Session 4 Lecture Notes for First Course in Java
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Writing Java Applets (Day 7)

What is an applet?
http://java.sun.com/j2se/1.3/docs/api/java/applet/Applet.html
An applet is a small program that is intended not to be run on its own, but rather to be embedded inside another application.
The Applet class must be the superclass of any applet that is to be embedded in a Web page or viewed by the Java Applet Viewer. The Applet class provides a standard interface between applets and their environment.

Applets versus Applications

Back in 1995-96, web applets were all the rage, and did much to make Java famous.

Can a single Java program include both an application and an applet? Why or why not?

"Sandbox" security restrictions

• no access to the client's file system, clipboard, print queue, or executables

• no access to or from web sites except the site that provided the html page with the current applet

• no ability to create a window that "spoofs" the appearance of another application

The web browser enforces this restrictions with its SecurityManager object:
http://java.sun.com/j2se/1.3/docs/api/java/lang/SecurityManager.html

Exception: the Signed Applet

Java 1.1 added the concept of a trusted (or signed) applet. 
A trusted applet is an applet that the user has taken action to put in the category of "trusted" applet. A trusted applet can do things outside of the security sandbox.

http://java.sun.com/docs/books/tutorial/jar/sign/intro.html
[explains signing, verification, and certificates]

http://java.sun.com/j2se/1.3/docs/api/java/security/package-frame.html
[details of the java.security package]

Applets for Java 1 and Java 1.1 versus Applets for Java 2

Whereas an Java 1.1 applet extends the java.applet.Applet class, a Java 2 Swing applet extends a subclass of java.applet.Applet: javax.swing.JApplet.

http://java.sun.com/j2se/1.3/docs/api/javax/swing/JApplet.html
The JApplet class is an extended version of java.applet.Applet that adds support for the JFC/Swing component architecture.

Sun's Tutorial on the Swing extension (javax):
http://java.sun.com/docs/books/tutorial/uiswing/index.html
includes this visual presentation of the Swing components:
http://java.sun.com/docs/books/tutorial/uiswing/components/components.html

Sun's Tutorial on AWT:
http://java.sun.com/docs/books/tutorial/information/download.html#OLDui

Details of the Swing package:
http://java.sun.com/j2se/1.3/docs/api/javax/swing/package-frame.html

Details of the AWT package:
http://java.sun.com/j2se/1.3/docs/api/java/awt/package-frame.html

The applets in the Day 7 chapter of Teach Yourself Java are Swing applets. That is, they extend javax.swing.applet. However, these applets show how to make an <applet> tag, which won't work in most browsers because the browser's virtual machine will not have the javax.swing.applet class. 

The applet viewer that the Java 2 SDK provides does have the javax.swing.applet class and does allow you do to testing for Java 2 applets. However, if you are creating Swing applets for real world users, your users will need a browser with the Java plug-in.

Using the Plug-in (Java 2 Applets) in a Web browser

What browser runs what kind of applet?

AWT applets and the <APPLET> tag

http://java.sun.com/docs/books/tutorial/applet/appletsonly/html.html
<APPLET CODE=AppletSubclass.class WIDTH=anInt HEIGHT=anInt> </APPLET>

Swing applets and the <OBJECT> tag

http://java.sun.com/docs/books/tutorial/uiswing/components/applet.html#plugin
If your applet's users don't have a Swing-enabled browser, then they'll need to use Java Plug-in to run the applet. Java Plug-in requires that an applet be included in an HTML page with an <OBJECT> tag or <EMBED> tag. You can download a free tool that automatically generates the necessary tags from an <APPLET> tag.

HTML Converter

Look at this simple applet posted on this web page: 
http://java.sun.com/docs/books/tutorial/uiswing/start/HelloSwingApplet.html

If you right-click to view source, you will discover that the HTML is longer than the source code for the applet.

This tool edits a web page so that its applets run with the Java Plug-in.
http://java.sun.com/products/plugin/1.3/docs/htmlconv.html

Creating Applets

A basic template for an applet:

import java.awt.* // for 1.1 applets
import javax.swing.* // for Swing applets
public class myApplet extends javax.swing.JApplet  {
   // Applet code
   // public void init() {
      // setBackground(Color.black); // you might set the background color here
      // or use an HTML page to pass a parameter
   // public void start() { // restarts the applet if the browser returns to page
   }
   // public  void stop() { // you can stop threads the applet started
   }
   }
}

A screenshot of an applet that keeps time.

Code with color:

Line 27 calls the paint() method because this applet does not have any Swing components.

Line 30 takes advantage of calendar functionality imported from the java.util package.

Line 31 converts the the time of day into a string.

http://java.sun.com/j2se/1.3/docs/api/java/util/Calendar.html#getTime()

getTime

public final Date getTime()

Gets this Calendar's current time.

Returns:

the current time.

Line 36 calls the Graphics2D method drawString(), which takes three arguments, a string to render, and the x- and y-coordinates on which to render.

Line 38 specifies the updating of the day every 1000 milliseconds (every second).
Line 43 repaints the screen so that the new time appears.

Note: The book on page 171 says that the paint() method is called automatically whenever the applet window must be redrawn. 
The book might be referring to a Java 1.1 method of the Canvas class:
       http://java.sun.com/products/jdk/1.1/docs/api/java.awt.Canvas.html#paint(java.awt.Graphics)
but this would be for AWT rather than a Swing.

In your Swing applets, you call repaint() if you want to specify it is time to refresh the graphic.
Swing components already have a paint() implementation. 
Sun's Swing Tutorial explains the concept of painting: http://java.sun.com/docs/books/tutorial/uiswing/overview/draw.html

Posting Your Applet on the Web

To make your applet available to a web browser, you need:

• a web page that has HTML that refers to the applet's class file
• the applet's class file to be where the web page page says it is

Example HTML code for a Java 1.1 applet:

The same example as output of the HTML converter for the Java Plug-in, which creates and populates the <OBJECT> tag.

The <OBJECT> tag informs Netscape Navigator 4.0 and higher and Microsoft Internet Explorer 4.0 and higher to look for an object reference in the HTML. The reference can be to an applet class file, or to some other object a web browser might support, such as an Active X control. 

Note: The <APPLET> tag has a CODE attribute that indicates the full name of the applet.class file. If you applet class file is on another machine, use the CODEBASE attribute to give the complete URL.

JARs (Java archives)

Network latency to establish a connection from the web client to the web server is a significant portion of the "world wide wait". A Java archive (JAR) allows the client to perform a single download operation for an active that might include many files.

http://java.sun.com/products/jdk/1.1/docs/guide/jar/
JAR (Java Archive) is a platform-independent file format that aggregates many files into one. Multiple Java applets and their requisite components (.class files, images and sounds) can be bundled in a JAR file and subsequently downloaded to a browser in a single HTTP transaction, greatly improving the download speed. The JAR format also supports compression, which reduces the file size, further improving the download time. 

If you use a JAR, the applet tag in your HTML file must specify a value for the archive attribute so that the browser's VM can find the archive.

<applet code="MyClass.class" archive="MyClass.jar" ...

Parameter Passing

The HTML for our example applet (line 8) does pass a parameter for the background color.

The applet uses this parameter (line 15), which it parses with the getParameter() method in the applet's init() method.

You could also pass parameters for other values.

Homework Assignments for 15 April:  

1. p. 191, 2nd bullet
2. p. 221, 2nd bullet

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