I realize the ticket title may be confusing.  Consider this example.  We can already provided hard-coded values to attributes such as ResponseHeaderAttribute.

%{o,my-header}

But what if I wanted to make the name of the header I was setting dynamic based upon the value of another attribute?

%{o,%{q,headerName}} 

That example would look for a query param named "headerName" and then use its value as the name of the response header.

There are a handful of exchange attribute which accept text which is currently limited to a static string, that I'd like to see start parsing the input to treat it as an attribute (which may very well be a ConstantAttribute).  This will make the predicate language more flexible and also allow for transformations as defined in UNDERTOW-2120.

• ResponseHeaderAttribute
• RequestHeaderAttribute
• QueryParameterAttribute
• CookieAttribute
• The new string transformer attributes spec'd in UNDERTOW-2120

Please note, the exchange attribute parser needs a fix applied to allow parsing of an attribute that nests another attribute. https://issues.redhat.com/browse/UNDERTOW-2273

The builder for these attributes would still accept a string token, but would run it through the ExchangeAttributeParser and internally would store an ExchangeAttribute instead of a String which would be read at the point of use.