This method serializes the specified object into its equivalent Json representation.
   * This method should be used when the specified object is not a generic type. This method uses
   * {@link Class#getClass()} to get the type for the specified object, but the
   * {@code getClass()} loses the generic type information because of the Type Erasure feature
   * of Java. Note that this method works fine if the any of the object fields are of generic type,
   * just the object itself should not be of a generic type. If the object is of generic type, use
   * {@link #toJson(Object, Type)} instead. If you want to write out the object to a
   * {@link Writer}, use {@link #toJson(Object, Appendable)} instead.
   *
   * @param src the object for which Json representation is to be created setting for Gson
   * @return Json representation of {@code src}.
   */
  public String toJson(Object src) {
    if (src == null) {
      return toJson(JsonNull.INSTANCE);
    }
    return toJson(src, src.getClass());
  }

它意味着获取一个对象示例，并将其转换为json字符串格式。
您要做的是将一个json字符串传递到该方法中。这行不通
你想要的是

gson.fromJson("yourStringJSONHere", YourClassWhichMatchesJSONVariables.class)

或者，如果您没有Map到json的类，只需要一个泛型类：

JsonElement jelement = new JsonParser().parse(yourJSONString);

使用gson for java解析json