Book Image

Tomcat 6 Developer's Guide

Book Image

Tomcat 6 Developer's Guide

Overview of this book

While Tomcat is one of the most popular servlet containers, its inner workings still remain a mystery to many developers. If you only have a superficial familiarity of how this container actually functions, much of its power remains untapped and underutilized. This book will provide you with all that you need to undertand how to effectively use Apache Tomcat. This book begins by providing detailed instructions on building a Tomcat distribution. The next few chapters introduce you to the conceptual underpinnings of web servers, the Java EE and servlet specifications, and the Tomcat container. Subsequent chapters address the key Tomcat components, taking care to provide you with the information needed to understand the internal workings of each component. Detailed examples let you walk through a Tomcat installation, stepping into key Tomcat components, as well as into your own custom servlets. During the course of the book you will encounter various structural components such as the Server and Service; containers such as the Engine, Host, Context, and Wrapper; and helpers such as the Loader, Manager, and Valve. You will also see how Tomcat implements the JNDI API to provide both a directory service for storage agnostic access to its resources, as well as a naming service that implements the Java EE Environment Naming Context. Along the way you will learn how various elements of the servlet 2.5 specification, as well as the HTTP RFCs are implemented by a servlet container. By the end of your journey, you will be able to count yourself as part of the elite minority of Java EE web developers who truly understand what goes on under the covers of a servlet container.
Table of Contents (17 chapters)
Tomcat 6 Developer's Guide
Credits
About the author
Acknowledgement
About the reviewers
Preface

Chapter 10. The Wrapper Component

We are finally at the workhorse of the Tomcat component hierarchy—the Wrapper. This unassuming component is what breathes life into your servlets, and consequently is a very critical component to a web developer.

In the default case, where Tomcat is not running in a clustered configuration, the container is expected to use only one servlet instance per<servlet> element in the web deployment descriptor. It is this single instance that is wrapped by a Wrapper component.

The servlet specification requires each servlet to implement the javax.servlet.Servlet interface, which defines the init(), service(), and destroy() life cycle methods. The primary role of a Wrapper component is to invoke these methods at appropriate times in the lifetime of a servlet instance.

When a servlet implements the SingleThreadModel interface, the Wrapper might actually manage a pool of servlet instances, while ensuring that only one thread is executing within a servlet instance...