EDIT: as pointed out by commenters, module is being deprecated, not maintained. So, Caveat Emptor etc
One such tool is Jackson JSON Schema module:
https://github.com/FasterXML/jackson-module-jsonSchema
which uses Jackson databind’s POJO introspection to traverse POJO properties, taking into account Jackson annotations, and produces a JSON Schema object, which may then be serialized as JSON or used for other purposes.