Gson基本操作，JsonObject，JsonArray，String，JavaBean，List互转

String、JsonObject、JavaBean 互相转换
    User user = new Gson() .fromJson(jsonObject, User .class);
    User user = new Gson() .fromJson( string, User .class);
    String string = new Gson() .toJson(user);
    JsonObject jsonObject = new Gson() .toJsonTree(user) .getAsJsonObject();
    JsonObject jsonObject = new JsonParser() .parse( string) .getAsJsonObject();
String、JsonArray、 List互相转换
    List <User > userList = gson .fromJson( string, new TypeToken < List <User >>() {} .getType());
    List <User > userList = gson .fromJson(jsonArray, new TypeToken < List <User >>() {} .getType());
    String string = new Gson() .toJson(userList);
    JsonArray jsonArray = new Gson() .toJsonTree(userList, new TypeToken < List <User >>() {} .getType()) .getAsJsonArray();
    JsonArray jsonArray = new JsonParser() .parse( string) .getAsJsonArray();