About

An extensible application is one that you can extend without modifying its original code base.

By designing an extensible application, you provide a way to upgrade or enhance specific parts of a product without changing the core code.

See also:

• Software Design - Extensibility (Modularity without compilation)
• Application - Service Provider Framework

It's what characterized a famework (versus a library)

Implementation

An extensible application implements a mechanism of extensions with:

• extension points,
• plug-ins
• or modules

that allow to extend it without modifying its base code.

A extension point declares a contract:

• with the help of XML markup
• or interfaces)

that extensions must conform to.

Plug-ins that want to connect to that extension point must implement that contract in their extension.

By adding a the plugin code implementing this extension points new functionality are added.