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Introduction to Java Applets and Distributed Computing

Learn about Java applets, their execution process, security concerns, and HTML tags for embedding them in web pages. Understand distributed computing concepts.

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Introduction to Java Applets and Distributed Computing

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  1. Java applets M. L. Liu Distributed Computing, M. L. Liu

  2. Introduction Java applets are one of three kinds of Java programs: • An application is a standalone program that can be invoked from the command line. • An applet is a program that runs in the context of a browser session. • A servlet is a program that is invoked on demand on a server program and that runs in the context of a web server process. Distributed Computing, M. L. Liu

  3. Applets, web page, client, server • Applets are programs stored on a web server, similar to web pages. • When an applet is referred to in a web page that has been fetched and processed by a browser, the browser generates a request to fetch (or download) the applet program, then executes the program in the browser’s execution context, on the client host. Distributed Computing, M. L. Liu

  4. Applet Execution - 1 • An applet program is a written as a subclass of the java.Applet class or the javax.swing.Japplet class. There is no main method: you must override the start method. Applet objects uses AWT for graphics. JApplet uses SWING. • It is a Grapics object that runs in a Thread object, so every applet can perform graphics, and runs in parallel to the browser process. Distributed Computing, M. L. Liu

  5. Applet Execution • When the applet is loaded, these methods are automatically invoked in order: • the init( ) method is invoked by the Java Virtual Machine. • The start( ) method • The paint( ) method. • The applet is now running and rendered on the web page. • You program the start( ) method and the paint( ) method for your application, and invoke a repaint call to re-render the graphics, if necessary. • At the end of the execution, the stop( ) method is invoked, followed by the destry( ) method to deallocate the applet’s resources. Distributed Computing, M. L. Liu

  6. Applet Securityhttp://java.sun.com/docs/books/tutorial/applet/overview/ For security reasons, applets that are loaded over the network have several restrictions. • an applet cannot ordinarily read or write files on the computer that it's executing on. • an applet cannot make network connections except to the host that it came from. Distributed Computing, M. L. Liu

  7. HTML tags for applets - 1 <APPLET specifies the beginning of the HTML applet code CODE="demoxx.class" is the actual name of the applet (usually a 'class' file) CODEBASE="demos/" is the location of the applet (relative as here, or a full URL) NAME="smily" the name you want to give to this instance of the applet on this page WIDTH="100" the physical width of the applet on your page HEIGHT="50"  the physical height of the applet on your page ALIGN="Top" where to align the applet within its page space (top, bottom, center) Distributed Computing, M. L. Liu

  8. HTML tags for applets - 2 <PARAM  specifies a parameter that can be passed to the applet NAME=“name1" the name known internally by the applet in order to receive this parameter VALUE="000000" the value you want to pass for this parameter > end of this parameter <PARAM  specifies a parameter that can be passed to the applet (applet specific) NAME=“name2" the name known internally by the applet in order to receive this parameter VALUE="ffffff" the value you want to pass for this parameter > end of this parameter </APPLET> specifies the end of the HTML applet code Distributed Computing, M. L. Liu

  9. <HTML> <BODY> <APPLET code=hello.class width=900 height=300> </APPLET> </BODY> </HTML> // applet to display a message in a window import java.awt.*; import java.applet.*; public class hello extends Applet{ public void init( ){ setBackground(Color.yellow); } public void paint(Graphics g){ final int FONT_SIZE = 42; Font font = new Font("Serif", Font.BOLD, FONT_SIZE); // set font, and color and display message on // the screen at position 250,150 g.setFont(font); g.setColor(Color.blue); // The message in the next line is the one you will see g.drawString("Hello, world!",250,150); } } The HelloWorld Applet Distributed Computing, M. L. Liu

  10. Advanced Applets • You can use threads in an applet. • You can make socket calls in an applet, subject to the security constraints. Distributed Computing, M. L. Liu

  11. Proxy server A proxy server can be used to circumvent the security constraints. Distributed Computing, M. L. Liu

  12. Summary • An applet is a Java class • Its code is downloaded from a web server • It is run in the browser’s environment on the client host • It is invoked by a browser when it scans a web page and encounters a class specified with the APPLET tag • For security reason, the execution of an applet is normally subject to restrictions: • applets cannot access files in the file system on the client host • Applets cannot make network connection exception to the server host from which it originated Distributed Computing, M. L. Liu

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