g6dicp - lecture 27
DESCRIPTION
G6DICP - Lecture 27. Java Applets. Types of Java programs. Applications Standalone programs Applets Delivered over WWW to client (browser) Servlets Run on WWW server. Java Application. Local Computer. Bytecode (.class). Interpreter (java or jre). VM. Java Applet. Server. - PowerPoint PPT PresentationTRANSCRIPT
School of Computer Science & Information Technology
School of Computer Science & Information Technology
G6DICP - Lecture 27G6DICP - Lecture 27
Java AppletsJava Applets
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Types of Java programsTypes of Java programs
ApplicationsApplications Standalone programsStandalone programs
AppletsApplets Delivered over WWW to client (browser)Delivered over WWW to client (browser)
ServletsServlets Run on WWW serverRun on WWW server
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Bytecode(.class)
Java ApplicationJava Application
Interpreter(java or jre)
VM
Local Computer
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ServerBytecode(.class)
Java AppletJava Applet
Web Browser VM
Client
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Java ServletJava Servlet
Web Server(eg Apache with jserv)
VM
HTML
Bytecode(.class)
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Feasibility of AppletsFeasibility of Applets
Java has platform independent binariesJava has platform independent binaries
WWW is designed to be data neutralWWW is designed to be data neutral
Security is Security is thethe major issue major issue
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The HTML APPLET TagThe HTML APPLET Tag
<HTML> <HEAD> <TITLE>An Applet Test</TITLE> </HEAD>
<BODY> <H1>An Applet Test</H1> <APPLET CODE=”myApplet.class" WIDTH=360 HEIGHT=250> </APPLET> </BODY> </HTML>
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About AppletsAbout Applets
Applets are always graphicalApplets are always graphical Applets must be subclasses of the Applet class (Panel)Applets must be subclasses of the Applet class (Panel) The Applet container is drawn in the window of the web browser.The Applet container is drawn in the window of the web browser.
Class Applet contains the init() methodClass Applet contains the init() method Invoked automatically by a browser when a web page containing the Invoked automatically by a browser when a web page containing the
<APPLET> tag is loaded<APPLET> tag is loaded Thus main method is not needed(!) Thus main method is not needed(!)
Security issuesSecurity issues Signed Applets - full functionality of JavaSigned Applets - full functionality of Java Unsigned Applets - substantially restricted functionalityUnsigned Applets - substantially restricted functionality
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A Hello World AppletA Hello World Applet
import java.awt.*;import java.applet.*;
public class demo extends Applet{ public void init() { setSize(360,250); }
public void paint(Graphics g) { g.drawString("Hello World!",50,50); }}
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Typical Conversion of an Application to an AppletTypical Conversion of an Application to an Applet Make sure that all i/o goes through the AWT Make sure that all i/o goes through the AWT
interface.interface. eg use g.drawString() instead of System.out.println()eg use g.drawString() instead of System.out.println()
Remove any means for stopping the programRemove any means for stopping the program eg remove System.exit()eg remove System.exit()
Subclass Applet instead of FrameSubclass Applet instead of Frame Override the Frame.init() method, and put the Override the Frame.init() method, and put the
functionality of the applications constructor in functionality of the applications constructor in there.there.
Remove the main() method of the application.Remove the main() method of the application.
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Applet Security Restrictions (1)Applet Security Restrictions (1) Applied by web browser (ie do not usually apply Applied by web browser (ie do not usually apply
in appletviewer)in appletviewer)
Cannot use native codeCannot use native code
Cannot read Cannot read somesome system properties system properties
Cannot start other programsCannot start other programs
Applet windows are clearly labelled - this cannot Applet windows are clearly labelled - this cannot be disabled or removed.be disabled or removed.
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Applet Security Restrictions (2)Applet Security Restrictions (2) Cannot read or write files on the client.Cannot read or write files on the client.
No direct file access to the server (although No direct file access to the server (although network connections can be made).network connections can be made).
Network connections can only be made to the Network connections can only be made to the machine from which the applet is being served.machine from which the applet is being served.
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Version IssuesVersion Issues
Classes used must be availableClasses used must be available This can be a problem with SwingThis can be a problem with Swing
Can be serious compatibility problems between Can be serious compatibility problems between software developed with a newer JDK and on software developed with a newer JDK and on earlier browser VMearlier browser VM
Modern browsers usually have a “plugin” VMModern browsers usually have a “plugin” VM Many older ones do notMany older ones do not