Tomcat Architecture Series-1.Overview & Basic Concepts

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Deep Dive into Tomcat Architecture: A Comprehensive Guide

Introduction

Apache Tomcat, as one of the most popular Java web servers and servlet containers, powers millions of web applications worldwide.This article provides a comprehensive overview of Tomcat’s architecture, helping developers understand its internal workings.

Table of Contents

  1. What is Tomcat?
  2. Architectural Overview
  3. Core Components
  4. Request Processing Flow
  5. Key Features
  6. Best Practices

1. What is Tomcat?

1.1 Brief History

Tomcat was first released in 1999, and it has evolved significantly over the years, the latest stable version is Tomcat 11.0.

1.2 Role and Responsibilities

  • Web server
  • Servlet Container functionality
  • JSP processing
  • WebSocket support

2. Architectural Overview

2.1 High-Level Architecture 

//Simplified representation of Tomcat<span>'s architecture Server (Top-level container) └── Service ├── Connector (HTTP/AJP) └── Container (Engine) └── Host └── Context └── Wrapper </span>
//Simplified representation of Tomcat<span>'s architecture Server (Top-level container) └── Service ├── Connector (HTTP/AJP) └── Container (Engine) └── Host └── Context └── Wrapper </span>
//Simplified representation of Tomcat's architecture Server (Top-level container) └── Service ├── Connector (HTTP/AJP) └── Container (Engine) └── Host └── Context └── Wrapper

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2.2 Key Design Principles

  • Modular design
  • Hierarchical structure
  • Component-based architecture
  • Extensibility

3. Core Components

3.1 Server Components 

<span>public</span> <span>interface</span> <span>Server</span> <span>{</span>
    <span>// The main server component</span>
    <span>public</span> <span>Service</span><span>[]</span> <span>findServices</span><span>();</span>
    <span>public</span> <span>void</span> <span>addService</span><span>(</span><span>Service</span> <span>service</span><span>);</span>
    <span>public</span> <span>void</span> <span>removeService</span><span>(</span><span>Service</span> <span>service</span><span>);</span>
<span>}</span>
<span>public</span> <span>interface</span> <span>Server</span> <span>{</span>
    <span>// The main server component</span>
    <span>public</span> <span>Service</span><span>[]</span> <span>findServices</span><span>();</span>
    <span>public</span> <span>void</span> <span>addService</span><span>(</span><span>Service</span> <span>service</span><span>);</span>
    <span>public</span> <span>void</span> <span>removeService</span><span>(</span><span>Service</span> <span>service</span><span>);</span>
<span>}</span>
public interface Server {
    // The main server component
    public Service[] findServices();
    public void addService(Service service);
    public void removeService(Service service);
}

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3.2 Service Component 

<span>public</span> <span>interface</span> <span>Service</span> <span>{</span>
    <span>// Combines one or more Connectors with a Container</span>
    <span>public</span> <span>Container</span> <span>getContainer</span><span>();</span>
    <span>public</span> <span>void</span> <span>setContainer</span><span>(</span><span>Container</span> <span>container</span><span>);</span>
    <span>public</span> <span>Connector</span><span>[]</span> <span>findConnectors</span><span>();</span>
<span>}</span>
<span>public</span> <span>interface</span> <span>Service</span> <span>{</span>
    <span>// Combines one or more Connectors with a Container</span>
    <span>public</span> <span>Container</span> <span>getContainer</span><span>();</span>
    <span>public</span> <span>void</span> <span>setContainer</span><span>(</span><span>Container</span> <span>container</span><span>);</span>
    <span>public</span> <span>Connector</span><span>[]</span> <span>findConnectors</span><span>();</span>
<span>}</span>
public interface Service {
    // Combines one or more Connectors with a Container
    public Container getContainer();
    public void setContainer(Container container);
    public Connector[] findConnectors();
}

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3.3 Connector Component 

<span>public</span> <span>interface</span> <span>Connector</span> <span>{</span>
    <span>// Handles communication with clients</span>
    <span>public</span> <span>void</span> <span>setPort</span><span>(</span><span>int</span> <span>port</span><span>);</span>
    <span>public</span> <span>void</span> <span>setProtocol</span><span>(</span><span>String</span> <span>protocol</span><span>);</span>
    <span>public</span> <span>Container</span> <span>getContainer</span><span>();</span>
<span>}</span>
<span>public</span> <span>interface</span> <span>Connector</span> <span>{</span>
    <span>// Handles communication with clients</span>
    <span>public</span> <span>void</span> <span>setPort</span><span>(</span><span>int</span> <span>port</span><span>);</span>
    <span>public</span> <span>void</span> <span>setProtocol</span><span>(</span><span>String</span> <span>protocol</span><span>);</span>
    <span>public</span> <span>Container</span> <span>getContainer</span><span>();</span>
<span>}</span>
public interface Connector {
    // Handles communication with clients
    public void setPort(int port);
    public void setProtocol(String protocol);
    public Container getContainer();
}

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3.4 Container Hierarchy

  • Engine
  • Host
  • Context
  • Wrapper

4. Request Processing Flow

4.1 Step-by-Step Process

  1. Client sends HTTP request
  2. Connector receives and processes request
  3. Request passes through Container pipline
  4. Servlet processes quest
  5. Response returns through the same path 
<span>// Simplified request processing flow</span>
<span>public</span> <span>class</span> <span>RequestProcessor</span> <span>{</span>
    <span>public</span> <span>void</span> <span>process</span><span>(</span><span>Request</span> <span>request</span><span>,</span> <span>Response</span> <span>response</span><span>)</span> <span>{</span>
        <span>// 1. Parse HTTP request</span>
        <span>connector</span><span>.</span><span>parse</span><span>(</span><span>request</span><span>);</span>
        <span>// 2. Create request/response objects</span>
        <span>Request</span> <span>req</span> <span>=</span> <span>new</span> <span>Request</span><span>(</span><span>request</span><span>);</span>
        <span>Response</span> <span>res</span> <span>=</span> <span>new</span> <span>Response</span><span>(</span><span>response</span><span>);</span>
        <span>// 3. Process through container pipeline</span>
        <span>container</span><span>.</span><span>getPipeline</span><span>().</span><span>invoke</span><span>(</span><span>req</span><span>,</span> <span>res</span><span>);</span>
        <span>// 4. Send response</span>
        <span>response</span><span>.</span><span>send</span><span>();</span>
    <span>}</span>
<span>}</span>
<span>// Simplified request processing flow</span>
<span>public</span> <span>class</span> <span>RequestProcessor</span> <span>{</span>
    <span>public</span> <span>void</span> <span>process</span><span>(</span><span>Request</span> <span>request</span><span>,</span> <span>Response</span> <span>response</span><span>)</span> <span>{</span>
        <span>// 1. Parse HTTP request</span>
        <span>connector</span><span>.</span><span>parse</span><span>(</span><span>request</span><span>);</span>

        <span>// 2. Create request/response objects</span>
        <span>Request</span> <span>req</span> <span>=</span> <span>new</span> <span>Request</span><span>(</span><span>request</span><span>);</span>
        <span>Response</span> <span>res</span> <span>=</span> <span>new</span> <span>Response</span><span>(</span><span>response</span><span>);</span>

        <span>// 3. Process through container pipeline</span>
        <span>container</span><span>.</span><span>getPipeline</span><span>().</span><span>invoke</span><span>(</span><span>req</span><span>,</span> <span>res</span><span>);</span>

        <span>// 4. Send response</span>
        <span>response</span><span>.</span><span>send</span><span>();</span>
    <span>}</span>
<span>}</span>
// Simplified request processing flow
public class RequestProcessor {
    public void process(Request request, Response response) {
        // 1. Parse HTTP request
        connector.parse(request);

        // 2. Create request/response objects
        Request req = new Request(request);
        Response res = new Response(response);

        // 3. Process through container pipeline
        container.getPipeline().invoke(req, res);

        // 4. Send response
        response.send();
    }
}

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5. Key Feature

5.1 Lifecycle Management 

<span>public</span> <span>interface</span> <span>Lifecycle</span> <span>{</span>
    <span>public</span> <span>void</span> <span>init</span><span>();</span>
    <span>public</span> <span>void</span> <span>start</span><span>();</span>
    <span>public</span> <span>void</span> <span>stop</span><span>();</span>
    <span>public</span> <span>void</span> <span>destroy</span><span>();</span>
<span>}</span>
<span>public</span> <span>interface</span> <span>Lifecycle</span> <span>{</span>
    <span>public</span> <span>void</span> <span>init</span><span>();</span>
    <span>public</span> <span>void</span> <span>start</span><span>();</span>
    <span>public</span> <span>void</span> <span>stop</span><span>();</span>
    <span>public</span> <span>void</span> <span>destroy</span><span>();</span>
<span>}</span>
public interface Lifecycle {
    public void init();
    public void start();
    public void stop();
    public void destroy();
}

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5.2 Pipeline-Value Mechanism 

<span>public</span> <span>interface</span> <span>Pipeline</span> <span>{</span>
    <span>public</span> <span>Valve</span> <span>getBasic</span><span>();</span>
    <span>public</span> <span>void</span> <span>setBasic</span><span>(</span><span>Valve</span> <span>valve</span><span>);</span>
    <span>public</span> <span>void</span> <span>addValve</span><span>(</span><span>Valve</span> <span>valve</span><span>);</span>
<span>}</span>
<span>public</span> <span>interface</span> <span>Pipeline</span> <span>{</span>
    <span>public</span> <span>Valve</span> <span>getBasic</span><span>();</span>
    <span>public</span> <span>void</span> <span>setBasic</span><span>(</span><span>Valve</span> <span>valve</span><span>);</span>
    <span>public</span> <span>void</span> <span>addValve</span><span>(</span><span>Valve</span> <span>valve</span><span>);</span>
<span>}</span>
public interface Pipeline {
    public Valve getBasic();
    public void setBasic(Valve valve);
    public void addValve(Valve valve);
}

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5.3 Class Loading

  • Web application class loader
  • Common class loader
  • System class loader

6. Best Practices

6.1 Configuration Guidelines 

<<span>!</span><span>--</span> Example server.xml configuration <span>--</span><span>></span>
<Server <span>port</span><span>=</span><span>"8005"</span> <span>shutdown</span><span>=</span><span>"SHUTDOWN"</span><span>></span>
    <Service <span>name</span><span>=</span><span>"Catalina"</span><span>></span>
        <Connector <span>port</span><span>=</span><span>"8080"</span> <span>protocol</span><span>=</span><span>"HTTP/1.1"</span>/>
        <Engine <span>name</span><span>=</span><span>"Catalina"</span> <span>defaultHost</span><span>=</span><span>"localhost"</span><span>></span>
            <Host <span>name</span><span>=</span><span>"localhost"</span> <span>appBase</span><span>=</span><span>"webapps"</span>/>
        </Engine>
    </Service>
</Server>
<<span>!</span><span>--</span> Example server.xml configuration <span>--</span><span>></span>
<Server <span>port</span><span>=</span><span>"8005"</span> <span>shutdown</span><span>=</span><span>"SHUTDOWN"</span><span>></span>
    <Service <span>name</span><span>=</span><span>"Catalina"</span><span>></span>
        <Connector <span>port</span><span>=</span><span>"8080"</span> <span>protocol</span><span>=</span><span>"HTTP/1.1"</span>/>
        <Engine <span>name</span><span>=</span><span>"Catalina"</span> <span>defaultHost</span><span>=</span><span>"localhost"</span><span>></span>
            <Host <span>name</span><span>=</span><span>"localhost"</span> <span>appBase</span><span>=</span><span>"webapps"</span>/>
        </Engine>
    </Service>
</Server>
<!-- Example server.xml configuration -->
<Server port="8005" shutdown="SHUTDOWN">
    <Service name="Catalina">
        <Connector port="8080" protocol="HTTP/1.1"/>
        <Engine name="Catalina" defaultHost="localhost">
            <Host name="localhost" appBase="webapps"/>
        </Engine>
    </Service>
</Server>

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6.2 Performance Optimization

  • Connector thread pool settings
  • Memory configuration
  • Connection timeout settings

Conclusion

Understanding Tomcat’s architecture is crucial for Java developers working with web application.
The knowledge helps in:

  • Efficient application deployment
  • Performance optimization
  • Custom component development

References

  • Apache Tomcat Official Document
  • Expert One-on-One J2EE Development without EJB
  • Tomcat: Thw Definitive Guide

原文链接：Tomcat Architecture Series-1.Overview & Basic Concepts

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