把一个
AccessLogValve在
Host或
Context元素如：
<Host name="www.mysite.com" appbase="..." > <Valve className="org.apache.catalina.valves.AccessLogValve" directory="logs" prefix="mysitelog." suffix=".txt" pattern="..." resolveHosts="false" /></Host>
该
pattern属性可以采用两个速记值之一（ common ， combined
）或使用多个常量和替换字符串的自定义模式。让我引用Tomcat文档：
https://tomcat.apache.org/tomcat-8.0-doc/config/valve.html#Access_Log_Valve
pattern属性的值由文字字符串组成，并以“％”字符作为前缀的模式标识符组合在一起，以替换为来自当前请求和响应的相应变量值。支持以下模式代码：
> %a - Remote IP address> %A - Local IP address> %b - Bytes sent, excluding HTTP headers, or '-' if zero> %B - Bytes sent, excluding HTTP headers> %h - Remote host name (or IP address if enableLookups for the connector> is false)> %H - Request protocol> %l - Remote logical username from identd (always returns '-')> %m - Request method (GET, POST, etc.)> %p - Local port on which this request was received. See also %{xxx}p> below.> %q - Query string (prepended with a '?' if it exists)> %r - First line of the request (method and request URI)> %s - HTTP status pre of the response> %S - User session ID> %t - Date and time, in Common Log Format> %u - Remote user that was authenticated (if any), else '-'> %U - Requested URL path> %v - Local server name> %D - Time taken to process the request, in millis> %T - Time taken to process the request, in seconds> %F - Time taken to commit the response, in millis> %I - Current request thread name (can compare later with stacktraces)还支持编写信息传入或传出的标头，cookie，会话或请求属性以及特殊的时间戳格式。它是根据Apache HTTP Server日志配置语法建模的。它们每个都可以使用不同的xxx键多次使用：
> %{xxx}i write value of incoming header with name xxx> %{xxx}o write value of outgoing header with name xxx> %{xxx}c write value of cookie with name xxx> %{xxx}r write value of ServletRequest attribute with name xxx> %{xxx}s write value of HttpSession attribute with name xxx> %{xxx}p write local (server) port (xxx==local) or remote (client) port> (xxx=remote)> %{xxx}t write timestamp at the end of the request formatted using the> enhanced SimpleDateFormat pattern xxx如您所见，可以使用很多字段，但是如果仍然需要更多字段，则必须编写自己的
AccessLogValve实现。
