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log4j2 double dollar $$ sign meaning in configuration

I am reading the configuration part of Log4j2.

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

What is the meaning of double $$ sign? e.g. $${sd:type}?



It seems that $ is used as an escape character. As stated in Log4J documentation, Log4j configuration file parser uses Apache Commons Lang’s StrSubstitutor, and this documentation for StrSubstitutor says:

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.

I guess they want to set the value to "${sd:type}" so that this variable can be evaluated later at run-time. There is a good example/explanation here: