Download What are Applets?

What are Applets?
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• Applets are "small" Java programs to be
integrated in web pages.
• Downloaded from a web server together
with their web page.
• Execution within a web browser, using
the browser's VM.
• Execution platform unknown at
programming time.
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Why Applets?
– Design facilities beyond HTML
– Interaction beyond HTML forms
– Program interaction between
web client and web server
– GUI for distributed applications
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Applet Programming
• Extend java.applet.Applet (orr javax.swing.JApplet)
• class Applet extends Panel {
public void init();
public void start();
public void stop();
}
• An applet is showing a panel within a browser window.
• init() is called when the applet is loaded. Design your panel, using
init() to add components and listeners - instead of the constructor.
• start() is called when the Applet is started – use it instead of main().
• Use the paint() method for drawing
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Applet Design
public class Foto extends Applet implements ActionListener{
private JButton button = new JButton("show photo");
private JLabel img = new JLabel();
private int imageIndex = 0;
ImageIcon [] foto = new ImageIcon[2];
public void init() {
// c.f. init() in applications
setLayout(new BorderLayout());
foto[0] = new ImageIcon("images/P8040106.png");
foto[1] = new ImageIcon("images/P8040112.png");
img.setIcon(foto[0]);
add(button, BorderLayout.NORTH);
add(img);
button.addActionListener(this);
}
public void start() {
// erase image at new start
imageIndex = 0;
img.setIcon(foto[imageIndex]);
}
public void actionPerformed(ActionEvent e) {
imageIndex = (imageIndex +1) % foto.length;
img.setIcon(foto[imageIndex]);
}
}
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Example: Clock
import
import
import
import
java.applet.*;
java.awt.*;
java.util.*;
java.text.*;
public class Clock1 extends Applet
implements Runnable
{
int width, height;
Thread t = null;
int hours=0, minutes=0, seconds=0;
String timeString = "";
public void init()
// from params, c.f. later slides
{
width = getSize().width;
height = getSize().height;
setBackground( Color.lightGrey );
}
from: http://www.dgp.toronto.edu/~mjmcguff/learn/java/09-clocks/
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public void start()
{ if ( t == null ) {
t = new Thread( this );
t.setPriority( Thread.MIN_PRIORITY );
t.start();
}
public void run() {
while (true) { // thread does some work
Calendar cal = Calendar.getInstance();
hours = cal.get( Calendar.HOUR_OF_DAY );
if ( hours > 12 ) hours -= 12;
minutes = cal.get( Calendar.MINUTE );
seconds = cal.get( Calendar.SECOND );
Date date = cal.getTime();
timeString = date.toString();
}
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void drawHand( double angle, int radius, Graphics g )
{
angle -= 0.5 * Math.PI;
int x = (int)( radius*Math.cos(angle) );
int y = (int)( radius*Math.sin(angle) );
g.drawLine( width/2, height/2, width/2 + x, height/2 + y );
}
void drawWedge( double angle, int radius, Graphics g ) { ... }
public void paint( Graphics g )
{
g.setColor( Color.blue );
drawWedge( 2*Math.PI * hours / 12, width/5, g );
drawWedge( 2*Math.PI * minutes / 60, width/3, g );
drawHand( 2*Math.PI * seconds / 60, width/2, g );
g.setColor( Color.black );
g.drawString( timeString, 10, height-10 );
}
} // end of class
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Applet Execution
• Applets are integrated into HTML pages
• The simplest Applet Tag is:
• <Applet Code="Clock.class"
Width=300
Height=300>
</Applet>
• Store this HTML file in the same directory as the
compiled applet class and run it by double clicking.
• Add a header to this HTML file and store it on the
web server. Download and execute it via the www.
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The complete Applet Tag
<APPLET
[CODEBASE = codebaseURL]
CODE = appletFile
[ALT = alternateText]
[NAME = appletInstanceName]
WIDTH = pixels
HEIGHT = pixels
[ALIGN = alignment]
[VSPACE = pixels]
[HSPACE = pixels] >
[< PARAM NAME = appletParameter1 VALUE = value >]
[< PARAM NAME = appletParameter2 VALUE = value >]
...[alternateHTML]
</APPLET>
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Applet Positioning
• Position attributes like in IMG tag:
• Align:
left, right, top, texttop, middle, absmiddle,
baseline, bottom, absbottom.
• VSpace, HSpace:
Distance from neighbouring elements (pixels)
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Parameter
Parameter:
• Free choice of paramNames
• Applets can read parameters using
getParameter(String paramName)
• Parameters are HTML page configuration data (not
user input)
alternateHTML:
• Any text – displayed if the browser does not support
Java
• i.e. has no java plugin.
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Parameter Example
<-- File AppletTest.html -->
<Applet Code=" ConfiguredFoto.class"
Width=200
Height=300>
<Param Name="Language" Value="Englisch">
<Param Name="ImageSource" Value="images/foto3.png">
Your Browser is not Java-enabled.<br>
You should install the Java plugin. <br>
<Href "http://www.javaplugin.htm"> Install Instructions for Plugin</Href>
</Applet>
public class ConfiguredFoto extends Foto {
public void init() {
foto = new ImageIcon(getParameter("ImageSource"));
if (getParameter("Language")!="English")
button.setText("-->");
super.init();
}
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}
Downloading Applets from Anywhere
• Use the Codebase attribute to hold the Download-URL
• Any URL permitted.
• Default: "DocumentBase" – i.e. download URL of the
HTML page
• <Applet Code="Bild.class"
CodeBase="http://www.tfh-berlin.de/...../"
Width=200 Height=300>
</Applet>
• read CodeBase and DocumentBase inside the applet:
getCodeBase(), getDocumentBase()
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Applets in Packages
• Rules similar to javac:
– applet name is a qulified class name
– separator is dot "." (not slash!)
– CodeBase denotes the root directory of the package
structure
• <Applet Code="fotos.Foto.class"
Width=700
Height=600>
</Applet>
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Wecker.htm
<html>
<body>
<applet code=wecker.WeckerApplet.class
archive=cs101-lib.jar, Alarmimages.jar
width="200" height="200" >
</applet>
</body>
</html>
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Applets in Jar Archives
• Bundle applet and resources in a single jar
•
easy deployment and administration (e.g.moving)
•
saves download time.
• Several jars possible – download the second while
displaying content from the first.
• <Applet Code="Bild.class"
Archive=Applets1.jar, Applets2.jar
... </Applet>
• Bild.class will be searched first in Applets1.jar, then in
Applets2.jar, then in the codebase directory
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Applet Security
An applet plays
in its "sandbox"
An applet comes in handcuffs:
It cannot read from the local file system
It can only establish network connections to its
codebase or document base.
It cannot start or stop programs.
It cannot execute "native" code.
It can only read non-critical, java relevant system
properties like version.
• If executed locally from hard disk, using appletviewer.exe, it has
less restrictions. So, if your applet works in the appletviewer, it
might still not run in a browser!
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Applet to Server Communication
Network connections permitted to CodeBase or
DocumentBase
Never use absolute server URL.
Always use relative URL:
URL serverUrl = getCodeBase();
Socket netConnection =
new Socket
(serverUrl.getHost(),55555);
OutputStream out =
netConnection.getOutputStream();
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a2a Communication
• An applet can directly access other applets which
– run in the same browser window and
– have the same CodeBase
• Such applets can call each other:
– use getAppletContext() to obtain the context object,
which keeps track of all applets in a browser.
– use getApplet("appname") to get the applet that was
loaded with the name attribut "appname"
– or use getApplets() to obtain an enumeration of all
accessable applets
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Example:
• Applet Runner has two methods:
• goLeft()
makes a stick figure walk to the left edge and then,
after a short pause, calls goRight
• goRight() is the reverse.
• Controlling a stick figure from another applet:
Runner right = (Runner)
(getAppletContext().
getApplet("rightRunner"));
right.goLeft();
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Tutorials
• Use google to find numerous tutorials!
• http://www.realapplets.com/tutorial/index.html
deals with applet basics
• http://www.dgp.toronto.edu/~mjmcguff/learn/java
tells you how to do all sorts of graphics and
imaging in an applet
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