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Introduction

Overview

Welcome to

How Tomcat Works

. This book dissects Tomcat 4.1.12 and 5.0.18 and

explains the internal workings of its free, open source, and most popular servlet

container code-named Catalina. Tomcat is a complex system, consisting of many

different components. Those who want to learn how Tomcat works often do know

where to start. What this book does is provide the big picture and then build a

simpler version of each component to make understanding that component easier.

Only after that will the real component be explained.

You should start by reading this Introduction as it explains the structure of the book

and gives you the brief outline of the applications built. The section "Preparing the

Prerequisite Software" gives you instructions on what software you need to download,

how to make a directory structure for your code, etc.

Who This Book Is for

This book is for anyone working with the Java technology.

 This book is for you if you are a servlet/JSP programmer or a Tomcat user

and you are interested in knowing how a servlet container works.

 It is for you if you want to join the Tomcat development team because you

need to first learn how the existing code works.

 If you have never been involved in web development but you have interest in

software development in general, then you can learn from this book how a

large application such as Tomcat was designed and developed.

 If you need to configure and customize Tomcat, you should read this book.

To understand the discussion in this book, you need to understand object-oriented

programming in Java as well as servlet programming. If you are not familiar with the

latter, there are a plethora of books on sendets, including Budi's

Java for the Web

with Servlets, JSP, and EJB

. To make the material easier to understand, each

chapter starts with background information that will be required to understand the

topic in discussion.

How A Servlet Container Works

A servlet container is a complex system. However, basically there are three things

that a servlet container does to service a request for a servlet:

 Creating a request object and populate it with information that may be used

by the invoked servlet, such as parameters, headers, cookies, query string,

URI, etc. A request object is an instance of the javax.servlet.ServletRequest

interface or the javax.servlet.http.ServletRequest interface.

 Creating a response object that the invoked servlet uses to send the

response to the web client. A response object is an instance of the

javax.servlet.ServletResponse interface or the

javax.servlet.http.ServletResponse interface.

 Invoking the service method of the servlet, passing the request and response

objects. Here the servlet reads the values from the request object and writes

to the response object.

As you read the chapters, you will find detailed discussions of Catalina servlet

container.

Catalina Block Diagram

Catalina is a very sophisticated piece of software, which was elegantly designed and

developed. It is also modular too. Based on the tasks mentioned in the section "How

A Servlet Container Works", you can view Catalina as consisting of two main

modules: the connector and the container.

The block diagram in Figure I.1 is, of course, simplistic. Later in the following

chapters you will unveil all smaller components one by one.

Figure I.1: Catalina's main modules

Now, back to Figure I.1, the connector is there to

connect

a request with the

container. Its job is to construct a request object and a response object for each

HTTP request it receives. It then passes processing to the container. The container

receives the request and response objects from the connector and is responsible for

invoking the servlet's service method.

Bear in mind though, that the description above is only the tip of the iceberg. There

are a lot of things that a container does. For example, before it can invoke a servlet's

service method, it must load the servlet, authenticate the user (if required), update

the session for that user, etc. It's not surprising then that a container uses many

Chapter 3 presents a simplified version of Tomcat 4's default connector. The

application built in this chapter serves as a learning tool to understand the connector

discussed in Chapter 4.

Chapter 4 presents Tomcat 4's default connector. This connector has been

deprecated in favor of a faster connector called Coyote. Nevertheless, the default

connector is simpler and easier to understand.

Chapter 5 discusses the container module. A container is represented by the

org.apache.catalina.Container interface and there are four types of containers:

engine, host, context, and wrapper. This chapter offers two applications that work

with contexts and wrappers.

Chapter 6 explains the Lifecycle interface. This interface defines the lifecycle of a

Catalina component and provides an elegant way of notifying other components of

events that occur in that component. In addition, the Lifecycle interface provides an

elegant mechanism for starting and stopping all the components in Catalina by one

single start/stop.

Chapter 7 covers loggers, which are components used for recording error messages

and other messages.

Chapter 8 explains about loaders. A loader is an important Catalina module

responsible for loading servlet and other classes that a web application uses. This

chapter also shows how application reloading is achieved.

Chapter 9 discusses the manager, the component that manages sessions in session

management. It explains the various types of managers and how a manager can

persist session objects into a store. At the end of the chapter, you will learn how to

build an application that uses a StandardManager instance to run a servlet that uses

session objects to store values.

Chapter 10 covers web application security constraints for restricting access to

certain contents. You will learn entities related to security such as principals, roles,

authenticator valve in the StandardContext object and uses basic authentication to

authenticate users.

Chapter 11 explains in detail the org.apache.catalina.core.StandardWrapper class

that represents a servlet in a web application. In particular, this chapter explains how

filters and a servlet's service method are invoked. The application accompanying this

chapter uses StandardWrapper instances to represents servlets.

Chapter 12 covers the org.apache.catalina.core.StandardContext class that

represents a web application. In particular this chapter discusses how a

StandardContext object is configured, what happens in it for each incoming HTTP

request, how it supports automatic reloading, and how Tomcat 5 shares a thread that

executes periodic tasks in its associated components.

Chapter 13 presents the two other containers: host and engine. You can also find the

standard implementation of these two containers:

org.apache.catalina.core.StandardHost and

org.apache.catalina.core.StandardEngine.

Chapter 14 offers the server and service components. A server provides an elegant

start and stop mechanism for the whole servlet container, a service serves as a

holder for a container and one or more connectors. The application accompanying

this chapter shows how to use a server and a service.

Chapters 15 explains the configuration of a web application through Digester, an

exciting open source project from the Apache Software Foundation. For those not

initiated, this chapter presents a section that gently introduces the digester library

and how to use it to convert the nodes in an XML document to Java objects. It then

explains the ContextConfig object that configures a StandardContext instance.

Chapter 16 explains the shutdown hook that Tomcat uses to always get a chance to

do clean-up regardless how the user stops it (i.e. either appropriately by sending a

shutdown command or inappropriately by simply closing the console.)

Chapter 17 discusses the starting and stopping of Tomcat through the use of batch

files and shell scripts.

Chapter 18 presents the deployer, the component responsible for deploying and

installing web applications.

Chapter 19 discusses a special interface, ContainerServlet, to give a servlet access

to the Catalina internal objects. In particular, it discusses the Manager application

that you can use to manage deployed applications.

Chapter 20 discusses JMX and how Tomcat make its internal objects manageable by

creating MBeans for those objects.

The Application for Each Chapter

Each chapter comes with one or more applications that focus on a specific

component in Catalina. Normally you'll find the simplified version of the component

being explained or code that explains how to use a Catalina component. All classes

and interfaces in the chapters' applications reside in the ex[

chapter

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