Spring Portlet MVC Implicit Model

Do you use Spring Portlet MVC? Have you ever noticed the ImplicitModel request parameter in your URL? It looks something like:

org.springframework.web.portlet.mvc.ImplicitModel=true

Well, it’s a Spring Portlet MVC 2.5 feature and I was scratching my head trying to figure out what it does. It’s set by Spring under the covers, so I dug into their source code to understand it and thought I would share.

The ImplicitModel is defined in Spring’s AnnotationMethodHandlerAdapter (which I describe in another post on annotation-driven portlet development w/Spring). In short, the AnnotationMethodHandlerAdapter is primarily responsible for scanning the @RequestMapping annotations on a given controller and executing the best matching method. It also sets and retrieves the ImplicitModel on portlet requests.

So what does the ImplicitModel do? Well, because most portal implementations redirect to the render phase after an action request (applying the post-redirect-get pattern), request attributes set in the action can’t be used in the subsequent render phase.

Spring Portlet MVC uses the ImplicitModel to overcome this limitation. On an action request, if the request’s model (see ModelMap class) isn’t empty, the AnnotationMethodHandlerAdapter will automatically store the ModelMap in portlet’s session for use on the upcoming render request. It also sets the render parameter (org.springframework.web.portlet.mvc.ImplicitModel), to tell the render phase to pre-load its model from the ImplicitModel stored in the session.

One additional tip: after a successful action method call, you may want to manually clear the model to prevent the action model data from being stored in the ImplicitModel. As an example, say you submit a form in an action request and render the same form with a success message after a successful submission. If you don’t want to display the originally submitted data on the success view - call ModelMap.clear() to prevent it from being stored as an ImplicitModel.