COMP201 Java Programming

1
COMP201 Java Programming Part II GUI Programming
Topic 11 Applets Chapter 10
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Outline

• Introduction
• What are applets?
• How to run?
• Applets basics
• Class loader and JAR files
• Security basics
• Applet life cycle
• Converting applications to applets
• Resources for applets
• Communication with browser

3
Introduction

• So far, weve only covered topics related to
  stand-alone applications
• An applet is designed to run within a browser
• Bring web pages to life
• Reason behind hype in Java
• Note Java is not a language for designing web
  pages. It is a tool for bring them to life.

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Introduction/JApplet Class

• An applet is a Java class which extends
  java.applet.Applet
• If Swing components are used, the applet must
  extend from javax.swing.JApplet
• We will only discuss applets that extend JApplet
• JApplet is a sub-class of Applet, which in turn
  is a subclass of Panel
• Event handling exactly the same as before

5
Introduction/First Applet
public class NotHelloWorldApplet extends
JApplet public void init()
Container contentPane getContentPane()
JLabel label new JLabel("Not a Hello,
World applet", SwingConstants.CENTER)
contentPane.add(label)
//NotHelloWorldApplet.java
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Introduction/First Applet

• Compare with
• class NotHelloWorldFrame extends JFrame
• public NotHelloWorldFrame()
• setTitle("NotHelloWorld")
• setSize(300, 200)
• Container contentPane getContentPane()
• contentPane.add(new NotHelloWorldPanel())
• public class NotHelloWorld
• public static void main(String args)
• NotHelloWorldFrame frame
  new NotHelloWorldFrame()
• frame.setDefaultCloseOperation(JFrame.EXIT_ON_
  CLOSE)
• frame.show()

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Introduction/First Applet

• Applets are created, run, and destroyed by web
  browser
• Dont set size for an applet determined by HTML
  file.
• Dont set title for an applet applets cannot
  have title bars.
• No need to explicitly construct an applet.
  Construction code placed inside the init method.
• There is no main method.
• An applet cannot be closed. It terminates
  automatically when the browser exit.
• No need to call method show. An applet is
  displayed automatically.

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Introduction/Running An Applet

• Compile the .java file
• 2. Create a HTML file that tells the browser
  which file to load and how to size the applet
• lthtmlgtltbodygt This is an example.
• ltapplet codeNotHelloWorldApplet.class
  width300 height300gt
• This text shown if browser doesnt do Java.
• lt/appletgt
• lt/bodygtlt/htmlgt
• View the HTML file with a browser or the command
  appletviewer (for testing).
• This works for applets extending Applet,
  but doesnt work for those that extend JApplet

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Introduction/Running An Applet

• Browser retrieves class file and automatically
  runs it using its JVM.
• Problem JVM of browsers does not support newest
  versions of Java (i.e. IDK 1.2 JDK 1.3)
• Suns solution require users to install the
  latest Java plug-in.
• (see http//java.sun.com/products/plugin/index.h
  tml)

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Introduction/Running An Applet

• Special HMTL tags needed for running applets with
  Java plug-in.
• The use of those special tags is not
  straightforward.
• Solution
• Write HTML file using traditional tags
• Feeds it to HTML converter to produces a new HTML
  file with the special tags.
• View the new HTML file.
• Get the HMTL converter from course page (Online
  resources).
• Example of HTML file produced by converter
  (NotHelloWordApplet.html)

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Applet Basics/Class Loader and JAR Files

• In the previous example, we have one classes
• NotHelloWorldApplet.class,
• Name of the applet class is placed in HTML file
• Class loader first fetches NotHelloWorldApplet.cla
  ss
• In the process, if the class loader notices that
  some other classes are also needed. It then makes
  another net connection to get them.
• Many connections might be needed in general,
  especially when there are associated resources
  such as images and sounds.
• Java Archive (JAR) files allow one to bundle a
  set of classes and resources into one file that
  can be downloaded via one net connection.

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Applet Basics/Class Loader and JAR Files

• Create JAR file with command jar
• jar cf myJarFile.jar .class .gif
• pack all files ending with .class or .gif
• Refer to JAR files in the APPLET tag
• ltapplet archivemyJarFile.jar
• codeMyApplet.class width300 height300gt
• (see TetrixAppletJar.html and run
  TetrixAppletJar)
• JAR file is downloaded via one net connection.
• Class loader tries to find necessary files in JAR
  file before attempting to get them from the net.

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Diversion/Self-Running Jar File

• jar cfm creates a jar file and a manifest
  file ./META_INF/MANIFEST.MF
• It describes features of the archive.
• To make an executable jar file, we need to
  indicate the main class in the manifest file.
• Make mainclass.txt with one line (no class and
  ended by return)
• Main-Class MyApplet
• Update the manifest variable
• jar umf mainclass.txt MyJarFile.jar
• Run
• java -jar MyJarFile.jar
• Or click on file icon

Self-Running Tetrix Jar
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Applet Basics/Security

• Applets are downloaded from the net and executed
  by a browsers JVM immediately.
• User never gets a chance to confirm or to stop an
  applet from running.
• Consequently, applets are restricted in what they
  can do.
• The applet security manager is responsible for
  enforcing access rules and throws an
  SecurityException when an access rule is violated.

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Applet Basics/Security

• By default, an applet is restricted to run
  inside the sandbox. Strictest security
  restrictions.
• Signed applets can have more access privileges.
• For now, we consider only applets playing in the
  sandbox.

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• Access rights for Applets and Java Applications
  (JA)
• BR applets running inside a browser with
  default applet security model
• AV applets running insider Applet viewer

BR
AV
JA
Read local file
N
Y
Y
Write local file
N
Y
Y
Get file info.
N
Y
Y
Delete file
N
N
Y
Run another program
N
Y
Y
Read the
user.name property
N
Y
Y
Y
Y
Y
Connect to network port on home server
Connect to network port on other server
N
Y
Y
Load Java library
N
Y
Y
Call exit
N
Y
Y
Create a pop-up window
warning
Y
Y
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Applet Basics/Applet Life Cycle

• An application starts from main and runs until
  exit
• Applets are controlled by browser
• Born when their pages are first loaded
• Respond to messages and user inputs when visible
• Turned off when not visible
• Disposed by browser either when changing page or
  quitting
• Change behavior by overriding methods
• init() start(), stop() destroyed()

All those methods are called automatically
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Applet Basics/Applet Life Cycle

• public void init()
• One-time initialization when first loaded
• Good place to process parameters and add
  interface components.
• public void start()
• Called whenever user returns to the page
  containing the applet after having gone off to
  other pages.
• Can be called multiple times.
• Good place to resume animation or game

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Applet Basics/Applet Life Cycle

• public void stop()
• Called when user moves off page
• Good place to stop time-consuming activities such
  as animation and audio playing.
• public void destroy()
• Called when browser shuts down.
• Good place to reclaim non-memory-dependent
  resources such as graphics contexts.
• Normally, no need to worry.

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Converting Applications to Applets

• Java applications can easily to converted into
  applets
• Non-IO codes stay essentially the same.
• Inputs from home server are easy to handle.
• Outputs to home server are more advanced (CGI,
  Servlet).
• (Applets cannot communicate with other servers.
  Applets can only phone home. If applets are to
  communicate with other servers, including the
  local server, more access rights have to be given
  using a security policy file.)
• Non-IO applications first.
• Pop up a window for application
• Place top-level frame of application inside a
  browser

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Converting Applications to Applets

• Popping up a window for application.
• What to do
• Assume Separate class for creating and
  showing a top-level frame. (If this class also
  does some other things, move the other things to
  other classes.)
• class NotHelloWorldFrame extends JFrame
• public class NotHelloWorld
• public static void main(String args)
• JFrame frame new NotHelloWorldFrame()
• frame.show()

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Converting Applications to Popup Applets

• Delete the class for creating and showing the
  top-level frame
• Add an applet class whose init method contains
  the same instructions as main method of deleted
  class.
• public class NHWApplet extends JApplet
• public void init()
• JFrame frame new NotHelloWorldFrame()
• frame.show()
• //NHWApplet.class
• The popup window coming with a warning message
  for security reasons, (which can be avoided for
  signed applets).

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Converting Applications to embedded Applets

• Placing top-level frame of application inside
  browser.
• What to do (Assume main is in the definition
  of the top-level frame its only purpose is to
  create a window and show it. NotHelloWorld.java)
• JFrame class gt JApplet class must be public
• Remove setSize set in HTML file
• Remove addWindowListener Applet cannot be closed
• Remove setTitle Applet cannot have title bar
• Replace constructor with init.
• Delete the main method.

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Converting Applications to embedded Applets
class NotHelloWorldFrame extends JFrame
public NotHelloWorldFrame()
setTitle("NotHelloWorld") setSize(300,
200) Container contentPane
getContentPane() contentPane.add(new
NotHelloWorldPanel()) public class
NotHelloWorld public static void main(String
args) NotHelloWorldFrame frame
new NotHelloWorldFrame()
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOS
E) frame.show()
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Converting Applications to Applets
public class NotHelloWorldApplet extends
JApplet public void init()
Container contentPane getContentPane()
JLabel label new JLabel("Not a Hello,
World applet", SwingConstants.CENTER)
contentPane.add(label)
//NotHelloWorldApplet.java
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Resources for Applets

• One can provide information to applets in HTML
  file
• Applets can access resources at home server
• Text
• Multimedia

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Passing Info to Applets via HTML File

• In HTML file, use PARAM, NAME, VALUE tags
• ltAPPLET CODE"Chart.class" WIDTH400 HEIGHT300gt
• ltPARAM NAME"title" VALUE"Diameters of the
  Planets"gt
• ltPARAM NAME"values" VALUE"9"gt
• . lt/Appletgt
• In applet, use the getParameter method of the
  Applet class
• getParameter("title") // returns "Diameters of
  the Planets
• String vString getParameter(values) //
  returns 9
• if (vString null )
• do something // precaution
• else
• int vInteger.parseInt(vString)//must parse
  to get numbers

Chart.java, Chart.html
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Accessing Resources at Home Server

• Where is home?
• Inside a subclass of Applet
• getDocumentBase returns URL of the html file that
  calls the applet
• getCodeBase returns URL of the applet itself
• Inside any other class x
• x.class gives one an object of the Class class
  that contain information of x.
• (Class is a special class and has method
  getResource. C.f. Object class)
• x.class.getResource( resourceName ) returns URL
  of resource
• Need the URL class in java.net package
• import java.net.

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Accessing Text Files at Home Server

• Find the URL of text file and the create an
  InputStream using the openStream method of URL
  class
• InputStream in url.openStream()
• Or create an InputStream directly using the
  getResourceAsStream method of the Class class.
• InputStream in x.class.getResoruceAsStream(
• fileName)
• The InputStream can be nested with other streams
  in the normal way (see Topic 4)

ResourceTest.java, ResourceTest.html
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Accessing Images at Home Server

• Applets can handle images in GIF or JPEG format
• Load images
• Inside an subclass Applet, use
• getImage(url), getImage(url, name)
• Inside other classes java.awt.Toolkit
• Toolkit.getDefaultToolkit().getImage( url )
• How to show an image?

ImageLoadApplet.java
Exercise Load the images in applet class
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Accessing Audio Files at Home Server

• Applets can handle audio files in AU, AIFF, WAV,
  or MIDI format.
• Audio Clips (java.applet.Applet)
• Load
• AudioClip getAudioClip(url),
• AudioClip getAudioClip(url, name)
• Then use play method of AudioClip to play
• and the stop method to stop
• Play without first loading
• void play(url),
• void play(url, name)
• //SoundApplet.java

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Communication with Browser

• To establish a communication channel between an
  applet and browser, use the getAppletContext
  method of the Applet class
• The method returns an object of the
  AppletContext, which is an interface.
• Two useful methods of interface AppletContext
• showDocument( URL url )
• showDocument(URL url, String target )
• ShowPageApplet.java

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Weird Trick

• To avoid the additional HTML file, one can add an
  applet tag as a comment inside the source file.
• /
• ltAPPLET CODE MyApplet.class WIDTH300
  HEIGHT300gt
• lt/APPLETgt
• /
• public class MyApplet extends Japplet
• Then run the applet viewer with the source file
  as its command line arguments
• Appletviewer MyApplet.java
• Not necessarily recommending this as standard
  practice. But it is handy during testing.