code: protected static ExpressionFactory getUserDefinedExpressionFactory(ExternalContext externalContext)
      {
          String expressionFactoryClassName
                  = WebConfigParamUtils.getStringInitParameter(externalContext, EXPRESSION_FACTORY);
          if (expressionFactoryClassName != null
                  && expressionFactoryClassName.trim().length() > 0)
          {
              if (log.isLoggable(Level.FINE))
              {
                  log.fine("Attempting to load the ExpressionFactory implementation "
                          + "you've specified: '" + expressionFactoryClassName + "'.");
              }
  
              return loadExpressionFactory(expressionFactoryClassName);
          }
  
          return null;
      }
docs: Try to load user-definied ExpressionFactory. Returns <code>null</code>,
  if no custom ExpressionFactory was specified.
  
  @param externalContext the current ExternalContext
  @return User-specified ExpressionFactory, or
  <code>null</code>, if no no custom implementation was specified