这里对tomcat7的server.xml文件进行解释一下，方便在分析启动源码的时候理解Digester做的事情。

<?xml version='1.0' encoding='utf-8'?>
<Server port="8005" shutdown="SHUTDOWN">
 <Listener className="org.apache.catalina.startup.VersionLoggerListener" />
 <Listener className="org.apache.catalina.security.SecurityListener" />
 <Listener className="org.apache.catalina.core.AprLifecycleListener" SSLEngine="on" />
 <Listener className="org.apache.catalina.core.JasperListener" />
 <Listener className="org.apache.catalina.core.JreMemoryLeakPreventionListener" />
 <Listener className="org.apache.catalina.mbeans.GlobalResourcesLifecycleListener" />
 <Listener className="org.apache.catalina.core.ThreadLocalLeakPreventionListener" />

 <GlobalNamingResources>
  <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>

 <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" />

  <Connector port="8443" protocol="org.apache.coyote.http11.Http11Protocol"
        maxThreads="150" SSLEnabled="true" scheme="https" secure="true"
        clientAuth="false" sslProtocol="TLS" />

  <Connector port="8009" protocol="AJP/1.3" redirectPort="8443" />

  <Engine name="Catalina" defaultHost="localhost">

   <Cluster className="org.apache.catalina.ha.tcp.SimpleTcpCluster"/>

   <Realm className="org.apache.catalina.realm.LockOutRealm">
    <Realm className="org.apache.catalina.realm.UserDatabaseRealm"
        resourceName="UserDatabase"/>
   </Realm>

   <Host name="localhost" appBase="webapps"
      unpackWARs="true" autoDeploy="true">

    <Valve className="org.apache.catalina.authenticator.SingleSignOn" />

    <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>

Server

tomcat中Server代表一个tomcat实例，所以只会存在一个Server，而在配置文件中也是作为顶级元素出现，代码如下：

<Server port="8005" shutdown="SHUTDOWN">
。。。
</Server>

port，监听shutdown命令的端口，-1表示禁用shutdown命令。 shutdown，关闭tomcat的指令。

Listener

监听器，用来监听某些事件的发生。