Exceptions

A servlet can propagate only exceptions that subclass IOException, ServletException or RuntimeException (because of service()'s signature).

The ServletException constructor takes a root cause as argument:

javax.servlet.ServletException(Throwable rootCause);
javax.servlet.ServletException(String msg, Throwable rootCause);

So, you can pass it on up, then fetch it with:

public Throwable ServletException.getRootCause();

The UnavailableException is the only one that subclasses ServletException. Used to indicate servlet is unavailable:

javax.servlet.UnavailableException(String msg); //permanently unavailable
javax.servlet.UnavailableException(String msg, int seconds); //be back in seconds