java applets

Topic 11: Applets
Chapter 10
Advanced Programming Techniques

Introduction
• Applets are java programs that run within browsers
• Example:
– Jmol applet
• http://jmol.sourceforge.net/applet/
– NASA 3D real-time satellite tracker
• http://liftoff.msfc.nasa.gov/realtime/jtrack/3d/JTrack3d.html
Applet
Client Browser ServerApplet
User

Outline
• An example
• Creation
– Converting applications to applets
• Transportation
– Jar files: Move applets from servers to browsers quickly
• Operation
– Applet life cycle
– Security restrictions
– Getting resources from home
– Communicating with browser
Applet
User

An Example
 An applet is a Java class which extends java.applet.Applet
 If Swing components are used, the applet must extend from
javax.swing.JApplet
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

• Compile and run
– Compile: javac NotHelloWorldApplet.java
– Run:
• Create a HTML file that tells the browser which file to load and how to size the applet
<html><body> This is an example.
<applet code=“NotHelloWorldApplet.class” width=300
height=300>
This text shown if browser doesn’t do Java.</applet>
</body></html>
• View the HTML file with a browser or the command appletviewer
– Note:
• Textpad: Cntrl+3 – creates a simple html file and show it with appletviewer
An Example

An Example
• More notes
– To view applet, one needs Java 2 enabled browser (recent version of IE
and Netscape, e.g. IE 6, Netscape 6, Netscape 7. Netscape 4 is not Java
2 enabled)
– Class files are cached by browser.
• Restart browser after each modification
• Alternatively, one can clear the cache from Java console, which can be
accessed from Netscape or control panel on Windows

• Compare with application: NotHelloWorld.java
class NotHelloWorldFrame extends JFrame
{
public NotHelloWorldFrame()
{
setTitle("NotHelloWorld");
setSize(300, 200);
JLabel label = new JLabel("Not a Hello,
World application", SwingConstants.CENTER);
}
}
public class NotHelloWorldApplication
{ public static void main(String[] args)
{ NotHelloWorldFrame frame =
new NotHelloWorldFrame();
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.show();
}}
An Example

• Applets are created, run, and destroyed by web browser
– Don’t set size for an applet: determined by HTML file.
– Don’t set title for an applet: applets cannot have title bars.
• Can have menus.
– 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.
An Example

Outline
• An example
• Creation
– Converting applications to applets
• Transportation
– Jar files: Move applets from servers to browsers quickly
• Operation
– Applet life cycle
– Security restrictions
– Getting resources from home
– Communicating with browser

Creating Applets from Applications
• Non-IO applications for now
– Pop up window for application
– Embed top-level frame of application inside
browser

• Popping up a window for application.
– 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 NotHelloWorldApplication
{ public static void main(String[] args)
{ JFrame frame = new NotHelloWorldFrame();
frame.show();
}
}

• Steps of conversion:
– 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.
– Remove code for closing window
public class NHWApplet extends JApplet
{ public void init()
{ JFrame frame = new NotHelloWorldFrame();
frame.show();
}
} //NHWApplet.java
– The popup window coming with a warning message for security reasons,
(which can be avoided for signed applets).

• Placing top-level frame of application inside browser.
– 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.)
{
{
setSize(300, 200);
JLabel label = new JLabel("Not a Hello, World applet", SwingConstants.CENTER);
}}
public class NotHelloWorld
{ public static void main(String[ ] args)
{ NotHelloWorldFrame frame = new NotHelloWorldFrame();
frame.show();
}}

• Steps of conversion
– Delete the class for creating and showing the top-level frame
– Convert the top-level frame class into an applet
• JFrame class => JApplet class; must be public
• Remove setSize: set in HTML file
• Remove setTitle: Applet cannot have title bar
• Replace constructor with init.

• Let’s do it now
{
{
setSize(300, 200);
JLabel label = new JLabel("Not a Hello, World applet", SwingConstants.CENTER);
}}
public class NotHelloWorld
{ public static void main(String[ ] args)
{ NotHelloWorldFrame frame = new NotHelloWorldFrame();
frame.show();
}}

Transportation of Applets via Jar Files
• PopupCalculatorApplet involve three classes
– CalculatorFrame.class, CalculatorPanel.class
PopupCalculatorApplet.class
• HTML file contains name of the applet class
– <APPLET CODE=" PopupCalculatorApplet.class " WIDTH = 60
HEIGHT = 20 > </APPLET>
• Class loader
– First fetches PopupCalculatorApplet.class
– In the process, it notices that some other classes are also needed. It then makes net
connections to get them.
– Many connections might be needed in general, especially when there are associated
resources such as images and sounds.

Jar Files
• Jar files
– A jar file is simply a zip file that contains a manifest file, which
describes features of the archive
– 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.

Jar Files
• Creating jar files
– jar cf PopupCalculatorAppletClasses.jar *class
• In general:
jar cf myJarFile.jar *.class *.gif
pack all files ending with .class or .gif

Jar Files
 Refer to JAR files in the APPLET tag
<APPLET CODE="PopupCalculatorApplet.class"
ARCHIVE="PopupCalculatorAppletClasses.jar,swing.jar"
WIDTH = 65 HEIGHT = 20 > </APPLET>
 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.

Diversion/Self-Running Jar File
• “To make an executable jar file, we need to indicate the main class in the
manifest file.
– Create “mainclass.mf” with one line (no “class” and ended by “return”)
Main-Class: MyApplet
– Create jar file with the manifest file
jar cvfm MyJarFile.jar mainclass.mf *class
– Also, one can update the manifest files of an existing jar file
jar umf mainclass.mf MyJarFile.jar
• Run:
– java -jar MyJarFile.jar
– Or click on file icon
Self-Running Calculator

Applet Life Cycle
• An application starts from main and runs
until exit
• Applets are controlled by browser through
4 methods
– init()
• Called when loaded by browser
– start()
• Called right after init and when user return
to page
– stop()
• Called when user moves off page
– destroy()
• Called when browser shuts down.
• Overwrite the methods to control applet
behavior
non-existent on page
off page
init( )
destroy( )
stop( )destroy( ) start( )

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

Applet Life Cycle
• public void stop()
– Called when user moves off page (to other pages)
– 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.
• Example: sound (Stop Playing when going out of page)
Compare with the one of the two other versions.

Outline
 An example
 Creation
 Converting applications to applets
 Transportation
 Jar files: Move applets from servers to browsers quickly
 Operation
 Applet life cycle
 Security restrictions
 Getting resources from home
 Communicating with browser

Security Restrictions
 Applets are downloaded from the net and executed by a
browser’s 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.

Security Restriction
 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.

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

Outline
 An example
 Creation
 Converting applications to applets
 Transportation
 Jar files: Move applets from servers to browsers quickly
 Operation
 Applet life cycle
 Security restrictions
 Getting resources from home
 Communicating with browser
Applet
User

Resources for Applets
 One can provide information to applets in HTML file
 Applets can access resources at home server:
 Text
 Multimedia

Passing Info to Applets via HTML File
 In HTML file, use PARAM, NAME, VALUE tags
<APPLET CODE="Chart.class" WIDTH=400 HEIGHT=300>
<PARAM NAME="title" VALUE="Diameters of the Planets">
<PARAM NAME="values" VALUE="9">
…. </Applet>
 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 v=Integer.parseInt(vString);//must parse to get numbers
Chart.java, Chart.html

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.*

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

 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
Accessing Images at Home Server
Exercise: Load the images in applet class

 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
Accessing Audio Files at Home Server

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

Java Web Start
• A technology for simplifying deployment of Java applications
– Gives you the power to launch full-featured applications with a single
click from your Web browser.
– The Java Web Start software is the reference implementation for the
Java Network Launching Protocol (JNLP)
– http://java.sun.com/products/javawebstart/docs/developersguide.html

Java Web Start
• What do you need?
– Jar files that contain class files & resources.
– A jnlp file for the application
– A link from the Web page to the JNLP file
– Configure the Web server so that the .jnlp file extension invokes Web Start
.(http://java.sun.com/products/javawebstart/docs/developersguide.html#web
site)
• Client side:
– Install Java Web Start, included in Download J2SE 5.0 JRE/SDK (jdk1.5.1)

Java Web Start
• Example 1 (javaWebStartExamples.zip):
– NotHelloWorld.jar generated from
NotHelloWorld.java
– NotHelloWorld.jnlp
See next page
– index.html
<a href="http://www.cs.ust.hk/~lzhang/teach/java03/webStart/NotHelloWorld.jnlp">
NotHelloWorld Application</a>

Java Web Start
– NotHelloWorld.jnlp
<?xml version="1.0" encoding="utf-8"?>
<jnlp
codebase="http://www.cs.ust.hk/~lzhang/teach/java03/webStart"
href="NotHelloWorld.jnlp">
<information>
<title>NotHelloWorld Application</title>
<vendor>Sun Microsystems, Inc.</vendor>
<homepage href="docs/help.html"/>
<description>NotHelloWorld Application</description>
<description kind="short">A demo of nothing useful</description>
<offline-allowed/>
</information>
<resources>
<j2se version="1.3"/>
<jar href="NotHelloWorld.jar"/>
</resources>
<application-desc main-class="NotHelloWorld"/>
</jnlp>

Java Web Start
• Unlike applets, web-start applications have a main() like normal Java
applications. There are a few special requirements:
• The application must be contained in a jar file
• By default restricted to Sandbox as Applets (cannot call standard IO libaries to
access the disk, you can only connect back the source host etc).
• Resources (files, images) must also be in a jar file and must be accessed using the
getResource() method.
• Like applets users can grant more access if they trust your code
• A JNLP API is required for some applications.

Java Web Start
• Web-start applications differ from applets in several ways:
– They are stored in the local disk so do not need to be downloaded each time.
– They can call System.exit().
– They do not have the same lifecycle.
– A web-start application can use a special class library which allows the
application to prompt users to approve reading and writing to/from the local
disk.
– Rather than HMTL tags in a web-page, XML (JNLP) is used to describe web-
start applications.

Java Web Start
• Example 2: ImageTest
– ImageTest.java
• Loading image using the getResource method
– ImageTest.jar
• Includes class files & image files
– ImageTest.jnlp
– Index.html
<a href="http://www.cs.ust.hk/~lzhang/teach/java03/webStart/ImageTest.jnlp">
ImageTest Application</a>

java applets

Recommended

Recommended

More Related Content

What's hot

What's hot (20)

Viewers also liked

Viewers also liked (20)

Similar to java applets

Similar to java applets (20)

More from Waheed Warraich

More from Waheed Warraich (10)

Recently uploaded

Recently uploaded (20)

java applets