先看一个简单的Bean的实例化过程，使用默认的构造函数
@Service public class UserService { public void query(){ System.out.println("query"); } }从AbstractApplicationContext#refresh中的finishBeanFactoryInitialization(beanFactory)进入 AbstractApplicationContext # finishBeanFactoryInitialization
protected void finishBeanFactoryInitialization(ConfigurableListableBeanFactory beanFactory) { // Initialize conversion service for this context. // 1.初始化此上下文的转换服务 if (beanFactory.containsBean(CONVERSION_SERVICE_BEAN_NAME) && beanFactory.isTypeMatch(CONVERSION_SERVICE_BEAN_NAME, ConversionService.class)) { beanFactory.setConversionService( beanFactory.getBean(CONVERSION_SERVICE_BEAN_NAME, ConversionService.class)); } // Register a default embedded value resolver if no bean post-processor // (such as a PropertyPlaceholderConfigurer bean) registered any before: // at this point, primarily for resolution in annotation attribute values. // 2.如果beanFactory之前没有注册嵌入值解析器，则注册默认的嵌入值解析器：主要用于注解属性值的解析。 if (!beanFactory.hasEmbeddedValueResolver()) { beanFactory.addEmbeddedValueResolver(new StringValueResolver() { @Override public String resolveStringValue(String strVal) { return getEnvironment().resolvePlaceholders(strVal); } }); } // Initialize LoadTimeWeaverAware beans early to allow for registering their transformers early. // 3.初始化LoadTimeWeaverAware Bean实例对象 String[] weaverAwareNames = beanFactory.getBeanNamesForType(LoadTimeWeaverAware.class, false, false); for (String weaverAwareName : weaverAwareNames) { getBean(weaverAwareName); } // Stop using the temporary ClassLoader for type matching. beanFactory.setTempClassLoader(null); // Allow for caching all bean definition metadata, not expecting further changes. // 4.冻结所有bean定义，注册的bean定义不会被修改或进一步后处理，因为马上要创建 Bean 实例对象了 beanFactory.freezeConfiguration(); // Instantiate all remaining (non-lazy-init) singletons. // 5.实例化所有剩余（非懒加载）单例对象 beanFactory.preInstantiateSingletons(); }2、从第5步进入DefaultListableBeanFactory # preInstantiateSingletons
@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. // 1.创建beanDefinitionNames的副本beanNames用于后续的遍历，以允许init等方法注册新的bean定义 List<String> beanNames = new ArrayList<String>(this.beanDefinitionNames); // Trigger initialization of all non-lazy singleton beans... // 2.遍历beanNames，触发所有非懒加载单例 bean的初始化 for (String beanName : beanNames) { // 3.获取beanName对应的 MergedBeanDefinition RootBeanDefinition bd = getMergedLocalBeanDefinition(beanName); // 4.bd对应的Bean实例：不是抽象类 && 是单例 && 不是懒加载 if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) { // 5.判断beanName对应的bean是否为FactoryBean if (isFactoryBean(beanName)) { // 5.1 通过beanName获取FactoryBean实例 // 通过getBean(&beanName)拿到的是FactoryBean本身；通过getBean(beanName)拿到的是FactoryBean创建的Bean实例 final FactoryBean<?> factory = (FactoryBean<?>) getBean(FACTORY_BEAN_PREFIX + beanName); // 5.2 判断这个FactoryBean是否希望急切的初始化 boolean isEagerInit; if (System.getSecurityManager() != null && factory instanceof SmartFactoryBean) { isEagerInit = AccessController.doPrivileged(new PrivilegedAction<Boolean>() { @Override public Boolean run() { return ((SmartFactoryBean<?>) factory).isEagerInit(); } }, getAccessControlContext()); } else { isEagerInit = (factory instanceof SmartFactoryBean && ((SmartFactoryBean<?>) factory).isEagerInit()); } if (isEagerInit) { // 5.3 如果希望急切的初始化，则通过beanName获取bean实例 getBean(beanName); } } else { // 6.如果beanName对应的bean不是FactoryBean，只是普通Bean，通过beanName获取bean实例 getBean(beanName); } } } //省略部分代码this.beanDefinitionNames 定义在DefaultListableBeanFactory中 AbstractBeanFactory # getBean(java.lang.String) AbstractBeanFactory # doGetBean DefaultSingletonBeanRegistry # getSingleton，从下面的代码中可以看出返回值为null 简单看一下DefaultSingletonBeanRegistry # isSingletonCurrentlyInCreation singletonsCurrentlyInCreation定义在DefaultSingletonBeanRegistry 中，是一个Set集合，用于存放当前正在创建的Bean的名称 回到 AbstractBeanFactory # doGetBean isPrototypeCurrentlyInCreation(beanName)这个方法还有点不明确，后面再讨论 AbstractBeanFactory # doGetBean继续往下执行到if(!typeCheckOnly)判断 进入判断的方法，将UserService的BD从mergedBeanDefinitions中移除，将beanName：userService添加到alreadyCreated集合中，至于这些BD在什么时候被加入到mergedBeanDefinitions，有空回头再看看前面的代码 AbstractBeanFactory # clearMergedBeanDefinition AbstractBeanFactory # doGetBean继续向下执行到getMergedLocalBeanDefinition 最后返回一个bd，从mergedBeanDefinitions中拿，没有，又从beanDefinitionMap中拿，那前面为啥删除呢？… checkMergedBeanDefinition() 会检查是否是抽象的，是就会抛出异常 执行到这一行就是真正的去创建一个Bean
