This is an ideal java socket client class It contains a socket client class and a RequestManager
You can just add the library file located in the bin directory to your project and enjoy the power of the library.
For those who want to modify the code and adapt it to their case,they can do so.

You need to have jackson library to be able to use those files, the jackson library is used to serialize and deserialize data to and from json

The communication between client and server is done using json, a request to the server is like {hash:(number>0),command:"",files?:[MediaFile],...others fields}, where the hash is the number associated to the request, and the command is the request command, and we can have more other fields depending on the command. The server so need to parse the request string , and based on the commands, return a response in json string format of course, and the client parse that response which must be like {hash:(number >=0),status:(number>=0),data:any,files?:[MediaFile]}.
Below is the classes diagram used

This class represents a request to send to the server, requests are made by the client and are managed by a RequestManager Object
To each request is assigned a hash code (hash field) , which becomes usefull when a response is received from the server. Each request has a required command field, so the server response is based on that value, and a data field (of ObjectNode type) which can contains an unlimited number of fields.

That class represents a response from the server, response are created by the Client from the json string recived from the server and are delivered to the RequestManager in case of request response, or to the message listener in case of server messages. To each response is assigned a status code and a hash code, the request and it's response will have the same hash code, so the request manager will know wich callback to call.

Free are you to defined your status code in a class, generaly to will share that class between your server and the client app, because they need the same codes.

This interface represents a call back on a request response.

that is an abstract class and has 3 abstract methods:

• send(String s): should be implemented (it is implemented by the Client class) by the class that communicated by the server
• getOutputStream(): should be implemented by the class that communicated by the server (here by the Client class), it returns the OutputStream underlying the connected socket to the server
• getInputStream(): should be implemented by the class that communicated by the server (here by the Client class), it returns the InputStream underlying the connected socket to the server

Thats is an interface and has just one method:

• onResponse(RequestResultCallBack callBack,Response response) : should be implemented.

That interface is responsible of handling ftp operation like sending or receiving a media file.
It contains just 2 method :

• send(OutputStream out,MediaFile file): permits sending a media file
• receive(InputStream in,MediaFile mediaFile): permits to receive a media file from the server You need to implements those two methods according to how your ftp server works

This class is responsibe of request management, all request to server pass bay it, and it store them in a list, so on request result it try to find the corresponding request and call its callback if any. That means this class need a RequestHandler object and a ResponseHandle to work.

This is the main class that represent a socket client, it's a bit complex, but very simple to understand
It inherits from the RequestHandler , so it can handle request and send them to the server.
It contains two main threads:

• readThread: in the loop of that thread we listen to the server and and treats so each incomming message Each message is readand translated to a json object

    String line=mDin.readUTF();
    if(line==null) break;
    JsonNode result=mapper.readTree(line);

  You will note that each response from the server should be a json and may have hash and status fields necessarily, else it's ignored

  if(!result.has("hash") || !result.has("status"))
    continue;

  A Response Object is then created and dispatched to the Request Manager or to the Message Listener according to the value of the hash field in the response, when this value is greater than 0 so that means the client has sent the message passing by the request manager and may have saved a callback on the request, so we send the response to the request manager, else that means we juste receive a message from the server without making any request, we transmit so the response to the listener if any!
  • writeThread: that thread is responsible for queued request string and send them to the server. it use the inner class named BlockingQueue, so if there is no request, it goes in sleep.

That interface is listener to server message, by server message I mean , messages send by the server without any request. Those message has the hash fiels equals 0.

This class represents a media file to send or receive to/from the server. It just gives a description a file name,size,absolutePath,type. Each request can contains many MediaFile, when executing the request those media files are sent to the server. When a request with media files is sent, the server may ask the media files , if possible by sending a response with status equals to 1, so the RequestManager will not remove the request from request queue, but will send the media files associated to that request.

To instanciate a client you juste need the new operator and call the open method to open connection to your server

  client=new Client();
   client.setListener(new MessageListener() {
       @Override
       public void onReceiveNewMessage(Response message) {

       }
   });
  ...
  new Thread(() -> {
   ....
   try {
       client.open(HOST,PORT);
       client.start();
      ...
   } catch (IOException e) {
       //retry to open
       //addressInput.show();
   }
 })
 .start();

And of course we need to configure our RequestManager, since it's a pure static class (contains only static methods), to do that we just new to call the configure method of RequestManager class like this

  RequestManager.configure(client, new ResponseHandler() {
      @Override
      public void onResponse(RequestResultCallBack callBack, Response response) {
          //runOnUiThread(() -> callBack.onSuccess(response)); //if the callback is created  in an android activity for example
          callBack.onSuccess(response)//this will fail if the callback is created  in an android activity and the callback change something about the views
      }
  }, new MediaFileHandler() {
      @Override
      public void send(OutputStream out, MediaFile file) {
          MFtp.ftpPut(file.absolutePath, (DataOutputStream) out);
      }

     @Override
     public File receive(InputStream in, MediaFile mediaFile) {
         File f=new File(Config.TEMP_DIR+"/"+mediaFile.name);
         if(!f.exists()){
             f=MFtp.ftpGetFile((DataInputStream) in, f.getParent());
         }
         return  f;
     }
  });

To perform a request to the server you just need to instaciate a new Request object, and fill its data object with you params

  //making a request to the server
  Request req=new Request("authenticate");
  if(!token.isEmpty())
      req.data.put("with",token);
  req.callback=new RequestResultCallBack() {
      @Override
      public void onSuccess(Response r) {
          JsonNode tmp = (JsonNode) r.data;
          pref.edit().putString("token",tmp.get("token").asText()).apply();
          groupeView.setText(tmp.get("name").asText());
          scoreView.setText(getString(R.string.score,tmp.get("score").asInt()));
      }
      @Override
      public void onFail(Response r) {

      }
  };
  req.execute();