Writing Applets

1
Writing Applets
Base on The Java Tutorial
Object-Oriented Programming for the Internet

• Overview of Applets
• Creating an Applet User Interface
• Communicating with Other Programs
• Understanding Applet Capabilities and
  Restrictions
• Finishing an Applet
• Common Applet Problems (and Their Solutions)

2
Overview of Applets

• Every applet is implemented by creating a
  subclass of the Applet class.
• The following figure shows the inheritance
  hierarchy of the Applet class.
• java.lang.Object
• ----java.awt.Component
• ----java.awt.Container
• ----java.awt.Panel
• ----java.applet.Ap
  plet

3
A Simple Applet

• A Simple Applet
• import java.applet.Applet
• import java.awt.Graphics
• public class Simple extends Applet
• StringBuffer buffer
• public void init()
• buffer new StringBuffer()
• addItem("initializing... ")
• public void start()
• addItem("starting... ")
• public void stop()

public void destroy() addItem("preparing for
unloading...") void addItem(String newWord)
System.out.println(newWord) buffer.append(new
Word) repaint() //AWT.Component publ
ic void paint(Graphics g) //Draw a Rectangle
around the applet's display area. g.drawRect(0,
0, size().width - 1, size().height - 1) //Draw
the current string inside the rectangle.
g.drawString(buffer.toString(), 5, 15)
4
The Life Cycle of an Applet

• Loading the Applet When an applet is loaded,
  here's what happens
• An instance of the applet's controlling class (an
  Applet subclass) is created.
• The applet initializes itself.
• The applet starts running.

5
The Life Cycle of an Applet

• Leaving and Returning to the Applet's Page
• When the user leaves the page, the applet has the
  option of stopping itself.
• When the user returns to the page, the applet can
  start itself again.
• The same sequence occurs when the user iconifies
  and then reopens the window that contains the
  apple.

6
The Life Cycle of an Applet

• Reloading the Applet
• Some browsers let the user reload applets, which
  consists of unloading the applet and then loading
  it again.
• Before an applet is unloaded, it's given the
  chance to stop itself and then to perform a final
  cleanup, so that the applet can release any
  resources it holds.
• After that, the applet is unloaded and then
  loaded again.

7
The Life Cycle of an Applet

• Quitting the Browser
• When the user quits the browser, the applet has
  the chance to stop itself and do final cleanup
  before the browser exits.
• Summary - An applet can react to major events in
  the following ways
• It can initialize itself.
• It can start running.
• It can stop running.
• It can perform a final cleanup, in preparation
  for being unloaded.

8
Methods for Milestones

• init()
• To initialize the applet each time it's loaded
  (or reloaded).
• The init() class is useful for one-time
  initialization that doesn't take very long.
• In general, the init() method should contain the
  code that you would normally put into a
  constructor.

9
Methods for Milestones

• start()
• To start the applet's execution, such as when the
  applet's loaded or when the user revisits a page
  that contains the applet.
• Every applet that does something after
  initialization (except in direct response to user
  actions) must override the start() method.
• The start() method either performs the applet's
  work or (more likely) starts up one or more
  threads to perform the work.

10
Methods for Milestones

• stop()
• To stop the applet's execution, such as when the
  user leaves the applet's page or quits the
  browser.
• Most applets that override start() should also
  override the stop() method.
• The stop() method should suspend the applet's
  execution, so that it doesn't take up system
  resources when the user isn't viewing the
  applet's page.

11
Methods for Milestones

• destroy()
• To perform a final cleanup in preparation for
  unloading.
• Many applets don't need to override the destroy()
  method, since their stop() method (which is
  called before destroy()) does everything
  necessary to shut down the applet's execution.
• However, destroy() is available for applets that
  need to release additional resources

12
Methods for Drawing and Event Handling

• There are two display methods that applets can
  override
• paint()
• The basic display method. Many applets implement
  the paint() method to draw the applet's
  representation within a browser page.
• update()
• A method you can use along with paint() to
  improve drawing performance.

13
Methods for Drawing and Event Handling

• Applets inherit their paint() and update()
  methods from the Applet class, which inherits
  them from the Abstract Window Toolkit (AWT)
  Component class.
• class Simple extends Applet
• . . .
• public void paint(Graphics g) . . .
• . . .

14
Methods for Drawing and Event Handling

• From the Component class, applets inherit a group
  of methods for event handling.
• The Component class defines several methods (such
  as mouseDown() and action()) for handling
  particular types of events, and then one
  catch-all method, handleEvent().

15
Methods for Drawing and Event Handling

• To react to an event, an applet must override
  either the handleEvent() method or the
  appropriate specialized method.
• For example, adding the following code to the
  Simple applet makes it respond to mouse clicks.
• import java.awt.Event
• . . .
• public boolean mouseDown(Event event, int x, int
  y)
• addItem("click!... ")
• return true

16
Methods for Adding UI Components

• The AWT supplies the following UI components
• Buttons (java.awt.Button)
• Checkboxes (java.awt.Checkbox)
• Single-line text fields (java.awt.TextField)
• Larger text display and editing areas
  (java.awt.TextArea)
• Labels (java.awt.Label)
• Lists (java.awt.List)
• Pop-up lists of choices (java.awt.Choice)
• Sliders and scrollbars (java.awt.Scrollbar)
• Drawing areas (java.awt.Canvas)
• Menus (java.awt.Menu, java.awt.MenuItem,
  java.awt.CheckboxMenuItem)
• Containers (java.awt.Panel, java.awt.Window and
  its subclasses)

17
Methods for Using UI Components in Applets

• The Applet class inherits from the AWT Container
  class, it's easy to add components to applets and
  to use layout managers to control the their
  onscreen positions. Some of the Container methods
  an applet can use
• add()
• Adds the specified Component.
• remove()
• Removes the specified Component.
• setLayout()
• Sets the layout manager.

18
Adding a Non-Editable Text Field to an Applet

• To make the Simple applet use a scrolling,
  non-editable text field, we can use the TextField
  class.
• Here is the revised source code.
• The revised init() method creates an uneditable
  text field (a TextField instance).
• It sets the applet's layout manager to one that
  makes the text field as wide as possible and then
  adds the text field to the applet.

19
Threads in Applets

• A thread -- sometimes known as an execution
  context or a lightweight process -- is a single
  sequential flow of control within a process.
• Even the simplest applets run in multiple
  threads, although it's not always apparent.
• Many applets create and use their own threads, so
  that they perform well without affecting the
  performance of the application they run in or of
  other applets.

20
Threads in Applets

• Every applet can run in multiple threads.
• Applet drawing methods (paint() and update()) are
  always called from the AWT drawing and event
  handling thread.
• The threads from which the major milestone
  methods -- init(), start(), stop(), and destroy()
  -- are called depends on the application that's
  running the applet.

21
Threads in Applets

• PrintThread is a modified version of SimpleApplet
  that prints the thread and thread group that its
  init(), start(), stop(), destroy(), and update()
  methods are called from.

22
Threads in Applets

• Rule of Thumb An applet performing a
  time-consuming task should create and use its own
  thread to perform that task.
• Applets typically perform two kinds of
  time-consuming tasks tasks that they perform
  once, and tasks that they perform repeatedly.

23
Threads in Applets Examples

• The first applet, AnimatorApplet, shows how to
  use a thread to perform repeated tasks.
• The second applet, SoundExample, shows how to use
  threads for one-time initialization tasks.

24
Using a Thread to Perform Repeated Tasks

• Applets typically create threads for repetitive
  tasks in the applet start() method.
• Creating the thread there makes it easy for the
  applet to stop the thread when the user leaves
  the page.
• All you need to do is implement the stop() method
  so that it stops the applet's thread. An Animator
  example

25
Using a Thread to Perform Repeated Tasks

• public void start()
• if (frozen)
• //Do nothing. The user has requested that we
• //stop changing the image.
• else
• //Start animating!
• if (animatorThread null)
• animatorThread new Thread(this)
• animatorThread.start()
• public void stop()
• animatorThread null

No garbage collect on this.
Click Here to see the source
26
Using a Thread to Perform One-Time Initialization

• If an applet needs to perform some initialization
  task that can take a while, it should consider
  performing the initialization in a thread.
• For example, anything that requires making a
  network connection should generally be done in a
  background thread.

27
Using a Thread to Perform One-Time Initialization

• Fortunately, GIF and JPEG image loading is
  automatically done in the background (using
  threads that you don't need to worry about).
• Sound loading unfortunately, is not guaranteed to
  be done in the background.
• The Applet getAudioClip methods don't return
  until they've loaded all the audio data.

28
Using a Thread to Perform One-Time Initialization

• SoundExample adheres closely to the model
  presented in Synchronizing Threads.
• The producer SoundLoader, a Thread subclass.
• The consumer SoundExample, an Applet subclass.
• The storage object SoundList, a Hashtable
  subclass.

29
What Applets Can and Can't Do

• For security reasons, applets that are loaded
  over the network have several restrictions.
• One is that an applet can't ordinarily read or
  write files on the computer that it's executing
  on.
• Another is that an applet can't make network
  connections except to the host that it came from.

30
Security Restrictions

• Current browsers impose the following
  restrictions on any applet that's loaded over the
  network
• An applet can't load libraries or define native
  methods.
• It can't ordinarily read or write files on the
  host that's executing it.
• It can't make network connections except to the
  host that it came from.

31
Security Restrictions

• Current browsers impose the following
  restrictions on any applet that's loaded over the
  network
• It can't start any program on the host that's
  executing it.
• It can't read certain system properties.
• Windows that an applet brings up look different
  than windows that an application brings up.

32
Security Restrictions

• Every browser implements security policies to
  keep applets from doing damage.
• Each browser has a SecurityManager object that
  implements its security policies.
• When a SecurityManager detects a violation, it
  throws a SecurityException.
• Your applet can catch this SecurityException and
  react appropriately.

33
Applet Capabilities

• The java.applet package provides an API that
  gives applets some capabilities that applications
  don't have
• Applets can make network connections to the host
  they came from.
• Applets running within a Web browser can easily
  cause HTML documents to be displayed.
• Applets can invoke public methods of other
  applets on the same page.

34
Applet Capabilities

• The java.applet package provides an API that
  gives applets some capabilities that applications
  don't have
• Applets that are loaded from the local file
  system (from a directory in the user's CLASSPATH)
  have none of the restrictions that applets loaded
  over the network do.
• Although most applets stop running once you leave
  their page, they don't have to.

35
Adding an Applet to an HTML Page

• Here's the simplest form of the ltAPPLETgt tag
• ltAPPLET CODEAppletSubclass.class WIDTHanInt
  HEIGHTanIntgt
• lt/APPLETgt

36
Specifying the Applet Directory with CODEBASE

• Add a CODEBASE attribute to tell the
  browser/viewer which directory the Applet
  subclass bytecodes are in.
• ltAPPLET CODEAppletSubclass.class CODEBASEaURL
  WIDTHanInt HEIGHTanIntgt
• lt/APPLETgt
• By making aURL an absolute URL, you can make a
  document loaded from your HTTP server run an
  applet from another HTTP server.
• When aURL is a relative URL..

37
Specifying Parameters with the ltPARAMgt Tag

• You can customize the applet's configuration with
  parameters. For example, you can set a button's
  text by specifying the value of a parameter named
  BUTTONTEXT.
• Note that ltPARAMgt tags must appear between the
  ltAPPLETgt and lt/APPLETgt tags for the applet they
  affect.

38
Specifying Parameters with the ltPARAMgt Tag

• ltAPPLET CODEAppletSubclass.class WIDTHanInt
  HEIGHTanIntgt
• ltPARAM NAMEparameter1Name VALUEaValuegt
• ltPARAM NAMEparameter2Name VALUEanotherValuegt
• lt/APPLETgt
• ltapplet codeAppletButton.class codebaseexample
  width350 height60gt
• ltparam namewindowType valueBorderWindowgt
• ltparam namewindowText value"BorderLayout"gt
• ltparam namebuttonText value"Click here to see a
  BorderLayout in action"gt
• . . .
• lt/appletgt

39
Specifying Text to be Displayed by Java-Deficient
Browsers

• ltapplet codeAppletButton.class codebaseexample
  width350 height60gt
• ltparam namewindowType valueBorderWindowgt
• ltparam namewindowText value"BorderLayout"gt
• ltparam namebuttonText value"Click here to see a
  BorderLayout in action"gt
• ltblockquotegt
• lthrgt
• ltemgtYour browser can't run 1.0 Java applets, so
  here's a picture of the window the program brings
  up
• lt/emgt
• ltpgt
• ltimg srcimages/BorderEx1.gif width302
  height138gt
• lthrgt
• lt/blockquotegt
• lt/appletgt