code: public static String json(Object obj, boolean writeClass,
                                Converter<Object, Map<String, Object>> mc) throws IOException {
          if (obj == null)
              return NULL;
          StringWriter sw = new StringWriter();
          try {
              json(obj, sw, writeClass, mc);
              return sw.getBuffer().toString();
          } finally {
              sw.close();
          }
      }
docs: json string.
  
  @param obj object.
  @return json string.
  @throws IOException.