Tomcat Configuration

A Very, Very, Very Brief Overview

Topics

Tomcat – Creating a new server.

– Adding servlets to the web.xml file.

– Initiating requests both with and without the translations inside the web.xml file.

– Added initialization parameters to the web.xml file.

– Starting and stopping Tomcat.

Servlet Engines

In order to execute a servlet, you must be running a servlet engine.– This is software that accepts and processes requests for

servlets.

– A servlet engine is usually a key component in an application server.

The application server, and hence the servlet engine, generally exists independently of the web server.– Application servers are tied to web servers via

connectors.

Web Connectors

When a web server receives a request for static content such as an HTML document, it can handle the request itself.

When a request for dynamic content, such as a servlet, is received, the web server must hand that request off to the application server for processing.

The software that coordinates this effort is called a web connector.

Jakarta - Tomcat

Tomcat is the servlet and JavaServer Pages engine from the Apache group.– http://jakarta.apache.org/tomcat

It’s small, installs quickly, and free.

For the purposes of this class, Tomcat is acting as both a web and application server.

Tomcat Root

The directory where you installed Tomcat is called the Tomcat Root. For instance:– c:\program files\apache group\tomcat-3.1

We’ll use the Tomcat Root as our point-of-reference for all Tomcat configuration stuff.

One installation of Tomcat can host multiple web applications. This is discussed on the next slide.

Document Root

Change to the webapps directory under the Tomcat Root. For example:– c:\program files\apache group\tomcat-3.1\webapps

Create a new subdirectory called the document root. For example:– SE491

A separate document root is created for each application we wish to host.

Directory Structure

Within your document root, create the following directory structure:– WEB-INF– WEB-INF/classes/– WEB-INF/lib/

WEB-INF is where Tomcat looks for elements relating to your application.

/classes is used for all of your servlets.

/lib is used for third-party APIs and classes.

Server Configuration

Once you have chosen a document root, it’s a good idea to make Tomcat aware of it.– In theory Tomcat can locate your new document root

implicitly, but you’re always better off controlling the configuration explicitly.

If any part of your Tomcat configuration is incorrect, Tomcat will fail to start and will record the problem in the <tomcat-root>/logs subdirectory. For instance:– c:\program files\apache group\tomcat-3.1\logs

Server Configuration

Find and edit the following file:<tomcat root>/conf/server.xml

Add a new entry at the bottom of the file:

<Context path="/se491" docBase="webapps/se491" debug="0" reloadable="true"></Context>

Server Configuration (cont.)

The context path should be the document root directory you just created. This is the place that Tomcat will use to derive all other locations.

The docBase is where Tomcat looks for non-dynamic elements such as HTML.

The debug suggests that you aren’t debugging the servlet request. Change this value to “1” to get more information in the Tomcat logs.

Server Configuration (cont.)

The reloadable entry suggests that if Tomcat notices that a class file has changed, it should reload that class the next time it is requested.

– This feature of application servers allows us to have 7 x 24 availability; we don’t need to shut down the server to deploy enhancements and fixes to the application.

Server Configuration (cont.)

You should add a new entry to server.xml for each document root you create.

– We typically create a single document root for each application being hosted by the application server.

– In a development environment we might host multiple application environments. For instance we might have document roots for development, string testing, system testing, acceptance testing, and performance testing.

– We can use a similar approach if we’re doing both enhancements and regular maintenance concurrently.

Servlet Installation

Before we can execute the servlet, we need to install it on the application server.

Each application server has its own method for performing this installation.– The following instructions apply to Tomcat only.

Class File Location

Compile the servlet like any other Java program.

Copy the servlet’s.class file to the WEB-INF/classes subdirectory.

If you are using packages, make certain that you use the same directory structure under WEB-INF/classes that you did when developing your source code.– This ensures that the class loader is able to find all of

the required resources.

web.xml

As with the Tomcat server configuration, when we add a new servlet to our application we should make Tomcat aware of it.– This is accomplished through the web.xml file.

Every application has its own web.xml file.

This file contains important configuration information that Tomcat uses to translate a browser request into a servlet execution.

web.xml (cont.)

<?xml version="1.0" encoding="ISO-8859-1"?>

<!DOCTYPE web-app PUBLIC "-//Sun Microsystems, Inc.//DTD Web Application 2.2//EN" "http://java.sun.com/j2ee/dtds/web-app_2.2.dtd">

<web-app> <servlet> <servlet-name> howdy </servlet-name> <servlet-class> HelloWorldExample </servlet-class>

</servlet>

<servlet-mapping> <servlet-name> howdy </servlet-name> <url-pattern> /howdy </url-pattern> </servlet-mapping></web-app>

web.xml (cont.)

The first part of this file provides a translation from the “pretty” name that users see in the browser to the servlet’s class name:

<servlet> <servlet-name> howdy </servlet-name> <servlet-class> HelloWorldExample </servlet-class>

</servlet>

web.xml (cont.)

The second part of this file provides a translation from groups of names to the servlet name identified in the first part of the file:

<servlet-mapping> <servlet-name> howdy </servlet-name> <url-pattern> /greetings </url-pattern>

</servlet-mapping>

Bouncing the Server

Whenever you make configuration changes to Tomcat you should “bounce” the server.– “Bouncing” is a euphemism for shutting the server

down and then immediately restarting it.

To bounce Tomcat, follow these steps:– Change to the <tomcat root>/bin directory– At the command line enter tomcat stop to stop the

Tomcat server.– At the command line enter tomcat start to start

the Tomcat server.

Running a Servlet

To execute a servlet you need to issue a request to Tomcat using your browser.

By default Tomcat listens for requests on port 8080. This can be configured, but for this class this value is fine.

Running a Servlet (cont.)

To execute the HelloWorldExample servlet, use the following URI:– http://localhost:8080/se491/greetings

Because we have a servlet mapping from /greetings to howdy, and howdy has been associated with the HelloWorldExample servlet in our web.xml file, Tomcat can translate this request into a servlet call.

Running a Servlet (cont.)

All of these URIs invoke the same servlet:– http://localhost:8080/se491/greetings

– http://localhost:8080/se491/servlet/howdy

– http://localhost:8080/se491/servlet/HelloWorldExample

This works because of the translations we defined in the web.xml file.

Servlet Lifecycle Reprise

You issue a request to the application server (Tomcat) using the client of your choice.

Tomcat looks at your request and attempts to locate the servlet within the URI.

Tomcat then looks at the servlet mappings that were defined in the web.xml file to determine the class name of the servlet that was requested.

Initialization Parameters

Sometimes we want to initialize a servlet when it’s first loaded by the servlet engine.– For instance, we may wish to load some configuration

settings before allowing the servlet to process requests.

This can be useful for testing when you don’t want to configure a web page to initiate the test.

Tomcat Initialization Parameters

Tomcat allows you to place the initialization parameters within the web.xml file.

It requires a slight modification from the original format:

<servlet> <servlet-name> message </servlet-name> <servlet-class> MessageServlet </servlet-class> <init-param> <param-name>msg</param-name> <param-value>this is a message....</param-value> </init-param> </servlet>

Changing Initialization Parameters

If you change the initialization parameters for a servlet, you should bounce the servlet engine to ensure that those changes are accepted.

Recycling the server forces it to reload the server.xml and web.xml files.