The best replacement for gson is jackson.

The best alternatives to gson are rated as following:
★★★★★  jackson
★★★★  json
★★★  jackson android
★★★  json simple
★★  xstream
★  json-smart
★  protobuf
  flexjson

Your alternative for gson is not listed? Suggest!
Speaking about gson, one probably shouldn't forget about jersey.

Other search terms related to gson and their ratings are:
★★★★★ jersey
★★★★ volley
You have in mind a companion for gson and it is not yet here? Propose!



Google Trends

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