There are web sites like http://mvnrepository.com/ letting to search for an "artifact" and giving a definition I can add to my project for a library to be downloaded automatically from a public repository.
But this particular example (mvnrepository.com) (the only working I know actually) is by-definition limited to "search by group, artifact or description" while I would like to search by a class full name. For example I'd like to enter org.apache.commons.lang.NotImplementedException to get <dependency org="commons-lang" name="commons-lang" rev="2.6"/> (or alike) as the answer. Is there a facility for this?
The answer of @khmarbaise is straight and correct, but if you possibly want to search over few additional public Maven repos (like JBoss), not only Maven Central, I recommend using http://grepcode.com.
EDIT: Unfortunately it seems http://grepcode.com, I used to strongly recommend for such a stuff, is dead now. Currently I don't know a better place than https://search.maven.org, already answered by @khmarbaise.