signed applet

Video Note

Run applets standalone

18.4 Applet Security Restrictions

Java uses the so-called “sandbox security model” for executing applets to prevent destructive programs from damaging the system on which the browser is running. Applets are not allowed to use resources outside the “sandbox.” Specifically, the sandbox restricts the following activities:

■Applets are not allowed to read from, or write to, the file system of the computer. Otherwise, they could damage the files and spread viruses.

■Applets are not allowed to run programs on the browser’s computer. Otherwise, they might call destructive local programs and damage the local system on the user’s computer.

■Applets are not allowed to establish connections between the user’s computer and any other computer, except for the server where the applets are stored. This restriction prevents the applet from connecting the user’s computer to another computer without the user’s knowledge.

Note

You can create signed applets to circumvent the security restrictions. See java.sun.com/ developer/onlineTraining/Programming/JDCBook/signed.html for detailed instructions on how to create signed applets.

18.5 Enabling Applets to Run as Applications

Despite some differences, the JFrame class and the JApplet class have a lot in common. Since they both are subclasses of the Container class, all their user-interface components, layout managers, and event-handling features are the same. Applications, however, are invoked from the static main method by the Java interpreter, and applets are run by the Web browser. The Web browser creates an instance of the applet using the applet’s no-arg constructor and controls and executes the applet.

In general, an applet can be converted to an application without loss of functionality. An application can be converted to an applet as long as it does not violate the security restrictions imposed on applets. You can implement a main method in an applet to enable the applet to run as an application. This feature has both theoretical and practical implications. Theoretically, it blurs the difference between applets and applications. You can write a class that is both an applet and an application. From the standpoint of practicality, it is convenient to be able to run a program in two ways.

How do you write such program? Suppose you have an applet named MyApplet. To enable it to run as an application, you need only add a main method in the applet with the implementation, as follows:

public static void main(String[] args) { // Create a frame

JFrame frame = new JFrame("Applet is in the frame");

// Create an instance of the applet MyApplet applet = new MyApplet();

// Add the applet to the frame frame.add(applet, BorderLayout.CENTER);

// Display the frame frame.setSize(300, 300);

frame.setLocationRelativeTo(null); // Center the frame frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); frame.setVisible(true);

}

create frame

create applet

add applet

show frame

LISTING 18.6 DisplayMessageApp.java

1 import javax.swing.*;

2 import java.awt.Font;

3 import java.awt.BorderLayout;

4

5 public class DisplayMessageApp extends JApplet {

6 private String message = "A default message"; // Message to display 7 private int x = 20; // Default x-coordinate

8 private int y = 20; // Default y-coordinate 9

10/** Determine whether it is an application */

11private boolean isStandalone = false;

12

13/** Initialize the applet */

14public void init() {

15if (!isStandalone) {

16// Get parameter values from the HTML file

17message = getParameter("MESSAGE");

18x = Integer.parseInt(getParameter("X"));

19y = Integer.parseInt(getParameter("Y"));

20}

21

22// Create a message panel

23MessagePanel messagePanel = new MessagePanel(message);

24messagePanel.setFont(new Font("SansSerif", Font.BOLD, 20));

25messagePanel.setXCoordinate(x);

26messagePanel.setYCoordinate(y);

27

28// Add the message panel to the applet

29add(messagePanel);

30}

31

32/** Main method to display a message

33@param args[0] x-coordinate

34@param args[1] y-coordinate

35@param args[2] message

36*/

37public static void main(String[] args) {

38// Create a frame

39JFrame frame = new JFrame("DisplayMessageApp");

41// Create an instance of the applet

42DisplayMessageApp applet = new DisplayMessageApp();

44// It runs as an application

45applet.isStandalone = true;

46

47// Get parameters from the command line

48applet.getCommandLineParameters(args);

49

50// Add the applet instance to the frame

51frame.add(applet, BorderLayout.CENTER);

52

53// Invoke applet's init method

54applet.init();

55applet.start();

56

57// Display the frame

58frame.setSize(300, 300);

isStandalone

applet params

standalone

command params