How does simply adding slf4j to the pom.xml wrap log4j?

Question

From what I have seen in example spring pom.xml files is that they add a few entries for slf4j and log4j and somehow when you use log4j in your spring application it will be

Answer 1

slf4j is a logging API, which doesn't do anything, just bunch of interfaces. log4j is a logging system, with concrete classes. there is a slf4j-log4j library which uses log4j as a backend for the slf4j API.

Some projects explicitly depend on log4j, they call concrete classes. So, you cannot use another backend (e.g. logback or j.u.l or apache commons or whatever) for your project which you wisely made using the slf4j API only.

There is a trick to substitute log4j classes by a mock implementation (the bridge) which just simply redirects all calls to the sl4j. In maven you just declare a dependency with very high version number and this mock considered as ultra-modern log4j library.