code: @Trivial
      private String dumpMap(Map<String, String[]> m) {
          StringBuffer sb = new StringBuffer();
          sb.append(" --- request parameters: ---\n");
          Iterator<String> it = m.keySet().iterator();
          while (it.hasNext()) {
              String key = it.next();
              String[] values = m.get(key);
              sb.append(key + ": ");
              for (String s : values) {
                  sb.append("[" + s + "] ");
              }
              sb.append("\n");
          }
          return sb.toString();
      }
docs: dump parameter map for trace.