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 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.
What is Jackson?
Jackson is a:
Streaming (reading, writing)
FAST (measured to be faster than any other Java json parser and data binder)
Powerful (full data binding for common JDK classes as well as any Java bean class, Collection, Map or Enum)
Zero-dependency (does not rely on other packages beyond JDK)
Open Source (LGPL or AL)
Fully conformant
Extremely configurable
JSON processor (JSON parser + JSON generator) written in Java. Beyond basic JSON reading/writing (parsing, generating), it also offers full node-based Tree Model, as well as full OJM (Object/Json Mapper) data binding functionality.