我们都知道web项目启动前要先初始化servlet，执行servlet的init方法，因此我们以Spring的DispatcherServlet的init方法为切入点。

在DispatcherServlet类中并没有找到init方法，继续从其父类中搜索，DispatcherServlet -> FrameworkServlet -> HttpServletBean，最终在其父类HttpServletBean中找到了init方法。

	@Override
	public final void init() throws ServletException {
		if (logger.isDebugEnabled()) {
			logger.debug("Initializing servlet '" + getServletName() + "'");
		}

		// Set bean properties from init parameters.
		PropertyValues pvs = new ServletConfigPropertyValues(getServletConfig(), this.requiredProperties);
		if (!pvs.isEmpty()) {
			try {
				// 定位资源
				BeanWrapper bw = PropertyAccessorFactory.forBeanPropertyAccess(this);
				// 加载配置信息
				ResourceLoader resourceLoader = new ServletContextResourceLoader(getServletContext());
				bw.registerCustomEditor(Resource.class, new ResourceEditor(resourceLoader, getEnvironment()));
				initBeanWrapper(bw);
				bw.setPropertyValues(pvs, true);
			}
			catch (BeansException ex) {
				if (logger.isErrorEnabled()) {
					logger.error("Failed to set bean properties on servlet '" + getServletName() + "'", ex);
				}
				throw ex;
			}
		}

		// Let subclasses do whatever initialization they like.
		// 让子类自定义实现初始化
		initServletBean();

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

可以在方法中看到，最重要的初始化方法是initServletBean()方法，具体实现逻辑交给子类实现。

在其子类FrameworkServlet中实现了initServletBean()方法：

	@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();
			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");
		}
	}

继续跟进initWebApplicationContext()方法

	/**
	 * 初始化并发布为servlet发布web容器
	 * Initialize and publish the WebApplicationContext for this servlet.
	 */
	protected WebApplicationContext initWebApplicationContext() {
		// 先从ServletContext中获得父容器
		WebApplicationContext rootContext =
				WebApplicationContextUtils.getWebApplicationContext(getServletContext());
		// 声明子容器
		WebApplicationContext wac = null;
		// 建立父子容器之间的关联关系
		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);
		}
		// 触发容器初始化
		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;
	}

最终调用了由DispatherServlet实现的onRefresh()方法，在initStrategies()方法中初始化了SpringMVC的九大组件。

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

	/**
	 * 初始化策略
	 * 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) {
		// 多文件上传组件
		initMultipartResolver(context);
		// 初始化本地语言环境
		initLocaleResolver(context);
		// 初始化模板处理器
		initThemeResolver(context);
		// 初始化url映射
		initHandlerMappings(context);
		// 初始化参数适配器
		initHandlerAdapters(context);
		// 初始化异常拦截器
		initHandlerExceptionResolvers(context);
		// 初始化视图预处理器
		initRequestToViewNameTranslator(context);
		// 初始化视图转化器
		initViewResolvers(context);
		// 初始化FlashMap管理器
		initFlashMapManager(context);
	}