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Take control of the servlet environment, Part 1

Invisibly extend the functionality of the servlet API

By Thomas E. Davis and Craig Walker, JavaWorld.com, 11/22/00

The ubiquitous servlet flourishes in all areas of Web development. Authors devote entire books to servlets. Almost every major Web server and application server supports them, if not directly, then through a plethora of third-party plug-ins. An integral part of the Java 2 Platform, Enterprise Edition (J2EE) framework, they provide the foundation for JavaServer Pages. If you are a Java fanatic, and your project has a Web interface, you use servlets.

Servlets are a simple concept, and the Java Servlet API reflects that. As Figure 1 illustrates, a Request object encapsulates all the data that the client passes to the server; a Response object encapsulates all the data that the server passes to the client. And a Session object stores any stateful data that needs to persist between the handshakes. That's all there is to a servlet ... almost.

Figure 1. The flow of information in a servlet

Sun's Java Servlet API does an excellent job of hiding all the dirty work from the programmer, leaving the servlet engine to manage it instead. However, the programmer occasionally needs to peek behind the curtain and twiddle the knobs. Sometimes the programmer wants to override the engine's functionality. This series will cover these issues. Over the next few months, we will explain how you can take control of your servlet environment. We will also exploit that control and demonstrate some useful real-world tricks that can increase the performance and scalability of your projects.

TEXTBOX: TEXTBOX_HEAD: Take control of the servlet environment: Read the whole series!

• Part 1: Invisibly extend the functionality of the servlet API

• Part 2: Alternatives to servlet session management

• Part 3: Beware of the cookie monster
• :END_TEXTBOX
  
  

Most servlet engines store the user's session data in memory. This prevents you from implementing a truly load-balanced farm of Web servers because each visitor must be sticky to the server that assigned him or her a session. Once a load-balancing mechanism directs a visitor to a specific server, it keeps sending that visitor back to the same server; thus, the visitor becomes stuck to a single server, rather than being rotated or balanced across all servers. To solve this problem, do you need to migrate to another servlet engine vendor or purchase a third-party solution? Absolutely not. We'll explain why in this series.

Have you ever run into the cookie subdomain bug? For example, if you access a cookie-writing page at http://drum.rudiment.net/, then hit the same page using the address http://www.drum.rudiment.net/, you will have two cookies with the exact same name. When you return to http://drum.rudiment.net/ and the page requests the cookie that it originally wrote, it's a toss-up as to which version the engine will return. We will crack that problem as well in this series of articles.

In Part 1, we will introduce the foundation for inserting an invisible layer of logic between the servlet engine and your servlet code. This layer will use simple and common design patterns and open the doors to many clever and powerful enhancements.

 

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