Server

import java.io.*;
import java.net.*;
import java.util.*;
import java.util.concurrent.*;
import javafx.stage.FileChooser;


public class FlieServer extends Thread{

  Socket s;
    public FlieServer(Socket s) {
        this.s = s;
    }
    public void run(){
        try {
            InputStream in = s.getInputStream();
            OutputStream out = s.getOutputStream();
            BufferedReader br = new BufferedReader(
                                new InputStreamReader(
                                s.getInputStream()));
             PrintWriter pw = new PrintWriter(out);
             String msg = br.readLine();
             if(msg.equals("list")){
              File f = new File("D:\\papapo\\flie");
              String[] filename = f.list() ;
              for(int i =0 ; i < filename.length ; i++){
                        pw.println(filename[i]);}
             }
             else if(msg.equals("upload")){
              String filename = br.readLine();
              File f  = new File("D:\\papapo\\flie\\"+filename);
              pw.println("Uploading...");
              pw.flush();
              FileOutputStream fout = new FileOutputStream(f);
              byte[] b = new byte[65536];
              int size ;
              while((size = in.read(b)) > 0){
                      fout.write(b, 0 , size);
              }
              fout.flush();
              fout.close();
             }
             else if(msg.equals("download")){
                    String filename = br.readLine();
                    File f = new File("D:\\papapo\\flie\\"+filename);

            if(!f.exists()){
                    pw.println("NOK");
                    pw.flush();}
            else{
                    pw.println("OK");
                    pw.flush();
                    FileInputStream fin = new FileInputStream(f);
                    byte[] buffer = new byte[65536];
                    int size;
                    while((size = fin.read(buffer))> 0) {
                        out.write(buffer, 0, size);
                    }
                    out.flush();
             }
             }
             pw.flush();
             in.close();
             out.close();
             br.close();
             s.close();
            } catch(Exception e){e.printStackTrace();}
    }
    public static void main(String[] args) {

        ServerSocket serv = null ;
        ExecutorService es = Executors.newFixedThreadPool(10);

        try {
            serv = new ServerSocket(6789) ;
            while (true) {
                    Socket s = serv.accept();
                    FlieServer fs = new FlieServer(s);
                    es.execute(fs);
    }
            } catch(Exception e) {e.printStackTrace();}
    }

}