and use it to swap data
via RMI/RPC/web service/any blocking, sync call from one app to another. For PHP to Java you can pick from the various integration libraries listed above, or use some web services standards like SOAP.
via messaging/any non-blocking, async operation where one app sends a message to another app.
Each of these patterns has pros and cons, but a good rule of thumb is to pick the one with the loosest coupling that you can get away with. For example, if you selected #4 your Java app could crash without also taking down your PHP app.
I'd suggest before looking at specific libraries or technologies listed in the answers here that you pick the right pattern for you, then investigate your specific options.
I have tried PHP-Java bridge(php-java-bridge.sourceforge.net/pjb/) and it works quite well. Basically, we need to run a jar file (JavaBridge.jar) which listens on port(there are several options available like Local socket, 8080 port and so on). Your java class files must be availabe to the JavaBridge in the classpath. You need to include a file Java.inc in your php and you can access the Java classes.
Sure, there are lots of ways, but you said about the limited resource...
IMHO define your own lightweight RPC-like protocol and use sockets on TCP/IP to communicate. Actually in this case there's no need to use full advantages of RPC etc... You need only to define API for this particular case and implement it on both sides. In this case you can serialize your packets to quite small. You can even assign a kind of GUIDs to your remote methods and use them to save the traffic and speed-up your intercommunication.
The advantage of sockets usage is that your solution will be pretty scalable.
You could try the PHP/Java integration.
Also, if the communication is one-way (something like "sendmail for IM"), you could write out the PHP requests to a file and monitor that in your Java app.
I was also faced with this problem recently. The Resin solution above is actually a complete re-write of PHP in Java along the lines of JRuby, Jython and Rhino. It is called Quercus. But I'm guessing for you as it was for me, tossing out your Apache/PHP setup isn't really an option.
And there are more problems with Quercus besides: the free version is GPL, which is tricky if you're developing commercial software (though not as tricky as Resin would like you to believe (but IANAL)) and on top of that the free version doesn't support compiling to byte code, so its basically an interpreter written in Java.
What I decided on in the end was to just exchange simple messages over HTTP. I used PHP's json_encode()/json_decode() and Java's json-lib to encode the messages in JSON (simple, text-based, good match for data model).
Another interesting and light-weight option would be to have Java generate PHP code and then use PHP include() directive to fetch that over HTTP and execute it. I haven't tried this though.
If its the actual HTTP calls you're concerned about (for performance), neither of these solutions will help there. All I can say is that I haven't had problems with the PHP and Java on the same LAN. My feeling is that it won't be a problem for the vast majority of applications as long as you keep your RPC calls fairly course-grained (which you really should do anyway).
Sorry, this is a bit of a quick answer but: i heard the Resin app server has support for integrating java and PHP.
They claim they can smash php and java together: http://www.caucho.com/resin-3.0/quercus/
I've used resin for serving J2ee applications, but not for its PHP support.
I'd be interested to hear of such adventures.
Why not use web service?
Make a Java layer and put a ws access(Axis, SpringWS, etc...) and the Php access the Java layer using one ws client.
I think it's simple and useful.
I've come across this page which introduces a means to link the two layers. However, it still requires a middle layer (TCP/IP). Moreover, other services may exploit the Java service as well because it accepts all incoming connections.
http://www.devx.com/Java/Article/20509
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