Spring MVC DispatcherServlet的启动以及初始化

Spring MVC是一个MVC模式的实现，在使用Spring MVC 时，主要需要在web.xml配置文件中设置DispatcherServlet，这个Servlet是实现Spring mvc 的前端控制器，所有的Web请求都需要通过它来处理，进行匹配、转发、数据处理。DispatcherServlet是实现Spring MVC最核心的部分。

在使用SpringMVC 时我们通常需要如下配置：

<servlet>
   <servlet-name>mvc-dispatcher</servlet-name>
   <servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>
        <init-param>
		<param-name>contextConfigLocation</param-name>
		<param-value>classpath:springmvc.xml</param-value>
        </init-param>
       <load-on-startup>1</load-on-startup>
</servlet>

<servlet-mapping>
   <servlet-name>mvc-dispatcher</servlet-name>
   <url-pattern>/</url-pattern>
</servlet-mapping>

Web容器启动，并开始加载各个Servlet，DispatcherServlet将建立自己的上下文来持有Spring MVC的bean对象，在建立这个自己持有的IoC容器时，会从ServletContext中得到根上下文作为DispatcherServlet持有的上下午的双亲上下文。有了这个根上下文，在对自己持有的上下文进行初始化，从而完成SpringMVC运行环境。在配置DispatcherServlet时可以设置初始化参数指定Ioc容器bean定义配置文件，名：<param-name>contextConfigLocation</param-name>，值：<param-value>classpath:springmvc.xml</param-value>，如果不指定，默认使用/WEB-INF/DispatcherServlet名-servlet.xml。上述配置如不指定 <init-param>，将使用/WEB-INF/mvc-dispatcher-servlet.xml。

1、DispatcherServlet类继承体系

DispatcherServlet通过继承FrameworkServlet和HttpServletBean而继承了HttpServlet，通过使用ServletAPI来对HTTP请求进行处理成为了Spring MVC的前端处理器，同时也实现了MVC模块与WEB容器集成。

2、DispatcherServlet的启动和初始化

DispatcherServlet启动

首先作为一个Servlet，在其生命周期中，将调用init（）方法，完成初始化相关工作，init（）方法在HttpServletBean中进行了重写，代码如下：

@Override
public final void init() throws ServletException {
if (logger.isDebugEnabled()) {
logger.debug("Initializing servlet '" + getServletName() + "'");
}
//获取Servlet的初始化参数，并进行设置
// Set bean properties from init parameters.
try {
      PropertyValues pvs = new ServletConfigPropertyValues(getServletConfig(), this.requiredProperties);
BeanWrapper bw = PropertyAccessorFactory.forBeanPropertyAccess(this);
ResourceLoader resourceLoader = new ServletContextResourceLoader(getServletContext());
bw.registerCustomEditor(Resource.class, new ResourceEditor(resourceLoader, getEnvironment()));
initBeanWrapper(bw);
bw.setPropertyValues(pvs, true);
}
catch (BeansException ex) {
logger.error("Failed to set bean properties on servlet '" + getServletName() + "'", ex);
      throw ex;
}
//这里调用子类FrameWorkServlet中重写的initServletBean()方法继续进行初始化
// Let subclasses do whatever initialization they like.
initServletBean();

   if (logger.isDebugEnabled()) {
logger.debug("Servlet '" + getServletName() + "' configured successfully");
}
}

在FrameWorkServlet类中，initServletBean()方法如下：

@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 {
//这里通过调用initWebApplicationContext（），初始化web应用上下文
this.webApplicationContext = initWebApplicationContext();
initFrameworkServlet();
}
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");
}
}

继续看initWebApplicationContext（）函数：

protected WebApplicationContext initWebApplicationContext() {
  //这里通过WebApplicationContextUtils工具类来获取根web应用上下文，这个上下文就是之前提到的全局应用根上下文，保存在ServletContext中，这个根上下文将作为当前mvc servlet上下文的双亲上下文。
   WebApplicationContext rootContext =
         WebApplicationContextUtils.getWebApplicationContext(getServletContext());
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();
}
if (wac == null) {
    //这里真正创建需要与该Servlet相关联的WebApplicationContext
// No context instance is defined for this servlet -> create a local one
wac = createWebApplicationContext(rootContext);
}

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.
//这里，ioc容器已经创建初始化完成，执行onRefresh(wac)进一步初始化MVC其他模块，后面说明
onRefresh(wac);
}

if (this.publishContext) {
    //这里将当前建立的上下文保存到ServletContext中，attrName 是与当前servlet名相关联，保证唯一性
// Publish the context as a servlet context attribute.
String attrName = getServletContextAttributeName();
getServletContext().setAttribute(attrName, wac);
      if (this.logger.isDebugEnabled()) {
this.logger.debug("Published WebApplicationContext of servlet '" + getServletName() +
"' as ServletContext attribute with name [" + attrName + "]");
}
   }

return wac;
}

下面来看这个当前上下文是如何创建的，createWebApplicationContext()函数如下：

protected WebApplicationContext createWebApplicationContext(ApplicationContext parent) {
   Class<?> contextClass = getContextClass();
   if (this.logger.isDebugEnabled()) {
this.logger.debug("Servlet with name '" + getServletName() +
"' will try to create custom WebApplicationContext context of class '" +
            contextClass.getName() + "'" + ", using parent context [" + parent + "]");
}
if (!ConfigurableWebApplicationContext.class.isAssignableFrom(contextClass)) {
throw new ApplicationContextException(
"Fatal initialization error in servlet with name '" + getServletName() +
"': custom WebApplicationContext class [" + contextClass.getName() +
"] is not of type ConfigurableWebApplicationContext");
}
  //通过BeanUtils反射实例化具体的上下文对象，contextClass通过getContextClass()获取，取得的class是Class<?> DEFAULT_CONTEXT_CLASS = XmlWebApplicationContext.class;
   ConfigurableWebApplicationContext wac =
         (ConfigurableWebApplicationContext) BeanUtils.instantiateClass(contextClass);
 //对上下文对象进行设置
wac.setEnvironment(getEnvironment());
wac.setParent(parent);
wac.setConfigLocation(getContextConfigLocation());
//这里对刚刚创建的WebApplicationContext，进行配置和初始化操作，完成容器的最终初始化
configureAndRefreshWebApplicationContext(wac);

   return wac;
}

继续configureAndRefreshWebApplicationContext（）函数：

protected void configureAndRefreshWebApplicationContext(ConfigurableWebApplicationContext wac) {
if (ObjectUtils.identityToString(wac).equals(wac.getId())) {
// The application context id is still set to its original default value
      // -> assign a more useful id based on available information
if (this.contextId != null) {
         wac.setId(this.contextId);
}
else {
// Generate default id...
wac.setId(ConfigurableWebApplicationContext.APPLICATION_CONTEXT_ID_PREFIX +
               ObjectUtils.getDisplayString(getServletContext().getContextPath()) + "/" + getServletName());
}
   }
 //设置ServletContext引用，以及其他对象的设置
   wac.setServletContext(getServletContext());
wac.setServletConfig(getServletConfig());
wac.setNamespace(getNamespace());
wac.addApplicationListener(new SourceFilteringListener(wac, new ContextRefreshListener()));

// The wac environment's #initPropertySources will be called in any case when the context
   // is refreshed; do it eagerly here to ensure servlet property sources are in place for
   // use in any post-processing or initialization that occurs below prior to #refresh
ConfigurableEnvironment env = wac.getEnvironment();
   if (env instanceof ConfigurableWebEnvironment) {
      ((ConfigurableWebEnvironment) env).initPropertySources(getServletContext(), getServletConfig());
}

   postProcessWebApplicationContext(wac);
applyInitializers(wac);
//嗲用容器的refresh()方法，完成日期初始化
wac.refresh();
}
可以看到对上下文进行了相关配置，设置ServletContext，设置初始化参数等，最后通过调用容器的refresh()方法启动整个容器，就像一般的IoC容器初始化过程一样。

至此，web应用上下文的启动、初始化已经完成，DispatcherServlet运行MVC需要的IoC容器已经建立起来，但是DispatcherServlet作为Spring
MVC前端控制器，还需要初始化MVC需要的其他模块，回到initWebApplicationContext()函数，看到在wac
= createWebApplicationContext(rootContext)后面，调用了onRefresh(was)函数，onRefresh函数在DispatcherServlet类中重写，代码如下：

@Override
protected void onRefresh(ApplicationContext context) {
//调用initStrategies完成MVC模块初始化
   initStrategies(context);
}

可以看到MVC初始化是在DispatcherServlet的initStrategies()方法中完成，在该方法中初始化各种MVC框架实现元素，如至此request映射的HandlerMappings，以及handler适配处理器HandlerAdapters，以及视图生成器ViewResolver等。

protected void initStrategies(ApplicationContext context) {
   initMultipartResolver(context);
initLocaleResolver(context);
initThemeResolver(context);
initHandlerMappings(context);
initHandlerAdapters(context);
initHandlerExceptionResolvers(context);
initRequestToViewNameTranslator(context);
initViewResolvers(context);
initFlashMapManager(context);
}

比如初始化HandlerMappings，在SpringMVC中，HandlerMappings的作用是为Http请求找到对应的Controller控制器，来进一步处理请求。

private void initHandlerMappings(ApplicationContext context) {
this.handlerMappings = null;
 //这里根据detectAllHandlerMappings布尔值确定是否导入所有的handlerMappings ，默认导入所有handlerMappings bean，这些bean可以来自于当前上下文容器，也可以来自其双亲根web上下文容器
   if (this.detectAllHandlerMappings) {
// Find all HandlerMappings in the ApplicationContext, including ancestor contexts.
Map<String, HandlerMapping> matchingBeans =
            BeanFactoryUtils.beansOfTypeIncludingAncestors(context, HandlerMapping.class, true, false);
      if (!matchingBeans.isEmpty()) {
this.handlerMappings = new ArrayList<HandlerMapping>(matchingBeans.values());
// We keep HandlerMappings in sorted order.
OrderComparator.sort(this.handlerMappings);
}
   }
else {
try {
         HandlerMapping hm = context.getBean(HANDLER_MAPPING_BEAN_NAME, HandlerMapping.class);
         this.handlerMappings = Collections.singletonList(hm);
}
catch (NoSuchBeanDefinitionException ex) {
// Ignore, we'll add a default HandlerMapping later.
}
   }
//如果没有发现配置的handlerMappings ，需要设置默认的handlerMappings ，这些默认的handlerMappings 通过配置文件，在jar包中指定，在DispatcherServlet.properties配置文件中默认设置BeanNameUrlHandlerMapping、DefaultAnnotationHandlerMapping
// Ensure we have at least one HandlerMapping, by registering
   // a default HandlerMapping if no other mappings are found.
if (this.handlerMappings == null) {
this.handlerMappings = getDefaultStrategies(context, HandlerMapping.class);
      if (logger.isDebugEnabled()) {
logger.debug("No HandlerMappings found in servlet '" + getServletName() + "': using default");
}
   }
}

DispatcherServlet.properties默认配置文件：

# Default implementation classes for DispatcherServlet's strategy interfaces.
# Used as fallback when no matching beans are found in the DispatcherServlet context.
# Not meant to be customized by application developers.

org.springframework.web.servlet.LocaleResolver=org.springframework.web.servlet.i18n.AcceptHeaderLocaleResolver

org.springframework.web.servlet.ThemeResolver=org.springframework.web.servlet.theme.FixedThemeResolver

org.springframework.web.servlet.HandlerMapping=org.springframework.web.servlet.handler.BeanNameUrlHandlerMapping,   org.springframework.web.servlet.mvc.annotation.DefaultAnnotationHandlerMapping

org.springframework.web.servlet.HandlerAdapter=org.springframework.web.servlet.mvc.HttpRequestHandlerAdapter,   org.springframework.web.servlet.mvc.SimpleControllerHandlerAdapter,   org.springframework.web.servlet.mvc.annotation.AnnotationMethodHandlerAdapter

org.springframework.web.servlet.HandlerExceptionResolver=org.springframework.web.servlet.mvc.annotation.AnnotationMethodHandlerExceptionResolver,   org.springframework.web.servlet.mvc.annotation.ResponseStatusExceptionResolver,   org.springframework.web.servlet.mvc.support.DefaultHandlerExceptionResolver

org.springframework.web.servlet.RequestToViewNameTranslator=org.springframework.web.servlet.view.DefaultRequestToViewNameTranslator

org.springframework.web.servlet.ViewResolver=org.springframework.web.servlet.view.InternalResourceViewResolver

org.springframework.web.servlet.FlashMapManager=org.springframework.web.servlet.support.SessionFlashMapManager