Annotation Processing Tool To Scan A Java Project In A Different Directory

We want to write an annotation processor to generate a json file based on the data of certain custom annotations. Let's call this annotation processor project jsongenerator. We want jsongenerator to be in a separate repo (lets say as a standalone CLI tool) which we run on the java project which we want to generate a json for (based on the annotation it uses).

So the question is, provided a "directory path" as an argument (which would link to the path of a different java project). E.g.

jsongenerator --dirPath=/path/to/java/project/source

How would one read the third party java project source to run annotation processing on it?