LAB-22: Java Neworking

Through the classes in java.net, Java programs can use TCP or UDP to communicate over the Internet. The URL, URLConnection, Socket, and ServerSocket classes all use TCP to communicate over the network. The DatagramPacket, DatagramSocket, and MulticastSocket classes are for use with UDP.

Exercise 1: Writing Client and Server

(1.1) Build and run the server side code

0. Start NetBeans IDE if you have not done so yet.
1. Create a new NetBeans project

Select File->New Project (Ctrl+Shift+N). The New Project dialog box appears.
Under Choose Project pane, select Java under Categories and Java Application under Projects.
Click Next.

Under Name and Location pane, for the Project Name field, type in NetworkingServer as project name.
For Create Main Class field, type in NetworkingServer. (Figure-1.10 below)
Click Finish.

Figure-1.10: Create a new project

Observe that NetworkingServer project appears and IDE generated NetworkingServer.java is displayed in the source editor window of NetBeans IDE.

2. Modify the IDE generated NetworkingServer.java as shown in Code-1.11 below. Study the code by paying special attention to the bold fonted parts.

/* SERVER – may enhance to work for multiple clients */
import java.net.*;
import java.io.*;

public class NetworkingServer {

    public static void main(String [] args) {

        ServerSocket server = null;
        Socket client;

        // Default port number we are going to use
        int portnumber = 1234;
        if (args.length >= 1){
            portnumber = Integer.parseInt(args[0]);
        }

        // Create Server side socket
        try {
            server = new ServerSocket(portnumber);
        } catch (IOException ie) {
            System.out.println("Cannot open socket." + ie);
            System.exit(1);
        }
        System.out.println("ServerSocket is created " + server);

        // Wait for the data from the client and reply
        while(true) {

            try {

                // Listens for a connection to be made to
                // this socket and accepts it. The method blocks until
                // a connection is made
                System.out.println("Waiting for connect request...");
                client = server.accept();

                System.out.println("Connect request is accepted...");
                String clientHost = client.getInetAddress().getHostAddress();
                int clientPort = client.getPort();
                System.out.println("Client host = " + clientHost + " Client port = " + clientPort);

                // Read data from the client
                InputStream clientIn = client.getInputStream();
                BufferedReader br = new BufferedReader(new
                        InputStreamReader(clientIn));
                String msgFromClient = br.readLine();
                System.out.println("Message received from client = " + msgFromClient);

                // Send response to the client
                if (msgFromClient != null && !msgFromClient.equalsIgnoreCase("bye")) {
                    OutputStream clientOut = client.getOutputStream();
                    PrintWriter pw = new PrintWriter(clientOut, true);
                    String ansMsg = "Hello, " + msgFromClient;
                    pw.println(ansMsg);
                }

                // Close sockets
                if (msgFromClient != null && msgFromClient.equalsIgnoreCase("bye")) {
                    server.close();
                    client.close();
                    break;
                }

            } catch (IOException ie) {
            }
        }
    }
}

Code-1.11: NetworkingServer.java

3. Study ServerSocket class through context-sensitive Javadoc.

4. Build and run the project

Right click NetworkingServer project and select Run.
Observe that the server is waiting for a connection request from a client in the Output window. (Figure-1.13 below)

ServerSocket is created ServerSocket[addr=0.0.0.0/0.0.0.0,port=0,localport=1234]
Waiting for connect request...

Figure-1.13: Result of running NetworkingSever application

(1.2) Build and run the client side code

1. Create a new NetBeans project

Select File->New Project (Ctrl+Shift+N). The New Project dialog box appears.
Under Choose Project pane, select Java under Categories and Java Application under Projects.
Click Next.

Under Name and Location pane, for the Project Name field, type in NetworkingClient as project name.
For Create Main Class field, type in NetworkingClient.
Click Finish.

Observe that NetworkingClient project appears and IDE generated NetworkingClient.java is displayed in the source editor window of NetBeans IDE.

2. Modify the IDE generated NetworkingClient.java as shown in Code-1.21 below. Study the code by paying special attention to the bold fonted parts.

/* CLIENT */
import java.io.*;
import java.net.*;

public class NetworkingClient {

    public static void main(String args[]) {

        Socket client = null;

        // Default port number we are going to use
        int portnumber = 1234;
        if (args.length >= 1){
            portnumber = Integer.parseInt(args[0]);
        }

        for (int i=0; i <10; i++) {
            try {
                String msg = "";

                // Create a client socket
                client = new Socket(InetAddress.getLocalHost(), portnumber);
                System.out.println("Client socket is created " + client);

                // Create an output stream of the client socket
                OutputStream clientOut = client.getOutputStream();
                PrintWriter pw = new PrintWriter(clientOut, true);

                // Create an input stream of the client socket
                InputStream clientIn = client.getInputStream();
                BufferedReader br = new BufferedReader(new
                        InputStreamReader(clientIn));

                // Create BufferedReader for a standard input
                BufferedReader stdIn = new BufferedReader(new
                        InputStreamReader(System.in));

                System.out.println("Enter your name. Type Bye to exit. ");

                // Read data from standard input device and write it
                // to the output stream of the client socket.
                msg = stdIn.readLine().trim();
                pw.println(msg);

                // Read data from the input stream of the client socket.
                System.out.println("Message returned from the server = " + br.readLine());

                pw.close();
                br.close();
                client.close();

                // Stop the operation
                if (msg.equalsIgnoreCase("Bye")) {
                    break;
                }

            } catch (IOException ie) {
                System.out.println("I/O error " + ie);
            }
        }
    }
}

Code-1.21: NetworkingClient.java

3. Display and study Socket class through context-sensitive Javadoc'ing.

4. Build and run the project

Right click NetworkingClient project and select Run.
Observe the client is prompting you to enter your name. (Figure-1.23 below)

Client socket is created Socket[addr=Passion2/192.168.2.4,port=1234,localport=1775]
Enter your name. Type Bye to exit.

Figure-1.23: Waiting for the user to enter name

Trouble-shooting: If you see the following exception, it is highly likely that either you have not started the server or if you started the server, the firewall on your system blocks the incoming connection request.

I/O error java.net.ConnectException: Connection refused: connect
I/O error java.net.ConnectException: Connection refused: connect
I/O error java.net.ConnectException: Connection refused: connect
I/O error java.net.ConnectException: Connection refused: connect

Figure-1.24: Error condition

Solution: Make sure the server is run first. Also make sure the firewall on your system is turned off.

Enter your name under the line of "Enter your name. Type Bye to exit." in the Output window, like Sang Shin in this example, and press Enter key.

Observe that the server responds back with "Hello, Sang Shin".

Figure-1.25: Enter your name

Enter a few more names and observe that the server kept sending back responses.
Type Bye.

5. Observe the server side.

Click NetworkingServer (run) tab to see the Output window of the server side.
Observe that the server received a message, Sang Shin in this example. (Figure-1.27 and Figure-1.28 below.)

ServerSocket is created ServerSocket[addr=0.0.0.0/0.0.0.0,port=0,localport=1234]
Waiting for connect request...
Connect request is accepted...
Client host = 192.168.2.4 Client port = 1775
Message received from client = Sang Shin
Waiting for connect request...
Connect request is accepted...
Client host = 192.168.2.4 Client port = 1777

Figure-1.27: Client connection request is accepted and message is received

Figure-1.28: Client connection request is accepted and message is received

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Summary

Exercise 2: Writing Multicast Client and Server

(2.1) Build and run the server side code

1. Create a new NetBeans project

Select File->New Project (Ctrl+Shift+N). The New Project dialog box appears.
Under Choose Project pane, select Java under Categories and Java Application under Projects.
Click Next.

Under Name and Location pane, for the Project Name field, type in MulticastChatServer as project name.
For Create Main Class field, type in MulticastChatServer.

Observe that MulticastChatServer project appears and IDE generated MulticastChatServer.java is displayed in the source editor window of NetBeans IDE.

2. Modify the IDE generated MulticastChatServer.java as shown in Code-2.11 below. Study the code by paying special attention to the bold fonted parts.

import java.net.*;

public class MulticastChatServer {

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

        // Default port number we are going to use
        int portnumber = 5000;
        if (args.length >= 1) {
            portnumber = Integer.parseInt(args[0]);
        }

        // Create a MulticastSocket
        MulticastSocket serverMulticastSocket =
                new MulticastSocket(portnumber);
        System.out.println("MulticastSocket is created at port " + portnumber);

        // Determine the IP address of a host, given the host name
        InetAddress group =
                InetAddress.getByName("225.4.5.6");

        // getByName- returns IP address of given host
        serverMulticastSocket.joinGroup(group);
        System.out.println("joinGroup method is called...");
        boolean infinite = true;

        // Continually receives data and prints them
        while (infinite) {
            byte buf[] = new byte[1024];
            DatagramPacket data =
                    new DatagramPacket(buf, buf.length);
            serverMulticastSocket.receive(data);
            String msg =
                    new String(data.getData()).trim();
            System.out.println("Message received from client = " + msg);
        }
        serverMulticastSocket.close();
    }
}

Code-2.11: MulticastChatServer.java

3. Display and study Javadoc of the MulticastSocket class.

4. Build and run the project

Right click MulticastChatServer project and select Run.
Observe that the server is waiting for a datagram message from a client in the Output window. (Figure-2.12 below)

MulticastSocket is created at port 5000
joinGroup method is called...

Figure-2.12: Waiting for a message from a client

(2.2) Build and run the client side code

1. Create a new NetBeans project

Select File->New Project (Ctrl+Shift+N). The New Project dialog box appears.
Under Choose Project pane, select Java under Categories and Java Application under Projects.
Click Next.

Under Name and Location pane, for the Project Name field, type in MulticastChatClient as project name.
For Create Main Class field, type in MulticastChatClient.
Click Finish.

Observe that MulticastChatClient project appears and IDE generated MulticastChatClient.java is displayed in the source editor window of NetBeans IDE.

2. Modify the IDE generated MulticastChatClient.java as shown in Code-2.21 below. Study the code by paying special attention to the bold fonted parts.

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

public class MulticastChatClient {

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

        // Default port number we are going to use
        int portnumber = 5000;
        if (args.length >= 1) {
            portnumber = Integer.parseInt(args[0]);
        }

        // Create a MulticastSocket
        MulticastSocket chatMulticastSocket = new MulticastSocket(portnumber);

        // Determine the IP address of a host, given the host name
        InetAddress group =
                InetAddress.getByName("225.4.5.6");

        // Joins a multicast group
        chatMulticastSocket.joinGroup(group);

        // Prompt a user to enter a message
        String msg = "";
        System.out.println("Type a message for the server:");
        BufferedReader br =
                new BufferedReader(new InputStreamReader(System.in));
        msg = br.readLine();

        // Send the message to Multicast address
        DatagramPacket data = new DatagramPacket(msg.getBytes(), 0,
                msg.length(), group, portnumber);
        chatMulticastSocket.send(data);

        // Close the socket
        chatMulticastSocket.close();
    }
}

Code-2.21: MulticastChatClient.java

3. Build and run the project

Right click MulticastChatClient project and select Run.
Observe the client is prompting you to enter a message. (Figure-2.23 below)

Type a message for the server:

Figure-2.23: Waiting for the user to enter a message

Enter a message, like Hello Server in this example, and press Enter key. (Figure-2.24 below)

Figure-2.24: Enter a message

4. Observe the server side.

Click MulticastChatServer (run) tab to see the Output window of the server side.
Observe that the server received a multicast message, Hello, World! in this example. (Figure-2.25 below)

MulticastSocket is created at port 5000
joinGroup method is called...
Message received from client = Hello Server

Figure-2.25: Message is received from a client

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Java Networking

Resources

Exercises

Exercise 1: Writing Client and Server

(1.1) Build and run the server side code

(1.2) Build and run the client side code

Summary

Exercise 2: Writing Multicast Client and Server

(2.1) Build and run the server side code

(2.2) Build and run the client side code

Summary

Homework