Applet Architecture - Introducing Java Applets

Applet Architecture - Introducing Java Applets

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  Applet Architecture AmitKumar Saha B.Tech. (3 rd year, Computer Science & Engineering ) College Roll No. 04/CS/20 Department of Computer Science & Informatics Haldia Institute of Technology Haldia Presenting Java Applets
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  What are Applets?” Applets are small applications that are accessed on an Internet Server, transported over the Internet, automatically installed,and run as part of a Web document” , ( Schildt , Herbert ”Java 2: The complete Reference,2001 )‏ An applet is a program written in the Java programming language that can be included in an HTML page, much in the same way an image is included in a page. When you use a Java technology-enabled browser to view a page that contains an applet, the applet's code is transferred to your system and executed by the browser's Java Virtual Machine (JVM) ( http://java.sun.com/applets/ )‏
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  Functional Diagram ofApplets
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  Applet Architecture Eventdriven An applet waits until an event occurs. The AWT notifies the applet about an event by calling event handler that has been provided by the applet.The applet takes appropriate action and then quickly return control to AWT All Swing components descend from the AWT Container class User initiates interaction with an Applet ( and not the other way around )
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  The Applet APIThe API is provided by the javax.swing.JApplet class and the java.applet.AppletContext interface. ( Java SE 6 ) Applets can use these APIs to do the following: Be notified by the browser of milestones. Load data files specified relative to the URL of the applet or the page in which it is running. Make the browser display a document. Find other applets running in the same page. Play sounds.
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  Life cycle ofan Applet init : This method is intended for whatever initialization is needed for your applet. It is called after the param attributes of the applet tag. start : This method is automatically called after init method. It is also called whenever user returns to the page containing the applet after visiting other pages. stop : This method is automatically called whenever the user moves away from the page containing applets. You can use this method to stop an animation. destroy : This method is only called when the browser shuts down normally Thus, the applet can be initialized once and only once, started and stopped one or more times in its life, and destroyed once and only once.
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  Methods for MilestonesFollowing is the interface for these methods: public class Simple extends JApplet { . . . public void init() { . . . } public void start() { . . . } public void stop() { . . . } public void destroy() { . . . } . . . } Not every applet needs to override every one of these methods. Some very simple applets override none of them.
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  Applet Skeleton publicclass Simple extends JApplet { . . . public void init() { . . . } public void start() { . . . } public void stop() { . . . } public void destroy() { . . . } public void paint (Graphics g){.. } . . . } In some situations the Applet may override the method update( ) Applets do not need a main( ) method Applets must be run under an applet viewer or a Java-compatible browser User I/O is not accomplished with Java’s stream I/O classes. Instead, applets use the interface provided by the AWT
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  Program Execution Flow
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  Handling Events Appletsinherit a group of event-handling methods from the Container class The Container class defines several methods, such as processKeyEvent and processMouseEvent , for handling particular types of events, and then one catch-all method called processEvent To react to an event, an applet must override the appropriate event-specific method
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  UI Components Swingsupplies the following UI components (the class that implements each component is listed in parentheses): Buttons (javax.swing.JButton ) Checkboxes ( javax.swing.JCheckBox ) Single-line text fields ( javax.swing.JTextField ) Larger text display and editing areas ( javax.swing.JTextArea ) Labels ( javax.swing.JLabel ) Lists ( javax.swing.JList ) Pop-ups ( javax.swing.Popup ) Scrollbars ( javax.swing.JScrollBar ) Sliders ( javax.swing.JSlider ) Drawing areas ( java.awt.Canvas ) Menus ( javax.swing.JMenu,javax.swing.JMenuBar javax.swing.JMenuItem, javax.swing.JCheckBoxMenuItem ) Containers ( javax.swing.JPanel, javax.swing.JWindow and its subclasses)
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  Methods for UIComponents add Adds the specified Component remove Removes the specified Component setLayout Sets the layout manager
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  Deploying Applets DeployingApplets on the Internet Versus an Intranet Use the < applet > tag if the Web page is accessed through the Internet, or if it is accessed through an Intranet in which people use different browsers. Use the < object > or < embed > tag if the Web page is accessed through an Intranet and you know which browser people use Deploying Applets for Specific Browsers For Internet Explorer only, use the < object > tag. For the Mozilla family of browsers only, use the < embed > tag
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  Practical Considerations SecurityRestrictions Applets cannot load libraries or define native methods. An applet cannot start any program on the host that is executing it. An applet cannot make network connections except to the host that it came from Threads in Applets Applets are generally multi-threaded – one thread builds up the UI while the other fetches data or does some background processing Working with a server-side application
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  … .recap Whatare Applets? Architecture of Applets Applets API Deploying Applets Applets & Security
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  References “ Java2 : The Complete Reference ”, Schildt, Herbert, TMH, 2001 http://java.sun.com
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  Thank You