COMP201 Java Programming Part II: GUI Programming

COMP201 Java Programming

Part II: GUI Programming

Topic 11: Applets

Chapter 10

COMP201 Topic 11 / Slide 2

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

http://liftoff.msfc.nasa.gov/realtime/jtrack/3d/JTrack3d.html


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


An Example


Compare with application: NotHelloWorld.javaclass NotHelloWorldFrame extends JFrame{ public NotHelloWorldFrame() { setTitle("NotHelloWorld"); setSize(300, 200);

Container contentPane = getContentPane();

JLabel label = new JLabel("Not a Hello, World application", SwingConstants.CENTER);

contentPane.add(label);

}}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.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

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.)

class NotHelloWorldFrame extends JFrame{ public NotHelloWorldFrame() { setTitle("NotHelloWorld"); setSize(300, 200); Container contentPane = getContentPane(); JLabel label = new JLabel("Not a Hello, World applet", SwingConstants.CENTER); contentPane.add(label); }}

public class NotHelloWorld{ public static void main(String[ ] args) { NotHelloWorldFrame frame = new NotHelloWorldFrame(); frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); 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

class NotHelloWorldFrame extends JFrame{ public NotHelloWorldFrame() { setTitle("NotHelloWorld"); setSize(300, 200); Container contentPane = getContentPane(); JLabel label = new JLabel("Not a Hello, World applet", SwingConstants.CENTER); contentPane.add(label); }}

public class NotHelloWorld{ public static void main(String[ ] args) { NotHelloWorldFrame frame = new NotHelloWorldFrame(); frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); frame.show(); }}



Outline An example





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 *.gifpack 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 filejar umf mainclass.mf MyJarFile.jar

Run: java -jar MyJarFile.jar Or click on file icon

Self-Running Calculator


Outline An example





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( )


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.

Applet Life Cycle


Outline

An example





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




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} // precautionelse 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


Outline

An example




Applet


User


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 AppletContextshowDocument( 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#website)

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.jnlpSee 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>

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COMP201 Java Programming Part II: GUI Programming