This chapter describes Netscape Application Server extensions, which provide an important service in applications by allowing Java application components to access third-party solutions, legacy systems and shared services.

• What is a Netscape Application Server Extension?
• About the NAS Application Model
• Why Build an Extension?
• About Building a Netscape Application Server Extension

• Integrate Existing Functionality
• Integrate Third-Party Solutions
• Implement Shared Services

• Extensions are multi-threaded, whereas application components are single threaded. Therefore, multiple clients can access the same instance of an extension at the same time.
• Extensions are persistent, living in the server upon startup and until shutdown. This gives you the opportunity do expensive initiation operations at server startup instead of at application component initialization, when the client is waiting for a reply.
• Extensions provide a programmatic interface to clients, which are typically application components but could be other extensions. Application components rely on a ValList object, a list of value/key pairs, to pass information to clients. While great for HTML access, a ValList does not perform as well as a programmatic method call.
• Extensions have access to object pooling, an important value-added feature that is not easily accessible to application components. Object pooling provides shared use of expensive resources, such as connections, and is one of several Netscape Extension Builder runtime features that are pre-built for easy implementation in your extension. The Netscape Extension Builder runtime features are discussed in a later section.

1. Start by defining the name of the interfaces and the names of the methods and parameters exposed by each interface.
2. Define the names of the coclasses that will become the extension classes.
3. Specify the interface or interfaces that each coclass implements.