Struts 2 - Understanding the working between OGNL and params interceptor

Question

I am new to Struts 2. I am studying it from the book Struts2 In Action. I am having difficulty in understanding some concepts in OGNL which are as follows-

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Answer 1

Let us say you write Login page and corresponding Login Action. On page you have a text box which has a name that says that it is a name field of User object. You also have a password field on page that says it maps to pwd field on the User object.

Now Object graph here would mean that it has to tell the params interceptor that the name field is actually a name field of user object in the login action. The action context, valuestack and OGNL will together have the knowledge that: Well maybe there are other actions as well, like FileUploadAction. And these other actions may also have a user object reference. But for current action context, the name field from the login page actually maps to nothing else but the name field of Login Action's user object. It does not belong to the name field of user object of FileUploadAction. They have this knowledge and also behave as if the field itself (actually the entire object graph) sits on value stack and is directly accessible. (A little background on ThreadLocal will be better to understand this.)

The interceptors are chained and it becomes a chain of responsibility on the way forward and back. An interceptor may perform some action is forward direction or the return direction or both. So let us say the the flow is like below:

 Page 1-->Interceptor_1--> Interceptor_2 --> Interceptor_3 -->Action_1

Then the return flow would be like below:

Action_1 --> Interceptor_3  --> Interceptor_2 --> Interceptor_1 --> The result Page

So, to answer your question, params inteceptor will be called in the return flow as well.