Understanding Tomcat: Request/Response Processing and the Servlet Lifecycle

Tomcat, a widely used Java web server and servlet container, is introduced as a "three‑legged cat" that the author has known since university and continues to work with in daily development.

The relationship between a web application and Tomcat is described: the application is packaged as a WAR file, deployed to Tomcat, and URL mappings must be defined either in raw Servlets or modern Spring MVC controllers.

When a request arrives, Tomcat first provides a socket service that understands the HTTP protocol, then distributes the request to the appropriate web application based on the URL, and finally wraps the raw HTTP data into HttpServletRequest and HttpServletResponse objects for the application to use.

The article outlines three core steps:

1. Provide Socket Service – Tomcat starts a socket listener that speaks HTTP.

2. Distribute Requests – Tomcat can serve multiple web applications and routes each URL to the correct one.

3. Encapsulate Request/Response – Tomcat creates request and response objects so developers can work with high‑level APIs instead of raw streams.

Project screenshots are shown to illustrate the implementation (images omitted for brevity).

1) Encapsulate Request Object – An image demonstrates how Tomcat parses the HTTP request line and headers from the input stream.

2) Encapsulate Response Object – An image shows the HTTP response format being written back to the client.

3) Servlet Request Handling Base Class – The diagram displays the typical doGet , doPost , and service methods defined by the servlet specification.

4) Servlet Implementation Classes – Two concrete servlet classes are presented as examples for later testing.

5) Servlet Configuration – The web.xml entries ( <servlet> and <servlet-mapping> ) that bind URLs to specific servlet classes are shown.

6) Startup Class – Screenshots illustrate the main class that launches the embedded Tomcat instance.

The author emphasizes that the entire Tomcat processing flow—URL routing, HTTP parsing, request/response wrapping, and servlet invocation—can be reproduced in a simple seven‑step project, encouraging readers to type the code themselves for deeper understanding.

7) Testing – A final screenshot shows the running application, confirming that the minimal Tomcat implementation works as expected.

In conclusion, building a lightweight Tomcat clone through these steps helps developers demystify the server’s inner workings and gain confidence in Java web development.