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How can I connect an android with a local server?

sorry if my english isn't perfect.
I'm trying to make an app and I need to exchange information between more devices.
I thought that could be a solution connect the devices on a server but I really don't have the idea where start.
What language I need to study to make this? There is a better solution?

This highly depends on what you are trying to achieve in the first place. It would be helpful if you could tell what you are trying to do, but I will still outline some general aspects:
You need to decide, what information is going to be exchanged and how this should happen
What information: Figure out, what exactly needs to be sent and received. Generic text messages? Images? Byte Streams?
How should this be done: Generally spoken, there are two approaches of getting information as a client: Polling and subscribing.
Polling: This approach means to periodically check an endpoint for new data. For example, HTTP uses this way: A web browser or any other client (REST-Client for example) periodically requests information from a HTTP-Server, using a connection just for this single request.
Subscribing / Sync / Notification: In some way or another, the client tells the server that it is interested in the information and wants to get notified when there is something new. The connection is initiated at the beginning and held open for further usage. The benefit of this approach is that changes are received immediately, but on the other hand a permanent connection needs to be maintained.
Things to study
At the beginning, get a good understanding of the TCP/IP Protocol, how Sockets work, how common Protocols do their job (e.g. HTTP, WebSockets)
Take a look at specific Protocols working on top of the basic ones
Tip: REST: Most common WebServices Protocol, providing a common way to exchange stateless data. Uses Polling.
WebSockets: Socket connection using Web Browsers. Commonly used to update information without needing to poll.
There is no specific language to learn for connections. It's more about understanding what the difficulties are and what ways have been invented to address this. Once you get to this point and know what you want to do, it's possible in every language.
Recommendation: As you seem to use Java/Android, I would try to use REST. A really great client-side library for REST on Android is Retrofit. For the server side use what fits for you .. common Java way would be to use Jersey, but you are free to choose from a lot of choices. If using Jersey is too hard for the beginning, maybe take a look at the JS/NodeJS world, those guys invented Express, which allows you to create a REST service out of just a database, wihtout having to code a lot.

First you need to decide if you want to go for an Android or an iOS application. There are other various mobile operating systems as well, but these are widely used . If you want to go for android which is most widely used in my opinion, then you need to learn Java. If you want to go for iOS application, then you need to learn swift or objectiveC. These languages provide the API to connect with various types of services such as Facebook, Firebase and Amazon etc. If you want to connect to some other local server who’s IP is known to you, then you can use socket programming to send messages.

There could be many ways you can implement this. One way will be using Web services. Of course REST might be a better option, if you follow this approach. You can implement Your service(server side code) with any language. I will recommend you use java since you are already using android.
Aside from this You might need to go through the basics of REST, its specifications and
some reference implementations for language of your preference.

Streaming of media

I am trying to stream media from a server to a client. I was going to use RTSP, but in RTSP the client tells the server what to do.
I need to be able to control the media the client gets from the server. The Server tells the Client what music he wants to play.
Is there an other protocol I can use or an other way I can work around? Maybe I am wrong and you can still do this using RTSP. I need to use UDP, as I need to use multicasts, and I dont want to have to build my own protocol to keep the client synchronized.
If you need more infos, dont hestiate to use the comments field below!
Edit
This application is done in Java. I would be happy, if you could maybe tell me a mechanism that works well with Java. I am currently using the Netty library, which looks to be pretty useful (although I have not figured out how to use it yet). I don't need an answer that is based on pure network knowledge, I know how I could build a whole streaming ecosystem. I need to get this working on Java 8, without having to do the handling of the basic protocol stuff myself.

Take a look at libjitsi which is an open source Java audio/video library which can be used for streaming with encryption. It does not use Netty but it looks to be mature.

Using a DataOutput/Input Stream with Netty?

I'm attempting to venture into non-blocking networking using Java and I had decided on using the Netty Networking Library to do so. I currently have a fully working server/client application that uses Strings to handle the networking functions. The server/client will dissasemble the strings and get the data from it; However this is not what I need to be doing.
I've been looking at the DataOutputStream and it seems it has the functions that I would like to be able to use over a network.
writeByte/readByte
writeBytes/readBytes
writeChar/readChar
writeDouble/readDouble
writeShort/readShort
writeFloat/readFloat
writeInt/readInt
writeUTF/readUTF.
Currently everything I've been doing has been in UTF Format, basically. I've just been sending bytes using (bytes[])String
I'm having a very hard time figuring out how to implement a system like this. I don't want to continue using this inefficient string based networking. It's a waste of bandwidth and I'm not learning anything doing it.

So, I found what I was looking for built into netty, I've been using it for a few days now and haven't posted back.
Inside of netty there is a ChannelBufferInputStream and a ChannelBufferOutputStream these both take in the ChannelBuffer as a parameter and allow similar functions to that of a DataStream. It has really helped me with my networking.

How can I implement a good Client-Server approach?

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.

Connect PHP code to Java backend

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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