Running a client/server apllication

[Roger Fed]

Hell,
I've developed my first Client/Server application using socket programming
the application works as follows:the client sends a string in lower case to the server and the server replies with the same string in upper case letters
I've developed these two classes
Client.java

import java.io.*;
import java.net.*;
public class Client {
    public static void main(String [] args) throws UnknownHostException, IOException{
        String sentence;
        String modifiedSentence;
        //Create a standard input stream
        BufferedReader inFromUser=new BufferedReader(new InputStreamReader(System.in));
        //Create a client socket and connect to the server
        Socket clientSocket=new Socket("Moustafa-PC",6789 );
        //Create output Stream attached to socket
        DataOutputStream outToServer=new DataOutputStream(clientSocket.getOutputStream());
        //Create input stream and attached it to socket
        BufferedReader inFromServer=new BufferedReader(new InputStreamReader(clientSocket.getInputStream()));
        //Read text from user
        sentence=inFromUser.readLine();
        //Send line to server
        outToServer.writeBytes(sentence);
        //Read a modified sentence from the user
        modifiedSentence=inFromServer.readLine();
        System.out.println("FROM SERVER"+modifiedSentence);
        clientSocket.close();



    }

}

and the Server.java

import java.io.*;
import java.net.*;
public class Server {
    public static void main(String [] args) throws IOException{
        String clientSentence;
        String capitalizedSentence;
        //Create a welcoming socket at port 6789
        ServerSocket welcomeSocket=new ServerSocket(6789);
        //Wait on welcoming socket for contact by client
        while(true)
        {
            Socket connectionSocket=welcomeSocket.accept();
            //Create input stream attached to socket
            BufferedReader inFromClient=new BufferedReader(new InputStreamReader(connectionSocket.getInputStream()));
            //Create output stream attached to socket
            DataOutputStream outToClient=new DataOutputStream(connectionSocket.getOutputStream());
            //Read in line from socket
            clientSentence=inFromClient.readLine();
            capitalizedSentence=clientSentence.toUpperCase()+"\n";
            //Write out line to socket
            outToClient.writeBytes(capitalizedSentence);
        }//End of the while loop, loop back and wait for another client connection
    }

}

My problem is that i don't know how can i run this application through command prompt or NetBeans.
so, I'd be grateful if you can help me in this
thanks in advance...................

[Rob Spoor]

You run them as you run any application: "java Server" for the server, "java Client" for the client. You'll need two command windows of course.

But your protocol is flawed. The sending and receiving parts do not match. You send in byte form, then try to read using a Reader (text form). Also, you try to read a single line but the sent data never has any line breaks (BufferedReader.readLine strips away the line breaks). The receiving code hangs. You can solve this by sending "\n" as well (which you are already doing from the server):

outToServer.writeBytes(sentence + "\n");

This will work on most systems, but it's not guaranteed. You're still mixing binary and text data. Switch to a Writer for the sending instead:

//DataOutputStream outToServer=new DataOutputStream(clientSocket.getOutputStream());
Writer outToServer = new OutputStreamWriter(clientSocket.getOutputStream());
outToServer.write(sentence + "\n");
outToServer.flush(); // to ensure the data is sent

There is now one issue, and one warning.
The issue: you are assuming that the systems the client and server will run on have the same system encoding. They may not. It's better to use an explicit encoding when creating the InputStreamReader and OutputStreamWriter objects.
The warning: you are now using "\n" as an explicit line break. This will work with this little example since readLine() can handle both "\n" and "\r\n", but most known services (like web servers) use "\r\n". If you then send "\n" they will have problems.

[Roger Fed]

Thanks Rob Spoor for your valuable help
I've modified the two classes as you said
I've replaced DataOutputStream with Writer in the two classes and I've used flush() method but when i run the server class it throws an exception :
Exception in thread "main" java.net.SocketException: Unrecognized Windows Sockets error: 0: JVM_Bind
at java.net.PlainSocketImpl.socketBind(Native Method)
at java.net.PlainSocketImpl.bind(PlainSocketImpl.java:365)
at java.net.ServerSocket.bind(ServerSocket.java:319)
at java.net.ServerSocket.<init>(ServerSocket.java:185)
at java.net.ServerSocket.<init>(ServerSocket.java:97)
at Server.main(Server.java:8)
what is this?
how can i get out of this problem?

[Rob Spoor]

Is something else using that port? You can see by opening a command prompt and then executing "netstat -a". An alternative is to use CurrPorts.

It's definitely not something in your code, because I was able to run your server just fine yesterday.

[Roger Fed]

Thanks Rob , you really helped me
my apllication is now works well
but i want to improve this to send more strings and the server reply to each one
I've modified the code in client as follow

do{
        sentence=inFromUser.readLine();
        //Send line to server
        outToServer.write(sentence+"\n");
        outToServer.flush();
        //Read a modified sentence from the user
        modifiedSentence=inFromServer.readLine();
        System.out.println("FROM SERVER: "+modifiedSentence);
        } while (!sentence.equals("bye"));

and also in server

do{
            clientSentence=inFromClient.readLine();
            capitalizedSentence=clientSentence.toUpperCase()+"\n";
            //Write out line to socket
            outToClient.write(capitalizedSentence);
            outToClient.flush();
            }while(!clientSentence.equals("bye"));

but it is still sends one string and after this the client sends and the server doesn't respond
this is the output of using this modified code
hello
FROM SERVER: HELLO
how are you
here the server supposed to respond but it doesn't
Where is the problem?

[Rob Spoor]

I don't know; it works for me. The code I've used, taken from your initial post and your last update. Note the System.out.print in the client part; that makes it easier for me to see the client is waiting for my input.

public class Client
{
    public static void main(String [] args) throws UnknownHostException, IOException
    {
        String sentence;
        String modifiedSentence;
        //Create a standard input stream
        BufferedReader inFromUser = new BufferedReader(new InputStreamReader(System.in));
        //Create a client socket and connect to the server
        Socket clientSocket = new Socket("localhost",6789 );
        //Create output Stream attached to socket
        Writer outToServer = new OutputStreamWriter(clientSocket.getOutputStream());
        //Create input stream and attached it to socket
        BufferedReader inFromServer = new BufferedReader(new InputStreamReader(clientSocket.getInputStream()));
        do
        {
            System.out.print("Input: ");
            sentence = inFromUser.readLine();
            //Send line to server
            outToServer.write(sentence + "\n");
            outToServer.flush();
            //Read a modified sentence from the user
            modifiedSentence = inFromServer.readLine();
            System.out.println("FROM SERVER: " + modifiedSentence);
        }
        while (!sentence.equals("bye"));
        clientSocket.close();
    }
}

public class Server
{
    public static void main(String [] args) throws IOException
    {
        String clientSentence;
        String capitalizedSentence;
        //Create a welcoming socket at port 6789
        ServerSocket welcomeSocket = new ServerSocket(6789);
        //Wait on welcoming socket for contact by client
        while (true)
        {
            Socket connectionSocket = welcomeSocket.accept();
            //Create input stream attached to socket
            BufferedReader inFromClient = new BufferedReader(new InputStreamReader(connectionSocket.getInputStream()));
            //Create output stream attached to socket
            Writer outToClient = new OutputStreamWriter(connectionSocket.getOutputStream());
            do
            {
                clientSentence = inFromClient.readLine();
                capitalizedSentence = clientSentence.toUpperCase() + "\n";
                //Write out line to socket
                outToClient.write(capitalizedSentence);
                outToClient.flush();
            }
            while (!clientSentence.equals("bye"));
        }//End of the while loop, loop back and wait for another client connection
    }
}