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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.

Inter-process-communication between a Java application and a local server

Firstly Cheers to all PROGRAMMERS [ Today = Programmers day :) ]
Secondly,
I'm working on a project where the specifications require using a server as a front end and an application in the back end. The project is an advanced smart home system. The server will handle commands coming from the client through the internet (let's say like a remote control from outside the house) and send them (through a channel of communication) to the application (planning on using JAVA application) which will handle the main logic like controlling hardware stuff (lights ...) , reading from a microphone (local mic) and accessing a database to act as a speech recognition system (offline).
Now I'm still in the planning phase and I'm not sure which technologies are the best for this project. I'm thinking to use Node.js or Apache as the server and a JAVA application as the back end and any SQL database for the application's SRS.
I hope this illustration demonstrates clearly how the system works:
The main question is:
What is the best way to make the Java application communicate with the server (communication channel [must be bidirectional]) ?
and Do you recommend a specific server other than the mentioned ones for this job ?
What crossed my mind so far:
1- JSP and servlets (making the server is the application too). But I don't want a server to handle the offline stuff and I'm not sure if java servlets can access hardware interface. I also want the server to be separate from making critical decisions (different layer for security reasons and since it won't be used as frequently as the local [offline] system).
2- Communication channel :
A- A shared file, but it's a bad idea since I don't want the application to check if the file contents changed (command received) or not from time to time (excessive operations).
B- A an inter-process-communication through a port (socket communication) seems the best solution but I don't know how that would turn in terms of operation cost and communication errors.
OS used : Linux Raspbian
EDIT:
I'm sure ZMQ+Apache is good enough for this task, but how is it in comparison to WebServices (like SOAP) ? Would WebServices be a better solution in terms of standard implementation and security ?
All related suggestions are welcomed, TQ

ZeroMQ is great for internal communications, or any other similar communication solutions.
For specifically your case, I can see that ZeroMQ would be a best fit.
Reasons:
You offline server have to be agnostic to web solution.
Communication can be reliable and bi-directional, possibly another patterns like (pub>sub, req<>res, etc).
Restarting any of sides would not require to restart the sockets (connection) on other side, as messages are queued.
Possibility to scale not just on same hardware, but as well to local area network or even through internet.
Big community of support. It might look a bit hard to get into, but in reality it is dead simple, just go to examples and once concept understood - it is very easy and neat to work with.
ZeroMQ has lots drivers for most popular languages, that includes Java and Node.js.
Considerations:
You need to think over packets and data will be sent. So some popular data protocols like XML or JSON is good way of thinking.
Responsibilities over different services - make sure they are not dependant on each other too much. Or if main offline server - is a core of system, make sure it does not depend on web facing service, so that web face can be removed/replaced/improved etc.
Few more points to think about:
Why Java, and what about modular approach? For example if you want to expand and scale - add more sensors into smart home solutions, then having one giant application would require to change it, it is harder to maintain as well as maintain different clients with own needs. Think modular way - some core functionality for offline stuff, but many aggregator processes that would talk to different sensors. This makes easier to support different setups and environments, as well maintain the system as a whole by improving independent components.

Trigger functions from another java program

We've got a server app and two stand alone client apps (both with different functionality - one for front office and the other for back office). Everything is written in Java.
What we need right now:
If both apps are running - click on a button in one app -> checks to see if the other app is open and triggers some functionality (display a message, open a frame) on that app
if the other app isn't open -> it should display a message saying so.
Can anyone point me in the right direction to achieve this. The best real life example I can give is: how clicking on the an itunes link in the web browser opens the iTunes application if installed and to the relevant appstore page.
EDIT: Our applications don't deal with websites at all. Everything uses Swing.

There is no "best" way to achieve inter-app communications but there are many ways; the best one will be the one that fits best your environment: network conditions, firewalls, number of calls, synchronous vs asynchronous, etc...
Usually communication is achieved using either:
Remote Procedure Calls: an app basically calls a function/method on the other app and passes arguments. RPC are usually synchronous: the response is sent within the same communication/transaction
Messaging: an app sends messages to the other app which, maybe, replies with other messages. Messaging is usually asynchronous.
The frontier between the two can be pretty blur with some protocols like REST.
In the Java world,
RPC is usually achieved using either
RMI: Java only solution; easy to implement; does not like firwalls much.
SOAP Web services: not Java centric; hard to implement; full of traps; network friendly.
Messaging can be achieved using
JMS: Java only; rather easy to implement but asynchronous; extremely powerful on high loads
JSON/XML HTTP/s Messaging: there are many protocols here from the most secure like AS2 to RNIF, plain XML/Json POST etc... These are network and language agnostic but always require some work to implement.
An hybrid approach is REST which has become very popular due to the benefits of an easy implementation and network friendliness but has the drawbacks of not being very formalized. it is a technology rather than a specification. I would look at documentation around JAX-RS and frameworks like Restlet and Jersey to get you started.
(Edit)
I purposely did not mention developing your own with Java sockets. IO is by definition impure and often multithreaded: IO is very hard to get right. If you really insist going down that route, at least, use the help of a proper framework like Apache Mina or Netty.

Adding notification push from Mysql server to Java application

Hi I have designed a desktop application in Java SE with netbeans and Mysql.I do not have any server coding for now since all my operations were of pull type. I need a push notification functionality. Previously I had a button that refreshed the notification but now I want the notifications to be updated automatically.
What would be best way to add this functionality without changing much of my present code.
Any help appreciated.
Thanks a lot

You really will have to change a lot about your architecture to have your application go the other direction. However, I think the best approach is to use a light weight JMS provider. That way your client can code to the JMS API which is really pretty straight forward, and your server can use JMS and its really pretty easy. It's not nearly as much effort as plumbing other options in.
The harder part is setting up the container in your application. It's not tremendously hard, but JMS has lots of options for configuration. Figuring out the point to point vs topic, durable vs non-durable and what's right for your application is a lot of research you have to do if you haven't done JMS before.
But, what this affords you is a very expressive control over messaging in your application. You can under the covers without modifying your code swap between polling or direct connections, message 1:1 or 1:many. If you need to send a message to one client vs. sending a message to all clients. You can segment messages between clients and create groups. Your messages can be durable or non-durable (survive client shutdown or server shutdown). The possibilities are endless, but you have to make a lot of architectural decisions. You also don't have to handle network topology and connectivity issues as much than if you used UDP multicast or TCP connections in reverse.
ActiveMQ and RabbitMQ can be easily embedded within a server using spring in a matter of 20 minutes. They also provide hooks for other platforms. It might sound overkill, but I've tried to do without JMS in the past when I should've used it and regretted not using it.

Server in Client's computer to create a TCP persistent link

I want to create a Java server in the client's browser to collect and manage communication
and update the displayed material in the client browser and receive the Get Push Post statements and then communicate through a long life persistent link for security reasons.
I know some universities have started working on this type of service
But i would prefer to do this from another approach.
What services/plugins are there that can help me in this field.
Its taken me a long time just to get to this point to even know what I'm asking for
But now I'm hearing I just need the tools and implementation of them.

What you're looking for is probably known as COMET, and can be done purely in JavaScript, without requiring a Java applet: http://en.wikipedia.org/wiki/Comet_(programming)) The advantage being that it could be implemented on a mobile device/tablet as well.
There are a number of libraries that will help you accomplish this depending on your programming environment. I tend to work in JSF, and so I favor tools for that environment, (IceFaces, RichFaces, PrimeFaces) but your choice of tools will depend on your specific application environment, which you haven't provided much detail about.