Java Applets

1
Java Applets
2
What is an Applet?

• Its a Java application that is designed to be
  downloaded from the Internet and run in a browser
• There are security implications when running
  applets
• Specifically, they cant access protected
  resources
• More about applet security later in the course

3
How an Applet Works

• Embed code in an HTML document to tell the
  browser
• Where to get the Java class files
• How the applet should appear on a Web page
• An HTML document may have multiple applets
• Install the Java Plug-In to ensure maximum
  compatibility
• This Plug-In is supported by Sun rather than the
  browser vendor

4
Applet Essentials

• An Applet is just a class that derives from
  JApplet
• Create a method named init() which contains any
  initialization code
• init() is of type void
• Init accepts no arguments
• From there, create visual components as you would
  with any Swing application
• As always, class name must match the file name
• Thats really all there is to it

5
Creating a First Applet (Example)

• import java.awt.
• import javax.swing.
• public class FirstApplet extends JApplet
• public void init()
• Container contentPane getContentPane()
• JLabel label new
• JLabel("Greetings",
• SwingConstants.CENTER)
• contentPane.add(label)

6
Compiling and Deploying an Applet

• Compile the Java Applet into a class
• Call javac or use Netbeans to compile the source
  file
• Copy the class file(s) to the Web server as
  necessary

7
Viewing an Applet

• Embed tags into an HTML document
• Older ltappletgt tag has attributes to define an
  applet
• Newer ltobjectgt tag is part of HTML 4.0 and
  defined by the W3C
• Use the appletviewer program for testing
• Example
• appletviewer FirstApplet.html

8
The ltappletgt Tag

• code attribute supplies the named of the applets
  class file
• Standard package directory conventions apply
• codebase attribute is used to supply relative
  directory references
• width and height attributes define the applets
  size when rendered in the browser
• name attribute is used in scripting to reference
  the applet programmatically
• align attribute controls how the applet is
  aligned about the surrounding text
• See Figure 10-6 on page 503
• Plain text is displayed when the browser cannot
  display an applet

9
Relative Directories and the codebase attribute

• codebase attribute specifies relative directory
  containing class files
• Use to keep applets organized
• Name corresponds to directory name containing the
  class file(s)

10
The ltobjectgt Tag

• Similar semantics to the ltappletgt tag
• classid attribute contains the name of the class
  in the form
• ClassidjavaFirstApplet.class
• codetype attribute contains application/java

11
Sample HTML Applet Code (1)

• lthtmlgt
• ltbodygt
• ltapplet code"FirstApplet.class"
• width"300" height"300"gt
• Browser cannot display applets
• lt/appletgt
• lt/bodygt
• lt/htmlgt

12
Sample HTML Applet Code (2)

• lthtmlgt
• ltbodygt
• ltobject
• codetypeapplication.java
• classidjavaCalculatorApplet.class
• width"300" height"300"gt
• lt/bodygt
• lt/htmlgt

13
Passing Information to an Applet (1)

• Embed the ltparamgt tag inside of the applet
• Use one ltparamgt tag for each parameter
• name attribute contains the parameter name
• value attribute contains the parameters value
• Note that parameters are returned as a String
  data type
• If a parameter value is not found, null is
  returned

14
Passing Information to an Applet (2)

• Call getParameter(name) in init() to retrieve a
  parameters value
• Call GetParameterInfo() to return a 2 dimensional
  array of parameters
• Values for each row are
• Parameter name
• Parameter type
• Parameter description

15
Passing information to an Applet (HTML Example)

• lthtmlgt
• ltbodygt
• ltapplet code"FirstApplet.class"
• width"300" height"300"gt
• ltparam name"version" value"1.0.0"/gt
• lt/appletgt
• lt/bodygt
• lt/htmlgt

16
Getting a Parameter Value (Example)

• public class FirstApplet extends JApplet
• public void init()
• String version getParameter("version")

17
Applet Life Cycle

• Four methods of the Applet class control applet
  initialization and destruction
• init
• start
• stop
• destroy

18
Applet (init)

• Conceptually similar to a constructor
• Init runs when the JVM executes the applet the
  first time
• Parameter values are typically passed here
• Note that applets to have constructors but we
  seldom use them

19
Applet (start)

• Start is called after init
• Start is called when the user navigates back to a
  page containing the applet
• Restart an animation for example
• Think of start as a GotFocus or Enter event

20
Applet (stop)

• Stop is called when the user navigates to another
  page
• Create code to suspend visual animations or other
  resource intensive activity

21
Applet (destroy)

• Destroy is called when the browser closes the
  window containing the applet

22
Communication with the Browser

• An applet runs inside of a browser
• Call getAppletContext() to get information from
  the ambient browser