Apache Tomcat

Apache Tomcat is a server suite from the Apache Software Foundation designed for testing, debugging, and running Java-based web applications. It is commonly referred to as a servlet container—a set of additional components that extend the web server’s functionality and enable it to run Java applications.

The name Tomcat was chosen by James Duncan Davidson, the software’s architect. The choice stems from O’Reilly’s tradition of featuring animals on book covers. Knowing that he would be publishing the first Tomcat textbook, Davidson decided to name the software after a special animal—independent, resourceful, and capable of fending for itself.

Apache Tomcat should not be confused with the Apache HTTP Server —the two products offer different functionality. The Apache HTTP Server delivers static and dynamic web pages to the browser, typically generated by PHP applications, such as WordPress. However, the Apache HTTP Server does not support Java servlets or JSP pages. The Tomcat container, on the other hand, was originally designed to handle content generated by Java applications.

Like other servlet containers, Tomcat has three distinctive features:

1. Processes client HTTP requests. For this purpose, a dedicated HTTP server is included.
2. Delivers pages to another web server. Tomcat is typically used in conjunction with Nginx or Apache.
3. It is part of the WildFly and GlassFish application servers, platforms on which web applications are run. 

Servlet container functions

Apache Tomcat simplifies the development and deployment of web applications. It scans project folders to instantly detect and deploy new servlets. If a servlet is updated during application execution, Tomcat updates all dependent components. If a servlet is deleted, the server stops processing requests and removes the component. As a servlet container, Tomcat performs several important functions:

• creates an environment for running servlets;
• sets session parameters;
• provides data exchange between servlets and clients;
• carries out identification and authorization of clients;
• Manages metadata.

Tomcat Components

Apache Tomcat consists of three main components: Catalina, Jasper, and Coyote.

Catalina is a servlet container that provides a command-line interface and implements the servlet specifications, known as the Servlet API. These specifications form the foundation of Java web technologies. Catalina enables interaction with Java libraries, enabling the dynamic creation of any web content.

Jasper. Processes JSP pages. They can contain both static ( HTML, XML, WML) and dynamic components (scripts, directives, actions).

Using the Jasper component, the Tomcat container acts as a template engine, automatically generating the required HTML code using JSP components. Jasper monitors all changes in dynamic page fragments and compiles the JSP elements. This converts the JSP page into a static one, which can then be sent to the HTTP server for further processing.

Coyote. An embedded web server for handling HTTP requests. It listens on TCP ports, responds to client requests directly, or passes the request to Catalina for processing JSP elements.

Installing Tomcat

Install the latest version of the JRE virtual machine or JDK. This is required for Apache Tomcat to run. You can download the version for Linux, macOS, or Windows from the official website.

Checking the correctness of environment variables. After installing the Java JDK, you should check the correctness of your environment variables. To do this on Windows:

1. Press the key combination Win + R.
2. Enter cmd in the line.
3. Run the command echo %JAVA_HOME% in the terminal.

If the variable is set correctly, the package version will appear as a response, for example, C:\Program Files\Java\jdk-17.0.2. The same string should appear at the end of the response to the echo %PATH% prompt in cmd.

If the variables aren’t set automatically, add them manually. To do this:

1. Go to Control Panel.
2. Select System – Advanced system settings.
3. Click Environment Variables.
4. Find PATH and click Edit.
5. Enter the path to the JDK package in the Program Files directory.

Downloading and installing Tomcat. After configuring the variables, download the latest version of Tomcat. On Windows, the server is installed like any other application. During installation, you can select “Install as a Windows service,” which will cause the server to start automatically when the OS boots. For optimal security, it’s best to run it under a different user account with limited privileges.

After installation, the server’s start page will be available at http://localhost:8080.

How to create your first web application

For this purpose, you can use the IntelliJ IDEA integrated development environment, which automatically creates the necessary Java project structure: source code files are placed in the src folder, and the completed web.xml and index.JSP files are uploaded to the web folder. The web.xml file lists instructions for the Tomcat server, and index.jsp is the main page of the web application, accessible at http://localhost:8080.

Traditionally, the first program in any language should display the greeting “Hello, world!”To output this phrase, you need to edit the index.JSP file as shown below:

<@ page contentType="text/html;charset=UTF-8" language="java">; My first application Hello, world! The application runs on an Apache Tomcat server.

Building war files to run on Tomcat

Tomcat processes Java web applications with the .war extension, which contain servlets, JSP files, static resources, and other data. When working in IntelliJ IDEA, the required .war file structure is created automatically. To create the project structure, follow these steps:

1. Go from Project Structure to Artifacts.
2. Click Web Application: Archive.
3. Click the Create manifest button below the web application name entry field.
4. Specify the path to the project directory – a META-INF folder with a manifest file inside will be created in it.
5. Click the Apply and OK buttons.

To pack a project into a war file, you need to:

1. Go to the Build tab.
2. Select Build Artifact.
3. Click on the project name.

During the build process, the environment will create an out directory with a subfolder called artifacts and place in it a file of the finished web application with the war extension, which can be run on the Tomcat server.

Tomcat is an indispensable tool for Java web application developers. It can operate as a standalone server or serve as a servlet container within a larger web server.