This document discusses how to use a custom transformer in Mule to directly transform an XML payload to JSON. It provides an example of a Mule flow that uses a custom Java transformer class to take the XML from a file, convert it to a JSON string using Jackson libraries, and write the JSON output to a new file. The custom transformer allows transforming the payload in a single step without using multiple standard transformers. The example demonstrates how to implement and use a basic custom transformer to achieve direct XML to JSON conversion in Mule flows.