Applet (1)

Applet (1)

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  APPLET By M. Sri Nandhini
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  Introduction  Applet issmall java program that can be easily transported over the network from one computer to other.  Used in internet applications.  Embedded in an html page, can be downloaded from the server and run on the client, so as to do a specific kind of job.  An applet is a program that runs in the context of a browser session.
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  Applets, Webpage, 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 the applet program, the executes the program in the browser’s execution context on the client host.
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  Types of javaprograms  Stand Alone Program  Run on a single machine  Compiler-javac  Interpreter-java  Web Based Program  Compiler-javac  Interpreter-applet viewer or web browser  Must Subclass of Applet  Import java.awt
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  Application vs Applet JavaApplications • Stand-alone program • Runs independently • Has a main() method • No HTML file • Run using JDK’s java interpreter Java Applet • Embedded program • Runs in a Web browser • No main() method • Requires an HTML file • Run using JDK’s applet viewer.
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  Applet Class
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  Applet Example import java.applet.*; importjava.awt.*; public class FirstApplet extends applet{ public void paint(Graphics g) { g.drawString(“Hello World”,10,50); } }
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  Output
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  Applet Life Cycle An applet may move from one state to another dependent upon a set of default behaviours inherited in the form of methods from ‘Applet’ class.  These states are  Born  Running  Idle  Dead
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  Applet state diagram
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  Life Cycle ofApplet  Init()-  Creates the objects needed by the applet  Sets up initial values, load font and images or set up colors.  Called only once during the lifetime of an applet.  Start()-  Moves to this phase automatically after the initialization state.  If the applet is stopped or it goes to idle state start() method must be called in order to force the applet again to the running state.
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  Life Cycle ofApplet Paint()-  This method is called each time to draw and redraw the output of an applet. Stop()-  Idle state, once it is stopped from running.  Destroy()-  An applet goes to dead state when it is destroyed by invoking the destroy() of applet class.  It results in complete removal of applet from the memory.
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  Common Methods  drawString()- Member of graphics class used to output a string to an applet.  It is typically called from within the paint() or update() method.  Void drawString(String msg,int a, int b).  setBackground() & getBackground()-  Belongs to component class used to set and get the background color.  Void setBackground(Color anyColor)
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  Common Methods  Predefinedconstants for each color such as Color.red can be used.  setForeground()& getForeground()-  Set and gets the color of the text to be displayed on the foreground of the applet window.  Void setForeground(Color anyColor)  showStatus()-  Display any string In the status window of the browser.  Void showString(Stringtext).
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  Applet Execution  Anapplet 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 graphics object that runs in a thread object, so every applet can perform graphics, and runs in parallel to the browser process.
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  Applet Execution When theapplet 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.
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  Applet Security For securityreasons, applets that are loaded over the network have several restrictions.  An applet cannot ordinarily read or write files on the computer that its executing on.  An applet cannot make network  connections except to the host that it came from.
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  Summary  An appletis 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:
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   Applet cannotaccess files in the file system on the client host.  Applet cannot make network connection exception to the server host from which it originated.