The current design limits possible performance improvements that could be obtained by custom parser implementations. A cleaner design would let the implementation provide both parser and object model implementations that are better integrated and higher performing.
If we implement http://java.net/jira/browse/JSON_PROCESSING_SPEC-9 a json structure could consist of a mix between implementations from the api package and implementation defined classes.
Builders would be created by static methods of the Json class or a JsonBuilderFactory created via the Json class.
JsonReader would require a JsonReaderFactory with similar methods to JsonParserFactory.
Discussion at http://java.net/projects/json-processing-spec/lists/users/archive/2012-12/message/9 and http://java.net/projects/json-processing-spec/lists/users/archive/2012-12/message/54