A Java serialization library that can convert Java Objects into JSON and back.
Gson is a Java library that can be used to convert Java Objects into their JSON representation. It can also be used to convert a JSON string to an equivalent Java object. Gson can work with arbitrary Java objects including pre-existing objects that you do not have source-code of. 
There are a few open-source projects that can convert Java objects to JSON. However, most of them require that you place Java annotations in your classes; something that you can not do if you do not have access to the source-code. Most also do not fully support the use of Java Generics. Gson considers both of these as very important design goals. 
Gson Goals
- Provide simple toJson() and fromJson() methods to convert Java objects to JSON and vice-versa
 - Allow pre-existing unmodifiable objects to be converted to and from JSON
 - Extensive support of Java Generics
 - Allow custom representations for objects
 - Support arbitrarily complex objects (with deep inheritance hierarchies and extensive use of generic types)
 
Gson Downloads
- Gson 2.4 Download downloads at Maven Central
 
Gson Documentation
- Gson API: Javadocs for the current Gson release
 - Gson user guide: This guide contains examples on how to use Gson in your code.
 - Gson Roadmap: Details on upcoming releases
 - Gson design document: This document discusses issues we faced while designing Gson. It also include a comparison of Gson with other Java libraries that can be used for Json conversion
 
Please use the google-gson Google group to discuss Gson, or to post questions. 
Gson-related Content Created by Third Parties
- Gson Tutorial by 
StudyTrails 
from https://github.com/google/gson
https://github.com/dimzon/gson