code: public void init(ServletRequest request)
      {
          String[] vals = request.getParameterValues(ITEM_PARAM);
  
          if (vals != null) {
              _retrieveMap = new HashMap();
  
              for (int i = 0; i < vals.length; i++) {
                  String val = vals[i];
                  int delimPos = val.indexOf(DELIM);
                  if (delimPos != -1) {
                      String expressionToRetrieve = val.substring(0, delimPos);
                      String fieldID = val.substring(delimPos + 1);
                      _retrieveMap.put(fieldID, expressionToRetrieve);
                  }
              }
          }
  
          _callbackFunc = request.getParameter(CALLBACK_PARAM);
      }
docs: Initialize, based on request parameters we're looking for.