Remko Popma commented on LOG4J2-1597:

About the Appenders manual page documentation for this feature: it may be 
better to not use List in the example configuration.

Users are likely to use this feature for a specific use case. I think the 
original use case Gary mentioned is actually a very good use case illustrating 
when this feature can be useful, much more than the ability to choose between 
two Lists. The example configuration could show a simplified 
"CustomOS390FileAppender", which clearly should only be initialized when 
actually running on OS/390.

Also, List appender is a test appender and not mentioned anywhere else on the 
site (and no need to do so since it is only for testing).

> Add a ScriptAppenderSelector to create an Appender specified by a Script
> ------------------------------------------------------------------------
>                 Key: LOG4J2-1597
>                 URL: https://issues.apache.org/jira/browse/LOG4J2-1597
>             Project: Log4j 2
>          Issue Type: New Feature
>          Components: Appenders
>            Reporter: Gary Gregory
>            Assignee: Gary Gregory
>             Fix For: 2.7
> The use case is to create an OS-based dynamic configuration file.
> The script determines which appender to use (once), and instantiates that 
> appender (once). There is no need for one appender to delegate to another 
> appender.
> This feature helps you build your configuration dynamically, all from the 
> configuration file, to determine which appenders to configure. This is 
> different from using a RoutingAppender which creates a level of indirection 
> and decides what to do for each log event _at runtime_.
> You can use multiple ScriptSelector Appenders if needed.
> For example:
> {code:xml}
> <Configuration status="WARN" name="RoutingTest">
>   <Appenders>
>     <ScriptAppenderSelector name="InitAppender">
>       <Script language="JavaScript"><![CDATA[
>          "OSNameFoo".search("Foo") > -1 ? "List2" : "List1";]]>
>       </Script>
>       <AppenderSet>
>         <List name="List1" />
>         <List name="List2" />
>       </AppenderSet>
>     </ScriptAppenderSelector>
>   </Appenders>
>   <Loggers>
>     <Root level="error">
>       <AppenderRef ref="InitAppender" />
>     </Root>
>   </Loggers>
> </Configuration>
> {code}
> The new plugins are ScriptSelector (an Appender that builds a different 
> ScriptSelector) and AppenderSet, a plugin with deferChildren set to true.

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