CS4273: Distributed System Technologies and Programming I
Lecture 3: Java Applets and Animations
Java Applet
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Applet is a special kind of Java application programs that are embedded into web
pages. It must run in a web-browser (or appletviewer).
It is downloaded from the network, and executed locally. It is the idea of develop once
and run anywhere.
Applet tag in HTML:
<APPLET
CODEBASE = “DirOrURL”
CODE = “Local.class”
NAME = “ThisAppletName”
WIDTH=pixels HEIGHT=pixels
ALIGN = AlignmentValue //LEFT, RIGHT, TOP, ……
VSPACE=pixels HSPACE=pixels
ALT = “AlternativeTextIfAppletNotWorking” >
<PARAM NAME = ParameterName VALUE = ParameterValue>
...
</APPLET>
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Passing Parameters to Applets
• Applets get parameters from <PARAM> fields:
<APPLET CODE="JavaGreeting.class" WIDTH=100 HEIGHT=50>
<PARAM NAME=Greeting VALUE=”Hello, Good Morning”>
</APPLET>
• Code of getting the parameter value in the applet:
String GreetingWords = getParameter("Greeting");
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Applet Methods
Applets do not have a main routine. Applet class has many methods. You
need to override some of them to let applet perform your task. The
Applet methods are listed below in the order they are called:
• init(): called only once when the applet is loaded. It initializes
variables and initial screen display.
• start(): called after init() and every time returned to the page
containing the applet after gone off. It can be called repeatedly.
• paint(Graphics g): it is also called by repaint().
• stop(): called when moving off the page. When coming back, start()
method is called. It is used to stop time-consuming activities
(animation threads) when not seen.
• destroy(): called when the browser shuts down. It deletes the applet
and recalls all resources.
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Example of an Applet
class BtnAdapter implements ActionListener {
public class music extends JApplet {
private int id;
AudioClip musicOnce, musicLoop;
BtnAdapter( int buttonID) {
public void init() {
id = buttonID;
add (playBtn = new Button("Play"));
}
playBtn.addActionListener(new BtnAdapter (0));
add (loopBtn = new Button("Loop"));
public void actionPerformed (ActionEvent e) {
add (stopBtn = new Button("Stop"));
switch (id) {
stopBtn.addActionListener( new BtnAdapter (2));
case 0: musicOnce.play();
musicOnce = getAudioClip(getDocumentBase(), “TAM.au");
break;
musicLoop = getAudioClip(getDocumentBase(), "deng0.au");
case 1: musicLoop.loop();
}
break;
case 2: musicLoop.stop();
public void start() {
}
musicLoop.loop();
}
}
}
public void stop() {
musicLoop.stop();
}
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paint, repaint & paintComponent
• Whenever you need to update the display, call repaint()
(you cannot call paint() directly). repaint() invokes paint().
• In paint(), you usually start with super.paint(), which
performs some preparation work for you, e.g., clear the
display area. Without calling super.paint(), the paint() will
paint on top of the existing drawings.
• JComponent has method paintComponent(). For painting
a sub-region of applet’s display area (i.e., Jcomponent),
you need to override paintComponent(), instead of paint().
paint() invokes paintComponent().
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Applets and Applications
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JApplet is a subclass of JPanel.
Create a component and add it to an applet in the same way as adding it to a
container, e.g., add(new button(“start”)), add(new Canvas()), etc.
Object
Component
Container
Window
JPanel
Frame
JApplet
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Play Audio Files in Applets
• When playing music, you need to get an AudioClip object and
then play it. Get an audio clip by:
– AudioClip getAudioClip(URL url, String name)
• Three methods on AudioClip objects:
– play(): it plays the audio file once and stop.
– loop(): it plays the audio file repeatedly until the stop method is called.
– stop(): it stops a loop play of an audio clip.
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Example of Playing AudioClips
class BtnAdapter implements ActionListener {
public class audioPlayer extends JApplet {
private int id;
AudioClip musicOnce, musicLoop;
BtnAdapter( int buttonID) {
public void init() {
id = buttonID;
add (playBtn = new Button("Play"));
}
playBtn.addActionListener(new BtnAdapter (0));
add (loopBtn = new Button("Loop"));
public void actionPerformed (ActionEvent e) {
add (stopBtn = new Button("Stop"));
switch (id) {
stopBtn.addActionListener( new BtnAdapter (2));
case 0: musicOnce.play();
musicOnce = getAudioClip(getDocumentBase(), “sun.au");
break;
musicLoop = getAudioClip(getDocumentBase(), "deng.au");
case 1: musicLoop.loop();
}
break;
case 2: musicLoop.stop();
public void start() {
}
musicLoop.loop();
}
}
}
public void stop() {
musicLoop.stop();
}
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Play Video in Applets
Java Media Framework (JMF) API is used to play and edit media files. You need to:
1) download JMF package to IE to enable JMF functions
http://java.sun.com/javase/technologies/desktop/media/jmf/2.1.1/download.html and,
2) compile programs with JMF library.
The main JMF API includes:
• A Manager class contains methods for playing and manipulating media clips.
• Create a player object out of URL of a video clip by using Manager method:
Player player = Manager.createRealizedPlayer(mediaURL)
• Get a video display panel and a control panel, and add them to applet:
videoPanel = player.getVisualComponent();
controlPanel = player.getControlPanelComponent();
add(“center”, videoPanel);
add(“south”, controlPanel);
• Control methods on the player object:
player.start();
player.stop();
player.close()
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Example of Playing Video in Applets
public class videoPlayer extends JApplet {
Player player;
public void init() {
URL mediaURL = new URL(getDocumentBase(),"bailey.mpg");
player = Manager.createRealizedPlayer(mediaURL);
Component video = player.getVisualComponent();
Component controls = player.getControlPanelComponent();
add("Center", video);
add("South", controls);
}
public void start() {
if (player != null) player.start();
}
public void stop() {
if (player != null) player.stop();
}
}
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Display Images
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get an Image by:
Image getImage(URL url, String name)
paint the image by:
drawImage(image, x, y, observer)
An example:
public class ImageTest extends Applet {
{
Image theImage = null;
public void init() {
theImage = getImage(getDocumentBase(), "pg9.gif");
}
public void paint(Graphics g) {
g.drawImage(theImage, 0, 0, this);
}
}
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Eample of Animation using Thread
public class animation extends JApplet implements
Runnable {
Thread runner;
Vector frames = new Vector();
int cur_frame, frame_delay = 400;
public void init() {
URL base = getDocumentBase();
for (int i = 1; i <= 8; i++) {
Image img = getImage(base,
"image/bunny" + i +".gif");
frames.addElement(img);}
}
public void start() {
if (runner == null) {
runner = new Thread(this);
runner.start(); }
}
public void stop() {
runner = null;
}
public void run() {
cur_frame = 1;
while(runner != null) {
Thread.sleep(frame_delay);
repaint();
if(cur_frame >= frames.size())
cur_frame = 1;
else
cur_frame++; }
}
public void paint(Graphics g) {
super.paint(g);
Image img =
(Image)frames.elementAt(cur_frame-1);
g.drawImage(img, cur_frame*30, 0, this);
}
}
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Steps of a Simple Animation
Animation using Thread
• In “init()”, load the images
into frames (data structure
of a vector).
• In “start()”, start a thread to
do the animation.
• In “run()” of the thread,
paint the cur-frame, pause a
while, and move forward
the pointer of frames.
• In “paint()”, paint the image
of the cur-frame.
Animation using Timer class
• Create a Timer object:
new Timer(delay, new TimerListener());
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The timer generates an action
event in interval of delay and
TimerListener() is an
actionListener class that handles
this action event.
Methods for Timer control:
timer.setDelay(delay);
timer.start();
timer.stop();
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Example of Animation using Timer
public void paint(Graphics g) {
public class bounceBall extends JApplet {
………….......
int delay = 100, x = 0, y = 20, radius = 10, dx = 10, dy = 10;
x += dx;
public Timer timer = new Timer(delay, new TimerListener());
y += dy;
public void init() {
g.setColor(Color.red);
panel.add(btSuspend = new JButton("Suspend"));
g.fillOval(x-radius, y-radius, radius*2, radius*2);
btSuspend.addActionListener(new ButtonAdapter());
}
panel.add(btResume = new JButton("Resume"));
class TimerListener implements ActionListener {
btResume.addActionListener(new ButtonAdapter());
public void actionPerformed(ActionEvent e) {
JScrollBar scrollBar = new JScrollBar();
repaint();
scrollBar.setOrientation(JScrollBar.HORIZONTAL);
}
scrollBar.addAdjustmentListener(new ScrollAdapter());
}
class ButtonAdapter implements ActionListener {
add(scrollBar, BorderLayout.NORTH);
public void actionPerformed (ActionEvent e) {
add(displayArea, BorderLayout.CENTER);
if(e.getSource() == btSuspend)
add(panel, BorderLayout.SOUTH);
timer.stop();
if(e.getSource()
== btResume)
timer.start();
timer.start();
}
}
public void start() {
}
timer.start();
class ScrollAdapter implements AdjustmentListener {
}
public void adjustmentValueChanged(AdjustmentEvent e) {
public void stop() {
timer.setDelay(scrollBar.getMaximum() - e.getValue());
timer.stop();
}
}
}
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Media Tracker
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MediaTracker class provides methods to check whether the loading of
an image (a group of images) is complete or not, and get status of
image loading. It is particularly useful in managing a group of images.
Construct a MediaTracker object by:
MediaTracker myTracker = new MediaTracker(this);
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Useful methods of MediaTracker class:
addImage(img, id), add the img into the Tracker
isErrorAny(), if loading process has an error
checkID (id), check if the image is loaded
checkAll (), if all images are loaded
waitForID (id),
waitForAll (), block the current executing thread until all images are loaded.
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Example of using MediaTracker
public class ImageApplet extends JApplet {
Image myImg = null;
MediaTracker myTracker = null;
public void init() {
myTracker = new MediaTracker(this);
myImg = getImage(getDocumentBase(), “me.gif”);
myTracker.addImage(myImg, 0);
}
public void paint(Graphics g) {
if (myTracker.isErrorAny()) {
g.drawString(“errors in loading”, 10, 10);
return;
} else if (myTracker.checkAll(true)) {
g.drawImage(myImg, 0, 0, this);
} else {
g.drawString(“image loading…”, 10, 10);
repaint(100) // recursive call paint every 100ms
}
}
}
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Applet Network Access
Security of an applet:
• applets cannot run any local executable program.
• applets cannot read or write to the local computer’s file system.
• applets cannot communicate with any host other than the server from
which they are downloaded.
Getting data from originating server
• applets get data from their originating servers:
URL getDocumentBase() // URL of this web page
URL getCodeBase() // URL of this of this applet
• Examples
cat = getImage (getDocumentBase(), “images/cat.gif”);
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Message Passing between Applets
When a web page contains several applets, it is often required that
the applets communicate with each other. An applet can send
messages to another applet on the same page (communicating
applets must originate from the same server).
• An applet sends a message to another via invoking a method of
the other applet. It is “inter-applet method invocation”.
• Applets identify each other by using their
names (assigned in the HTML file).
Applet 2
• The message passing is done inside the ……Applet 1
……
……
Applet2.recvMsg(m)
browser (at the client side) without the ……
recvMsg(m) {
……
server involved.
}
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HTML File Containing two Applets with Names
<html>
<body>
<APPLET CODE="sender.class" WIDTH=450 HEIGHT=300 NAME=“Bob">
<PARAM NAME=RECEIVER VALUE=“Alice">
</APPLET>
<APPLET CODE="receiver.class" WIDTH=450 HEIGHT=300 NAME=“Alice">
</APPLET>
</body>
</html>
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Steps for sending a message to another applet
1. Sender gets receiver’s name by
recverName = getParameter("RECEIVER")
2. Sender gets the receiver applet by
recver = getAppletContext().getApplet(recverName)
3. Sender invokes the receiver’s method by
reply_msg = ((receiver) recver).recvMsg(msg)
// “recvMsg” is a method of the receiver (for receiving messages).
4. The receiver’s method can return a value (or string) that the sender can
get as the reply of its message.
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The sender program
// sender.java
public class sender extends Applet implements ActionListener {
public void init() {
recvName = getParameter("RECEIVER");
//... set the GUI
}
public void actionPerformed(ActionEvent event) {
String reply_msg, msg = “test of applet message passing";
Applet recver = null;
//Get the receiver applet
recver = getAppletContext().getApplet(recverName);
// call receiver applet’s method.
reply_msg = ((receiver)recver).recvMsg(myName, msg);
status.append(“Replied msg: " + reply_msg +"\n");
msg = reply_msg;}
}
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The receiver program
// receiver.java
public class receiver extends JApplet implements ActionListener {
private JButton button = new JButton("Clear");
private JTextArea status = new JTextArea(5, 60);
public void init() {
button.addActionListener(this); add("North", button);
add("Center", status);
add("South", new JLabel("My name is "+ getParameter("NAME")));
}
public String recvMsg(String senderName, String msg) {
status.append("Received from " + senderName +": " + msg +"\n");
i++;
return msg+i;
// echo received message back }
public void actionPerformed(ActionEvent event) {
status.setText("");
i = 0;
}
}
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