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The biggest addition that XWork provides on top of OGNL is the support for the ValueStack. While OGNL operates under the assumption there is only one "root", XWork's ValueStack concept requires there be many "roots".
For example, suppose we are using standard OGNL (not using XWork) and there are two objects in the OgnlContext map: "foo" -> foo and "bar" -> bar and that the foo object is also configured to be the single root object. The following code illustrates how OGNL deals with these three situations:
What this means is that OGNL allows many objects in the context, but unless the object you are trying to access is the root, it must be prepended with a namespaces such as @bar. Now let's talk about how XWork is a little different...
In XWork, the entire ValueStack is the root object in the context. Rather than having your expressions get the object you want from the stack and then get properties from that (ie: peek().blah), XWork has a special OGNL PropertyAccessor that will automatically look at the all entries in the stack (from the top down) until it finds an object with the property you are looking for.
For example, suppose the stack contains two objects: Animal and Person. Both objects have a "name" property, Animal has a "species" property, and Person has a "salary" property. Animal is on the top of the stack, and Person is below it. The follow code fragments help you get an idea of what is going on here:
In the last example, there was a tie and so the animal's name was returned. Usually this is the desired effect, but sometimes you want the property of a lower-level object. To do this, XWork has added support for indexes on the ValueStack. All you have to do is:
With expression like  ...  etc. Struts 2 will cut the stack and still return back a CompoundRoot object. To get the top of that particular stack cut, use 0.top
would get the top of the stack cut starting from element 0 in the stack (similar to top in this case)
would get the top of the stack cut starting from element 1 in the stack
OGNL supports accessing static properties as well as static methods.
By default, Struts 2 is configured to disallow this--to enable OGNL's static member support you must set the
struts.ognl.allowStaticMethodAccess constant to
true via any of the Constant Configuration methods.
OGNL's static access looks like this:
However, XWork allows you to avoid having to specify the full package name and call static properties and methods of your action classes using the "vs" prefix:
"vs" stands for "value stack". The important thing to note here is that if the class name you specify is just "vs", the class for the object on the top of the stack is used. If you specify a number after the "vs" string, an object's class deeper in the stack is used instead.
Besides the examples and descriptions given above, there are a few major changes in the EL since WebWork 1.x. The biggest one is that properties are no longer accessed with a forward slash (/) but with a dot (.). Also, rather than using ".." to traverse down the stack, we now use "[n]" where n is some positive number. Lastly, in WebWork 1.x one could access special named objects (the request scope attributes to be exact) by using "@foo", but now special variables are accessed using "#foo". However, it is important to note that "#foo" does NOT access the request attributes. Because XWork is not built only for the web, there is no concept of "request attributes", and thus "#foo" is merely a request to another object in the OgnlContext other than the root.
not directly supported, but #baz is similar
'top' or 
Struts 2 places request parameters and request, session, and application attributes on the OGNL stack. They may be accessed as shown below.
|#action['foo'] or #action.foo||current action getter (getFoo())|
#parameters['foo'] or #parameters.foo
request parameter ['foo'] (request.getParameter())
#request['foo'] or #request.foo
request attribute ['foo'] (request.getAttribute())
#session['foo'] or #session.foo
session attribute 'foo'
#application['foo'] or #application.foo
ServletContext attributes 'foo'
#attr['foo'] or #attr.foo
Access to PageContext if available, otherwise searches request/session/application respectively