Struts 2 - Understanding the working between OGNL and params interceptor

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-

1. We know that params interceptor moves the data from the request parameters to the action object in ValueStack. Now while reading, I came upon a line that says- "The tricky part of the job is mapping the name of the parameter to an actual property on the ValueStack. This is where OGNL comes in. The params interceptor interprets the request parameter name as an OGNL expression to locate the correct destination property on the ValueStack".

Ques1) Here, what does "interprets" mean? Is it that params interceptor translates the request parameter into some OGNL expression and then OGNL expression provides mapping to the properties in ValueStack OR does it mean something else?

1. When result start its rendering process, the Struts 2 tags retrieve data from the ValueStack by referencing specific values with OGNL expressions.

Ques 2) So the tags take OGNL expressions, but how is the data being moved? Earlier, params interceptor was the one that moved the data but now there is no params interceptor. So how is the data being moved?

The parameter names ARE OGNL expressions. It's a case of ConventionOverConfiguraiton. If we agree to make the names of the parameters valid OGNL expressions that can access a javabeans property, then it's easy to simply hand that name over to OGNL as an expression. This is done internally of course; you don't really need to know how it works unless you are hacking on that part of the Struts 2 code.

Answer #2

The action object is sitting on top of the ValueStack. The ValueStack is avaible, via it's existence as part of the ThreadLocal ActionContext, from any code executing on the same thread. Since a web app uses a single thread to handle the processing of a request, we know that the Result layer will be able to get to the ValueStack to retrieve the data, again going using the name in the tag as a OGNL expression.

Note:

The key part of all of this is the fact that the ValueStack is available to any code executing on the same thread. This allows all code processing a single request to have access to the ValueStack, which they can obtain via the ThreadLocal ActionContext ( read about Java's ThreadLocal class if you don't understand ).

The params interceptor can then try to use the param name as an OGNL expression to write data to the ValueStack ( which servers as the OGNL context -- again read about the OGNL API if you don't understand ). Then the code in the Result classes that handle the rendering of the response can interpret the various names and values from the tag libraries as OGNL expressions to READ data from the ValueStack.