How does slf4j bind to implementation? Does it really do so during compile time?

Question

In the documentation for slf4j it says that the binding happens during complie time:

\"SLF4J does not rely on any special class loader machinery. In fact, each SLF4J bi

Answer 1

It was my question too and I'd like to add my answer, since found the other two answer not clear enough (though are perfectly correct).

First, check this line in the implementation of LoggerFactory.bind() in slf4j-api (link)

// the next line does the binding
StaticLoggerBinder.getSingleton();

There is a class called org.slf4j.impl.StaticLoggerBinder. Check its implementation on github.

Now go ahead and download the slf4j-api.jar from the central maven repository, extract it and find StaticLoggerBinder.class file.

Don't try! You can't. In fact the whole org.slf4j.impl has been removed from the package. Check the pom.xml of the project:

  
    org.apache.maven.plugins
    maven-antrun-plugin
    
      
        process-classes
        
         run
        
      
    
    
      
        Removing slf4j-api's dummy StaticLoggerBinder and StaticMarkerBinder
        
      
    
  

Last, check one of the SLF4j's binding packages, for example slf4j-simple. Can you find org.slf4j.impl.StaticLoggerBinder class?

In sum, when you have slf4j-api.jar alongside one (and only one) of the binding packages in you runtime environment, you have only one org.slf4j.impl.StaticLoggerBinder class which does the binding.