How to call a Servlet/Filter before a JSP is executed in CQ5?

Question

In a Spring MVC application we have a Controller that would execute before calling the JSP. The Controller would prefetch some values from the database and set them in the model and forward the control to JSP.

How do I implement this feature in CQ 5? I want the SlingFilter to execute before the JSP is executed. And the JSP is not a page component but a component that appears in the side kick.

Note:

I can do this by writing my own SlingSerlvet that would prefetch my required values and use the RequestDispatcher to forward to the JSP. But by this method I would have to go through a URL like "/bin/.*". And this is again at a page level I want this kind of functionality at component level.

Answer 1

So to answer your specific question, if you want a filter to be executed before a component is called you would create a filter that is listening to Component level filter scope.

See http://sling.apache.org/documentation/the-sling-engine/filters.html

You would then have your filter change the incoming request to a SlingServletRequest and determine if the target resource is the one that you are looking for.

However this filter would be executed on every single component that is being included on a page. The reverse process of this that may be useful to you is the ResourceDecorator.

http://sling.apache.org/documentation/the-sling-engine/wrap-or-decorate-resources.html

These are executed when the resource is identified, prior to the servlet and filter calls, which would allow you to verify if a resource is a type that you are interested in, and then allows you to add additional information to the resource object.However this is, once again a service that would be applied to every resource that is identified.

However, if what you are looking for is a filter that is only executed for a specific path, then no. Sling doesn't do that. You mentioned Spring MVC and Spring MVC works on a completely different concept of MVC then what Slings version of MVC does.

EDIT

So in a traditional web app, the servlet would be at a fixed position and all filters are applied prior to the call to that servlet. In Sling you are dynamically wiring servlets together to generate the resulting page. So each time that you are in a servlet and call directly or indirectly the request dispatcher, it's executing the resolution process again and applying a series of filters again before the new servlet is executed.

To prevent a high level filter that needs to applied only to the main request being applied on every single internal dispatch, they came up with the idea of contexts, or chains of filters that are applied at different times and associated with different types of includes.

Here is a basic filter that will log a message when it's called. I did this from memory so you'll need to dink with it.

@SlingFilter(scope = SlingFilterScope.COMPONENT, order = Integer.MIN_VALUE)
public class SampleFilter implements Filter {

    private static final Logger LOG = LoggerFactory.getLogger(SampleFilter.class);

@Override
    public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException {
        SlingHttpServletRequest slingRequest = (SlingHttpServletRequest) request;
        Resource res = slingRequest.getResource();
        if (!(res == null || ResourceUtil.isNonExistingResource(res))) {
            LOG.error("this servlet is called before resource {} at path {}", res.getName(),res.getPath());
        }
        chain.doFilter(request, response);
    }

}

The important part of this is scope = SlingFilterScope.COMPONENT take a look at the page I had listed earlier and try out different combinations of slignfilterscope and you'll see how it's being applied at different times. scope = SlingFilterScope.REQUEST would be once at a top level on a per page basis.