code: private void evaluateExpressions() throws JspException {
  
          // 'basename' attribute (mandatory)
          basename = (String) ExpressionEvaluatorManager.evaluate(
                  "basename", basename_, String.class, this, pageContext);
  
          // 'prefix' attribute (optional)
          if (prefix_ != null) {
              prefix = (String) ExpressionEvaluatorManager.evaluate(
                      "prefix", prefix_, String.class, this, pageContext);
          }
      }
docs: Evaluates expressions as necessary