code: @Override
      public void flush() throws IOException {
          if (null != baos) {
              baos.flush();
              if (baos.size() > 0) {
                  if (charset == null)
                      sb.append(new String(baos.toByteArray()));
                  else
                      sb.append(new String(baos.toByteArray(), charset));
                  baos.reset();
              }
          }
      }
docs: 使用StringBuilder前,务必调用