Servlet是什麼

為了能讓Web伺服器與Web應用這兩個不同的軟體系統協作，需要一套標準介面，Servlet就是其中最主要的一個介面。

規定：

Web伺服器可以訪問任意一個Web應用中實現Servlet介面的類。

Web應用中用於被Web伺服器動態呼叫的程式程式碼位於Servlet介面的實現類中。

SUN公司（現在被Oracle收購了……）制定了Web應用於Web伺服器進行協作的一系列標準Java介面（統稱為Java Servlet API）。

SUN公司還對Web伺服器釋出及執行Web應用的一些細節做了規約。SUN公司把這一系列標準Java介面和規約統稱為Servlet規範。

Servlet是一種執行在伺服器上的小外掛。

Servlet容器是什麼

在Servlet規範中，把能夠釋出和執行JavaWeb應用的Web伺服器稱為Servlet容器，他的最主要特稱是動態執行JavaWeb應用中的Servlet實現類中的程式程式碼。

Tomcat是什麼

Tomcat是Servlet容器，同時也是輕量級的Web伺服器。這是它的兩個身份！

Apache Server、Microsoft IIS、Apache Tomcat都是Web伺服器。

Tomcat作為Web伺服器時，主要負責實現HTTP傳輸等工作。

Tomcat作為Servlet容器時，主要負責解析Request，生成ServletRequest、ServletResponse，將其傳給相應的Servlet（呼叫service( )方法），再將Servlet的相應結果返回。

Tomcat組成結構

Server，代表整個Servlet容器元件，是Tomcat的頂層元素。其中可以包含一到多個Service；

Service，包含一個Engine，以及一到多個Connector；

Connector，代表和客戶端程式實際互動的元件，負責接收客戶請求，以及向客戶返回響應結果；

Engine，處理同一個Service中所有Connector接收到的客戶請求；

Host，在Engine中可以包含多個Host，每個Host定義了一個虛擬主機，它可以包含一個到多個Web應用；

Context，一個Host中可以包含多個Context，每個Context代表了執行在虛擬主機上的單個Web應用。

這些欄位都在conf/server.xml中配置，下面是一段apache tomcat 6.0.36預設的server.xml：

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<?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

分為三種：

1、獨立的Servlet容器

2、其他Web伺服器程序內的Servlet容器

3、其他Web伺服器程序外的Servlet容器

第一種，Tomcat作為獨立的Web伺服器單獨執行，Servlet容器作為Web伺服器的一部分，這也是預設工作模式。

這種模式下，Tomcat是一個獨立執行的Java程式，執行在Java虛擬機器程序中。