Simple Java Client/Server Program

Question

I'm writing my first java client/server program which just establishes a connection with the server sends it a sentence and the server sends the sentence back all capitalized. This is actually an example straight out of the book, and it works well and fine when I'm running the client and server on the same machine and using localhost for the server address. But when I put the client program on a different computer, it times out and never makes a connection with the server. I'm not sure why this is and its kind of lame making a your first client/server program and not actually be able to use it on two different machines. Here is the client code:

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

public class TCPClient {
    public static void main(String argv[]) throws Exception {
        String sentence;
        String modifiedSentence;
        BufferedReader inFromUser = new BufferedReader(new InputStreamReader(System.in));

        Socket clientSocket = new Socket("localhost", 6789);
        DataOutputStream outToServer = new DataOutputStream(clientSocket.getOutputStream());
        BufferedReader inFromServer = new BufferedReader(new InputStreamReader(clientSocket.getInputStream()));

        sentence = inFromUser.readLine();
        outToServer.writeBytes(sentence + '\n');
        modifiedSentence = inFromServer.readLine();
        System.out.println(modifiedSentence);
        clientSocket.close();
    }
}

Here is the server code:

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

public class TCPServer {
    public static void main(String args[]) throws Exception {
        String clientSentence;
        String capitalizedSentence;
        ServerSocket welcomeSocket = new ServerSocket(6789);

        while(true) {
            Socket connectionSocket = welcomeSocket.accept();
            BufferedReader inFromClient = new BufferedReader(new InputStreamReader(connectionSocket.getInputStream()));
            DataOutputStream outToClient = new DataOutputStream(connectionSocket.getOutputStream());
            clientSentence = inFromClient.readLine();
            capitalizedSentence = clientSentence.toUpperCase() + '\n';
            outToClient.writeBytes(capitalizedSentence);
        }
    }
}

The only thing I change when I run it on two different machines is the client program makes its socket with the IP address of the machine with the server program (which I got from whatismyipaddress.com). Thanks a lot for any help.

Update: I am indeed on a campus and it seems that its probably not allowing me to use that random port. Any suggestions on finding out what port I can use and or a port that is more than likely allowed?

Answer 1

It's probably a firewall issue. Make sure you port forward the port you want to connect to on the server side. localhost maps directly to an ip and also moves through your network stack. You're changing some text in your code but the way your program is working is fundamentally the same.

Answer 2

There is a fundamental concept of IP routing: You must have a unique IP address if you want your machine to be reachable via the Internet. This is called a "Public IP Address". "www.whatismyipaddress.com" will give you this. If your server is behind some default gateway, IP packets would reach you via that router. You can not be reached via your private IP address from the outside world. You should note that private IP addresses of client and server may be same as long as their corresponding default gateways have different addresses (that's why IPv4 is still in effect) I guess you're trying to ping from your private address of your client to the public IP address of the server (provided by whatismyipaddress.com). This is not feasible. In order to achieve this, a mapping from private to public address is required, a process called Network Address Translation or NAT in short. This is configured in Firewall or Router. You can create your own private network (say via wifi). In this case, since your client and server would be on the same logical network, no private to public address translation would be required and hence you can communicate using your private IP addresses only.

Answer 3

If you got your IP address from an external web site (http://whatismyipaddress.com/), you have your external IP address. If your server is on the same local network, you may need an internal IP address instead. Local IP addresses look like 10.X.X.X, 172.X.X.X, or 192.168.X.X.

Try the suggestions on this page to find what your machine thinks its IP address is.

Answer 4

Instead of using the IP address from whatismyipaddress.com, what if you just get the IP address directly from the machine and plug that in? whatismyipaddress.com will give you the address of your router (I'm assuming you're on a home network). I don't think port forwarding will work since your request will come from within the network, not outside.

Answer 5

Outstream is not closed ... close the stream so that response goes back to test client. Hope this helps.

Answer 6

you can get ip of that computer runs server program from DHCP list in that router you connected to.

Answer 7

My try to do client socket program

server reads file and print it to console and copies it to output file

Server Program:

package SocketProgramming.copy;

import java.io.BufferedOutputStream;
import java.io.File;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.net.ServerSocket;
import java.net.Socket;

public class ServerRecieveFile {
    public static void main(String[] args) throws IOException {
        // TODO Auto-enerated method stub
        int filesize = 1022386;
        int bytesRead;
        int currentTot;
        ServerSocket s = new ServerSocket(0);
        int port = s.getLocalPort();
        ServerSocket serverSocket = new ServerSocket(15123);
        while (true) {
            Socket socket = serverSocket.accept();
            byte[] bytearray = new byte[filesize];
            InputStream is = socket.getInputStream();
            File copyFileName = new File("C:/Users/Username/Desktop/Output_file.txt");
            FileOutputStream fos = new FileOutputStream(copyFileName);
            BufferedOutputStream bos = new BufferedOutputStream(fos);
            bytesRead = is.read(bytearray, 0, bytearray.length);
            currentTot = bytesRead;
            do {
                bytesRead = is.read(bytearray, currentTot,
                        (bytearray.length - currentTot));
                if (bytesRead >= 0)
                    currentTot += bytesRead;
            } while (bytesRead > -1);
            bos.write(bytearray, 0, currentTot);
            bos.flush();
            bos.close();
            socket.close();
        }
    }
}

Client program:

package SocketProgramming.copy;

import java.io.BufferedInputStream;
import java.io.BufferedReader;
import java.io.File;
import java.io.FileInputStream;
import java.io.FileReader;
import java.io.IOException;
import java.io.OutputStream;
import java.net.InetAddress;
import java.net.ServerSocket;
import java.net.Socket;
import java.net.UnknownHostException;

public class ClientSendFile {
    public static void main(String[] args) throws UnknownHostException,
            IOException {
        // final String FILE_NAME="C:/Users/Username/Desktop/Input_file.txt";
        final String FILE_NAME = "C:/Users/Username/Desktop/Input_file.txt";
        ServerSocket s = new ServerSocket(0);
        int port = s.getLocalPort();
        Socket socket = new Socket(InetAddress.getLocalHost(), 15123);
        System.out.println("Accepted connection : " + socket);
        File transferFile = new File(FILE_NAME);
        byte[] bytearray = new byte[(int) transferFile.length()];
        FileInputStream fin = new FileInputStream(transferFile);
        BufferedInputStream bin = new BufferedInputStream(fin);
        bin.read(bytearray, 0, bytearray.length);
        OutputStream os = socket.getOutputStream();
        System.out.println("Sending Files...");

        os.write(bytearray, 0, bytearray.length);

        BufferedReader r = new BufferedReader(new FileReader(FILE_NAME));
        String as = "", line = null;
        while ((line = r.readLine()) != null) {
            as += line + "\n";
            // as += line;

        }
        System.out.print("Input File contains following data: " + as);
        os.flush();
        fin.close();
        bin.close();
        os.close();
        socket.close();

        System.out.println("File transfer complete");
    }
}

Answer 8

this is client code

first run the server program then on another cmd run client program

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

public class frmclient 
{
 public static void main(String args[])throws Exception
 {

   try
        {
          DataInputStream d=new DataInputStream(System.in);
          System.out.print("\n1.fact\n2.Sum of digit\nEnter ur choice:");

           int ch=Integer.parseInt(d.readLine());
           System.out.print("\nEnter number:");
           int num=Integer.parseInt(d.readLine());

           Socket s=new Socket("localhost",1024);

           PrintStream ps=new PrintStream(s.getOutputStream());
           ps.println(ch+"");
           ps.println(num+"");

          DataInputStream dis=new DataInputStream(s.getInputStream());
           String response=dis.readLine();
                 System.out.print("Answer:"+response);


                s.close();
        }
        catch(Exception ex)
        {

        }
 }


}

this is sever side code

import java.io.*;
import java.net.*;
public class frmserver {

  public static void main(String args[])throws Exception
  {

try
    {

    ServerSocket ss=new ServerSocket(1024);
       System.out.print("\nWaiting for client.....");
       Socket s=ss.accept();
       System.out.print("\nConnected");

       DataInputStream d=new DataInputStream(s.getInputStream());

        int ch=Integer.parseInt(d.readLine());
       int num=Integer.parseInt(d.readLine());
         int result=0;

        PrintStream ps=new PrintStream(s.getOutputStream());
        switch(ch)
        {
          case 1:result=fact(num);
                 ps.println(result);
                  break;
          case 2:result=sum(num);
                 ps.println(result);
                  break;
        }

          ss.close();
          s.close();
    }
catch(Exception ex)
{

}
  }

  public static int fact(int n)
  {
  int ans=1;
    for(int i=n;i>0;i--)
    {
      ans=ans*i;
    }
    return ans;
  }
  public static int sum(int n)
  {
   String str=n+"";
   int ans=0;
    for(int i=0;i<str.length();i++)
    {
      int tmp=Integer.parseInt(str.charAt(i)+"");
      ans=ans+tmp;
    }
    return ans;
  }
}

Answer 9

import java.io.*;
import java.net.*;
class serversvi1
{
  public static void main(String svi[]) throws IOException
  {
    try
    {
      ServerSocket servsock=new ServerSocket(5510);
      DataInputStream dis=new DataInputStream(System.in);

      System.out.println("enter the file name");

      String fil=dis.readLine();
      System.out.println(fil+" :is file transfer");

      File myfile=new File(fil);
      while(true)
      {
        Socket sock=servsock.accept();
        byte[] mybytearray=new byte[(int)myfile.length()];

        BufferedInputStream bis=new BufferedInputStream(new FileInputStream(myfile));

        bis.read(mybytearray,0,mybytearray.length);

        OutputStream os=sock.getOutputStream();
        os.write(mybytearray,0,mybytearray.length);
        os.flush();

        sock.close();
      }
    }
    catch(Exception saranvi)
    {
      System.out.print(saranvi);
    }
  }
}


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

class clientsvi1
{
  public static void main(String svi[])throws IOException
  {
    try
    {
      Socket sock=new Socket("localhost",5510);
      byte[] bytearray=new byte[1024];

      InputStream is=sock.getInputStream();
      DataInputStream dis=new DataInputStream(System.in);
      System.out.println("enter the file name");

      String fil=dis.readLine();
      FileOutputStream fos=new FileOutputStream(fil);
      BufferedOutputStream bos=new  BufferedOutputStream(fos);
      int bytesread=is.read(bytearray,0,bytearray.length);
      bos.write(bytearray,0,bytesread);
      System.out.println("out.txt file is received");
      bos.close();
      sock.close();
    }
    catch(Exception SVI)
    {
      System.out.print(SVI);
    }
  }
}