What is the use of filter and chain in servlet?

Question

chain.doFilter(req,res);
We used this in a servlet program. I want to know what is the use of the method doFilter() in a servlet? Also what is the use of filter and chain concept in Java servlets?

Answer 1

Servlet filters are implementation of the chain of responsibility pattern

The point is that each filter stays "in front" and "behind" each servlet it is mapped to. So if you have a filter around a servlet, you'll have:

void doFilter(..) { 
    // do stuff before servlet gets called

    // invoke the servlet, or any other filters mapped to the target servlet
    chain.doFilter(..);

    // do stuff after the servlet finishes
}

You also have the option not to call chain.doFilter(..) in which case the servlet will never be called. This is useful for security purposes - for example you can check whether there's a user logged-in.