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Lecture. Client-side Programming. Java Applets. Mini-applications. What is an applet?. Requirements to run an applet. Topics for Discussion. Life cycle of an applet. Java Applet Examples. Java Applets. Applicationlets - mini-applications. What is an applet ?.

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Java Applets

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Client-side Programming

Java Applets


What is an applet?

Requirements to run an applet

Topics for Discussion

Life cycle of an applet

Java Applet Examples

Java Applets

Applicationlets - mini-applications

What is an applet?

  • An applet is a special kind of Java program that a browser enabled with Java technology can download from the internet and run.

  • An applet is typically embedded inside a web page and runs in the context of a browser.

  • When a browser loads a Web page containing an applet, the applet downloads into the Web browser and executes. The browser that executes an applet is generically called the applet container.

  • Appletviewer is also an applet container that comes with the JDK. It can be used for testing applets as you develop them and before embedding them in Web pages.

  • You should be aware that some web browsers do not support J2SE by default.

Java Applets

  • Java’s first exposure to the mass market was via the Web

  • Applets was an early attempt to expand functionality of web pages

  • Ideally suited to the internet

    • Classes are small and fast

    • Java will run on any platform

    • Classes can be dynamically downloaded as required

    • Java is inherently secure

An applet inherits from a class

  • An applet must be a subclass of the java.applet.Applet class.

  • The Applet class provides the standard interface between the applet and the browser environment.

  • Swing provides a special subclass of the Applet class called javax.swing.JApplet.

  • The JApplet class should be used for all applets that use Swing components to construct their graphical user interfaces (GUIs).

  • The browser's Java Plug-in software manages the lifecycle of an applet.

Java applets are compiled

  • the Java programming language compiler (javac), takes your source file and translates its text into instructions that the Java virtual machine can understand. The instructions contained within this file are known as bytecodes.

  • bytecode.class files are generated



  • You will need a Compiler for Java, to translate your source code into something executable:

    • download Sun's Java Software Development Kit (abbreviated as JDK or SDK), which includes a compiler, utilities, example applets, and a boat-load of documentation

    • be sure to get the Java SDK and not the JRE(Java Runtime Environment) -- the former allows you to compile java programs, the latter only allows you to run them.

  • You can use Scite or NetBeans IDE to edit and compile your applets.

Note: If you don't see the example running, you might need to enable the JavaScript interpreter in your browser so that the Deployment Toolkit script can function properly.

Java Plug-ins

Java(TM) Platform SE 6 U19 - Version:

Description:Next Generation Java Plug-in 1.6.0_19 for Mozilla browsers

Location:C:\Program Files\Java\jre6\bin\new_plugin\npjp2.dll

Java Deployment Toolkit - Version:

Description:NPRuntime Script Plug-in Library for Java(TM) Deploy

Location:C:\Program Files\Mozilla Firefox\plugins\npdeploytk.dll

Applet internet life cycle

Browser requests and receives HTMLfrom server via HTTP.

Browser examines HTML for <applet> tag.

Browser requests and receives .class plus images, sounds, etc. via HTTP.

The JVM that comes with the browser executes the applet.

Exercise: compare with the PHP and Javascriptinternet life cycle.

import javax.swing.*;

import java.awt.*;

public class HelloWorldApplet extends JApplet


String message;

public void init()


message = "Hello World";


public void paint(Graphics artist)


artist.drawString(message, 20, 30);



‘Hello, World!’ applet

A web page will display an applet program in a display area that is embedded between the other contents on that page.

By default, this will appear as a blank canvas onto which text, Swing components, graphics and images can be painted by the applet program.

Knows how to render content onto the canvas.

Serves like a constructor for initializing variables and for adding Swing components to the applet’s interface.

The paint method is called spontaneously.

Applet inherits a paint() method from JApplet class that can be used to paint text, shapes and graphics into the applet’s display area.



<title>Hello World Applet</title>



<applet code = "HelloWorldApplet.class"

width = "300" height = "60">

You require a Java-enabled browser to view this applet.




‘Hello, World!’ applet

To embed a Java applet in a web page, the HTML code must specify the name of the applet file and the size of the applet’s display area that is to be allocated on the page.

The information is specified in the body of the web page with attributes of the

HTML <applet> tag.

Embedding in a web page

code attribute is assigned the name of the compiled applet file including its .class extension.

This pair of tags can surround a text message that

will only be displayed if the applet cannot be


Canvas size of 300x60 pixels in which the applet will run.

Default text

‘Hello, World!’ applet

Java SDK includes AppletViewer for testing

Compiled programs with the Java interpreter.

Both the HTML file and compiled applet file should be saved together in a directory.

Testing with Applet Viewer

Appletviewer will display the applet but not other contents in a web page.

The applet can now be tested from a command prompt:

Appletviewer HelloWorldApplet.html

Appletviewer will open a window that displays the HelloWorldApplet program:

Java Applications


JVM calls MyClass.main() as entry point


Small applications designed to run inside other applications

- usually Web browsers and appletviewers

They run in the JVM provided by Web browsers

Writing Java Applets

  • Applets must extend the Applet class

  • They do not have a main method

  • They have a number of methods called by the browser JVM

  • A selection of these methods are overridden in Java applets

Applet lifecycle

  • Various methods are called by the browser JVM at different times:

  • init() initializes the applet

  • start() starts the applet

  • paint(), repaint(), update() involved in drawing the applet

  • stop() stops the applets

  • destroy() closes the applet

  • By overriding these methods in your applet subclass, you decide what these methods do.

Use these method headers as they will be called by the applet container.

Applet lifecycle

public void init()

  • Called by the applet container when an applet is loaded for execution.

  • This method initialises an applet.

  • Typical actions performed in this function are:

    • initialising fields

    • creating GUI components

    • loading sounds to play

    • loading images to display

    • creating threads

  • Keep the init method short so that your applet can load quickly.

During the applet’s execution, the applet container creates an instance of the class indicated in the applet and calls it’s init() method.

Applet lifecycle

public void start()

  • Called by the applet container afterinit() completes execution.

  • If the user browses to another Web site and later returns to the applet’s HTML page, start() is called again.

  • The method performs tasks that must be completed when the applet is loaded for the first time and that must be performed every time the applet’s HTML page is revisited.

  • Typical actions performed in this function are:

    • starting an animation

    • starting other threads of execution for computationally-intensive tasks.

Applet lifecycle

public void paint( Graphics g)

  • Called by the applet container afterinit() and start().

  • paint() is also called when the applet needs to be repainted – whenever the applet is covered and uncovered by another open window.

  • Typical actions performed in this function are:

    • drawing with the g Graphics object. This is passed by the applet container.

Applet lifecycle

public void stop()

  • Called by the applet container when the user leaves the applet’s Web page by browsing to another Web page.

  • stop() performs tasks that might be required to suspend the applet’s execution, so that the applet does not use computer processing time when it is not displayed on screen.

  • Typical actions performed in this function are:

    • stop execution of animations and threads.

Applet lifecycle

public void destroy()

  • Called by the applet container when the applet is being removed from memory.

  • This occurs when the user exits the browsing session by closing all the browser windows and may also occur at the browser’s discretion when the user has browsed to other Web pages.

  • Typical actions performed in this function are:

    • tasks that are required to clean up resources allocated to the applet.

Applet inheritance


  • Container

    • paint()



  • ActionListener

    • actionPerformed()

  • Applet

    • init()

    • start()

    • stop()

    • destroy()




Running applets

You should compileyour MyApplet.javasource.

The browser gets instructions for loading and running applet class files (i.e. bytecodes) from its input HTML

<applet code=“MyApplet.class” height=50 width=400>



  • <html>

  • <head><title>Applet viewer</title></head>

  • <body>

  • <hr>

  • <applet code="HiThere.class" width=200 height=400>

    • <param name=Text value="Hello">

    • <param name=R value="23">

    • <param name=G value="54">

    • <param name=B value="75">

  • </applet>

  • <hr>

  • </body>

  • </html>

Reading Applet Parameters

See AppletParameterTest.htm


Applet tag

Despite <object> being officially a recommended tag, as of 2010, the support of the object tag was not yet consistent among browsers.

Sun kept recommending the older <applet> tag for deploying in multibrowser environments, as it remained the only tag consistently supported by the most popular browsers.

To support multiple browsers, the object tag currently requires JavaScript (that recognizes the browser and adjusts the tag), usage of additional browser-specific tags or delivering adapted output from the server side. Deprecating applet tag has been criticised. Oracle now provides a maintained JavaScript code to launch applets with cross platform workarounds.

You use the <object> tag to deploy applets that are to be used only with Internet Explorer.




Images can be downloaded via HTTP using applets

Place the URL of THIS applet into the URL object

URL urlBase = this.getCodeBase();

Applet fetches image via HTTP

myImage = this.getImage(urlBase, ”AnImage.jpg");

Draw the image on the graphics tablet

g.drawImage(myImage, 30, 110, 128, 96, this);

Applet security

  • Applets cannot

  • Read/write to or from the file system

  • Access your network

  • Launch applications

  • Launch new windows without a warning message

  • Go to other websites and download files to your machine

Sandbox Security Model

Client protection from malicious applets

  • The Java platform uses the sandbox security model to prevent code that is downloaded to your local computer from accessing local system resources, such as files.

  • Code executing in the sandbox is not allowed to “play outside the sandbox”

Sandbox Security Model

Client protection from malicious applets

  • Applets can be signedwith a digital signature (security certificate) to indicate that its from a trusted source) and certified to relax security. These applets have extensive capabilities to access the client, but only if the user accepts the applet’s security certificate.

  • Not commonly seen on the internet

  • On the other hand, unsigned applets operate within a security sandbox that allows only a set of safe operations.

Sandbox Security Model

Client protection from malicious applets

  • Note:

  • When a signed applet is accessed from JavaScriptcode in an HTML page, the applet is executed within the security sandbox.

  • This implies that the signed applet essentially behaves likes an unsigned applet.

Java Applet Examples



import java.awt.Graphics;

import javax.swing.JApplet;

import javax.swing.JOptionPane;

public class MyAppletextends JApplet


private double x;

public void init(){



public void paint(){




Input Dialog



import javax.swing.JOptionPane;


String strNum;

double num;


strNum = JOptionPane.showInputDialog("Enter first floating-point number");

You will have to extract the number from the string:

num = Double.parseDouble(firstNumber);



public void paint(Graphics g){

super.paint(g); //call the superclass version of method paint


g.drawString("The sum is " + sum, 25,25); //(25, 25) - coordinates





import javax.swing.JComboBox;


private JComboBoxsoundJComboBox;

String choices[] = { "Welcome", "Chimes", "Latina" };

soundJComboBox = new JComboBox( choices ); // create JComboBox

Responding to events

import java.awt.event.ItemListener;



new ItemListener() // anonymous inner class


// stop sound and change to sound to user's selection

public void itemStateChanged( ItemEvent e )




case 0: currentSound = sound1;


case 1: currentSound = sound2;

break; case 2: currentSound = sound3;

break; }

} // end method itemStateChanged

} // end anonymous inner class

); // end addItemListener method call

add( soundJComboBox ); // add JComboBox to applet




import javax.swing.JButton;

private JButtonplayJButton, loopJButton, stopJButton;

// set up button event handler and buttons

//ButtonHandler is a user-defined class

ButtonHandler handler = new ButtonHandler();

// create Play JButton

playJButton = new JButton( "Play" );

playJButton.addActionListener( handler );

add( playJButton );

// create Stop JButton

stopJButton = new JButton( "Stop" );

stopJButton.addActionListener( handler );

add( stopJButton );

//... and so on...

Events forButtons


import javax.swing.JButton;

// private inner class to handle button events

private class ButtonHandler implements ActionListener


// process play, loop and stop button events

public void actionPerformed( ActionEventactionEvent )


if ( actionEvent.getSource() == playJButton )

currentSound.play(); // play AudioClip once

else if ( actionEvent.getSource() == loopJButton )

currentSound.loop(); // play AudioClip continuously

else if ( actionEvent.getSource() == stopJButton )

currentSound.stop(); // stop AudioClip

} // end method actionPerformed

} // end class ButtonHandler




import java.applet.AudioClip;

private AudioClip sound1, sound2, sound3, currentSound;

// load sounds and set currentSound

sound1 = getAudioClip( getDocumentBase(), "welcome.wav" );

sound2 = getAudioClip( getDocumentBase(), "chimes.wav" );

sound3 = getAudioClip( getDocumentBase(), "Success.wav" );

currentSound = sound1;






…\Lectures\Applet\applet - Animation\components-TumbleItemProject

Do we still need applets?

In early days, not much could be on a web page without Java applets

Now with dynamic HTML4, Javascript, cascading style sheets, Java applets are not so necessary for many functions

But maybe applets will make a comeback

Current uses of applets

  • Online banking sites

  • Gaming sites

  • Sites which require real-time data transfer

  • Client-side applications that need to make use of Java’s powerful features

  • Applet examples/showcases

    • http://www.dgp.toronto.edu/~mjmcguff/learn/java/

    • javaboutique.internet.com

    • www.anfyjava.com

  • Also see java.sun.com for examples

Java Web plug-in

  • Version conflicts is one disadvantage of using applets

  • To avoid version problems, associated with the Java applets, there is the Java plugin

  • This is an up-to-date virtual machine that can be installed on your machine

  • See java.sun.com for more details

Recent Improvements

With recent improvements to the Java Plug-in software, unsigned applets launched using Java Network Launch Protocol (JNLP) can safely access the client with the user's permission.

You will have to use Java NetBeans IDE to use this properly

…\Lectures\Applet\applet - Animation\components-TumbleItemProject

Applet Deployment

Applets can be launched in two ways:

You can launch an applet by specifying the applet's launch properties directly in the <applet> tag. This old way of deploying applets imposes severe security restrictions on the applet.

Alternatively, you can launch your applet by using Java Network Launch Protocol (JNLP). Applets launched by using JNLP have access to powerful JNLP APIs and extensions.



Next-Generation Java Plug-in, Java Runtime Environment

  • With the next-generation Java Plug-in, the JRE no longer runs inside the browser.

  • Instead, the JREruns in a separate process. By default, all applets run in the same JRE instance.

  • However, applets can now specify the JRE version they require to run on.

  • More than one JRE instance will be launched when different versions of the JREare needed, or when the applet requires more resources than any currently extant instance can supply.



Next-Generation Java Plug-in, Java Runtime Environment

  • Compatibility

  • The browser and the applet can still communicate with one another.

  • However, existing APIshave been re-engineered to use process sockets, so things continue to work as they did before--only better



Next-Generation Java Plug-in, Java Runtime Environment

  • Benefits

  • Applets that require different versions of the JRE can run simultaneously.

  • Applets can specify JRE start-up parameters such as heap size. (A new applet uses an existing JRE if it's requirements are a subset of an existing JRE, otherwise, a new JRE instance is launched.)

  • The message-passing interfaces are written in Java, so they run on all supported platforms, in the same way, so cross-browser compatibility is enhanced.



Next-Generation Java Plug-in, Java Runtime Environment

  • Limitations:

  • If two applets each require a large amount of memory, they might both run in the same JRE, causing one of them to run out of memory. But that's only a concern when you have multiple applets running simultaneously.

  • The Java browser plug-in finds available JREs by inspecting the Java Control Panel.



  • Applet properties

  • Applet life cycle

  • Writing an applet

  • Passing parameters to an applet

  • Sandbox security model

  • Some Java components

  • Applet traffic over the internet

    • How does this compare with PHP and Javascript?


  • Applet Tag

    • http://download.java.net/jdk7/docs/technotes/guides/plugin/developer_guide/using_tags.html#object

  • Java applet tutorial:

    • http://download.oracle.com/docs/cd/E17409_01/javase/tutorial/deployment/applet/

  • Demonstration Applets

    • http://www.dgp.toronto.edu/~mjmcguff/learn/java/

    • http://download.oracle.com/docs/cd/E17476_01/javase/1.5.0/docs/relnotes/demos.html

    • http://java.sun.com/applets/jdk/1.4/index.html

    • http://www.walter-fendt.de/ph14e/projectile.htm

  • Netbeans IDE

    • http://download.oracle.com/docs/cd/E17409_01/javase/tutorial/information/examples.html#opening

    • http://download.oracle.com/docs/cd/E17409_01/javase/tutorial/getStarted/cupojava/netbeans.html


JAVA Swing components

  • http://www.java2s.com/Code/JavaAPI/javax.swing/JComboBoxaddItemListenerIte

    Best Practices

  • http://download.oracle.com/docs/cd/E17409_01/javase/6/docs/technotes/guides/jweb/applet/best_practices.html

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