code: private String _getRenderFlashMapTokenFromPreviousRequest(FacesContext facesContext)
      {
          ExternalContext externalContext = facesContext.getExternalContext();
          String tokenValue = null;
          ClientWindow clientWindow = externalContext.getClientWindow();
          if (clientWindow != null)
          {
              if (facesContext.getApplication().getStateManager().isSavingStateInClient(facesContext))
              {
                  Map<String, Object> sessionMap = externalContext.getSessionMap();
                  tokenValue = (String) sessionMap.get(FLASH_RENDER_MAP_TOKEN+
                          SEPARATOR_CHAR+clientWindow.getId());                
              }
              else
              {
                  FlashClientWindowTokenCollection lruMap = getFlashClientWindowTokenCollection(externalContext, false);
                  if (lruMap != null)
                  {
                      tokenValue = (String) lruMap.get(clientWindow.getId());
                  }
              }
          }
          else
          {
              HttpServletResponse httpResponse = ExternalContextUtils.getHttpServletResponse(externalContext);
              if (httpResponse != null)
              {
                  //Use a cookie
                  Cookie cookie = (Cookie) externalContext.getRequestCookieMap().get(FLASH_RENDER_MAP_TOKEN);
                  if (cookie != null)
                  {
                      tokenValue = cookie.getValue();
                  }
              }
              else
              {
                  //Use HttpSession or PortletSession object
                  Map<String, Object> sessionMap = externalContext.getSessionMap();
                  tokenValue = (String) sessionMap.get(FLASH_RENDER_MAP_TOKEN);
              }
          }
          return tokenValue;
      }
docs: Retrieve the map token of the render map from the previous request.
  
  Returns the value of _saveRenderFlashMapTokenForNextRequest() from
  the previous request.
  @param externalContext
  @return