@Override
public void refresh() throws BeansException, IllegalStateException {
    synchronized (this.startupShutdownMonitor) {
        StartupStep contextRefresh = this.applicationStartup.start("spring.context.refresh");

        // Prepare this context for refreshing. 准备刷新上下文
        prepareRefresh();

        // Tell the subclass to refresh the internal bean factory. 告诉子类刷新内部bean工厂
        ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();

        // Prepare the bean factory for use in this context. 准备bean工厂以用于此上下文
        prepareBeanFactory(beanFactory);

        try {
            // Allows post-processing of the bean factory in context subclasses. 允许在上下文子类中对bean工厂进行后置处理
            postProcessBeanFactory(beanFactory);

            StartupStep beanPostProcess = this.applicationStartup.start("spring.context.beans.post-process");
            // Invoke factory processors registered as beans in the context. 调用上下文中注册为bean的工厂处理器
            invokeBeanFactoryPostProcessors(beanFactory);

            // Register bean processors that intercept bean creation.  注册拦截器bean创建的bean处理器
            registerBeanPostProcessors(beanFactory);
            beanPostProcess.end();

            // Initialize message source for this context. 初始化此上下文的消息源
            initMessageSource();

            // Initialize event multicaster for this context. 为此上下文初始化事件多播
            initApplicationEventMulticaster();

            // Initialize other special beans in specific context subclasses. 初始化特定上下文子类中的其他特殊bean
            onRefresh();

            // Check for listener beans and register them. 检查监听器bean并注册
            registerListeners();

            // Instantiate all remaining (non-lazy-init) singletons. 实例化所有剩余的（非懒惰初始化）单例
            finishBeanFactoryInitialization(beanFactory);

            // Last step: publish corresponding event. 最后一步：发布相应的事件
            finishRefresh();
        }

        catch (BeansException ex) {
            if (logger.isWarnEnabled()) {
                logger.warn("Exception encountered during context initialization - " +
                        "cancelling refresh attempt: " + ex);
            }

            // Destroy already created singletons to avoid dangling resources. 销毁已创建的单例以避免悬空资源
            destroyBeans();

            // Reset 'active' flag. 重置active标志
            cancelRefresh(ex);

            // Propagate exception to caller. 将异常传播到调用方
            throw ex;
        }

        finally {
            // Reset common introspection caches in Spring's core, since we
            // might not ever need metadata for singleton beans anymore...
            //重置Spring核心中的公共内省缓存，因为我们可能不再需要单例bean的元数据
            resetCommonCaches();
            contextRefresh.end();
        }
    }
}

/**
 * Prepare this context for refreshing, setting its startup date and
 * active flag as well as performing any initialization of property sources.
 */
protected void prepareRefresh() {
    // Switch to active. 切换到激活
    this.startupDate = System.currentTimeMillis();
    this.closed.set(false);
    this.active.set(true);

    if (logger.isDebugEnabled()) {
        if (logger.isTraceEnabled()) {
            logger.trace("Refreshing " + this);
        }
        else {
            logger.debug("Refreshing " + getDisplayName());
        }
    }

    // Initialize any placeholder property sources in the context environment. 初始化上下文环境中的任何占位符属性源
    initPropertySources();

    // Validate that all properties marked as required are resolvable:验证标记为需要的所有属性是否可解析
    // see ConfigurablePropertyResolver#setRequiredProperties 主要看ConfigurablePropertyResolver#setRequiredProperties
    getEnvironment().validateRequiredProperties();

    // Store pre-refresh ApplicationListeners... 预刷新应用监听器
    if (this.earlyApplicationListeners == null) {
        this.earlyApplicationListeners = new LinkedHashSet<>(this.applicationListeners);
    }
    else {
        // Reset local application listeners to pre-refresh state. 重置应用监听器为预刷新状态
        this.applicationListeners.clear();
        this.applicationListeners.addAll(this.earlyApplicationListeners);
    }

    // Allow for the collection of early ApplicationEvents, 允许收集早期的应用事件在multicaster可用后一次性发布
    // to be published once the multicaster is available...
    this.earlyApplicationEvents = new LinkedHashSet<>();
}