The ValueStack is a context for the beans evaluated via the EL expressions.
The ValueStack is a context for the beans evaluated via the EL expressions. ValueStack allows multiple beans to be pushed in and dynamic EL expressions to be evaluated against it. When evaluating an expression, the stack will be searched down the stack, from the latest objects pushed in to the earliest, looking for a bean with a getter or setter for the given property or a method of the given name (depending on the expression being evaluated).
One of the known implementations of the ValueStack is OGNL (Object Graph Notation Language). OGNL allows for dynamic OGNL expressions to be evaluated against it.
The XWork provides extension to OGNL and support for the ValueStack. While OGNL operates under the assumption there is only one "root", XWork's ValueStack concept requires there be many "roots".
The value value stack internals page: http://struts.apache.org/docs/value-stack-internals
