Unveiling Tomcat’s Architecture: From Servlet Spec to Core Components

Tomcat Architecture Design

1. Servlet Specification

1.1 What Servlets Do

Servlet is a JavaEE specification that extends Java for web services, defining interfaces such as Servlet, HttpRequest, HttpResponse, and Filter, which are implemented by containers like Tomcat and Jetty.

The specification does not provide an implementation; a container receives an HTTP request, parses it, creates a servlet instance, calls its init method, wraps request data in HttpServletRequest, invokes the servlet’s service method, and finally uses HttpServletResponse to send the response back to the client.

1.2 Core Servlet APIs

Key servlet APIs that are essential for understanding Tomcat:

ServletConfig : obtain servlet initialization parameters and the ServletContext object.

ServletContext : share data across the entire web application.

ServletRequest : encapsulate HTTP request information.

ServletResponse : encapsulate HTTP response information.

ServletConfig is created by the container during servlet initialization and passed to the servlet via the init() method.

Obtain initialization information.

Obtain the ServletContext object.

ServletContext is a single object per web application, created when Tomcat starts and destroyed when it stops, allowing data sharing among servlets.

public class ServletTwoImpl extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        response.setContentType("text/html;charset=utf-8");
        // 1. Parameter passing
        ServletContext servletContext = this.getServletContext();
        String value = String.valueOf(servletContext.getAttribute("name"));
        System.out.println("value=" + value);
        // 2. Get init parameter
        String userName = servletContext.getInitParameter("user-name");
        System.out.println("userName=" + userName);
        // 3. Get application info
        String servletContextName = servletContext.getServletContextName();
        System.out.println("servletContextName=" + servletContextName);
        // 4. Get paths
        String pathOne = servletContext.getRealPath("/");
        String pathTwo = servletContext.getRealPath("/WEB-INF/");
        System.out.println("pathOne=" + pathOne + ";pathTwo=" + pathTwo);
        response.getWriter().print("执行：doGet; value：" + value);
    }
}

1.3 ServletRequest

HttpServletRequest extends ServletRequest and encapsulates request data. Core functions: obtain request message, network connection information, and request attribute data.

1.4 ServletResponse

HttpServletResponse extends ServletResponse and encapsulates response data. Core functions: set response headers, send status code, set response body, and perform redirects.

2. Tomcat Design

Tomcat implements the servlet specification by providing a container that receives HTTP requests, parses them, creates servlet instances, and invokes their service methods to generate responses.

2.1 What Is Tomcat

Tomcat is a servlet container that implements part of the J2EE/Jakarta EE specifications, serving as a server for Java web applications.

2.2 Tomcat Architecture Structure

The server.xml file defines Tomcat’s components; each XML element corresponds to a specific component.

Official documentation: https://tomcat.apache.org/tomcat-8.5-doc/config/server.html

<Server port="8005" shutdown="SHUTDOWN">
  <Service name="Catalina">
    <Executor name="tomcatThreadPool" namePrefix="catalina-exec-" maxThreads="150" minSpareThreads="4"/>
    <Connector port="8080" protocol="HTTP/1.1" connectionTimeout="20000" redirectPort="8443"/>
    <Connector executor="tomcatThreadPool" 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"/>
      <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"/>
      </Host>
    </Engine>
  </Service>
</Server>

2.3 Component Classification

Top‑level elements:

Server: the root element of the configuration file.

Service: groups an Engine and its Connectors.

Connector: the interface through which external clients send requests to a Service.

Container: processes requests from Connectors; Engine, Host, and Context are containers arranged in a parent‑child hierarchy.

Engine: handles all requests.

Host: handles requests for a specific virtual host.

Context: handles requests for a specific web application.

The components work together as follows: a Server contains one or more Services; each Service contains one Engine and one or more Connectors; the Engine contains Hosts; each Host contains Contexts.