java applets

04/12/23 Distributed Computing, M. L. Liu

1

Java applets

M. L. Liu

04/12/23 Distributed Computing, M. L. Liu 2

Introduction

Java applets are one of three kinds of Java programs: An application is a standalone program that can be

invoked from the command line. An applet is a program that runs in the context of a

browser session. A servlet is a program that is invoked on demand on a

server program and that runs in the context of a web server process.


Applets, web page, 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 (or download) the applet program, then executes the program in the browser’s execution context, on the client host.

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H e llo W o rld.c la s s

s e r ve r ho s t

we b s e rv e r

m y W e bPa g e .h tm l

br o w s e r ho s t

bro ws e r re qe u s t fo rm y W e bPa g e .h tm l

m y W e bPa g e .h tm l

re qu e s t fo rH e llo W o rldcla s s




Applet Execution - 1

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 Grapics 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. You program the start( ) method and the paint( ) method for

your application, and invoke a repaint call to re-render the graphics, if necessary.

At the end of the execution, the stop( ) method is invoked, followed by the destry( ) method to deallocate the applet’s resources.


Applet Security http://java.sun.com/docs/books/tutorial/applet/overview/

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 it's executing on. an applet cannot make network connections

except to the host that it came from.


HTML tags for applets - 1

<APPLET specifies the beginning of the HTML applet code

CODE="demoxx.class" is the actual name of the applet (usually a 'class' file)

CODEBASE="demos/" is the location of the applet (relative as here, or a full URL)

NAME="smily" the name you want to give to this instance of the applet on this page

WIDTH="100" the physical width of the applet on your page

HEIGHT="50" the physical height of the applet on your page

ALIGN="Top" where to align the applet within its page space (top, bottom, center)


HTML tags for applets - 2

<PARAM specifies a parameter that can be passed to the applet

NAME=“name1" the name known internally by the applet in order to receive this parameter

VALUE="000000" the value you want to pass for this parameter

> end of this parameter

<PARAM specifies a parameter that can be passed to the applet (applet specific)

NAME=“name2" the name known internally by the applet in order to receive this parameter

VALUE="ffffff" the value you want to pass for this parameter

> end of this parameter

</APPLET> specifies the end of the HTML applet code


The HelloWorld Applet<HTML><BODY><APPLET code=hello.class width=900

height=300></APPLET></BODY></HTML>

// applet to display a message in a window

import java.awt.*;import java.applet.*;

public class hello extends Applet{public void init( ){

setBackground(Color.yellow);

}

public void paint(Graphics g){ final int FONT_SIZE = 42;

Font font = new Font("Serif", Font.BOLD, FONT_SIZE);

// set font, and color and display message on// the screen at position 250,150

g.setFont(font);g.setColor(Color.blue);

// The message in the next line is the one you will see

g.drawString("Hello, world!",250,150);}

}


Advanced Applets You can use threads in an applet. You can make socket calls in an applet, subject to the security constraints.

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c o n n ec tio n r eq u es t

c o n n ec tio n r eq u es tf o r b id d en

allo w eds er v er Y

s er v er Z


Proxy server

A proxy server can be used to circumvent the security constraints.

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a pple t

H o s t X

ap p le t d o w n lo ad

c o n n ec tio n r eq u es ts er v er Y

s er v er Zc o n n ec tio n r eq u es t


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: applets cannot access files in the file system on the client host Applets cannot make network connection exception to the

server host from which it originated

java applets

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