Applets 

Introduction

• An applet  is a Java program embedded within a Web page and run by a Web browser.
• An applet has a graphical user interface (GUI ).
• Java applet programming is event-driven programming.
• Applets are much like java applications with the following exceptions:
• You use the Applet or JApplet classes instead of the JFrame class to build your application.
• Like JFrame,  Applet and JApplet already have a content pane you may set its layout manager
• You don't have to setup the size of the applet in the java code, instead, this is done in the HTML code.
• Applets are run within a web page by an Applet Plugin
  
• Creating an Applet in Netbeans
  
• Create your project as normal
  
• Right click on the package and select New File/Folder
• Select Java Gui forms and JApplet Form as the file type. This will create a JApplet with the form designer enabled.
• (if you select Java class then JApplet under that, then you won't get the form designer capability).
• Edit  your JApplet like you would a JFrame.
  
• To test,  right click on the applet file in the files window and select "run file". This will start up the applet viewer and allow you to test your applet.
• To test with an html,  create a new html file with an applet tag, use archive = "..\dist\projectname.jar" code = "appletname.class".  Right click on the html file and select view
  

Due to security ordinary Applets can't:

• read or write files on the user's system (without special permission)
• communicate with an internet site other than the one they originated on
• run or load any programs in the user's system
• You can read files from the server where the applet resides and communicate with that server

Trusted Applets

• User must have a relatively new browser, and have Java JDK 1.3 or higher installed (JDK 1.6 is the current version)
• Trusted applets are allowed access to the user's hard drive.
• To be a "true" trusted applet you have to pay money to a certificate service and provide them with lots of information. You can however, create your own key certificate and attach that to your applet.  When a user accesses the applet, they will be presented with a pop-up box asking if they want to trust you.
  
• There are two steps to making your applet a trusted applet
• first generate a key certificate (from a cmd window)
  

• To read more about trusted applets, click here
• To read more about security and plugins, click here
• Netbeans has the ability to sign the applets for you.  To create a trusted  Applet in Netbeans:
• Create your project with the JApplet inside it.
• In Netbeans,  right click on the project name inside the "projects" window (upper left), select "properties" from the popup menu
  
• Under the Application item, open WebStart
• Turn on the "Enable Web Start"
  
• Turn on the button labeled "Applet descriptor" and choose your JApplet class from the list below the radion button
  
• Press the button labeled "Customize" under the "Signing" item
• Select the "Self signed by generated key" option (Netbeans will create the key certificate)
• Select "Enable Software Projections" for the Mixed Code item
• The next time you do a clean and build, a signed Jar will be created, also in the distr folder you will find a "bare bones" html file for testing (launch.html) and a jnlp file that when double clicked will launch your JAppet inside the applet tester.  (You don't have to turn on the signing to get the bare bones html and jnlp files)
• JNLP files are XML text files that tell your JRE how to run your applet from the desktop.  You'll probably need to edit the file to set the width and height to make it look right.  To run using the JNLP you need both the JNLP file and the JAR file.
  

Other programming notes:

• you can add any of the following methods to control interaction if needed:
• init() - is called when the browser creates the applet object. 
  
• start() - this is called whenever the applet begins (after the init) and whenever the user leaves the applet page, then returns to it in the browser
• stop()  - called whenever a user leaves the applet page
• destroy() - allows the applet to perform cleanup before it is released from memory
• paint() is called whenever the applet needs to repaint itself, this occurs if the browser is minimized then restored, during scrolling and any other time the applet becomes visible. NOTE: you only have to provide a paint method if you are doing graphics with a Canvas or Graphics2D object. Buttons, lists, etc repaint themselves. For JApplet, override the update method instead.
• If you rebuild your applet you may have to exit the browser then restart it to get the new version loaded. Pressing the reload button on the browser may not due the trick.

The Applet HTML Tag

• The Applet tag was officially deprecated (made obsolete) in HTML version 4, and isn't supported in HTML version 5. But many browsers still support it.  Instead, you should use a the java deployment toolkit script (see below).
  

<applet CODE     = "TestApplet.class"
ARCHIVE  = "TestApplet.jar"
WIDTH    = 200
HEIGHT   = 200
HSPACE   = 0
VSPACE   = 0 
ALT = "alternate text to show if the applet cannot be run by the browser"
ALIGN    = Middle>
<PARAM NAME = "p1" VALUE="p1" />
<PARAM NAME = "p2" VALUE ="p2" />
 </applet>

• The CODE parameter identifies the name of the Applet class
• The Archive parameter identifies the name of a JAR file that holds all the class files, gifs, and jpgs associated with the applet. This parameter is omitted if you do not create a jar file. Instead, be sure to have all the class files in the same directory as the one specified in the code parameter.
• The Width and Height specify how big a space on the web page to allocate for the applet. this is in pixels
• the HSPACE and VSPACE specify the amount of space around the applet to separate from text on the web page
• the Align positions the applet window on the web page. This works much the same way as aligning text and images on a web page.
• the text in the ALT parameter will be displayed if the applet cannot be started (plugin not installed or other error like a bad class name in the applet tag).
  
• between the <applet> and </applet> tags can come optional  parameter tags in the form of
  

 <param NAME = "parametername" value = "parametervalue" />

Deployment Toolkit Script:

• In the old Java Plug-in, applets always run with the latest version of JRE installed on a client machine. With the next generation Java Plug-in introduced in the Java SE 6 update 10 release, you can specify that the Java kernel (core set of Java classes absolutely required by the JRE) or normal online installer be automatically downloaded, if the specified minimum JRE version is not already installed on the client. If the specified minimum JRE version does not exist, the latest version of the JRE is downloaded from www.java.com. In the case of a Java kernel download, any additional packages may also be specified for download as required by the applet.

• Applets can be deployed via the applet tag. Parameters to configure deployment may be specified as attributes and parameters to the applet tag. For more information, see http://docs.oracle.com/javase/7/docs/technotes/guides/jweb/deployment_advice.html
  

• Use the runApplet() function in deployJava to ensure that a minimum Java Runtime Environment is available on a client machine before launching the applet.

<script src="http://www.java.com/js/deployJava.js"></script>
<script>
    var attributes = {codebase:applet url',
                      code:'Applet.class',
                      archive:'Applet.jar',
                      width:710, height:540} ;
    var parameters = {fontSize:16} ;
    var version = '1.6' ;
    deployJava.runApplet(attributes, parameters, version);
</script>

• The above code will launch the Java 2D applet on JRE version 1.6.0 or higher with one parameter (fontSize). The bolded items would be what you would customize for your applet.
• To trigger the installation of Java kernel and additional packages, add the following before the deployJava.runApplet() function:

• This gives :

<script src="http://www.java.com/js/deployJava.js"></script>
<script>
    var attributes = {codebase:applet url',
                      code:'Applet.class',
                      archive:'Applet.jar',
                      width:710, height:540} ;
    var parameters = {fontSize:16} ;
    var version = '1.6' ;
  
    deployJava.setInstallerType('kernel');
    // include any required packages as shown below
    deployJava.setAdditionalPackages('javax.swing, javax.xml');

    deployJava.runApplet(attributes, parameters, version);
</script>

Example:

JAR Files

• Creating JAR files
• A JAR file takes all the files needed for an applet and zips them together into one file. This makes managing the files on the web site easier and downloading by clients faster.
• Its good practice to package all your applet class files,  gif, jpgs, etc. in a JAR file, in Netbeans just select Run-Clean and Build from the menu.
  
• The Jar file will be created in the dist sub folder of your project
• This jar file will be the only file that needs to reside on the web server.
  

Calculator Applet

Click here for a zip file with the code

WebMenu Example

• This applet shows several new things: how  to pass parameters to an applet, how to put messages in the java console, and how an applet can interact with the browser.
  
• Passing parameters.
  
• first add <param ,,,> tags between your <applet> and </applet> tags in the HTML file, e.g.
  
•  name must be there and is referenced by your code when getting the value.  Value is a string passed to the  applet. 
  

• Inside your applet code, add code to the init() method, and in the init method add calls to getParameter to retrieve the data. If getParameter returns nulll, then the HTMLdid not have a parameter by that name. getParameter cannot be called in the constructor of the applet.

   public void init()
    {
     ....
      if(getParameter("Target") != null)
           target = getParameter("Window");       
    }

• Java Console
  
• Java Console is a window built into most browsers that display error messages from applets and allow a very limited amount of debugging of applets.
  
• In FireFox look under the Tools menu  for "Web developer"
• In Chrome press the little wrench, the look for the"Tools-Developer Tools" menu
• In Internet Explorer, look under the "Tools-Develop Tools" menu

• Loading Web pages.    For an applet to tell the browser to go to a different page you must first create a URL object, then use the showDocument method of the applet context.
  
     try{
            URL link = new URL("http://www.msnbc.com");
            getAppletContext().showDocument(link,target); // link is a string that has the URL
       }

         catch(MalformedURLException ex) {
              showStatus("Bad URL: http://www.msnbc.com");
     }
• In order to create a URL object you must import java.net.*;
  
  
• the second parameter on showDocument method is optional, if it is there then it specifies where to show the document (similar to the target parameter on an HTML a href tag). e.g.
  
       getAppletContext().showDocument(link,"_blank");  // show the document in a new browser window.
 
• "_self"     Show in the window and frame that contain the applet.
  
• "_parent"   Show in the applet's parent frame. If the applet's frame has no parent frame, acts the same as  "_self".
  
• "_top"   Show in the top-level frame of the applet's window. If the applet's frame is the top-level frame,   acts the same as "_self".
  
• "_blank"   Show in a new, unnamed top-level window.
  
•  name     Show in the frame or window named name. If a target named name does not already exist, a  new top-level window with the specified name is created, and the document is shown there.
  

Example: An Applet that Sketches

• This program provides basic sketching features for a web page.
  
• The toolbar has buttons that change the current color
  
• In the center area is a JPanel object with a white background that is used to drawn on.  The Graphics object is used to drawn lines.
  
  
• New things:
  
• MouseListener - The MouseListener events let Sketcher know when a button was pressed, when it was released, and when the mouse entered and exited the Sketcher's boundaries.
  
• MousePressed - mouse button went down
  
• MouseReleased - mouse button went up
  
  
• MouseMotionListener The MouseMotionListener events let Sketcher know when the mouse moved (with or without a button down).
  
• MouseMoved indicates the mouse is moving
  
• MouseDragged indicates the mouse is moving with a button pressed
  
  
• MouseEvent - this object is generated and sent to event handlers.  It contains methods to determine the x,y location of the mouse at the time of the event and the state of the mouse buttons.
  
• getX()  - returns X coordinate
  
• getY() - returns Y coordinate
  
• getClickCount() - returns number of clicks (used for processing double clicks)
  
• getButton() - tells which button caused the event
  
  
• Graphics class - Sketcher creates a graphics class whenever it needs to draw something on the screen. Graphics class provides many functions to draw shapes, lines, and text.  In this case I used the setColor and drawLine functions. You can draw on top of most objects. To do so you ask the component to getGraphics this returns a Graphics object, then you tell the object to perform graphics functions. In this case I am drawing on a JPanel. The Color object is used to access common colors or to create specific colors.  
  

      Graphics g = sketchPanel.getGraphics();       // create a graphics object
      g.setColor(currentColor);             // set its color
      g.drawLine(downX,downY, evt.getX(), evt.getY()); // tell it to draw a line

Slide Viewer