I prefer Log4J2 built-in JSON Template Layout to Logback's various JSON output solution.
The API of Log4J2 and SLF4J v2 are similar enough that it does not matter for basic usage, I don't see the benefit of SLF4J in a project that's already using Log4J2.
The API of Log4J2 and SLF4J v2 are similar enough that it does not matter for basic usage, I don't see the benefit of SLF4J in a project that's already using Log4J2.