vy commented on issue #2666:
URL:
https://github.com/apache/logging-log4j2/issues/2666#issuecomment-2176884248
@ngocnhan-tran1996, I collaborated on this issue with @ppkarwasz, and I
decided to not update JTL (JSON Template Layout) to perform property
substitution on resolver configuration objects. Yet the issue you reported
about the trailing `}` is indeed a `StrSubstitutor` bug, which I will address
in another ticket.
### JTL and property substitution
JTL supports property substitution on string members of a JSON object,
granted they are not part of a resolver configuration. Let me make this clear
with an example:
```json
{
"x": "${bar}",
"y": {
"$resolver": "...",
"z": "${bar}"
}
}
```
`${bar}` for `x` will be substituted at *configuration-time* (i.e., while
compiling JTL templates), whereas `${bar}` of `z` will not be substituted,
since it is part of a *template configuration object*. In #2667, I tried
various approaches to improve the situation, but it always caused other issues.
It is important to understand property substitution gets performed at three
levels in your example:
* For literals in `log4j2.xml` (at configuration-time)
* For literals in a JTL template (at configuration-time)
* By Pattern Layout wired by `pattern` resolver (at runtime)
When you use `${env:TZ:-${TIMEZONE}}`, you actually want this to happen at
configuration-time, since
1. For performance reasons, it better be executed once instead of for every
log event
2. It accesses to a *configuration* property: `TIMEZONE`
Hence, it doesn't make sense to access to a Log4j configuration property
(`TIMEZONE`) from an external JSON file, i.e., event template provided by the
`eventTemplateUri` attribute. That said, as I tried to explain, if you would
have inlined your template into `log4j2.xml` using the `eventTemplate`
attribute, `${env:TZ:-${TIMEZONE}}` would have just worked fine.
I advise you to inline your _"JTL template fields accessing to configuration
properties"_ using [event template additional
fields](https://logging.staged.apache.org/log4j/2.x/manual/json-template-layout.html#plugin-element-EventTemplateAdditionalField):
```json
<JsonTemplateLayout eventTemplateUri="classpath:MyLayout.json">
<EventTemplateAdditionalField
key="instant"
format="JSON"
value='{"$resolver": "pattern", "pattern": "${env:TZ:-%d{yyyy-MM-dd
HH:mm:ss}}"}'/>
```
All `${foo}` in the above example will be subject to property substitution
at configuration-time.
I will repurpose #2667 to improve JTL docs on this subject.
### Notes on time zone
It is also peculiar that you need to change the time zone in your
application depending on an environment variable. Would you mind sharing a
little bit more detail about your use case, please? Where do you eventually
persist/forward these logs? Why not sticking to UTC doesn't work for you?
### Notes on trailing `}`
You stated
> I dont know why `message1` and `message3` have brace `}` at the end
It is a `StrSubstitutor` bug, you can also reproduce it with `PatternLayout`
too:
```xml
<PatternLayout pattern="${env:TZ:-%d{yyyy-MM-dd HH:mm:ss}{GMT+00}}"/>
```
I will create a separate issue for this.
### Conclusion
In short,
* We don't think JTL should be fixed
* I will update JTL docs on this subject
* You can/should use `EventTemplateAdditionalField`s
* Trailing `}` is a bug, which I will address in another ticket/PR
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