Java Plug-in 1.3 and RSA Signed Applets
Pages: 1, 2, 3

The SimpleScannerApplet Source Code
Now that you know what the scanner does, let's see how it works. Listing 1 shows the source code of the SimpleScannerApplet. It extends the Swing JApplet class and defines two inner classes: Scanner and ScannerException. The Scanner class handles the clicking of the Scan button and performs the actual scanning. The ScannerException class is used to throw scanner-related exceptions.

Listing 1. The SimpleScanner Applet

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import javax.swing.border.*;
import java.net.*;

public class SimpleScannerApplet extends JApplet {

    // Declare the GUI components used by the applet
    JTextField hostTextField = new JTextField("localhost",20);
    JTextField portTextField = new JTextField("80",5);
    JTextField resultsTextField = new JTextField(40);
    JButton scanButton = new JButton("Scan");
    JLabel hostLabel = new JLabel("Host name or IP address: ");
    JLabel portLabel = new JLabel("Port number: ");
    JLabel resultsLabel = new JLabel("Results: ");
    JPanel mainPanel = new JPanel(new FlowLayout(FlowLayout.LEFT));
    JPanel topPanel = new JPanel(new FlowLayout(FlowLayout.LEFT));
    JPanel bottomPanel = new JPanel(new FlowLayout(FlowLayout.LEFT));

    public SimpleScannerApplet() {
        // Layout the applet's components
        Border etched = BorderFactory.createEtchedBorder();
        // Add an etched border to the main panel
        mainPanel.setBorder(BorderFactory.createTitledBorder(etched, "Simple Port Scanner"));
        // Host and port fields in top panel
        topPanel.add(hostLabel);
        topPanel.add(hostTextField);
        topPanel.add(portLabel);
        topPanel.add(portTextField);
        // Scan button and result field in bottom panel
        bottomPanel.add(scanButton);
        bottomPanel.add(resultsLabel);
        bottomPanel.add(resultsTextField);
        // Add top and bottom panels to main panel
        mainPanel.add(topPanel);
        mainPanel.add(bottomPanel);
        // Set up the button's event handler
        scanButton.addActionListener(new Scanner());
        // Add the main panel to the applet's content pane
        getContentPane().add(mainPanel);
    }

    // The Scan button's event handler does the actual scanning
    private class Scanner implements ActionListener {

        // Declare fields used as scan parameters
        String host;
        InetAddress address;
        int port;
        String ip;

        // Handle the button's action event
        public void actionPerformed(ActionEvent ev) {
            out("Checking scan parameters ...");
            if(validParameters()) {
               out("Scanning ...");
               try {
                    // Try to connect to the host/port
                    Socket s = new Socket(address, port);
                    // No exception: SUCCESS
                    out("Port "+port+" is open on "+host+" ("+ip+").");
                    s.close();
                }catch(Exception ex) {
                    // Could not connect
                    if(ex instanceof SecurityException) out(ex.getMessage());
                    else out("Port "+port+" is closed on "+host+" ("+ip+").");
                }
            }
        }

        // Validate scan parameters
        private boolean validParameters() {
            try {
                host = hostTextField.getText();
                String portString = portTextField.getText();
                // Convert host name to an InetAddress object
                address = InetAddress.getByName(host);
                // Get the host's IP address
                ip = address.getHostAddress();
                // Convert the portString to an int
                port = Integer.decode(portString).intValue();
                // Make sure that it's in range
                if(port > 65535) throw new ScannerException("Invalid port.");
            }catch(Exception e) {
                // Handle any validation-related exceptions
                out(e.getMessage());
                return false;
            }
            return true;
        }

        // Convenience method for writing results
        private void out(String msg) {
            resultsTextField.setText(msg);
        }
    }

    // Used to identify invalid scan parameters
    private class ScannerException extends Exception {

        public ScannerException(String msg) {
            super(msg);
        }
    }
}

SimpleScannerApplet
The SimpleScannerApplet begins by declaring the GUI components that it displays. It uses Swing components for maximum compatibility across browser platforms.

// Declare the GUI components used by the applet
JTextField hostTextField = new JTextField("localhost",20);
JTextField portTextField = new JTextField("80",5);
JTextField resultsTextField = new JTextField(40);
JButton scanButton = new JButton("Scan");
JLabel hostLabel = new JLabel("Host name or IP address: ");
JLabel portLabel = new JLabel("Port number: ");
JLabel resultsLabel = new JLabel("Results: ");
JPanel mainPanel = new JPanel(new FlowLayout(FlowLayout.LEFT));
JPanel topPanel = new JPanel(new FlowLayout(FlowLayout.LEFT));
JPanel bottomPanel = new JPanel(new FlowLayout(FlowLayout.LEFT));

The SimpleScannerApplet constructor lays out the applet's GUI and creates an instance of Scanner to act as the Scan button's event handler.

public SimpleScannerApplet() {
  // Layout the applet's components
  Border etched = BorderFactory.createEtchedBorder();
  // Add an etched border to the main panel
mainPanel.setBorder(BorderFactory.createTitledBorder(etched,
"Simple Port Scanner"));
  // Host and port fields in top panel
  topPanel.add(hostLabel);
  topPanel.add(hostTextField);
  topPanel.add(portLabel);
  topPanel.add(portTextField);
  // Scan button and result field in bottom panel
  bottomPanel.add(scanButton);
  bottomPanel.add(resultsLabel);
  bottomPanel.add(resultsTextField);
  // Add top and bottom panels to main panel
  mainPanel.add(topPanel);
  mainPanel.add(bottomPanel);
  // Set up the button's event handler
  scanButton.addActionListener(new Scanner());
  // Add the main panel to the applet's content pane
  getContentPane().add(mainPanel);
}

Scanner
The Scanner inner class performs the actual scanning. It begins by declaring field variables that store the scan parameters.

// Declare fields used as scan parameters
String host;
InetAddress address;
int port;
String ip;

It then implements the actionPerformed() method of java.awt.event.ActionListener to handle the clicking of the Scan button. It uses the validParameters() method to determine whether the user supplied a valid host name/IP address and port number and to convert the users inputs into an InetAddress object and int port value.

A try-catch statement is used to catch any exceptions that result from the scanning. An attempt is made to create a TCP socket to the specified address and port. If the attempt is successful, then the port is identified as being open and then the socket is closed. If the attempt is unsuccessful, then an exception is thrown by the Socket constructor. The catch block handles this exception by checking to see if the exception is a SecurityException or some other type of exception.

If a SecurityException is thrown, then the message associated with the exception is displayed in the Results field. Otherwise, the port is identified as being closed for the specified host/IP address.

// Handle the button's action event
public void actionPerformed(ActionEvent ev) {
   out("Checking scan parameters ...");
   if(validParameters()) {
      out("Scanning ...");
      try {
         // Try to connect to the host/port
         Socket s = new Socket(address, port);
         // No exception: SUCCESS
         out("Port "+port+" is open on "+host+" ("+ip+").");
         s.close();
      }catch(Exception ex) {
         // Could not connect
         if(ex instanceof SecurityException) 
out(ex.getMessage());
         else 
out("Port "+port+" is closed on "
+host+" ("+ip+").");
      }
   }
}

The Scanner class defines two other methods that support the scanning process. The validParameters() method checks to see if the user has entered a valid host/IP address and port number and then converts those values from String objects to an InetAddress object and int port value.

// Validate scan parameters
private boolean validParameters() {
  try {
    host = hostTextField.getText();
    String portString = portTextField.getText();
    // Convert host name to an InetAddress object
    address = InetAddress.getByName(host);
    // Get the host's IP address
    ip = address.getHostAddress();
    // Convert the portString to an int
    port = Integer.decode(portString).intValue();
    // Make sure that it's in range
    if(port > 65535)
throw new ScannerException("Invalid port.");
  }catch(Exception e) {
    // Handle any validation-related exceptions
    out(e.getMessage());
    return false;
  }
  return true;
}

The out() method simplifies the display of output to the Results field.

// Convenience method for writing results
private void out(String msg) {
  resultsTextField.setText(msg);
}

ScannerException
The ScannerException class extends java.lang.Exception to provide a mechanism for identifying scanner-related exceptions.

// Used to identify invalid scan parameters
private class ScannerException extends Exception {

  public ScannerException(String msg) {
    super(msg);
  }
}

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