Log4j vs. Logback 性能比较Inside

看过源码有段时间了，对logback中对判定是否记录一条日志记录的优化印象比较深刻，整理如下，希望对你有所帮助。

Java的logger框架有一个重要的层级（hierarchy）的概念，按约定，是通过小数点来区别父子关系的，也就是名字为“x.y”的logger是“x.y.z”logger的parent。

Hierachy的命名规则：

Named Hierarchy: A logger is said to be an ancestor of another logger if its name followed by a dot is a prefix of the descendant logger name. A logger is said to be a parent of a child logger if there are no ancestors between itself and the descendant logger.

这样做的好处是我们可以灵活的指定不同logger的log level和Appender targets。

Logger Name	Assigned Level	Inherited Level	Added Appenders	Additivity Flag	Output Targets	Comment
root	warn	warn	A1	not applicable	A1	The root logger is anonymous but can be accessed with the Logger.getRootLogger() method. There is no default appender attached to root.
x	info	info	A-x	true	A1, A-x	Appenders of "x" and root.
x.y	none	info	A-xy	false	A1, A-x, A-xy	Appenders in "x.y" , and its parents "x" and root. Log level is info which is inherited from "x"
x.y.z	debug	debug	A-xyz	true	A-xyz	Appenders of "x.y.z". becuase its parent "x.y" addivitity is false, so no more appenders will be added

因为这个hierachy的关系，在做日志输出时，logger不仅要check自己是不是满足输出条件，还要遍历其所有的Additivity为非false的parent，看parents们能不能输出日志。

下图是一个Filter和Handler在Logger Hierarchy中运作的示意图，其运行逻辑是：

When a message is passed to a Logger, the message is passed through the Logger's Filter, if the Loggerhas a Filter set. The Filter can either accept or reject the message. If the message is accepted, the message is forwarded to the Handler's set on the Logger. If no Filter is set, the message is always accepted.

If a message is accepted by the Filter, the message is also forwarded to the Handler's of the parentLogger's. However, when a message is passed up the hierarchy, the message is not passed through theFilter's of the parent Logger's. The Filter's are only asked to accept the message when the message is passed directly to the Logger, not when the message comes from a child Logger.