Restore the public access level of the Log4jMarker class to its pre-2.19.0 state

[AlexxeyKa]

Hello,

I would like to request that the org.apache.logging.slf4j.Log4jMarker class be made public once again, as it was prior to version 2.19.0.

Having markers in the LogEvent is a powerful mechanism for passing custom markers with custom fields inside. Previously, it was possible to implement a custom marker and pass it through the log4j logger as a Log4jMarker, which could then be caught in a custom FieldProvider, analyzed, and acted upon.

However, with the release of version 2.19.0, this functionality was removed by making the Log4jMarker class package-private. As a result, all custom markers are converted into Log4jMarker inside the Log4jMarkerFactory, and only the name and parents fields are retained, which is unacceptable for our use case.

Therefore, I request that the Log4jMarker class be made public once again, in order to allow for the same level of flexibility and customization that was available prior to version 2.19.0. Additionally, I would like to note that the Log4jMarker constructor remains public, so making the class itself public should not pose any security concerns.

Thank you for your consideration.

[vy]

Even though I don't agree with the way you use Log4jMarker, we should indeed not break the backward compatibility. Hence, I have fixed the issue. (Had a talk with @ppkarwasz and he will drop our further remarks here.)

All in all, feedback is feedback. Thanks for sharing the issue with us. 🙇

[ppkarwasz]

@AlexxeyKa,

The API breakage in 2.19.0 was unintentional and we'll revert it in the 2.x branch. As for the 3.x branch I would keep the class hidden.

The reasons behind this are largely explained in #770: using markers to convey additional data is IMHO a hack to workaround SLF4J's limitations. Unlike the Log4j API that allows to log any kind of Message, for a very long time SLF4J only allowed to log Strings (or the equivalent of ParameterizeMessage).

The situation changed with the release of SLF4J 2.x:

• using LoggingEventBuilder#addKeyValue you can attach any kind of additional data to the logging event.

As discussed in #1813 the bridges between SLF4J and Log4j API will (probably) map KeyValuePairs to entries in a MapMessage and viceversa.

@rgoers, am I missing something?