public final void init() throws ServletException {
    // Set bean properties from init parameters.
    // 从 ServletConfig 中取出初始化参数到 PropertyValues。ServletConfigPropertyValues 的构造器中将会检查是否缺失了必要属性
    PropertyValues pvs = new ServletConfigPropertyValues(getServletConfig(), this.requiredProperties);
    if (!pvs.isEmpty()) {
        try {
            // 将 servlet 对象包装成 BeanWrapper ，从而能够以 Spring 的方式（反射）来注入参数
            BeanWrapper bw = PropertyAccessorFactory.forBeanPropertyAccess(this);
            ResourceLoader resourceLoader = new ServletContextResourceLoader(getServletContext());
            // 注册 PropertyEditor，遇到 Resource 类型的属性时，用 ResourceEditor 解析
            bw.registerCustomEditor(Resource.class, new ResourceEditor(resourceLoader, getEnvironment()));
            // 初始化 BeanWrapper，空方法
            initBeanWrapper(bw);
            // 注入属性，忽略没有 setter 的属性
            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.
    // 由子类实现初始化逻辑
    initServletBean();
}

private static class ServletConfigPropertyValues extends MutablePropertyValues {
    public ServletConfigPropertyValues(ServletConfig config, Set<String> requiredProperties)
            throws ServletException {
        // 将 requiredProperties 拷贝到新的 Set missingProps
        Set<String> missingProps = (!CollectionUtils.isEmpty(requiredProperties) ?
                new HashSet<>(requiredProperties) : null);

        // 将 ServletConfig 中的初始化参数取出，添加到 MutablePropertyValues 中
        Enumeration<String> paramNames = config.getInitParameterNames();
        while (paramNames.hasMoreElements()) {
            String property = paramNames.nextElement();
            Object value = config.getInitParameter(property);
            addPropertyValue(new PropertyValue(property, value));
            if (missingProps != null) {
                missingProps.remove(property);
            }
        }

        // Fail if we are still missing properties.
        if (!CollectionUtils.isEmpty(missingProps)) {
            // 存在必须出现的条件没出现
            throw new ServletException(
                    "Initialization from ServletConfig for servlet ‘" + config.getServletName() +
                    "‘ failed; the following required properties were missing: " +
                    StringUtils.collectionToDelimitedString(missingProps, ", "));
        }
    }
}

protected final void initServletBean() throws ServletException {
    getServletContext().log("Initializing Spring " + getClass().getSimpleName() + " ‘" + getServletName() + "‘");
    if (logger.isInfoEnabled()) {
        logger.info("Initializing Servlet ‘" + getServletName() + "‘");
    }
    long startTime = System.currentTimeMillis();

    try {
        // 初始化 webApplicationContext
        this.webApplicationContext = initWebApplicationContext();
        // 在容器被加载后执行，由子类来实现一些必要的初始化
        initFrameworkServlet();
    }
    catch (ServletException | RuntimeException ex) {
        logger.error("Context initialization failed", ex);
        throw ex;
    }
    // 略去打印日志的部分
    ... 
}

protected WebApplicationContext initWebApplicationContext() {
    // 获取 Root WebApplicationContext
    WebApplicationContext rootContext =
            WebApplicationContextUtils.getWebApplicationContext(getServletContext());
    WebApplicationContext wac = null;

    if (this.webApplicationContext != null) {
        // A context instance was injected at construction time -> use it
        // 一个上下文已经被注入进来
        wac = this.webApplicationContext;
        if (wac instanceof ConfigurableWebApplicationContext) {
            // 如果是 ConfigurableWebApplicationContext，
            ConfigurableWebApplicationContext cwac = (ConfigurableWebApplicationContext) wac;
            if (!cwac.isActive()) {
                // 没有激活，设置父容器，配置并且刷新容器
                if (cwac.getParent() == null) {
                    cwac.setParent(rootContext);
                }
                configureAndRefreshWebApplicationContext(cwac);
            }
        }
    }
    if (wac == null) {
        // 尝试从 ServletContext 中获取一个容器
        wac = findWebApplicationContext();
    }
    if (wac == null) {
        // 创建一个新的容器并初始化
        wac = createWebApplicationContext(rootContext);
    }

    if (!this.refreshEventReceived) {
        // 没有触发过刷新时间
        synchronized (this.onRefreshMonitor) {
            // 手动触发刷新事件
            onRefresh(wac);
        }
    }

    if (this.publishContext) {
        // Publish the context as a servlet context attribute.
        // 将容器发布到 ServletContext 的属性上
        String attrName = getServletContextAttributeName();
        getServletContext().setAttribute(attrName, wac);
    }

    return wac;
}

@Override
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);
}