Spring bean 如何实例化

bean实例化结论：

getBean -> 实例化对象 -> 进行依赖注入 -> 调用 BeanPostProcessor.postProcessBeforeInitialization -> InitializingBean.afterPropertiesSet() -> 自定义init方法 -> BeanPostProcessor.postProcessAfterInitialization

1、核心入口


AbstractApplicationContext#refresh


tips：此时已完成xml、注解中自定义的bean的解析，放在beanDefinitionMap中，还未进行实例化

@Override
public void refresh() throws BeansException, IllegalStateException {
   synchronized (this.startupShutdownMonitor) {
      // Prepare this context for refreshing.
      prepareRefresh();

      // Tell the subclass to refresh the internal bean factory.
      ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();

      // Prepare the bean factory for use in this context.
      prepareBeanFactory(beanFactory);

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

         // Invoke factory processors registered as beans in the context.
         invokeBeanFactoryPostProcessors(beanFactory);

         // Register bean processors that intercept bean creation.
         registerBeanPostProcessors(beanFactory);

         // Initialize message source for this context.
         initMessageSource();

         // Initialize event multicaster for this context.
         initApplicationEventMulticaster();

         // Initialize other special beans in specific context subclasses.
         onRefresh();

         // Check for listener beans and register them.
         registerListeners();

         // Instantiate all remaining (non-lazy-init) singletons.
         finishBeanFactoryInitialization(beanFactory);

         // Last step: publish corresponding event.
         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);

         // Propagate exception to caller.
         throw ex;
      }

      finally {
         // Reset common introspection caches in Spring's core, since we
         // might not ever need metadata for singleton beans anymore...
         resetCommonCaches();
      }
   }
}

2、refresh分析


2.1 prepareRefresh();


设置启动标志位


2.2 ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();

获取beanFactory，beanFactory是在创建ApplicationContext时生成的，GenericApplicationContext构造函数中

/**
 * Create a new GenericApplicationContext.
 * @see #registerBeanDefinition
 * @see #refresh
 */
public GenericApplicationContext() {
   this.beanFactory = new DefaultListableBeanFactory();
}

2.3 prepareBeanFactory(beanFactory);


设置beanFactory参数，比如beanClassLoader，系统默认的beanPostProcessors等


2.4 postProcessBeanFactory(beanFactory);


如果需要增加、处理BeanPostProcessor，子类可重写此方法


2.5 invokeBeanFactoryPostProcessors(beanFactory);


初始化并调用BeanFactoryPostProcessors,不是Bean的后处理器，是BeanFactory的后处理器


2.6 registerBeanPostProcessors(beanFactory);


捞取所有实现了BeanPostProcessor接口的bean，依次注册到beanFactory，如果实现PriorityOrdered接口的，会放前面并会进行排序，其他的放后面


2.7 initMessageSource();


初始化MessageSource，国际化处理


2.8 initApplicationEventMulticaster();


初始化ApplicationEventMulticaster，用来进行事件广播


2.9 onRefresh();


初始化其他特殊bean，可在子类覆盖此方法


2.10 registerListeners();


注册事件监听器，将实现ApplicationListener的bean，注册到2.8中的ApplicationEventMulticaster


2.11 finishBeanFactoryInitialization(beanFactory);


初始化所有剩下的非lazy-init的singleton bean，真正开始实例化自定义的bean（下面详解）


2.12 finishRefresh();


初始化并调用LifecycleProcessor，获取并调用Lifecycle的beans的start，发送ContextRefreshedEvent事件等


如果使用springBoot的话，此处会启动容器（如tomcat）

@Override
protected void finishRefresh() {
   super.finishRefresh();
   EmbeddedServletContainer localContainer = startEmbeddedServletContainer();
   if (localContainer != null) {
      publishEvent(
            new EmbeddedServletContainerInitializedEvent(this, localContainer));
   }
}

private EmbeddedServletContainer startEmbeddedServletContainer() {
   EmbeddedServletContainer localContainer = this.embeddedServletContainer;
   if (localContainer != null) {
      localContainer.start();
   }
   return localContainer;
}

3、finishBeanFactoryInitialization(beanFactory) 详解

核心方法是在DefaultListableBeanFactory.preInstantiateSingletons(); 先遍历beanDefinitionNames，只有对非抽象、单例、非懒加载的bean才进行实例化

@Override
public void preInstantiateSingletons() throws BeansException {
   if (this.logger.isDebugEnabled()) {
      this.logger.debug("Pre-instantiating singletons in " + this);
   }

   // Iterate over a copy to allow for init methods which in turn register new bean definitions.
   // While this may not be part of the regular factory bootstrap, it does otherwise work fine.
   List beanNames = new ArrayList(this.beanDefinitionNames);

   // Trigger initialization of all non-lazy singleton beans...
   for (String beanName : beanNames) {
      RootBeanDefinition bd = getMergedLocalBeanDefinition(beanName);
      if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) {
         if (isFactoryBean(beanName)) {
            final FactoryBean factory = (FactoryBean) getBean(FACTORY_BEAN_PREFIX + beanName);
            boolean isEagerInit;
            if (System.getSecurityManager() != null && factory instanceof SmartFactoryBean) {
               isEagerInit = AccessController.doPrivileged(new PrivilegedAction() {
                  @Override
                  public Boolean run() {
                     return ((SmartFactoryBean) factory).isEagerInit();
                  }
               }, getAccessControlContext());
            }
            else {
               isEagerInit = (factory instanceof SmartFactoryBean &&
                     ((SmartFactoryBean) factory).isEagerInit());
            }
            if (isEagerInit) {
               getBean(beanName);
            }
         }
         else {
            getBean(beanName);
         }
      }
   }

   // Trigger post-initialization callback for all applicable beans...
   for (String beanName : beanNames) {
      Object singletonInstance = getSingleton(beanName);
      if (singletonInstance instanceof SmartInitializingSingleton) {
         final SmartInitializingSingleton smartSingleton = (SmartInitializingSingleton) singletonInstance;
         if (System.getSecurityManager() != null) {
            AccessController.doPrivileged(new PrivilegedAction