Advanced Programming, Java Programming, Applets.ppt

Introduction
• Part of Java’s success comes from the
fact that it is the first language specifically
designed to take advantage of the power
of the World-Wide Web
• In addition to more traditional application
programs, Java makes it possible to write
small interactive programs called applets
that run under the control of a web
browser

• Applet
– Program that runs in
• Web browser (IE, Communicator)
• appletviewer
– a program designed to run an applet as a stand-alone program
– Executes when HTML (Hypertext Markup Language)
document containing applet is opened and downloaded
– Can not run independently
– normally run within a controlled environment (sandbox)
– Used in internet computing.
– Every browser implements security policies to keep
applets from compromising system security
Introduction

Applications vs. Applets
• Similarities
– Both applets and applications are Java
programs
– Since JFrame and JApplet both are
subclasses of the Container class, all the
user interface components, layout managers,
and event-handling features are the same for
both classes.

Applications vs. Applets
• Differences
– Applications 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 through the init, start, stop, and destroy methods.
– Applets have security restrictions
– Applications run in command windows whereas
applets run in web browsers
– Web browser creates graphical environment for
applets, GUI applications are placed in a frame.
– There is no main() method in an Applet.

Security Restrictions on Applets
• Applets are not allowed to read from, or write to,
the file system of the computer viewing the
applets.
• Applets are not allowed to run any programs on
the browser’s computer.
• Applets are not allowed to establish connections
between the user’s computer and another
computer except with the server where
the applets are stored.
• An applet cannot load libraries or define native
methods.
• It cannot read certain system properties

Applet class
• From Component, an applet inherits the ability to draw
and handle events
• From Container, an applet inherits the ability to include
other components and to have a layout manager control
the size and position of those components
• Every applet is implemented by creating a subclass of
the Applet class

Life Cycle of Applet
• An applet actually has a life cycle
– It can initialize itself.
– It can start running.
– It can stop running.
– It can perform a final cleanup, in preparation for
being unloaded.

The Applet Class
•When the applet is loaded, the Web
browser creates an instance of the applet
by invoking the applet’s no-arg
constructor.
•The browser uses the init, start, stop, and
destroy methods to control the applet.
•By default, these methods do nothing. To
perform specific functions, they need to be
modified in the user's applet so that the
browser can call your code properly.

Browser Calling Applet Methods
Browser
invokes start()
Destroyed
Browser invokes
destroy()
Browser
invokes stop()
Loaded
Initialized
Browser
invokes init()
Started Stopped
Created
Browser creates
the applet
JVM loads the
applet class
Browser
invokes stop()
Browser
invokes start()

The init() Method
Invoked when the applet is first loaded
and again if the applet is reloaded.
A subclass of Applet should override this
method if the subclass has an initialization
to perform. The functions usually
implemented in this method include
creating new threads, loading images,
setting up user-interface components, and
getting string parameter values from the
<applet> tag in the HTML page.

The start() Method
Invoked after the init() method is executed;
also called whenever the applet becomes active
again after a period of inactivity (for example,
when the user returns to the page containing the
applet after surfing other Web pages).
A subclass of Applet overrides this method if
it has any operation that needs to be
performed whenever the Web page
containing the applet is visited. An applet
with animation, for example, might use the
start method to resume animation.

The stop() Method
The opposite of the start() method, which is called
when the user moves back to the page containing the
applet; the stop() method is invoked when the user
moves off the page.
A subclass of Applet overrides this method if it
has any operation that needs to be performed
each time the Web page containing the applet is
no longer visible. When the user leaves the
page, any threads the applet has started but not
completed will continue to run. You should
override the stop method to suspend the running
threads so that the applet does not take up
system resources when it is inactive.

The destroy() Method
Invoked when the browser exits normally
to inform the applet that it is no longer
needed and that it should release any
resources it has allocated.
A subclass of Applet overrides this method if it
has any operation that needs to be performed
before it is destroyed. Usually, you won't need to
override this method unless you wish to release
specific resources, such as threads that the
applet created.

The JApplet Class
•The Applet class is an AWT class and is not
designed to work with Swing components.
•To use Swing components in Java applets, it is
necessary to create a Java applet that extends
javax.swing.JApplet, which is a subclass of
java.applet.Applet.
•JApplet inherits all the methods from the Applet
class. In addition, it provides support for laying
out Swing components.

Writing Applets
• Always extends the JApplet class, which is
a subclass of Applet for Swing components.
• Override init(), start(), stop(), and
destroy() if necessary. By default, these
methods are empty.
• Add your own methods and data if necessary.
• Applets are always embedded in an
HTML page.

Java Applet Skeleton
/*
Program MyFirstApplet
An applet that displays the text "I
Love Java"
and a rectangle around the text.
*/
import java.applet.*;
import java.awt.*;
public class MyFirstApplet extends JApplet
{
public void paint( Graphics graphic)
{
graphic.drawString("I Love
Java",70,70);
graphic.drawRect(50,50,100,30);
}
}
Comment
Import
Statements
Class Name
Method Body

First Simple Applet
// WelcomeApplet.java: Applet for
//displaying a message
import javax.swing.*;
public class WelcomeApplet extends
JApplet {
/** Initialize the applet */
public void init() {
add(new JLabel("Welcome to Java",
JLabel.CENTER));
}
}

First Simple Applet
<html>
<head>
<title>Welcome Java Applet</title>
</head>
<body>
<applet
code = "WelcomeApplet.class"
width = 350
height = 200>
</applet>
</body>
</html>

1 // Fig. 3.6: WelcomeApplet.java
2 // A first applet in Java.
3
4 // Java core packages
5 import java.awt.Graphics; // import class Graphics
6
7 // Java extension packages
8 import javax.swing.JApplet; // import class JApplet
9
10 public class WelcomeApplet extends JApplet {
11
12 // draw text on applet’s background
13 public void paint( Graphics g )
14 {
15 // call inherited version of method paint
16 super.paint( g );
17
18 // draw a String at x-coordinate 25 and y-coordinate 25
19 g.drawString( "Welcome to Java Programming!", 25, 25 );
20
21 } // end method paint
22
23 } // end class WelcomeApplet
import allows us to use
predefined classes (allowing
us to use applets and
graphics, in this case).
extends allows us to inherit the
capabilities of class JApplet.
Method paint is guaranteed to
be called in all applets. Its first
line must be defined as above.
A Simple Java Applet: Drawing a String

The <applet> HTML Tag
<applet
code=classfilename.class
width=applet_viewing_width_in_pixels
height=applet_viewing_height_in_pixels
[archive=archivefile]
[codebase=applet_url]
[vspace=vertical_margin]
[hspace=horizontal_margin]
[align=applet_alignment]
[alt=alternative_text]
>
<param name=param_name1
value=param_value1>
</applet>

Passing Parameters to Applets
<applet
code = "DisplayMessage.class"
width = 200
height = 50>
<param name=MESSAGE value="Welcome
to Java">
<param name=X value=20>
<param name=Y value=20>
alt="You must have a Java-enabled
browser to view the applet"
</applet>

Advanced Programming, Java Programming, Applets.ppt

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