servlet3.0+规范后，允许servlet，filter，listener不必声明在web.xml中，而是以硬编码的方式存在，实现容器的零配置。
为此，Spring为其提供了一个实现类SpringServletContainerInitializer 来处理启动容器时加载相关配置。
SpringServletContainerInitializer 主要是获取WebApplicationInitializer实现类中配置的servlet，filter，listener，并进行处理，加载进入容器中。
所以，WebApplicationInitializer才是我们需要关心的接口

package org.springframework.web;

import java.lang.reflect.Modifier;
import java.util.LinkedList;
import java.util.List;
import java.util.ServiceLoader;
import java.util.Set;
import javax.servlet.ServletContainerInitializer;
import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.annotation.HandlesTypes;

import org.springframework.core.annotation.AnnotationAwareOrderComparator;


@HandlesTypes(WebApplicationInitializer.class)
public class SpringServletContainerInitializer implements ServletContainerInitializer {

    @Override
    public void onStartup(Set<Class<?>> webAppInitializerClasses, ServletContext servletContext)
            throws ServletException {

        List<WebApplicationInitializer> initializers = new LinkedList<WebApplicationInitializer>();

        if (webAppInitializerClasses != null) {
            for (Class<?> waiClass : webAppInitializerClasses) {
                // Be defensive: Some servlet containers provide us with invalid classes,
                // no matter what @HandlesTypes says...
                if (!waiClass.isInterface() && !Modifier.isAbstract(waiClass.getModifiers()) &&
                        WebApplicationInitializer.class.isAssignableFrom(waiClass)) {
                    try {
                        initializers.add((WebApplicationInitializer) waiClass.newInstance());
                    }
                    catch (Throwable ex) {
                        throw new ServletException("Failed to instantiate WebApplicationInitializer class", ex);
                    }
                }
            }
        }

        if (initializers.isEmpty()) {
            servletContext.log("No Spring WebApplicationInitializer types detected on classpath");
            return;
        }

        servletContext.log(initializers.size() + " Spring WebApplicationInitializers detected on classpath");
        AnnotationAwareOrderComparator.sort(initializers);
        for (WebApplicationInitializer initializer : initializers) {
            initializer.onStartup(servletContext);
        }
    }

}

在spring mvc 3以后，它提供了一个基本的实现类AbstractAnnotationConfigDispatcherServletInitializer，我们可以继承使用。

package com.ypark.sys.config;

import javax.servlet.Filter;
import javax.servlet.ServletContext;
import javax.servlet.ServletException;

import org.springframework.web.context.request.RequestContextListener;
import org.springframework.web.filter.CharacterEncodingFilter;
import org.springframework.web.servlet.support.AbstractAnnotationConfigDispatcherServletInitializer;
import org.springframework.web.util.IntrospectorCleanupListener;
import org.springframework.web.util.Log4jConfigListener;

/**
 * web.xml的java config
 */
public class ServletInitializer extends AbstractAnnotationConfigDispatcherServletInitializer {

    @Override
    public void onStartup(ServletContext servletContext) throws ServletException {
        // Log4jConfigListener
        servletContext.setInitParameter("log4jConfigLocation", "classpath:conf/log4j.properties");
        servletContext.addListener(Log4jConfigListener.class);
        // 在springmvc中获取request的监听器
        servletContext.addListener(RequestContextListener.class);
        servletContext.addListener(IntrospectorCleanupListener.class);
        super.onStartup(servletContext);
    };

    @Override
    protected Class<?>[] getServletConfigClasses() {
        return new Class[] { SpringWebConfig.class };
    }

    @Override
    protected String[] getServletMappings() {
        return new String[] { "/" };
    }

    @Override
    protected Class<?>[] getRootConfigClasses() {
        return new Class[] { SpringCoreConfig.class };
    }

    @Override
    protected Filter[] getServletFilters() {
        CharacterEncodingFilter characterEncodingFilter = new CharacterEncodingFilter();
        characterEncodingFilter.setEncoding("UTF-8");
        characterEncodingFilter.setForceEncoding(true);
        return new Filter[] { characterEncodingFilter };
    }

}

它等价于下述的web.xml

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://java.sun.com/xml/ns/javaee" xmlns:web="http://java.sun.com/xml/ns/javaee"  xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd" id="WebApp_ID" version="3.0">
  <context-param>
    <param-name>log4jConfigLocation</param-name>
    <param-value>classpath:conf/log4j.properties</param-value>
  </context-param>
  <context-param>
    <param-name>contextConfigLocation</param-name>
    <param-value>classpath:conf/spring-core.xml</param-value>
  </context-param>
  <filter>
    <description>字符集过滤器</description>
    <filter-name>encodingFilter</filter-name>
    <filter-class>org.springframework.web.filter.CharacterEncodingFilter</filter-class>
    <init-param>
      <description>字符集编码</description>
      <param-name>encoding</param-name>
      <param-value>UTF-8</param-value>
    </init-param>
    <async-supported>true</async-supported>
  </filter>
  <filter-mapping>
    <filter-name>encodingFilter</filter-name>
    <url-pattern>/*</url-pattern>
  </filter-mapping>
  <listener>
    <listener-class>org.springframework.web.util.Log4jConfigListener</listener-class>
  </listener>
  <listener>
    <description>spring监听器</description>
    <listener-class>org.springframework.web.context.ContextLoaderListener</listener-class>
  </listener>
  <listener>
    <description>在springmvc中获取request的监听器</description>
    <listener-class>org.springframework.web.context.request.RequestContextListener</listener-class>  
  </listener> 
  <listener>
    <listener-class>org.springframework.web.util.IntrospectorCleanupListener</listener-class>
  </listener>
  <servlet>
    <description>spring mvc servlet</description>
    <servlet-name>springMvc</servlet-name>
    <servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>
    <init-param>
      <description>spring mvc 配置文件</description>
      <param-name>contextConfigLocation</param-name>
      <param-value>classpath:conf/spring-mvc.xml</param-value>
    </init-param>
    <load-on-startup>1</load-on-startup>
    <async-supported>true</async-supported>
  </servlet>
  <servlet-mapping>
    <servlet-name>springMvc</servlet-name>
    <url-pattern>/</url-pattern>
  </servlet-mapping>
</web-app>