Applet (1)

Introduction
 Applet is small java program that can be
easily transported over the network from
one computer to other.
 Used in internet applications.
 Embedded in an html page, can be
downloaded from the server and run on the
client, so as to do a specific kind of job.
 An applet is a program that runs in the
context of a browser session.

Applets, Webpage,
Client, Server
 Applets are programs stored on a web
server, similar to web pages.
 When an applet is referred to in a web page
that has been fetched and processed by a
browser, the browser generates a request
to fetch the applet program, the executes
the program in the browser’s execution
context on the client host.

Types of java programs
 Stand Alone Program
 Run on a single machine
 Compiler-javac
 Interpreter-java
 Web Based Program
 Compiler-javac
 Interpreter-applet viewer or web browser
 Must Subclass of Applet
 Import java.awt

Application vs Applet
Java Applications
• Stand-alone program
• Runs independently
• Has a main() method
• No HTML file
• Run using JDK’s
java interpreter
Java Applet
• Embedded program
• Runs in a Web
browser
• No main() method
• Requires an HTML
file
• Run using JDK’s
applet viewer.

Applet Example
import java.applet.*;
import java.awt.*;
public class FirstApplet extends applet{
public void paint(Graphics g)
{
g.drawString(“Hello World”,10,50);
}
}

Applet Life Cycle
 An applet may move from one state to
another dependent upon a set of default
behaviours inherited in the form of methods
from ‘Applet’ class.
 These states are
 Born
 Running
 Idle
 Dead

Life Cycle of Applet
 Init()-
 Creates the objects needed by the applet
 Sets up initial values, load font and images or
set up colors.
 Called only once during the lifetime of an
applet.
 Start()-
 Moves to this phase automatically after the
initialization state.
 If the applet is stopped or it goes to idle state
start() method must be called in order to force
the applet again to the running state.

Life Cycle of Applet
Paint()-
 This method is called each time to draw
and redraw the output of an applet.
Stop()-
 Idle state, once it is stopped from
running.
 Destroy()-
 An applet goes to dead state when it is
destroyed by invoking the destroy() of
applet class.
 It results in complete removal of applet
from the memory.

Common Methods
 drawString()-
 Member of graphics class used to output a
string to an applet.
 It is typically called from within the paint() or
update() method.
 Void drawString(String msg,int a, int b).
 setBackground() & getBackground()-
 Belongs to component class used to set
and get the background color.
 Void setBackground(Color anyColor)

Common Methods
 Predefined constants for each color such as
Color.red can be used.
 setForeground()& getForeground()-
 Set and gets the color of the text to be
displayed on the foreground of the applet
window.
 Void setForeground(Color anyColor)
 showStatus()-
 Display any string In the status window of
the browser.
 Void showString(Stringtext).

Applet Execution
 An applet program is a written as a
subclass of the java. Applet class or the
javax.swing.Japplet class. There is no main
method: you must override the start
method. Applet objects uses AWT for
graphics. Japplet uses SWING.
 It is a graphics object that runs in a thread
object, so every applet can perform
graphics, and runs in parallel to the browser
process.

Applet Execution
When the applet is loaded,these methods are
automatically invoked in order:
 The init() method is invoked by the Java
Virtual Machine.
 The start() method.
 The paint()method.
 The applet is now running and rendered on
the web page.

Applet Security
For security reasons, applets that are loaded
over the network have several restrictions.
 An applet cannot ordinarily read or write
files on the computer that its executing on.
 An applet cannot make network
 connections except to the host that it came
from.

Summary
 An applet is a java class.
 Its code is downloaded from a web server.
 It is run in the browser’s environment on the
client host.
 It is invoked by a browser when it scans a
web page and encounters a class specified
with the APPLET tag. For security reason ,
the execution of an applet is normally
subject to restrictions:

 Applet cannot access files in the file system
on the client host.
 Applet cannot make network connection
exception to the server host from which it
originated.

Applet (1)

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