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Official MongoDB Java Driver comes with utility methods for parsing JSON to BSON and serializing BSON to JSON.
import com.mongodb.DBObject;
import com.mongodb.util.JSON;
DBObject dbObj = ... ;
String json = JSON.serialize( dbObj );
DBObject bson = ( DBObject ) JSON.parse( json );
The driver can be found here: https://mongodb.github.io/mongo-java-driver/