spring集成servlet的疑惑

<servlet>
    <description></description>
    <display-name>UserActionImpl2</display-name>
    <servlet-name>UserActionImpl2</servlet-name>
    <servlet-class>com.action.MyAction</servlet-class>
  </servlet>
  <servlet-mapping>
    <servlet-name>UserActionImpl2</servlet-name>
    <url-pattern>/user</url-pattern>
  </servlet-mapping>

 

package com.action;

import java.io.IOException;
import javax.servlet.Servlet;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import org.springframework.web.context.WebApplicationContext;
import org.springframework.web.context.support.WebApplicationContextUtils;

/**
 * Servlet implementation class UserAction
 */
public class MyAction extends HttpServlet {
	private static final long serialVersionUID = 1L;

	
	protected Servlet servlet;
	/**
     * @see HttpServlet#HttpServlet()
     */
    public MyAction() {
        super();
    }

	/**
	 * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
	 */
	protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		servlet.service(request, response);
	}

	
	@Override
	public void init() throws ServletException {
		System.out.println("init");
		String servletName = getServletConfig().getServletName(); 
        WebApplicationContext webapplicationcontext = WebApplicationContextUtils
                .getRequiredWebApplicationContext(getServletContext());  
        this.servlet = (Servlet) webapplicationcontext.getBean(servletName); 
	}

	/**
	 * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
	 */
	protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		servlet.service(request, response);
	}

}

package com.action.impl;

import java.io.IOException;

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

import org.springframework.web.context.WebApplicationContext;
import org.springframework.web.context.support.WebApplicationContextUtils;

import com.action.MyAction;
import com.pojo.User;
import com.services.UserServices;

public class UserActionImpl2 extends MyAction {

	private static final long serialVersionUID = 1L;
	private static final int size = 5;
	private UserServices userServices;
	public void setUserServices(UserServices userServices) {
		this.userServices = userServices;
	}
	@Override
	protected void doGet(HttpServletRequest request,
			HttpServletResponse response) throws ServletException, IOException {
		
	}
	@Override
	protected void doPost(HttpServletRequest request,
			HttpServletResponse response) throws ServletException, IOException {
		this.doGet(request, response);
	}
	
	protected Servlet servlet;
	@Override
	public void init() throws ServletException {
		System.out.println("init children");
		String servletName = getServletConfig().getServletName(); 
        WebApplicationContext webapplicationcontext = WebApplicationContextUtils
                .getRequiredWebApplicationContext(getServletContext()); 
        this.servlet = (Servlet) webapplicationcontext.getBean(servletName); 
	}
}

为什么web.xml里面一定得配置com.action.Myaction才能获得spring提供的servlet对象，配置子类UserActonImpl2，也是通过init从spring获取servlet对象，为什么userServices为NULL？

我对配置子类UserActonImpl2的理解：因为UserActonImpl2已经从web服务器获取到，然后通过init方法来对servlet注入对象！这样就不会为userServices注入对象了！

对配置父类com.action.Myaction不能很理解，求javaeye的哥哥姐姐们指点下，解释下这2者是怎么获取servlet对象的，还有就是怎么样可以在web.xml里面直接配置子类UserActonImpl2能够获取userServices？

语言表达能力有限，描述了N久，才这样，我是hold不住了，哥哥姐姐们一定的hold住