web工程中servlet能够映射到一个http请求路径，且每个servlet有其自己的生命周期，其使用init函数进行初始化，用service函数提供服务。
SpringMVC利用servlet这些特点，将容器的初始化巧妙的融合在了servlet的初始化过程中。
所以容器的初始化触发点在servlet init中。

org.springframework.web.servlet.HttpServletBean.init()函数。
@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 {
                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) {
                if (logger.isErrorEnabled()) {
                    logger.error("Failed to set bean properties on servlet '" + getServletName() + "'", ex);
                }
                throw ex;
            }
        }

        // Let subclasses do whatever initialization they like.
        initServletBean();//这里被子类覆盖

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

initServletBean();//这里被子类覆盖

@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();//这里是关键位置
            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");
        }
    }

this.webApplicationContext = initWebApplicationContext();//这里是关键位置。定义在org.springframework.web.servlet.FrameworkServlet.initWebApplicationContext()，其中有root上下文和web上下文关系是如何建立的逻辑。