前言：

Servlet是什么？Servlet容器是什么？Tomcat是什么？Tomcat的组成结构以及Tomcat的工作模式。

最近没事在翻《Tomcat与Java Web开发技术详解》，本文随记一篇。

题外话：前几天做了一幅漫画，发到微博上，效果还不错~

回归正题……

正文：

Servlet是什么

为了能让Web服务器与Web应用这两个不同的软件系统协作，需要一套标准接口，Servlet就是其中最主要的一个接口。

规定：

Web服务器可以访问任意一个Web应用中实现Servlet接口的类。

Web应用中用于被Web服务器动态调用的程序代码位于Servlet接口的实现类中。

SUN公司（现在被Oracle收购了……）制定了Web应用于Web服务器进行协作的一系列标准Java接口（统称为Java Servlet API）。

SUN公司还对Web服务器发布及运行Web应用的一些细节做了规约。SUN公司把这一系列标准Java接口和规约统称为Servlet规范。

Servlet是一种运行在服务器上的小插件。

Servlet容器是什么

在Servlet规范中，把能够发布和运行JavaWeb应用的Web服务器称为Servlet容器，他的最主要特称是动态执行JavaWeb应用中的Servlet实现类中的程序代码。

Tomcat是什么

Tomcat是Servlet容器，同时也是轻量级的Web服务器。这是它的两个身份！

Apache Server、Microsoft IIS、Apache Tomcat都是Web服务器。

Tomcat作为Web服务器时，主要负责实现HTTP传输等工作。

Tomcat作为Servlet容器时，主要负责解析Request，生成ServletRequest、ServletResponse，将其传给相应的Servlet（调用service( )方法），再将Servlet的相应结果返回。

Tomcat组成结构

Server，代表整个Servlet容器组件，是Tomcat的顶层元素。其中可以包含一到多个Service；

Service，包含一个Engine，以及一到多个Connector；

Connector，代表和客户端程序实际交互的组件，负责接收客户请求，以及向客户返回响应结果；

Engine，处理同一个Service中所有Connector接收到的客户请求；

Host，在Engine中可以包含多个Host，每个Host定义了一个虚拟主机，它可以包含一个到多个Web应用；

Context，一个Host中可以包含多个Context，每个Context代表了运行在虚拟主机上的单个Web应用。

这些字段都在conf/server.xml中配置，下面是一段apache tomcat 6.0.36默认的server.xml：

<?xml version='1.0' encoding='utf-8'?>
<!--
  Licensed to the Apache Software Foundation (ASF) under one or more
  contributor license agreements.  See the NOTICE file distributed with
  this work for additional information regarding copyright ownership.
  The ASF licenses this file to You under the Apache License, Version 2.0
  (the "License"); you may not use this file except in compliance with
  the License.  You may obtain a copy of the License at

      http://www.apache.org/licenses/LICENSE-2.0

  Unless required by applicable law or agreed to in writing, software
  distributed under the License is distributed on an "AS IS" BASIS,
  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  See the License for the specific language governing permissions and
  limitations under the License.
-->
<!-- Note:  A "Server" is not itself a "Container", so you may not
     define subcomponents such as "Valves" at this level.
     Documentation at /docs/config/server.html
 -->
<Server port="8005" shutdown="SHUTDOWN">

  <!--APR library loader. Documentation at /docs/apr.html -->
  <Listener className="org.apache.catalina.core.AprLifecycleListener" SSLEngine="on" />
  <!--Initialize Jasper prior to webapps are loaded. Documentation at /docs/jasper-howto.html -->
  <Listener className="org.apache.catalina.core.JasperListener" />
  <!-- Prevent memory leaks due to use of particular java/javax APIs-->
  <Listener className="org.apache.catalina.core.JreMemoryLeakPreventionListener" />
  <!-- JMX Support for the Tomcat server. Documentation at /docs/non-existent.html -->
  <Listener className="org.apache.catalina.mbeans.ServerLifecycleListener" />
  <Listener className="org.apache.catalina.mbeans.GlobalResourcesLifecycleListener" />

  <!-- Global JNDI resources
       Documentation at /docs/jndi-resources-howto.html
  -->
  <GlobalNamingResources>
    <!-- Editable user database that can also be used by
         UserDatabaseRealm to authenticate users
    -->
    <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>

  <!-- A "Service" is a collection of one or more "Connectors" that share
       a single "Container" Note:  A "Service" is not itself a "Container", 
       so you may not define subcomponents such as "Valves" at this level.
       Documentation at /docs/config/service.html
   -->
  <Service name="Catalina">
  
    <!--The connectors can use a shared executor, you can define one or more named thread pools-->
    <!--
    <Executor name="tomcatThreadPool" namePrefix="catalina-exec-" 
        maxThreads="150" minSpareThreads="4"/>
    -->
    
    
    <!-- A "Connector" represents an endpoint by which requests are received
         and responses are returned. Documentation at :
         Java HTTP Connector: /docs/config/http.html (blocking & non-blocking)
         Java AJP  Connector: /docs/config/ajp.html
         APR (HTTP/AJP) Connector: /docs/apr.html
         Define a non-SSL HTTP/1.1 Connector on port 8080
    -->
    <Connector port="8080" protocol="HTTP/1.1" 
               connectionTimeout="20000" 
               redirectPort="8443" />
    <!-- A "Connector" using the shared thread pool-->
    <!--
    <Connector executor="tomcatThreadPool"
               port="8080" protocol="HTTP/1.1" 
               connectionTimeout="20000" 
               redirectPort="8443" />
    -->           
    <!-- Define a SSL HTTP/1.1 Connector on port 8443
         This connector uses the JSSE configuration, when using APR, the 
         connector should be using the OpenSSL style configuration
         described in the APR documentation -->
    <!--
    <Connector port="8443" protocol="HTTP/1.1" SSLEnabled="true"
               maxThreads="150" scheme="https" secure="true"
               clientAuth="false" sslProtocol="TLS" />
    -->

    <!-- Define an AJP 1.3 Connector on port 8009 -->
    <Connector port="8009" protocol="AJP/1.3" redirectPort="8443" />


    <!-- An Engine represents the entry point (within Catalina) that processes
         every request.  The Engine implementation for Tomcat stand alone
         analyzes the HTTP headers included with the request, and passes them
         on to the appropriate Host (virtual host).
         Documentation at /docs/config/engine.html -->

    <!-- You should set jvmRoute to support load-balancing via AJP ie :
    <Engine name="Catalina" defaultHost="localhost" jvmRoute="jvm1">         
    --> 
    <Engine name="Catalina" defaultHost="localhost">

      <!--For clustering, please take a look at documentation at:
          /docs/cluster-howto.html  (simple how to)
          /docs/config/cluster.html (reference documentation) -->
      <!--
      <Cluster className="org.apache.catalina.ha.tcp.SimpleTcpCluster"/>
      -->        

      <!-- The request dumper valve dumps useful debugging information about
           the request and response data received and sent by Tomcat.
           Documentation at: /docs/config/valve.html -->
      <!--
      <Valve className="org.apache.catalina.valves.RequestDumperValve"/>
      -->

      <!-- This Realm uses the UserDatabase configured in the global JNDI
           resources under the key "UserDatabase".  Any edits
           that are performed against this UserDatabase are immediately
           available for use by the Realm.  -->
      <Realm className="org.apache.catalina.realm.UserDatabaseRealm"
             resourceName="UserDatabase"/>

      <!-- Define the default virtual host
           Note: XML Schema validation will not work with Xerces 2.2.
       -->
      <Host name="localhost"  appBase="webapps"
            unpackWARs="true" autoDeploy="true"
            xmlValidation="false" xmlNamespaceAware="false">

        <!-- SingleSignOn valve, share authentication between web applications
             Documentation at: /docs/config/valve.html -->
        <!--
        <Valve className="org.apache.catalina.authenticator.SingleSignOn" />
        -->

        <!-- Access log processes all example.
             Documentation at: /docs/config/valve.html -->
        <!--
        <Valve className="org.apache.catalina.valves.AccessLogValve" directory="logs"  
               prefix="localhost_access_log." suffix=".txt" pattern="common" resolveHosts="false"/>
        -->

      </Host>
    </Engine>
  </Service>
</Server>

Tomcat工作模式

分为三种：

1、独立的Servlet容器

2、其他Web服务器进程内的Servlet容器

3、其他Web服务器进程外的Servlet容器

第一种，Tomcat作为独立的Web服务器单独运行，Servlet容器作为Web服务器的一部分，这也是默认工作模式。

这种模式下，Tomcat是一个独立运行的Java程序，运行在Java虚拟机进程中。

其他两种模式我暂时没有用到，也不太明确其构成，暂且不表了……

Author：Pirate Leo
blog：http://blog.csdn.net/pirateleo
email：codeevoship@gmail.com
转载请注明出处。
由于博文在发布后难免会有勘误或补充，推荐到本博客中阅读此文。