ActionContext、ValueStack、Stack Context之间的关系

                                 |
                                 |--application
                                 |
                                  |--session
        context map---|
                                 |--value stack(root)
                                 |
                                 |--request
                                 |
                                 |--parameters
                                 |
                                 |--attr (searches page, request, session, then application scopes)

 

 

 

The framework uses a standard naming context to evaluate OGNL expressions. The top level object dealing with OGNL is a Map (usually referred as a context map or context). OGNL has a notion of there being a root (or default) object within the context. In expression, the properties of the root object can be referenced without any special "marker" notion. References to other objects are marked with a pound sign ( #).

The framework sets the OGNL context to be our ActionContext, and the value stack to be the OGNL root object. (The value stack is a set of several objects, but to OGNL it appears to be a single object.) Along with the value stack, the framework places other objects in the ActionContext, including Maps representing the application, session, and request contexts. These objects coexist in the ActionContext, alongside the value stack (our OGNL root).

 

The Action instance is always pushed onto the value stack. Because the Action is on the stack, and the stack is the OGNL root, references to Action properties can omit the # marker. But, to access other objects in the ActionContext, we must use the # notation so OGNL knows not to look in the root object, but for some other object in the ActionContext.

 

----------------from  Struts2 docs

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