A simple web-app code - user registration. Is the layering ok?
I know you're asking strictly for feedback about the layers (which look fine to me), but in case you're looking for other feedback too:

if(username.equals("")
        || password.equals("")
            || email.equals("")){


You need to null check these values. getParameter can return null so you will get a NullPointerException if they don't exist.

List&lt;String&gt; errors = new ArrayList&lt;String&gt;();
errors.add("Registiration was not successful. Maybe username/email was already taken?");
httpServletRequest.setAttribute("errors",errors);


Can be done in one line if you want:

errors.add(Arrays.asList("Registiration was not successful. Maybe username/email was already taken?"));