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Lecture
Client-side Programming
Java Applets
Mini-applications
1
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What is an applet?
Requirements to run an applet
Life cycle of an applet
Java Applet
Examples
Topics
for Discussion
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Java Applets
Applicationlets - mini-applications
What is an applet?
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• 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
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An applet inherits from a class
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• 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
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• 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
Requirements
JAVA SDK
1. 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.
2. 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.
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Java Plug-ins
Java(TM) Platform SE 6 U19 - Version: 6.0.190.4
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 6.0.190.4 Version: 6.0.190.4
Description:NPRuntime Script Plug-in
Library for Java(TM) Deploy
Location:C:\Program Files\Mozilla
Firefox\plugins\npdeploytk.dll
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Applet internet life cycle
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1. Browser requests and receives HTML from server via HTTP.
2. Browser examines HTML for <applet> tag.
3. Browser requests and receives .class plus images, sounds, etc. via
HTTP.
4. The JVM that comes with the browser executes the applet.
Exercise: compare with the PHP and Javascript internet life cycle.
‘Hello, World!’ applet
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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.
import javax.swing.*;
import java.awt.*;
public class HelloWorldApplet extends JApplet
{
String message;
Serves like a constructor for
initializing variables and for
adding Swing components to
the applet’s interface.
public void init()
{
message = "Hello World";
}
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.
Knows how to
render content
onto the canvas.
public void paint(Graphics artist)
{
artist.drawString(message, 20, 30);
}
}
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‘Hello, World!’ applet
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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.
code attribute is assigned
the name of the compiled
applet file including its .class
extension.
<html>
Embedding in a web page
<head>
<title>Hello World Applet</title>
</head>
<body>
<applet code = "HelloWorldApplet.class"
width = "300" height = "60">
This pair of tags can surround a text message that
will only be displayed if the applet cannot be
You require a Java-enabled browser to view this
applet.
executed.
</applet>
Default text
</body>
</html>
Canvas size of
300x60 pixels in
which the applet
will run.
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‘Hello, World!’ applet
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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
The applet can now be tested from a command prompt:
Appletviewer HelloWorldApplet.html
Appletviewer will
display the applet
but not other
contents in a web
page.
Appletviewer will open a window that displays the HelloWorldApplet program:
14
Java Applications
Standalone
JVM calls MyClass.main() as entry point
Applets
Small applications designed to run inside other applications
- usually Web browsers and appletviewers
They run in the JVM provided by Web browsers
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Writing Java Applets
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• 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
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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
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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
During the applet’s execution, the
applet container creates an instance
• loading sounds to play
of the class indicated in the applet
• loading images to display
and calls it’s init() method.
• creating threads
• Keep the init method short so that your applet can load
quickly.
Applet lifecycle
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public void start()
• Called by the applet container after init() 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 computationallyintensive tasks.
Applet lifecycle
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public void paint( Graphics g)
• Called by the applet container after init() 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
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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
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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
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Component
Container
–paint()
EventListener
Panel
ActionListener
–actionPerformed()
Applet
–init()
–start()
–stop()
–destroy()
implements
extends
HiThere
Running applets
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You should compile your MyApplet.java source.
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>
</applet>
More HTML
<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>
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Reading Applet Parameters
See AppletParameterTest.htm
http://www.devdaily.com/java/edu/pj/pj010003
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Applet tag
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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.
http://download.java.net/jdk7/docs/technotes/guides/plugin/developer_guide/using_tags.html#object
http://www.answers.com/topic/java-applet
Images
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);
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Applet security
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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”
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Sandbox Security Model
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Client protection from malicious applets
• Applets can be signed with 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
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Client protection from malicious applets
Note:
• When a signed applet is accessed from JavaScript
code in an HTML page, the applet is executed within
the security sandbox.
• This implies that the signed applet essentially
behaves likes an unsigned applet.
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JApplet
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MyApplet.java
import java.awt.Graphics;
import javax.swing.JApplet;
import javax.swing.JOptionPane;
public class MyApplet extends JApplet
{
private double x;
public void init(){
//…
}
public void paint(){
//…
}
}
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JApplet
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MyApplet.java
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);
JApplet
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MyApplet.java
public void paint(Graphics g){
super.paint(g); //call the superclass version of method paint
g.drawRect(15,10,270,20);
g.drawString("The sum is " + sum, 25,25); //(25, 25) - coordinates
}
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Combobox
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MyApplet.java
import javax.swing.JComboBox;
...
private JComboBox soundJComboBox;
String choices[] = { "Welcome", "Chimes", "Latina" };
soundJComboBox = new JComboBox( choices ); // create JComboBox
Responding to events
import java.awt.event.ItemListener;
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MyApplet.java
soundJComboBox.addItemListener(
new ItemListener() // anonymous inner class
{
// stop sound and change to sound to user's selection
public void itemStateChanged( ItemEvent e )
{
currentSound.stop();
switch(soundJComboBox.getSelectedIndex()){
case 0: currentSound = sound1;
break;
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
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Buttons
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MyApplet.java
import javax.swing.JButton;
private JButton playJButton, 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 for Buttons
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MyApplet.java
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( ActionEvent actionEvent )
{
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
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Sounds
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MyApplet.java
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;
...
currentSound.play();
currentSound.loop();
currentSound.stop();
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…\Lectures\Applet\applet - Animation\components-TumbleItemProject
Do we still need applets?
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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
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Java Web plug-in
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• 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
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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\componentsTumbleItemProject
Applet Deployment
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Applets can be launched in two ways:
1. 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.
2. 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.
http://download.oracle.com/docs/cd/E17409_01/javase/tutorial/deployment/applet/deployingApplet.html
Architecture
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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 JRE runs 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 JRE are needed,
or when the applet requires more
resources than any currently extant
instance can supply.
http://download.oracle.com/docs/cd/E17409_01/javase/6/docs/technotes/guides/jweb/applet/applet_execution.html
Architecture
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Next-Generation Java Plug-in, Java Runtime Environment
Compatibility
• The browser and the applet can
still communicate with one
another.
• However, existing APIs have
been re-engineered to use process
sockets, so things continue to work
as they did before--only better
http://download.oracle.com/docs/cd/E17409_01/javase/6/docs/technotes/guides/jweb/applet/applet_execution.html
Architecture
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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.
http://download.oracle.com/docs/cd/E17409_01/javase/6/docs/technotes/guides/jweb/applet/applet_execution.html
Architecture
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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.
http://download.oracle.com/docs/cd/E17409_01/javase/6/docs/technotes/guides/jweb/applet/applet_execution.html
Summary
•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?
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References
•
•
•
•
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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/a
pplet/
Demonstration Applets
– http://www.dgp.toronto.edu/~mjmcguff/learn/java/
– http://download.oracle.com/docs/cd/E17476_01/javase/1.5.0/docs/relnotes/d
emos.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/e
xamples.html#opening
– http://download.oracle.com/docs/cd/E17409_01/javase/tutorial/getStarted/cu
pojava/netbeans.html
References
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JAVA Swing components
•
http://www.java2s.com/Code/JavaAPI/javax.swing/JComboBoxaddItemListe
nerIte
Best Practices
http://download.oracle.com/docs/cd/E17409_01/javase/6/docs/technotes/guides/j
web/applet/best_practices.html