Understanding Tomcat and Its Integration with Spring Boot

Many junior Java developers start directly with Spring Boot and never encounter the underlying servlet container, typically Apache Tomcat.

Apache Tomcat is an open‑source Java Servlet container and web server that implements the Servlet, JSP, EL and WebSocket specifications. Its core functions are:

Servlet container – manages servlet lifecycle.

Web server – serves static HTTP resources.

JSP engine – compiles JSP to servlets.

A minimal pure‑Spring (non‑Boot) web application can be built by writing a servlet and configuring web.xml :

@WebServlet("/hello")
public class HelloServlet extends HttpServlet {
    protected void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        response.getWriter().println("Hello, Tomcat!");
    }
}

<web-app>
    <servlet>
        <servlet-name>HelloServlet</servlet-name>
        <servlet-class>com.example.HelloServlet</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>HelloServlet</servlet-name>
        <url-pattern>/hello</url-pattern>
    </servlet-mapping>
</web-app>

Spring Boot simplifies this by embedding Tomcat automatically through the spring-boot-starter-web dependency, so developers usually do not need to manage the container themselves.

If you prefer another embedded server such as Jetty, you can exclude the default Tomcat starter and add the Jetty starter in your pom.xml :

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-web</artifactId>
    <exclusions>
        <exclusion>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-tomcat</artifactId>
        </exclusion>
    </exclusions>
</dependency>

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-jetty</artifactId>
</dependency>

Understanding Tomcat’s role helps developers appreciate what Spring Boot abstracts away and gives them the flexibility to switch containers when needed.