ServletContainerInitializer vs ServletContextListener

I'm trying to register a servlet using servletContainerInitializer but it doesn't seem to work, Maybe it's my code (kindly review it), but I came to wonder about the difference between ServletContainerInitializer and ServletContextListener, because the follwing code runs fine when used as ServletContextListener instead.

From servlet 3.0 specification:

4.4

Configuration methods (adding servlets dynamically):

... or from the onStartup method of a ServletContainerInitializer implementation ...

The ServletContainerInitializer:

package com.marmoush.javaexamples.nullhaus.servlet;

import java.util.Set;
import javax.servlet.ServletContainerInitializer;
import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.ServletRegistration;

public class MyInit implements ServletContainerInitializer {
    public void onStartup(Set<Class<?>> c, ServletContext ctx) throws ServletException {
        System.out.println("hello");
        ServletRegistration reg = ctx.addServlet("q31","com.marmoush.javaexamples.nullhaus.servlet.Q31");
        reg.addMapping("/q31/*");
    }
}

The servlet which I'm trying to auto-register:

package com.marmoush.javaexamples.nullhaus.servlet;

import java.io.IOException;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

public class Q31 extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse