How to show error message if Server does not have a file requested by client TCP using java

Question

I have to write java project for TCP Client/Server. The goal of the code is the Client asks server about a file. if the file is available it will be saved into client directory and show message the file is found and saved otherwise, the file is not found.

the code works when the file is in the Server directory and it will be saved in the client directory but if the file is not available it will show message in server side the file not available but in Client side it will save a new empty file with the name of the file that not available and message the File received

Can you help please

I have Server code

import java.io.*;
import java.net.*;
import java.util.*; 

//********************
public class Server
{
   static ServerSocket  ServSock;
   static int           QLen = 6;

   //***
   public static void main(String args[])
   {
      try
      {
         //*** establish server socket
         ServSock = new ServerSocket(Integer.parseInt(args[0]), QLen);

         while (true)
         {
               //***  accept a client
               Socket Sock = ServSock.accept();

               //*** pass the client to a new thread
               new Worker(Sock).start();
          }
      }
      catch(IOException e)
          {System.out.println(e);}
    }
}





//*************************
class Worker extends Thread
{
   Socket          Sock;
   PrintWriter     PW;
   BufferedReader  DIS;
   String          Question = "Enter the file name to transfer from server:";


   //**************
   Worker(Socket S)
   {Sock=S;}


   //**************
   public void run()
   {
     try
     {
        //*** thread identifies itself
        System.out.println("Thread: " + getName());


        //*** auto-flush
        //*** set up socket I/O streams
        PW = new PrintWriter (new BufferedWriter (new OutputStreamWriter(Sock.getOutputStream())),true);
        DIS  = new BufferedReader (new InputStreamReader (Sock.getInputStream()));



        //*** send server question
        PW.println(Question);


        //*** wait for client response
        String R = DIS.readLine();
        Scanner in = null; 
         try{
                BufferedOutputStream put=new BufferedOutputStream(Sock.getOutputStream());
                 BufferedReader st=new BufferedReader(new InputStreamReader(Sock.getInputStream()));
                  String str = "/Users/Batool/Desktop/HW/Server/";
                 String path = str + R; 
                 System.out.println("The requested file is path: "+path);
                 System.out.println("The requested file is : "+R);

                 File f=new File(path);
                  System.out.println(f.isFile());


                 if(f.isFile())
                 { 

            //System.out.println("inside if");

                     FileInputStream fis=new FileInputStream(f);
             System.out.println("File transfered");
System.out.println("———————————————————————————————");

                   byte []buf=new byte[1024];
                     int read;


                     while((read=fis.read(buf,0,1024))!=-1)
                     {
                         put.write(buf,0,read);
                         put.flush();
                     }




        }//end if 



                else{ System.out.println("File not found");
              System.out.println("———————————————————————————————");}//end else   

            }//end try


         catch(FileNotFoundException e)
            {
               System.out.println(e.getMessage());
               //System.exit(0);

             }//end catch


        //*** close this socket connection
        Sock.close();
        }

     catch(IOException e)
        {System.out.println(e);}
   }
}

and I have Client code

import java.io.*;
import java.net.*;
import java.util.*; 

public class Client
{
   static Socket          sock;
   static PrintWriter     pw;
   static BufferedReader  br;
   static String          response = "";

   //************************************
   public static void main(String args[]) throws IOException
   {
      //*** establish connection to remote server
      sock = new Socket(args[0],
                       Integer.parseInt(args[1]));  //*** provide server name & port


      //*** set up socket I/O streams
      pw = new PrintWriter (new BufferedWriter (new OutputStreamWriter(sock.getOutputStream())),true);
      br  = new BufferedReader (new InputStreamReader (sock.getInputStream()));


      //*** wait for server question
      String r = br.readLine();    //*** THIS IS A BLOCKING CALL
      System.out.println("Server asks: " + r);

      //*** respond to server
      /*Scanner in = new Scanner(System.in); 
     String s = in.nextLine();
      pw.println(s);
      pw.flush(); 
     */
     Socket s=null;
        BufferedInputStream get=null;
        PrintWriter put=null;
        try
        { 
             get=new BufferedInputStream(sock.getInputStream());
            put=new PrintWriter(sock.getOutputStream(),true);

            String f;
            int u;
            //System.out.println("Enter the file name to transfer from server:");
            DataInputStream dis=new DataInputStream(System.in);
            f=dis.readLine();
            put.println(f);
            File f1=new File(f);
            String str = "/Users/Batool/Desktop/HW/Client";
            FileOutputStream  fs=new FileOutputStream(new File(str,f1.toString()));

        System.out.println(new File(str,f1.toString()));

System.out.println(f1.isFile());

            byte jj[]=new byte[1024];

            while((u=get.read(jj,0,1024))!=-1)
            { 
                fs.write(jj,0,u);
            } 
            fs.close();
            System.out.println("File received");
System.out.println("———————————————————————————————");
             sock.close();
        }catch(Exception e)
        {
                       System.out.println("File not found");
            System.out.println("———————————————————————————————");
            e.printStackTrace();
            System.exit(0);
        }


      //*** close this socket connection
      //sock.close();
   }
}

Answer 1

Basically you need to implement a protocol.

Client asks server: do you have file X?

a) Servers says: yes, I got xyz bytes for you, and the md5sum is zyx

Then your client asks for the file; and it can verify it got the correct amount of bytes and that no transmission errors happened. (of course you don't need that verification part; but depending on your context, those are things that you should consider to have in place, too)

b) Server says: no, I don't have that file

Then your client gives an error message to its user.

In other words: the server just reading files and pushing bytes out will not do!

You want to turn your server into something that offers services. And a service has a contract, such as "input" and "output". And services need ways to "negotiate" what should happen next. In your current solution, everything that happen is hardcoded - meaning there is already a protocol in place, but that protocol is defined by the instructions that you put into your code!