Servlet和Tomcat的基礎知識

前言：

Servlet是什麼？Servlet容器是什麼？Tomcat是什麼？Tomcat的組成結構以及Tomcat的工作模式。

最近沒事在翻《Tomcat與Java Web開發技術詳解》，本文隨記一篇。

題外話：前幾天做了一幅漫畫，發到微博上，效果還不錯~

迴歸正題……

正文：

Servlet是什麼

爲了能讓Web服務器與Web應用這兩個不同的軟件系統協作，需要一套標準接口，Servlet就是其中最主要的一個接口。

規定：

Web服務器可以訪問任意一個Web應用中實現Servlet接口的類。

Web應用中用於被Web服務器動態調用的程序代碼位於Servlet接口的實現類中。

SUN公司（現在被Oracle收購了……）制定了Web應用於Web服務器進行協作的一系列標準Java接口（統稱爲Java Servlet API）。

SUN公司還對Web服務器發佈及運行Web應用的一些細節做了規約。SUN公司把這一系列標準Java接口和規約統稱爲Servlet規範。

Servlet是一種運行在服務器上的小插件。

Servlet容器是什麼

在Servlet規範中，把能夠發佈和運行JavaWeb應用的Web服務器稱爲Servlet容器，他的最主要特稱是動態執行JavaWeb應用中的Servlet實現類中的程序代碼。

Tomcat是什麼

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

Apache Server、Microsoft IIS、Apache Tomcat都是Web服務器。

Tomcat作爲Web服務器時，主要負責實現HTTP傳輸等工作。

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

Tomcat組成結構

Server，代表整個Servlet容器組件，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，代表和客戶端程序實際交互的組件，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代表了運行在虛擬主機上的單個Web應用。

這些字段都在conf/server.xml中配置，下面是一段apache tomcat 6.0.36默認的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

分爲三種：

1、獨立的Servlet容器

2、其他Web服務器進程內的Servlet容器

3、其他Web服務器進程外的Servlet容器

第一種，Tomcat作爲獨立的Web服務器單獨運行，Servlet容器作爲Web服務器的一部分，這也是默認工作模式。

這種模式下，Tomcat是一個獨立運行的Java程序，運行在Java虛擬機進程中。

其他兩種模式我暫時沒有用到，也不太明確其構成，暫且不表了……

Author：Pirate Leo
blog：http://blog.csdn.net/pirateleo
email：[email protected]
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