springmvc采用最常见的前端控制器模式（Front Controller Pattern）
是用来提供一个集中的请求处理机制，所有的请求都将由一个单一的处理程序处理。该处理程序可以做认证/授权/记录日志，或者跟踪请求，然后把请求传给相应的处理程序。
springmvc中所有请求都会到达DiapatcherServlet,具体由 doDispatch(HttpServletRequest request, HttpServletResponse response)方法进行分发，
分发到具体的controller层。首先我们来看看DispatcherServlet继承关系.
HttpServlet继承了HttpServlet,并重写了init 方法：
@Override
public final void init() throws ServletException {
if (logger.isDebugEnabled()) {
logger.debug("Initializing servlet '" + getServletName() + "'");
}
// Set bean properties from init parameters.
try {
PropertyValues pvs = new ServletConfigPropertyValues(getServletConfig(), this.requiredProperties);
<servlet>
<servlet-name>dispatcherServlet</servlet-name>
<servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>
<init-param>
<param-name>contextConfigLocation</param-name>
<!-- 指定SpringMVC框架的配置文件的路径和名称 -->
<param-value>classpath*:conf/mvc/mvc-context.xml</param-value>
</init-param>
<load-on-startup>1</load-on-startup>
</servlet>
//构造ServletConfigPropertyValues过程中会查找web.xml配置,读取nit-param中的配置，并设置到ServletConfigPropertyValues中
BeanWrapper bw = PropertyAccessorFactory.forBeanPropertyAccess(this);//BeanWrapper 是spring内部实例化bean的方法,使用BeanWrapper实例化DispatcherServlet
ResourceLoader resourceLoader = new ServletContextResourceLoader(getServletContext());
bw.registerCustomEditor(Resource.class, new ResourceEditor(resourceLoader, getEnvironment()));//注册响应的属性编辑器（资源加载器），当类型是Resource的用ResourceEditor进行类型转换
initBeanWrapper(bw);//初始化bean
bw.setPropertyValues(pvs, true);//进行属性赋值
}
catch (BeansException ex) {
logger.error("Failed to set bean properties on servlet '" + getServletName() + "'", ex);
throw ex;
}
// Let subclasses do whatever initialization they like. //让子类做一写具体的处理
initServletBean();
if (logger.isDebugEnabled()) {
logger.debug("Servlet '" + getServletName() + "' configured successfully");
}
}
比如上面这段配置，传递了contextConfigLocation参数，之后构造BeanWrapper，这里使用BeanWrapper，有2个理由：1. contextConfigLocation属性在FrameworkServlet中定义，HttpServletBean中未定义 2. 利用Spring的注入特性，只需要调用setPropertyValues方法就可将contextConfigLocation属性设置到对应实例中，也就是以依赖注入的方式初始化属性。然后设置DispatcherServlet中的contextConfigLocation属性(FrameworkServlet中定义)为web.xml中读取的contextConfigLocation参数，该参数用于构造SpringMVC容器上下文。
下面看下FrameworkServlet这个类，FrameworkServlet继承自HttpServletBean。
首先来看下该类覆写的initServletBean方法：
/**
* Overridden method of {@link HttpServletBean}, invoked after any bean properties
* have been set. Creates this servlet's WebApplicationContext.
*/
@Override
protected final void initServletBean() throws ServletException {
getServletContext().log("Initializing Spring FrameworkServlet '" + getServletName() + "'");
if (this.logger.isInfoEnabled()) {
this.logger.info("FrameworkServlet '" + getServletName() + "': initialization started");
}
long startTime = System.currentTimeMillis();
try {
this.webApplicationContext = initWebApplicationContext();//将servlet属性与spring上下文绑定,webApplicationContext作为上下文环境。
initFrameworkServlet();//不做任何处理，DispatcherServlet也不做任何处理
}
catch (ServletException ex) {
this.logger.error("Context initialization failed", ex);
throw ex;
}
catch (RuntimeException ex) {
this.logger.error("Context initialization failed", ex);
throw ex;
}
if (this.logger.isInfoEnabled()) {
long elapsedTime = System.currentTimeMillis() - startTime;
this.logger.info("FrameworkServlet '" + getServletName() + "': initialization completed in " +
elapsedTime + " ms");
}
}
来看看initWebApplicationContext具体代码实现：
protected WebApplicationContext initWebApplicationContext() {
WebApplicationContext rootContext =WebApplicationContextUtils.getWebApplicationContext(getServletContext());//获取根上下文,一般放在ServletContext上下文中，
键 WebApplicationContext.class.getName() + ".ROOT"，值webApplicationContext
WebApplicationContext wac = null;
if (this.webApplicationContext != null) {
// A context instance was injected at construction time -> use it
wac = this.webApplicationContext;
if (wac instanceof ConfigurableWebApplicationContext) {
ConfigurableWebApplicationContext cwac = (ConfigurableWebApplicationContext) wac;
if (!cwac.isActive()) {
// The context has not yet been refreshed -> provide services such as
// setting the parent context, setting the application context id, etc
if (cwac.getParent() == null) {
// The context instance was injected without an explicit parent -> set
// the root application context (if any; may be null) as the parent
cwac.setParent(rootContext);
}
configureAndRefreshWebApplicationContext(cwac);
}
}
}
if (wac == null) {
// No context instance was injected at construction time -> see if one
// has been registered in the servlet context. If one exists, it is assumed
// that the parent context (if any) has already been set and that the
// user has performed any initialization such as setting the context id
wac = findWebApplicationContext();//contextAttribute以这个属性名去servletContext中查找springmvc上下文。
//org.springframework.web.servlet.FrameworkServlet.CONTEXT.dispatcherServlet attrname作为键名保存在servletContext上下文中
}
if (wac == null) {
// No context instance is defined for this servlet -> create a local one
wac = createWebApplicationContext(rootContext);//创建WebApplicationContext上下文，并将相关配置加入其中
}
if (!this.refreshEventReceived) {
// Either the context is not a ConfigurableApplicationContext with refresh
// support or the context injected at construction time had already been
// refreshed -> trigger initial onRefresh manually here.
onRefresh(wac); //模板方法在DispatcherServlet中具体实现，加载一些初始化设置。
}
if (this.publishContext) {
// Publish the context as a servlet context attribute.
String attrName = getServletContextAttributeName();
getServletContext().setAttribute(attrName, wac);//将先创建的上下文放到ServletContext中去。
if (this.logger.isDebugEnabled()) {
this.logger.debug("Published WebApplicationContext of servlet '" + getServletName() +
"' as ServletContext attribute with name [" + attrName + "]");
}
}
return wac;
}
这里的根上下文是web.xml中配置的ContextLoaderListener监听器中根据contextConfigLocation路径生成的上下文
<!--加载spring上下文对象 -->
<context-param>
<param-name>contextConfigLocation</param-name>
<param-value>classpath:conf/cpic/root-context.xml</param-value>
</context-param>
<!--default enviroment for setting -->
<context-param>
<param-name>spring.profiles.active</param-name>
<param-value>development</param-value>
</context-param>
比如这段配置文件中根据classpath :conf/applicationContext.xml下的xml文件生成的根上下文。
最后看下DispatcherServlet。
DispatcherServlet覆写了FrameworkServlet中的onRefresh方法：
/**
* This implementation calls {@link #initStrategies}.
*/
@Override
protected void onRefresh(ApplicationContext context) {
initStrategies(context);//初始化各种策略的接口
}
/**
* Initialize the strategy objects that this servlet uses.
* <p>May be overridden in subclasses in order to initialize further strategy objects.
*/
protected void initStrategies(ApplicationContext context) {
initMultipartResolver(context);
initLocaleResolver(context);
initThemeResolver(context);
initHandlerMappings(context);
initHandlerAdapters(context);
initHandlerExceptionResolvers(context);
initRequestToViewNameTranslator(context);
initViewResolvers(context);
initFlashMapManager(context);
}
//很明显，initStrategies方法内部会初始化各个策略接口的实现类。
总结一下各个Servlet的作用：
1. HttpServletBean
主要做一些初始化的工作，将web.xml中配置的参数设置到Servlet中。比如servlet标签的子标签init-param标签中配置的参数。
2. FrameworkServlet
将Servlet与Spring容器上下文关联。其实也就是初始化FrameworkServlet的属性webApplicationContext，这个属性代表SpringMVC上下文，它有个父类上下文，既web.xml中配置的ContextLoaderListener监听器初始化的容器上下文。
3. DispatcherServlet
初始化各个功能的实现类。比如异常处理、视图处理、请求映射处理等。