code: @Override
      public String readObjectB(final Class clazz) {
          this.fieldIndex = 0; //必须要重置为0
          if (this.text.length == 0) return null;
          char ch = this.text[++this.position];
          if (ch == '{') return "";
          if (ch <= ' ') {
              for (;;) {
                  ch = this.text[++this.position];
                  if (ch > ' ') break;
              }
              if (ch == '{') return "";
          }
          if (ch == 'n' && text[++position] == 'u' && text[++position] == 'l' && text[++position] == 'l') return null;
          if (ch == 'N' && text[++position] == 'U' && text[++position] == 'L' && text[++position] == 'L') return null;
          throw new ConvertException("a json object text must begin with '{' (position = " + position + ") but '" + ch + "' in (" + new String(this.text) + ")");
      }
docs: 判断下一个非空白字符是否为{
  
  @param clazz 类名
  
  @return 返回 null 表示对象为null， 返回空字符串表示当前class与返回的class一致，返回非空字符串表示class是当前class的子类。