Diffrerence between ActionContext & ValueStack in struts 2

OGNL stands for Object-Graph Navigation Language; it is an expression language for getting and setting properties of Java objects. You use the same expression for both getting and setting the value of a property.

The struts 2 framework uses a standard naming context to evaluate OGNL expressions. 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.

  |

                     |--application

                     |

                     |--session

       context map---|

                     |--value stack(root)

                     |

                     |--request

                     |

                     |--parameters

                     |

                     |--attr (searches page, request, session, then application scopes)

                     |

Referencing an Action property

<s:property value="name"/>

Other (non-root) objects in the ActionContext can be rendered use the # notation.

<s:property value="#session.mySessionKey"/> or

<s:property value="#request['myRequestKey']"/>

The ActionContext is also exposed to Action classes via a static method.

ActionContext.getContext().getSession().put("mySessionPropKey", mySessionObject);