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

Request processing


As we saw in the last chapter, the context's basic valve first notifies any ServletRequestListener instances by invoking their requestInitialized() method, and then calls invoke() on the servlet wrapper's pipeline.

A wrapper's pipeline, by default, only has a single valve, the StandardWrapperValve. It calls its wrapper's allocate() method to obtain an instance of its wrapped servlet to process the incoming request.

If this is a normal servlet (that is, it does not implement the deprecated SingleThreadModel interface), then there will always be only one instance of a given servlet per wrapper, and it is this instance that is returned to service every incoming request for this servlet.

While servlets that are marked to be loaded on startup are instantiated and initialized by the context within its start() method, the rest are lazily initialized on first use.

The wrapper is requested to allocate a servlet instance to handle this request. If an instance has not yet been allocated...