A lot of warnings are emitted since Spring Boot 3.4.0-M3 which messages follow this form: The Logger XXX was created with the message factory org.apache.logging.log4j.message.ReusableMessageFactory@YYY and is now requested with a null message factory (defaults to org.apache.logging.log4j.message.ParameterizedMessageFactory), which may create log events with unexpected formatting.
To reproduce, just create a simple new demo project with Spring Initializr and use Log4j 2 for logging like this one: https://github.com/romainmoreau/log4j-demo Here's an execution log of the described demo with the warnings: https://github.com/romainmoreau/log4j-demo/actions/runs/10973215278/job/30470356576#step:4:37 The warnings don't appear with the same demo with Spring Boot 3.4.0-M2: https://github.com/romainmoreau/log4j-demo/actions/runs/10973225142/job/30470377223#step:4:882
Edit 1: with the 3.4.0-M3 demo, if I rollback to Log4j 2.23.1, the warnings are gone too. I'm not sure the issue is about Spring Boot misusing the new Log4j version or a bug in Log4j itself.
Edit 2: well it seems to be a Log4j issue: https://github.com/apache/logging-log4j2/issues/2933