java - log4j2 双美元 $$ 符号在配置中的含义

Question

我正在阅读 Log4j2 的配置部分。 http://logging.apache.org/log4j/2.x/manual/configuration.html

<Appenders>
    <Console name="STDOUT">
      <PatternLayout pattern="%m%n"/>
    </Console>
    <List name="List">
      <ThresholdFilter level="debug"/>
    </List>
    <Routing name="Routing">
      <Routes pattern="$${sd:type}">
        <Route>
          <RollingFile name="Rolling-${sd:type}" fileName="${filename}"
                       filePattern="target/rolling1/test1-${sd:type}.%i.log.gz">
            <PatternLayout>
              <pattern>%d %p %c{1.} [%t] %m%n</pattern>
            </PatternLayout>
            <SizeBasedTriggeringPolicy size="500" />
          </RollingFile>
        </Route>
        <Route ref="STDOUT" key="Audit"/>
        <Route ref="List" key="Service"/>
      </Routes>
    </Routing>
  </Appenders>

双$$符号是什么意思？例如$${sd:type}?

Answer 1

好像是用$作为转义字符。如 Log4J documentation 中所述, Log4j 配置文件解析器使用Apache Commons Lang's StrSubstitutor ，而这个 StrSubstitutor 的文档说:

The other possibility is to use the escape character, by default '$'. If this character is placed before a variable reference, this reference is ignored and won't be replaced. For example:

The variable $${${name}} must be used.

我猜他们想将值设置为 "${sd:type}" 以便稍后可以在运行时评估此变量。这里有一个很好的例子/解释:http://logging.apache.org/log4j/2.x/manual/lookups.html#ContextMapLookup