[ITC521 only] Topic 10 Java Socket Programming (for Networking and Communication)

1. The Client/Server Computing

Two or multiple programs can be connected to each other via Internet. If one of them is considered as server and others are considered as client then the communication among themselves happen through server socket and client sockets.

Data transmission through socket:

At the first step the server creates a socket using a port which can be connected by a client socket. Once the protocol establishes a connection, they are able to exchange information. These basic connection mechanisms to create an I/O stream is depicted in the following figure which along with detail description can be found in chapter 33 of the textbook.

 

Server to serve multiple sockets:

 A server can serve multiple sockets as illustrated in following figure:

An example code of the server serving multiple clients is as follows:

1  import java.io.*;
  2  import java.net.*;
  3  import java.util.*;
  4  import java.awt.*;
  5  import javax.swing.*;
  6  
  7  public class MultiThreadServer extends JFrame {
  8    // Text area for displaying contents
  9    private JTextArea jta = new JTextArea();
 10  
 11    public static void main(String[] args) {
 12      new MultiThreadServer();
 13    }
 14  
 15    public MultiThreadServer() {
 16      // Place text area on the frame
 17      setLayout(new BorderLayout());
 18      add(new JScrollPane(jta), BorderLayout.CENTER);
 19  
 20      setTitle("MultiThreadServer");
 21      setSize(500, 300);
 22      setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
 23      setVisible(true); // It is necessary to show the frame here!
 24  
 25      try {
 26        // Create a server socket
 27        ServerSocket serverSocket = new ServerSocket(8000);
 28        jta.append("MultiThreadServer started at " + new Date() + '\n');
 29  
 30        // Number a client
 31        int clientNo = 1;
 32  
 33        while (true) {
 34          // Listen for a new connection request
 35          Socket socket = serverSocket.accept();
 36  
 37          // Display the client number
 38          jta.append("Starting thread for client " + clientNo +
 39            " at " + new Date() + '\n');
 40  
 41          // Find the client's host name, and IP address
 42          InetAddress inetAddress = socket.getInetAddress();
 43          jta.append("Client " + clientNo + "'s host name is "
 44            + inetAddress.getHostName() + "\n");
 45          jta.append("Client " + clientNo + "'s IP Address is "
 46            + inetAddress.getHostAddress() + "\n");
 47  
 48          // Create a new thread for the connection
 49          HandleAClient task = new HandleAClient(socket);
 50  
 51          // Start the new thread
 52          new Thread(task).start();
 53  
 54          // Increment clientNo
 55          clientNo++;
 56        }
 57      }
 58      catch(IOException ex) {
 59        System.err.println(ex);
 60      }
 61    }
 62  
 63    // Inner class
 64    // Define the thread class for handling new connection
 65    class HandleAClient implements Runnable {
 66      private Socket socket; // A connected socket
 67  
 68      /** Construct a thread */
 69      public HandleAClient(Socket socket) {
 70        this.socket = socket;
 71      }
 72  
 73      /** Run a thread */
 74      public void run() {
 75        try {
 76          // Create data input and output streams
 77          DataInputStream inputFromClient = new DataInputStream(
 78            socket.getInputStream());
 79          DataOutputStream outputToClient = new DataOutputStream(
 80            socket.getOutputStream());
 81  
 82          // Continuously serve the client
 83          while (true) {
 84            // Receive radius from the client
 85            double radius = inputFromClient.readDouble();
 86  
 87            // Compute area
 88            double area = radius * radius * Math.PI;
 89  
 90            // Send area back to the client
 91            outputToClient.writeDouble(area);
 92  
 93            jta.append("radius received from client: " +
 94              radius + '\n');
 95            jta.append("Area found: " + area + '\n');
 96          }
 97        }
 98        catch(IOException e) {
 99          System.err.println(e);
100        }
101      }
102    }
103  }

Note: you are highly encouraged to read chapter 33 thoroughly to understand this topic well.