Does any has any experience with converting string geojson into com.vividsolutions.jts geometry object (point, polygon, etc). Maybe there is some java library I cannot find.
For a much more lightweight alternative to GeoTools, check out jts2geojson:
GeoJSONReader reader = new GeoJSONReader(); Geometry geometry = reader.read(json);
There seems to be nowadays also native GeoJSON reader/writer code in JTS trunk:
OpenJUMP can read and write GeoJSON with code that is in https://sourceforge.net/p/jump-pilot/code/HEAD/tree/core/trunk/src/com/vividsolutions/jump/io/geojson/. Perhaps it could be reused.