code: public String getSourceBody(UnitRequest msg) {
          String result;
          String source = msg.get("$url", String.class);
          if (source == null || source.trim().length() <= 0) {
              result = msg.argJson().toJSONString();
          } else {
              result = msg.get("$body", String.class);
          }
          return result;
      }
docs: 获取请求内容