Any array used as parameter shows up unreadable.

It should instead use the same toString logic used by MessageFormatter.

I'd have submitted a patch/branch for this issue but - unfortunately - SLF4J MessageFormatter does not have a method that's performing just the String conversion.

Please see
http://github.com/huxi/lilith/blob/master/lilith-data/logging/src/main/java/de/huxhorn/lilith/data/logging/MessageFormatter.java
especially the method
String deepToString(Object o)

Because of this, a fix (providing and using said method) would span over both SLF4J and Logback.