前言

<servlet>
    <servlet-name>spring-mvc</servlet-name>
    <servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>
    <init-param>
        <param-name>contextConfigLocation</param-name>
        <param-value>classpath:config/spring-mvc.xml</param-value>
    </init-param>
    <load-on-startup>1</load-on-startup>
</servlet>
<servlet-mapping>
    <servlet-name>spring-mvc</servlet-name>
    <url-pattern>/</url-pattern>
</servlet-mapping>

根据web.xml的配置，我们可以知道springMVC的主体控制器部分就是DispatcherServlet，同时也不难看出DispatcherServlet的本质其实就是一个Servlet。那我们这次来剖析一下DispatcherServlet的初始化流程。

DispatcherServlet类图

DispatcherServlet类图

其中HttpServletBean、FrameworkServlet和DispatcherServlet都是springMVC框架中的东西。

HttpServletBean类的init方法

写java web时，我们知道如果需要初始化，那我们就需要重载HttpServlet的init方法。那么我们首先来寻找init方法的位置，可以发现它在HttpServletBean类中。

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);
      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;
   }

   // Let subclasses do whatever they like.
   initServletBean();

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

该方法主要做了如下两件事：

1. 将初始化参数设置为该servlet的属性
2. 调用子类的initServletBean方法

附：以我的理解来解释一下初始化参数设置为属性这一步骤，

<servlet>
    <servlet-name>spring-mvc</servlet-name>
    <servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>
    <init-param>
        <param-name>contextConfigLocation</param-name>
        <param-value>classpath:config/spring-mvc.xml</param-value>
    </init-param>
    <load-on-startup>1</load-on-startup>
</servlet>
<servlet-mapping>
    <servlet-name>spring-mvc</servlet-name>
    <url-pattern>/</url-pattern>
</servlet-mapping>

在配置文件中我们可以发现DispatcherServlet的初始化参数为contextConfigLocation=classpath:config/spring-mvc.xml，我们可以在FrameworkServlet的属性列表中找到contextConfigLocation这个属性。那么这块地方做的工作就是将初始化参数中contextConfigLocation的值配置给DispatcherServlet的contextConfigLocation属性。

FrameworkServlet类的initServletBean方法

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();
      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");
   }
}

该函数中主要调用了两个方法：initWebApplicationContext()和initFrameworkServlet()。

initFrameworkServlet()是用来给子类做初始化扩展的，事实上FrameworkServlet中是一个空实现，DispatchServlet也并没有重载该方法。

FrameworkServlet类的initWebApplicationContext方法

初始化WebApplicationContext，并且将根WebApplicationContext设置为其父context。

protected WebApplicationContext initWebApplicationContext() {
   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) {
      // 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.
      onRefresh(wac);
   }

   if (this.publishContext) {
      // 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;
}

DispatcherServlet类的onRefresh方法

我们可以发现，无论怎样，initWebApplicationContext方法最终都会调用到onRefresh方法。onRefresh在DispatcherServlet中被重载了，那我们直接看onRefresh部分的源码。

protected void onRefresh(ApplicationContext context) {
   initStrategies(context);
}

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

我们可以看到onRefresh调用了initStrategies方法，而initStrategies则对MultipartResolver, LocaleResolver, ThemeResolve, HandlerMappings, HandlerAdapters, HandlerExceptionResolvers, RequestToViewNameTranslator, ViewResolvers和FlashMapManager做了初始化。

不难知道，我们之前阅读的HandlerMapping, HandlerAdapter及ViewResolver等都是在这一步进行初始化的操作。

initHandlerMappings

我们取其中的HandlerMapping的初始化来继续分析初始化流程,

private void initHandlerMappings(ApplicationContext context) {
   this.handlerMappings = null;

   // 如果detectAllHandlerMappings属性设置为真，则从用户配置文件中查找所有的HandlerMapping，并排序
   if (this.detectAllHandlerMappings) {
      Map<String, HandlerMapping> matchingBeans =
            BeanFactoryUtils.beansOfTypeIncludingAncestors(context, HandlerMapping.class, true, false);
      if (!matchingBeans.isEmpty()) {
         this.handlerMappings = new ArrayList<HandlerMapping>(matchingBeans.values());
         OrderComparator.sort(this.handlerMappings);
      }
   }
   // 否则只使用名称为"handlerMapping"的HandlerMapping
   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.
      }
   }

   // 如果没有找到HandlerMapping，则使用默认配置
   if (this.handlerMappings == null) {
      this.handlerMappings = getDefaultStrategies(context, HandlerMapping.class);
      if (logger.isDebugEnabled()) {
         logger.debug("No HandlerMappings found in servlet '" + getServletName() + "': using default");
      }
   }
}

从源码中可以知道，HandlerMapping的初始化流程主要分为两种，

1. 使用用户自己的配置
2. 使用默认配置

对于自己的配置没有什么可说的，那我们接下来来看默认配置，

protected <T> List<T> getDefaultStrategies(ApplicationContext context, Class<T> strategyInterface) {
   String key = strategyInterface.getName();
   String value = defaultStrategies.getProperty(key);
   if (value != null) {
      String[] classNames = StringUtils.commaDelimitedListToStringArray(value);
      List<T> strategies = new ArrayList<T>(classNames.length);
      for (String className : classNames) {
         try {
            Class<?> clazz = ClassUtils.forName(className, DispatcherServlet.class.getClassLoader());
            Object strategy = createDefaultStrategy(context, clazz);
            strategies.add((T) strategy);
         }
         catch (ClassNotFoundException ex) {
            throw new BeanInitializationException(
                  "Could not find DispatcherServlet's default strategy class [" + className +
                        "] for interface [" + key + "]", ex);
         }
         catch (LinkageError err) {
            throw new BeanInitializationException(
                  "Error loading DispatcherServlet's default strategy class [" + className +
                        "] for interface [" + key + "]: problem with class file or dependent class", err);
         }
      }
      return strategies;
   }
   else {
      return new LinkedList<T>();
   }
}

我们可以看到里面有一个关键的属性defaultStrategies。我们来看看这个defaultStrategies到底是怎么一回事，

private static final String DEFAULT_STRATEGIES_PATH = "DispatcherServlet.properties";

private static final Properties defaultStrategies;

static {
   // Load default strategy implementations from properties file.
   // This is currently strictly internal and not meant to be customized
   // by application developers.
   try {
      ClassPathResource resource = new ClassPathResource(DEFAULT_STRATEGIES_PATH, DispatcherServlet.class);
      defaultStrategies = PropertiesLoaderUtils.loadProperties(resource);
   }
   catch (IOException ex) {
      throw new IllegalStateException("Could not load 'DispatcherServlet.properties': " + ex.getMessage());
   }
}

可以发现defaultStrategies里的属性来自于DispatcherServlet.properties这个文件，那么我们继续找到该文件，

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

到这里我们的疑惑应该就已经全部解清了。也知道之前阅读的HandlerMapping、HandlerAdater等默认值为什么是那些了。当然了，如果我们只需要其中的部分HandlerMapping等，也并不需要修改该文件，从getDefaultStrategies函数中我们可以知道，只需要我们在配置文件中有定义相应的属性，那么就不会使用默认的。