Spring Boot 应用上下文刷新过程

前面讲了Spring Boot基于JavaConfig的执行流程，主要讲了应用环境的准备和初始化、应用上下文的实例化和准备，在应用上下文实例化时只注册了为后续解析配置必要的bean，应用上下文中基本上没有任何东西。本文通过分析AbstractApplicationContext#refresh()的执行来对Spring Boot的应用上下文的配置进行讲解，同时说明Spring Boot自动配置的实现。

AbstractApplicationContext#refresh()的代码及关键代码和注释如下 ：

public void refresh() throws BeansException, IllegalStateException {
  synchronized (this.startupShutdownMonitor) {
    // Prepare this context for refreshing.
    // 为应用上下文的刷新做准备--设置时间、记录刷新日志、初始化属性源中的占位符(事实上什么都没做)和验证必
    // 要的属性等
    prepareRefresh();

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

    // Prepare the bean factory for use in this context.
    // 准备在这个应用上下文中使用的bean factory
    prepareBeanFactory(beanFactory);

    try {
      // Allows post-processing of the bean factory in context subclasses.
      // bean factory 后置处理
      postProcessBeanFactory(beanFactory);

      // Invoke factory processors registered as beans in the context.
      // 调用应用上下文中作为bean注册的工厂处理器
      invokeBeanFactoryPostProcessors(beanFactory);

      // Register bean processors that intercept bean creation.
      // 注册拦截创建bean的bean处理器
      registerBeanPostProcessors(beanFactory);

      // Initialize message source for this context.
      // 初始化消息源
      initMessageSource();

      // Initialize event multicaster for this context.
      // 初始化事件广播
      initApplicationEventMulticaster();

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

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

      // Instantiate all remaining (non-lazy-init) singletons.
      // 实例化所有的单例bean
      finishBeanFactoryInitialization(beanFactory);

      // Last step: publish corresponding event.
      // 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.
      destroyBeans();

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