code: public JSONObject getMessage() throws ActivityException {
          String message = null;
          String slackMessageName = getAttributeValueSmart(SLACK_MESSAGE);
          if (slackMessageName == null)
              throw new ActivityException("slack message attribute is not set");
          Asset template = AssetCache.getAsset(slackMessageName);
  
          if (template == null) {
              message = slackMessageName;
          }
          else {
              message = context.evaluateToString(template.getStringContent());
          }
  
          JSONObject json = new JSONObject();
          String env = ApplicationContext.getRuntimeEnvironment().toUpperCase();
          json.put("text", env + " - " + getSlackPrefix() + " - " + message);
  
          String altText = null;
          if (json.has("text")) {
              String text = json.getString("text");
              if (text.length() > 200)
                  altText = text.substring(0, 197) + "...";
          }
          if (altText != null)
              json.put("text", altText);
          return json;
      }
docs: /*
  The source of Slack message can be either Json asset or  process variable.