code: private void evaluateExpressions() throws JspException {
          /* 
           * Note: we don't check for type mismatches here; we assume
           * the expression evaluator will return the expected type
           * (by virtue of knowledge we give it about what that type is).
           * A ClassCastException here is truly unexpected, so we let it
           * propagate up.
           */
  
          xml = ExpressionUtil.evalNotNull(
                  "parse", "xml", xml_, Object.class, this, pageContext);
          systemId = (String) ExpressionUtil.evalNotNull(
                  "parse", "systemId", systemId_, String.class, this, pageContext);
  
          try {
              filter = (XMLFilter) ExpressionUtil.evalNotNull(
                      "parse", "filter", filter_, XMLFilter.class, this, pageContext);
          } catch (NullAttributeException ex) {
              // explicitly let 'filter' be null
              filter = null;
          }
      }
docs: /* Evaluates expressions as necessary