I would like to have a left navigation tree menu on my website.
When I click on a tree node a respective page should appear on the body of the website.
The state of node should appear to be open under a tree control used in the left navigation menu.
Can anyone please advice how to build this with Struts 2 ?
Struts is an implementation of the Front Controllerpattern. It has nothing to do with the way your client-side page looks or functions.
You can start with this tutorial (though it is for a horizontal menu, the principals are the same for a left-side menu).