If we require to make changes in the OOTB servlet, then we may require to look for the script making the ajax servlet call and there we can call our custom servlet.
Was thinking if its possible to trigger custom servlet instead of existing servlet ootb for given paths/ resource type. Similar to using service ranking property. What does Adobe recommends when there is a need to customize ootb servlet.
You can create your own servlets using the API. We have many HELPX artilces that show use of creating Sling Servlets - like this one:
This Servlet invokes a Workflow - but it shows you how to create an AEM Sling Servlet for AEM 6.4.
akashdeepm40900243 the general recommendation is to avoid customizing the OOTB servlets. What is your requirement? Perhaps there is some other way to solve this.
However, if you find you absolutely need to override the OOTB servlet then this article does a good job of explaining how to do so:
The best way is to create your own servlet using API's as Scott recommended. Also we have so many HELPX articles on this topics. Here the couple of below.
Hope this helps!!