How to write code in java TCP network programming

Question

1.00/5 (1 vote)

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Hello. I am beginner in network programming. I am gonna write code for client and ask user for some question in client and user will also input the choice based on integer. Supposedly, the server should return the user's input to the client. But it not works. I hope you guys can help me and give example. Thank you in advance.

What I have tried:

Server code

Java

import java.io.IOException;
import java.io.PrintWriter;
import java.net.ServerSocket;
import java.net.Socket;
import java.util.Scanner;

public class TestServer
{
    private static ServerSocket servSock;
    private static final int PORT = 1234;

    public static void main(String[] args)
    {
        System.out.println("Opening port...\n");
        try
        {
            servSock = new ServerSocket(PORT);      //Step 1.
        }
        catch(IOException ioEx)
        {
            System.out.println("Unable to attach to port!");
            System.exit(1);
        }
        do
        {
            handleClient();
        }
        while (true);
    }

    private static void handleClient()
    {
        Socket link = null;                     			//Step 2.
        try
        {
            link = servSock.accept();        				//Step 2.

            Scanner input = new Scanner(link.getInputStream()); 	//Step 3.
            PrintWriter output = new PrintWriter(link.getOutputStream(),true); 	//Step 3.

            int numMessages = 50;
            String message = input.nextLine();      			//Step 4.
            while (!message.equals("***CLOSE***"))
            {
                System.out.println("Message received.");
                numMessages++;
                output.println("Message " + numMessages + ": " + message);   		//Step 4.
                message = input.nextLine();
            }
            output.println(numMessages + " messages received."); 	//Step 4.
        }
        catch(IOException ioEx)
        {
            ioEx.printStackTrace();
        }
        finally
        {
            try
            {
                System.out.println( "\n* Closing connection... *");
                link.close();                    //Step 5.
            }
            catch(IOException ioEx)
            {
                System.out.println("Unable to disconnect!");
                System.exit(1);
            }
        }
    }
}

Client code

Java

package testclient;

import java.io.IOException;
import java.io.PrintWriter;
import java.net.InetAddress;
import java.net.Socket;
import java.net.UnknownHostException;
import java.util.Scanner;

public class TestClient
{

    private static InetAddress host;
    private static final int PORT = 1234;
    
    public static void main(String[] args)
    {
       try
       {
           host = InetAddress.getLocalHost();
       }
       catch(UnknownHostException uhEx)
       {
           System.out.println("Host ID not found");
           System.exit(1);
       }
       accessServer();
    }
    
    private static void accessServer()
    {
        Socket link = null;
        
        try
        {
            link = new Socket(host,PORT);
            Scanner input = new Scanner(link.getInputStream());
            
            PrintWriter output = new PrintWriter(link.getOutputStream(),true);
            
            Scanner userEntry = new Scanner(System.in);
            
            String message, response;
            do
            {
                System.out.println("Are you in healthy condition?");
                message = userEntry.nextLine();
                System.out.println("Are you in happy mode?");
                message = userEntry.nextLine();
                System.out.println("Are you in sad condition?");
                message = userEntry.nextLine();
                System.out.println("Are you in sick condition?");
                message = userEntry.nextLine();
                response = input.nextLine();
                System.out.println("\nServer " +response);
                output.println(message);
                response = input.nextLine();
                System.out.println("\nServer " +response);
            }while(!message.equals("***CLOSE***"));
        }
        catch(IOException ioEx)
        {
            ioEx.printStackTrace();
        }
        
        finally
        {
            try
            {
                System.out.println("\n*Closing connection...*");
                link.close();
            }
            catch(IOException ioEx)
            {
                System.out.println("Unable to disconnect...");
                System.exit(1);
            }
        }
    }     
}

Posted 8-Jan-18 1:44am

eddyfy

Updated 8-Jan-18 3:13am

CPallini

v2

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Comments

ZurdoDev 8-Jan-18 8:00am

Why does it not work?

eddyfy 8-Jan-18 8:23am

After, i have added another question, it doesn't work for me. it cannot attach to the port

eddyfy 8-Jan-18 8:24am

After, i had added another question it doesn't work in the server.

ZurdoDev 8-Jan-18 8:42am

You answered what is not working. You need to answer why. And since we can't run your code, you need to do some debugging and find out what is happening and why. Then you can fix it.

You can't just call up a mechanic and tell them that you can't drive to work. You have to figure out why.

eddyfy 8-Jan-18 9:46am

I am sorry. But the server cannot attach to the port. So, that it can't return back the user input's to the client

1 solution

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Jochen Arndt · Answer 1 · 2018-01-08T03:13:00

Solution 1

The sequence on the client for each question should be:

Print on screen
Read user input
Send to server
Receive response from server
Print response on screen

But your code does not do that. It first prints questions and reads the user input for four questions and then waits for a server response but without sending to the server.

It should be like

Java

System.out.println("Are you in healthy condition?");
message = userEntry.nextLine();
output.println(message);
response = input.nextLine();
System.out.println("\nServer " +response);

// Next question goes here repeating the above

Posted 8-Jan-18 3:13am

Jochen Arndt

Comments

eddyfy 8-Jan-18 9:40am

But the server still doesn't work. It said that unable to attach to the port

Jochen Arndt 8-Jan-18 9:52am

That is probably because you are not closing the socket and the port is therefore blocked from the previous execution.

How do you have stopped your server application?
You have an infinite while loop in main and System.Exit is only called when there is an exception when closing the link socket. Even when System.Exit is called, it is recommended to close the socket before.

Try to add
servSock.close();
after closing the link socket or do that in your main() function and remove the infinite loop (you already have a loop in your handling function).