SpringMVC的启动入口在SpringServletContainerInitializer类，它是ServletContainerInitializer实现类(Servlet3.0新特性)。在实现方法中使用WebApplicationInitializer创建ApplicationContext、创建注册DispatcherServlet、初始化ApplicationContext等。

SpringMVC已经将大部分的启动逻辑封装在了几个抽象WebApplicationInitializer中，开发者只要继承这些抽象类实现抽象方法即可。

本文将详细分析ServletContainerInitializer、SpringServletContainerInitializer和WebApplicationInitializer的工作流程。

Servlet3.0的ServletContainerInitializer

ServletContainerInitializer接口

ServletContainerInitializer是Servlet3.0的接口。

该接口在web应用程序启动阶段接收通知，注册servlet、filter、listener等。

该接口的实现类可以用HandlesTypes进行标注，并指定一个Class值，后续会将实现、扩展了这个Class的类集合作为参数传递给onStartup方法。

以tomcat为例，在容器配置初始化阶段，将使用SPI查找实现类，在ServletContext启动阶段，初始化并调用onStartup方法来进行ServletContext的初始化。

SpringServletContainerInitializer实现类

在onStartup方法创建所有的WebApplicationInitializer对并调用onStartup方法。

以下为SPI配置，在spring-web/src/main/resources/META-INF/services/javax.servlet.ServletContainerInitializer文件：

WebApplicationInitializer接口

WebApplicationInitializer接口概述

Interface to be implemented in Servlet 3.0+ environments in order to configure the ServletContext programmatically -- as opposed to (or possibly in conjunction with) the traditional web.xml-based approach.

Implementations of this SPI will be detected automatically by SpringServletContainerInitializer, which itself is bootstrapped automatically by any Servlet 3.0 container. See its Javadoc for details on this bootstrapping mechanism.

示例：

public class MyWebAppInitializer implements WebApplicationInitializer {

   @Override
   public void onStartup(ServletContext container) {
     // Create the 'root' Spring application context
     AnnotationConfigWebApplicationContext rootContext =
       new AnnotationConfigWebApplicationContext();
     rootContext.register(AppConfig.class);

     // Manage the lifecycle of the root application context
     container.addListener(new ContextLoaderListener(rootContext));

     // Create the dispatcher servlet's Spring application context
     AnnotationConfigWebApplicationContext dispatcherContext =
       new AnnotationConfigWebApplicationContext();
     dispatcherContext.register(DispatcherConfig.class);

     // Register and map the dispatcher servlet
     ServletRegistration.Dynamic dispatcher =
       container.addServlet("dispatcher", new DispatcherServlet(dispatcherContext));
     dispatcher.setLoadOnStartup(1);
     dispatcher.addMapping("/");
   }
}

开发者可以编写类继承AbstractAnnotationConfigDispatcherServletInitializer抽象类，这个抽象类已经将大部分的初始化逻辑进行了封装。

WebApplicationInitializer实现和继承关系

AbstractContextLoaderInitializer类：

Convenient base class for WebApplicationInitializer implementations that register a ContextLoaderListener in the servlet context. The only method required to be implemented by subclasses is createRootApplicationContext(), which gets invoked from registerContextLoaderListener(ServletContext).

AbstractDispatcherServletInitializer类：

Base class for WebApplicationInitializer implementations that register a DispatcherServlet in the servlet context. Most applications should consider extending the Spring Java config subclass AbstractAnnotationConfigDispatcherServletInitializer.

AbstractAnnotationConfigDispatcherServletInitializer类：

WebApplicationInitializer to register a DispatcherServl