Explain the internal working of Grails' convention-based URL mapping?

Grails resolves an incoming URL against a prioritized set of mapping sources, checking explicit application-defined patterns before falling back to the generic controller/action convention, so both approaches can coexist in the same application.

Explicit patterns declared in UrlMappings.groovy are checked first, since they represent an intentional override of the default behavior; a URL segment like /products/$id in such a mapping automatically extracts id into the request's params map. If no explicit pattern matches, Grails falls back to parsing the path directly as /controller/action/id, looking for a controller class and action method whose names match those segments, ultimately falling back to an index action if no specific action segment was given at all.

This two-tier resolution — explicit mappings first, convention-based fallback second — is what lets an application define a handful of clean, custom URLs for its most important routes while every other controller keeps working automatically under the default convention without needing its own explicit mapping entry.