Applets

Applets
in Java

Prof. David Bernstein
James Madison University

Computer Science Department

bernstdh@jmu.edu

What Are They?

Program (in an Object-Oriented Language):
- A group of cooperating classes with a well-defined entry point (i.e., a method that should be executed first)and, perhaps, a re-entry point and/or an exit point.
Applet:
- Runs inside of a client (e.g., a browser)
- Entry point is init() and then start()
- Top-level container is a JApplet (or Applet)
- Is often delivered using the Internet

Applets in Java

The Applet Lifecycle in Java

When an HTML page containing an <applet> element is loaded for the first time, the appropriate object (i.e., the descendent of the Applet class referred to in the <applet> element is constructed, and its init() and start() methods are called in a thread other than the event dispatch thread.
Each time the user leaves the page containing the applet, the stop() method is called (again, not in the event dispatch thread).
Similarly, each time the user re-loads the page containing the applet, the start() method is called.
When the browser is shut down, the destroy() method is called (again, not in the event dispatch thread).

Embedding Applets in WWW Pages

An Applet is embedded in a WWW page using the <applet> tag in HTML
The name of the Applet is passed to the browser using the code attribute
The CLASSPATH is passed to the browser using the codebase attribute
To pass parameters to an Applet you use the <param> tag in HTML. Each <param> tag has two attributes, a name and a value. These name-value pairs can be retrieved in the Applet using the getParameter() method.

Typical Limitations and Restrictions (of Unsigned Applets)

An Applet can not run a local executable
An Applet can not communicate with any machine other that the originating host
An Applet can not read or write files
An Applet can not obtain information about the local machine other than the version of Java that is being used, the name and version of the operating system, and information about special character usage