Demystifying Tomcat: 7 Steps to Build a Simple Java Web Server

Tomcat is a widely used web server that also implements the Servlet specification, acting as the container for Java web applications. When a web app is packaged as a WAR file and deployed to Tomcat, the URL mapping determines which class and method handle each request, whether using raw Servlets or modern Spring MVC.

Because the web application runs inside Tomcat, every request first reaches Tomcat, which processes it in several stages.

1. Provide Socket Service

Tomcat starts by opening a socket that speaks the HTTP protocol, essentially a specialized socket service. It can be based on BIO, NIO, or AIO.

2. Dispatch Requests

Tomcat can host multiple web applications and routes incoming URLs to the appropriate one.

3. Encapsulate Request/Response

Tomcat creates HttpServletRequest and HttpServletResponse objects, so developers can work with high‑level abstractions instead of raw sockets.

4. Servlet Request Handling Base Class

The container provides the standard Servlet API with methods such as doGet, doPost, and service.

5. Servlet Implementation Classes

Two concrete Servlet examples are shown to illustrate how custom Servlets are written and later used for testing.

6. Servlet Configuration

In web.xml, <servlet> and <servlet-mapping> elements associate specific URLs with the corresponding Servlet classes.

7. Startup Class

The main class launches Tomcat, initializing the server and loading the configured Servlets.

The following images illustrate each step:

By following these seven steps and writing the code yourself rather than copying, you gain a deeper understanding of Tomcat’s inner workings.