Tomcat生成空的访问日志文件

Question

由于我需要由Tomcat处理的每个请求的运行时信息，因此已在文件{$CATALINA_HOME}/conf/server.xml中为Tomcat配置了访问日志输出，如下所示。但是在运行应用程序时，访问日志文件只是保持为空。有人知道原因吗？非常感谢！

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

    <!-- 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
         Note: The pattern used is equivalent to using pattern="common" -->
    <Valve className="org.apache.catalina.valves.AccessLogValve"
              directory="/home/admin/shopcenter/logs/"
              prefix="localhost_access_log"
              suffix=".log"
              buffered="false"
              encoding="GB18030"
              pattern="common"/>

</Host>