Hi Benjamin,
On Thu, 10 Aug 2023 at 14:45, Benjamin Krahl <benjamin.kr...@gmail.com> wrote:
>
> Currently SLF4JLogger in Log4j 2 will format all messages before it passes
> these to SLF4J. I think this does not need to be the case.
>
> SLF4J FAQ <https://www.slf4j.org/faq.html#string_contents> states that:
> "Assuming complexObject is an object of certain complexity, for a log
> statement of level DEBUG, you can write:
>
> logger.debug("{}", complexObject);
>
> The logging system will invoke complexObject.toString() method only after
> it has ascertained that the log statement was enabled. Otherwise, the cost
> of complexObject.toString() conversion will be advantageously avoided."
>
> When Log4j 2 wraps the Message, SLF4J can invoke the formatting
> conditionally by a call of the overridden toString().
Both the Log4j API and SLF4J behave in the same way: the message is
formatted into a string if and only if it is going to be logged.
Therefore, in the current implementation `Object#toString()` is called
only if it is necessary.
Moreover, since the formatting performed by the Log4j API is
garbage-free (under certain conditions), while most SLF4J backends are
not, the overall user experience should be better.
The only disadvantage of the current implementation of
`log4j-to-slf4j` is that the argument array is not passed to the SLF4J
backend. If the backend makes use of this array (e.g. it transforms it
to JSON), the information is lost. I see two approaches to solve this:
* we can delegate formatting to SLF4J for some kinds of messages
(ParameterizedMessage, ReusableParameterizedMessage) by passing to
SLF4J `Message#getFormat()` and `Message#getParameters()`. I don't
like this solution since statistically it will generate more garbage
and we can not apply it to all messages: e.g. an ObjectMessage should
rather be logged with a format `{}` and a single object parameter,
* since the introduction of `LoggingEventAware` in SLF4J 2.x, we could
create our own `LoggingEvent` that wraps Log4j's `Message` and log it
directly to SLF4J.
If you are willing to submit a PR, I think this is the latter is the way to go.
Piotr
[1] https://www.slf4j.org/api/org/slf4j/spi/LoggingEventAware.html
