Mastering Servlets and Tomcat: Architecture, Components, and Request Flow

This article is part of a Java interview series that is continuously updated.

What is a Servlet?

Servlet is a Java EE specification that extends Java for web services via a unified interface, implemented by containers such as Tomcat and Jetty. When an HTTP request arrives, the container creates an HttpServletRequest object, invokes lifecycle methods like init() and service(), and returns a response wrapped as HttpServletResponse.

What is the Servlet Specification?

Two JAR files: servlet-api.jar and jsp-api.jar (JSP is also a Servlet).

Two packages: javax.servlet and javax.servlet.http.

Defines interfaces such as Servlet, Filter, Listener, ServletRequest, and ServletResponse.

Why is Tomcat called a Web Container or Servlet Container?

Tomcat implements the Servlet specification and provides a container that holds our Servlets.

Tomcat Architecture Overview

<?xml version="1.0" encoding="UTF-8"?>
<Server port="8005" shutdown="SHUTDOWN">
  <Listener className="org.apache.catalina.startup.VersionLoggerListener" />
  <Listener className="org.apache.catalina.core.AprLifecycleListener" SSLEngine="on" />
  <Listener className="org.apache.catalina.core.JreMemoryLeakPreventionListener" />
  <Listener className="org.apache.catalina.mbeans.GlobalResourcesLifecycleListener" />
  <Listener className="org.apache.catalina.core.ThreadLocalLeakPreventionListener" />
  <GlobalNamingResources>
    <Resource name="UserDatabase" auth="Container" type="org.apache.catalina.UserDatabase" description="User database that can be updated and saved" factory="org.apache.catalina.users.MemoryUserDatabaseFactory" pathname="conf/tomcat-users.xml" />
  </GlobalNamingResources>
  <Service name="Catalina">
    <Connector port="8080" protocol="HTTP/1.1" connectionTimeout="20000" redirectPort="8443" />
    <Engine name="Catalina" defaultHost="localhost">
      <Realm className="org.apache.catalina.realm.LockOutRealm">
        <Realm className="org.apache.catalina.realm.UserDatabaseRealm" resourceName="UserDatabase" />
      </Realm>
      <Host name="localhost" appBase="webapps" unpackWARs="true" autoDeploy="true">
        <Valve className="org.apache.catalina.valves.AccessLogValve" directory="logs" prefix="localhost_access_log" suffix=".txt" pattern="%h %l %u %t \"%r\" %s %b" />
        <Context docBase="F:/workspace/my-web-maven/target" path="/" reloadable="true" />
        <Context docBase="F:/workspace/my-web-maven/target" path="/tian" reloadable="true" />
      </Host>
    </Engine>
  </Service>
</Server>

The XML configuration mirrors the architectural diagram; classes such as Listener, Service, Host, and Engine correspond to Java implementations.

Tomcat Core Components

Server : Starts the Tomcat instance and provides lifecycle listeners and global naming resources.

Service : Groups one or more Connectors that share a Container.

Connector : Accepts client connections, creates request/response objects, and forwards them to the Engine.

Container : Manages Servlets and includes Engine, Host, Context, and Wrapper components.

Component Relationships

Tomcat Request Processing Flow

The request reaches port 8080 and is captured by the Coyote HTTP/1.1 Connector.

The Connector forwards the request to the Engine of its Service.

The Engine matches the request to a Host (default localhost).

The Host selects the appropriate Context based on the request path.

The Context finds the matching Servlet (e.g., JspServlet for *.jsp).

The Container creates HttpServletRequest and HttpServletResponse objects and invokes the Servlet's doGet or doPost.

The response propagates back through Context, Host, Engine, Connector, and finally to the client browser.