java - log4j2 访问 env 属性

Question

log4j2文档中提到读取env属性我们必须使用$${env:USERNAME}

我发现带有单个 $ 的 ${env:USERNAME} 仍然有效。

有谁知道我们为什么要使用$$？!

Answer 1

来自http://logging.apache.org/log4j/2.x/manual/configuration.html手册页:

An interesting feature of StrLookup processing is that when a variable reference is declared with multiple leading '$' characters each time the variable is resolved the leading '$' is simply removed. In the previous example the "Routes" element is capable of resolving the variable at runtime. To allow this the prefix value is specified as a variable with two leading '$' characters. When the configuration file is first processed the first variable is simply removed. Thus, when the Routes element is evaluated at runtime it is the variable declaration "${sd:type}" which causes the event to be inspected for a StructuredDataMessage and if one is present the value of its type attribute to be used as the routing key. Not all elements support resolving variables at runtime. Components that do will specifically call that out in their documentation.