1、google的Gson
http://code.google.com/p/google-gson/
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.
2、Json-lib
http://sourceforge.net/projects/json-lib/
Json-lib is a java library for transforming beans, maps, collections, java arrays and XML to JSON and back again to beans and DynaBeans. It is based on the work by Douglas Crockford in http://www.json.org/java.