聊聊AccessLogValve访问日志

Java class name of the implementation to use. This MUST be set to org.apache.catalina.valves.AccessLogValve to use the default access log valve.

可以定制访问日志的类，默认是AccessLogValve ，这个AccessLogValve 是按照自己的一套pattern的，在Tomcat中；

还有一个可选的Extended Access Log Valve 这个也是一个pattern，它主要针对于的是W3c规范的pattern，也就是https://www.w3.org/TR/WD-logfile.html 这个链接的规范； 

如果你足够牛B或者需要对默认的AccessLogValve进行修改，你自己也可以继承这个AccessLogValve  进行自定义的实现；

Absolute or relative pathname of a directory in which log files created by this valve will be placed. If a relative path is specified, it is interpreted as relative to $CATALINA_BASE. If no directory attribute is specified, the default value is "logs" (relative to $CATALINA_BASE).

访问日志的目录位置，默认就在logs下;

The prefix added to the start of each log file's name. If not specified, the default value is "access_log".

logfile文件名的前缀，默认就是access_log;

The suffix added to the end of each log file's name. If not specified, the default value is "" (a zero-length string), meaning that no suffix will be added.

logfile文件名的后缀，默认是“”；

Allows a customized timestamp in the access log file name. The file is rotated whenever the formatted timestamp changes. The default value is .yyyy-MM-dd
. If you wish to rotate every hour, then set this value to .yyyy-MM-dd.HH
. The date format will always be localized using the locale en_US
.

logfile文件名中会有基于年月日的部分，作为文件名中的中间部分；

Flag to determine if log rotation should occur. If set to false
, then this file is never rotated and fileDateFormat
 is ignored. Default value: true

日志轮转，目前的策略是按照天进行轮转，也就是每一天结束之后，第二天会创建一个新文件；

By default for a rotatable log the active access log file name will contain the current timestamp in fileDateFormat
. During rotation the file is closed and a new file with the next timestamp in the name is created and used. When setting renameOnRotate
to true
, the timestamp is no longer part of the active log file name. Only during rotation the file is closed and then renamed to include the timestamp. This is similar to the behavior of most log frameworks when doing time based rotation. Default value:false

如果设置了日志轮转，那么无论是原有文件和新文件中都有时间戳，而这个renameOnRotate 的意思是目前正在写入的访问日志中，名称中不包含这个时间戳，只有在轮转文件关闭，时间戳才被加上；

这个实际上是为了和一些日志框架做兼容，大多数日志框架为了区分正在写入的log，该文件通常都不会加时间戳；

A formatting layout identifying the various information fields from the request and response to be logged, or the word common
 orcombined
 to select a standard format. See below for more information on configuring this attribute.

对访问日志进行格式化，对于格式的写法，见Tomcat中有非常明确的定义：

Character set used to write the log file. An empty string means to use the system default character set. Default value: use the system default character set.

日志输出的字符集

The locale used to format timestamps in the access log lines. Any timestamps configured using an explicit SimpleDateFormat pattern (%{xxx}t
) are formatted in this locale. By default the default locale of the Java process is used. Switching the locale after the AccessLogValve is initialized is not supported. Any timestamps using the common log format (CLF
) are always formatted in the locale en_US
.

如果在访问日志中使用了预定义的 %{xxx}t，相当于使用timestamp，对于Locale时区的定义，就是通过这个属性；

Set to true
 to check for the existence of request attributes (typically set by the RemoteIpValve and similar) that should be used to override the values returned by the request for remote address, remote host, server port and protocol. If the attributes are not set, or this attribute is set to false
 then the values from the request will be used. If not set, the default value of false
 will be used.

如果该属性不设置，在访问日志中通过pattern记录的ip,port，协议等信息，都是从request中获得的；

而该属性的意思在于，可以通过RemoteIpValve对一些request的信息做一些手脚，是一个开关；

Turns on conditional logging. If set, requests will be logged only if ServletRequest.getAttribute()
 is not null. For example, if this value is set to important
, then a particular request will only be logged if ServletRequest.getAttribute("important") != null
. The use of Filters is an easy way to set/unset the attribute in the ServletRequest on many different requests.

对于reqeust的属性进行条件过滤，当request.getAttribute为null的时候，访问日志什么都不会记录；

或者你也可以指定，当reqeust的哪一个属性存在时，访问日志进行记录；

这个属性整体上来说，是一个条件记录的功能；

Turns on conditional logging. If set, requests will be logged only if ServletRequest.getAttribute()
 is null. For example, if this value is set to junk
, then a particular request will only be logged if ServletRequest.getAttribute("junk") == null
. The use of Filters is an easy way to set/unset the attribute in the ServletRequest on many different requests.

conditionIf的反过来；

The same as conditionUnless
. This attribute is provided for backwards compatibility.

和conditionUnless一样；

Flag to determine if logging will be buffered. If set to false
, then access logging will be written after each request. Default value:true

访问日志是否采用缓冲区的方式，实际上就是用Buffered缓冲输出流进行输出；

Log message buffers are usually recycled and re-used. To prevent excessive memory usage, if a buffer grows beyond this size it will be discarded. The default is 256
 characters. This should be set to larger than the typical access log message size.

如果设置了buffered 这个属性，那么需要找一个byte[]数组进行倒，这个属性就是设置这个数组大小的；

This attribute is no longer supported. Use the connector attribute enableLookups
 instead.

If you have enableLookups
 on the connector set to true
 and want to ignore it, use %a instead of %h in the value of pattern
.

该属性已不被支持了；