Listeners and Filters

This is a precis of topics covered in chapters 12 and 13 in Fields, Kolb and Bayern.

Listeners

• Listeners execute methods when key events occur
• Examples of events include the following:
  • Loading (i.e. initializing) a Web application
  • Starting a user session
  • Ending a user session
• Possible uses include the following:
  • Starting and terminating a database connection for a user session
  • Responding to user login and logout

Fields, Kolb and Bayern provide a database bean that uses a listener to create and terminate a database connection. Note that the valueBound and the valueUnbound methods are automatically called when the session is respectively started and terminated.

Filters

• Filters modify or transform content originating from a servlet or JSP
• A filter operates as a separate module
• Possible uses include:
  • Updating/modifying content without changing originating JSP
  • Reformatting content to appear in specific browsers
  • Transforming content into different formats (XML or WML)
  • Restricting content to those who are logged in