Re: Do Route-keys of Routing appender of Log4j2 need to use "$$" or "$" for unresolved expressions?

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Hi Thorsten,

On 6.05.2025 09:46, Thorsten Schöning wrote:
> I have the following two configs, which seem to behave the same:
>
> ```xml
> <Routing name="[...]">
> 	<Routes pattern="$${ctx:foo}">
> 		<Route key="true"        ref="RollingFileFoo" />
> 		<Route key="$${ctx:foo}" ref="RollingFile"    />
> 	</Routes>
> </Routing>
> ```
>
> vs.
>
> ```xml
> <Routing name="[...]">
> 	<Routes pattern="$${ctx:foo}">
> 		<Route key="true"       ref="RollingFileFoo" />
> 		<Route key="${ctx:foo}" ref="RollingFile"    />
> 	</Routes>
> </Routing>
> ```
>
> I'm a bit confused which the correct value is because both seem to work exactly the same. Which shouldn't happen if the `key` attribute implements an equals comparison to it's value.

There is subtle difference between the two configurations:

  * In the first case the value of `key` will always be a literal
    `${ctx:foo}`.
  * In the second case Log4j Core will expand `${ctx:foo}` at
    configuration time. Since the configuration of Log4j Core happens
    very early in the startup sequence, `foo` will almost certainly be
    undefined, so `key` will be a literal `${ctx:foo}`. In the very
    unlikely case `foo` is defined at configuration time, however, the
    value of `foo` will be substituted and your configuration will not
    work as expected.

To see the difference, try adding:

static {
ThreadContext.put("foo", "true");
}

private static final Logger logger = LogManager.getLogger();

at the very beginning of the main class of your application.

Piotr

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