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)