Applets

What is an Applet?  According to Sun “An applet is a small program that is intended not to be run on its own, but rather to be embedded inside another application.”

Applet  An applet is a java program that executes using an appletviewer or a java enabled web browser (such as Internet Explorer).  The appletviewer is a utility for applets that is included with the Java SDK.  The appletviewer or web browser only executes an applet when a HTML document containing the applet is opened in the appletviewer or web browser.

Hello World Applet import java.applet.*; import java.awt.*; public class AppletExample extends Applet { public void paint(Graphics g) { g.drawString("Hello World", 50, 25); }//end paint }

Applet  We compile the AppletExample applet using the javac command.  Now we need to create a HTML page file that will include the applet

Basic Applet start  All applets are started via a HTML file </APPLET>  This identifies the name of the main class and the portion of the browser window that should be made available for the applet.

HTML Java Applet This is a HTML file with an embeded applet

Running the Applet  There are two ways of executing the Java applet: 1.Simply double click on the html file which contains the applet. 2.Within the DOS console, use the appletviewer command to execute the applet. E.g. appletviewer Example1.html

OUTPUT

What can an Applet do?  An applet can: –Draw pictures on a web page –Create a new window and draw in it –Play sounds  Multimedia, jpegs’s etc. –Receive input from the user through the keyboard or the mouse –Make a network connection to the server from which it came and can send to and receive arbitrary data from that server.

Applet Security  Main concerns with applets is that the browser downloads code across the network, possibly from an unknown source and executes on the client’s system.  Breeding ground for viruses  As a security measure, the class loader in the Java VM checks the bytecode in the Java class file to make sure they haven’t been tampered with since compilation

Applet Security: What an applet cannot do  Write data on any of the host’s disks.  Read any data from the hosts disks without the users permission. In some environments, notably Netscape, an applet cannot read data from the users disks even with permission.  Delete files.  Read from or write to arbitrary blocks of memory, even on a non-memory protected operating system like the Mac OS. All memory access is strictly controlled.

Applet Security: What an applet cannot do  Java enabled browsers place security features on applets.  IE does not allow applets to access files on the local disk or communicate with systems on the network apart from the web server that provides the HTML page that contains the applet.

Applet Security: What an applet cannot do  An applet provider can sign a Java applet with a digital signature to verify that it comes from a trusted source. If an applet is downloaded by an enabled browser, the client has to decide whether he/she trusts the applet provider. If so, the browser may grant extended powers to the applet.

An applet cannot (continued)  Introduce a virus or trojan horse into the host system.  An applet is not supposed to be able to crash a host system.

Applets vs Applications

GUI Applets  Applets are capable of containing swing GUI components.  It is possible to convert a previously constructed Java application into a Java applet.

Running an Applet class  The applet class is loaded into the VM  The VM creates an object from this class  The VM calls the following methods in the following sequence: –init –start –paint

Entry point methods  init – Executed ONLY at the creation of the applet object. Generally used to setup user interface, variables, etc. (like a constructor)  start - executed each time an appletviewer window is selected as well as at the creation of the object  paint (Graphics) – Used to draw lines/ shapes on the appletviewer window. Can be called at any point by calling ‘repaint()’ which wipes the screen and executes paint again. paint is supplied an object of type graphics which is contains the screen. paint can use methods in this object to draw on the screen.

Converting an Application into an Applet  Remember that an init method of an applet is similar to the main method in an application.  Applications normally are derived from JFrame, whereas Applets are derived from JApplet.  Size and Visibility of the JFrame in an application is set within the application via the setSize and setVisible methods. The size of an Applet should be set within the HTML document  Applets must be declared public.

Example import Java.awt.*; import Java.awt.Window.*; import Java.awt.event.*; import javax.swing.*; class MyApplication extends JFrame { JButton north = new JButton ("North"); JButton south = new JButton ("South"); JButton east = new JButton ("East"); JButton west = new JButton ("West"); JTextArea textArea = new JTextArea(); MyApplication() { setUpGUI(); } public static void main(String[] args) { MyApplication m = new MyApplication(); } void setUpGUI() { Container c = getContentPane(); c.setLayout (new BorderLayout()); c.add (BorderLayout.NORTH, north); c.add (BorderLayout.SOUTH, south); c.add (BorderLayout.EAST, East); c.add (BorderLayout.WEST, West); c.add(BorderLayout.CENTER "Center", new JScrollPane(textArea)); setSize (200,200); setVisible(true); }

Example:Applet /*HTML code necessary to start the Applet <APPLET CODE = "MyJApplet.class" WIDTH= "200" HEIGHT="200"> */

Applet public class MyJApplet extends JApplet { JButton north = new JButton ("North"); JButton south = new JButton ("South"); JButton east = new JButton ("East"); JButton west = new JButton ("West"); JTextArea textArea = new JTextArea(); public void init() { setUpGUI(); } void setUpGUI() { Container c = getContentPane(); c.setLayout (new BorderLayout()); c.add (BorderLayout.NORTH, north); c.add (BorderLayout.SOUTH, south); c.add (BorderLayout.EAST, East); c.add (BorderLayout.WEST, West); c.add(BorderLayout.CENTER "Center", new JScrollPane(textArea)); setSize (200,200); setVisible(true); }