1. Client-side Programming
Lecture
Client-side Programming
Java Applets
Mini-applications

2. Topics for Discussion What is an applet? Requirements to run an applet
Life cycle of an applet
Topics for Discussion
Java Applet Examples

3. Applicationlets - mini-applications
Java Applets
Applicationlets - mini-applications

4. 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.

5. 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

6. 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.

7. 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

8. Requirements
JAVA SDK
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.

9. Requirements Google Chrome
Plug-ins are enabled by default. Use the Content Settings dialog to manage your plug-in settings. Follow these steps (for Windows):
Click the Tools menu.
Select Options.
Click the Under the Hood tab.
Click Content settings in the "Privacy" section.
Click the Plug-ins tab.
Select "Do not allow any site to use plug-ins." You can make exceptions for specific websites by clicking Exceptions.
Click Close to save your setting.

10. 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  - Version: 
Description:NPRuntime Script Plug-in Library for Java(TM) Deploy
Location:C:\Program Files\Mozilla Firefox\plugins\npdeploytk.dll

11. Applet internet life cycle
Browser requests and receives HTML from 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 Javascript internet life cycle.

12. ‘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.
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);
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.

13. ‘Hello, World!’ applet Embedding in a web page
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.
<html>
<head>
<title>Hello World Applet</title>
</head>
<body>
<applet code = "HelloWorldApplet.class"
width = "300" height = "60">
You require a Java-enabled browser to view this applet.
</applet>
</body>
</html>
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
executed.
Canvas size of 300x60 pixels in which the applet will run.
Default text

14. ‘Hello, World!’ applet Testing with Applet Viewer
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:

15. 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

16. 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

17. 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.

18. 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.

19. Applet lifecycle 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 computationally-intensive tasks.

20. Applet lifecycle 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.

21. 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.

22. 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.

23. Applet inheritance Component Container paint() EventListener Panel
ActionListener
actionPerformed()
Applet
init()
start()
stop()
destroy()
implements
extends
HiThere

24. Running applets 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>

25. 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>
</body>
</html>

26. Reading Applet Parameters
See AppletParameterTest.htm

27. 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.

28. 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);

29. 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

30. 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”

31. Sandbox Security Model
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.

32. Sandbox Security Model
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.

33. Java Applet Examples

34. JApplet import java.awt.Graphics; import javax.swing.JApplet;
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(){

35. Input Dialog

36. JApplet import javax.swing.JOptionPane; ... String strNum; double num;
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);

37. JApplet public void paint(Graphics g){
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 }

38. combobox

39. Combobox import javax.swing.JComboBox; ...
MyApplet.java
import javax.swing.JComboBox;
...
private JComboBox soundJComboBox;
String choices[] = { "Welcome", "Chimes", "Latina" };
soundJComboBox = new JComboBox( choices ); // create JComboBox

40. Responding to events import java.awt.event.ItemListener; 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

41. Buttons

42. Buttons import javax.swing.JButton;
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...

43. Events for Buttons import javax.swing.JButton;
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

44. sounds

45. Sounds import java.applet.AudioClip;
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();

46. animation
…\Lectures\Applet\applet - Animation\components-TumbleItemProject

47. 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

48. 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
javaboutique.internet.com
Also see java.sun.com for examples

49. 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

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

51. 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.

52. Next-Generation Java Plug-in, Java Runtime Environment
Architecture
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.

53. Next-Generation Java Plug-in, Java Runtime Environment
Architecture
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

54. Next-Generation Java Plug-in, Java Runtime Environment
Architecture
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.

55. Next-Generation Java Plug-in, Java Runtime Environment
Architecture
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.

56. 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?

57. References
Applet Tag
Java applet tutorial:
Demonstration Applets
Netbeans IDE

58. References JAVA Swing components Best Practices
Best Practices