Slf4j is not a real logging facade.
Slf4j does not support many features of its implementors.
For short, I mention log4j examples below.
- Slf4j cannot specify a user selected configuration file, but forces user to use default (log4j.properties or log4j.xml) at one of so many Java roots (each Jar has one root plus JVM root and classes or bin). If two JAR files have it, it is hard to control which one to use safely.
- Slf4j cannot support all Log4j levels, such as 'fatal'. When switch big code from Log4j to Slf4j, huge code change effort is needed (e.g. deciding how to rearrange levels).
- Two key Jar files (log4j-over-slf4j.jar or slf4j-log4j12.jar) must be chosen. If classpath both, won't work. If choose one randomly, lose unexpected features (e.g. log4j-over-slf4j.jar does not support multiple log files for same classes; e.g. one for events log and one for raw data log).
