补充：关于扫描的逻辑

/*** Scan the class path for candidate components.* @param basePackage the package to check for annotated classes* @return a corresponding Set of autodetected bean definitions*/
public Set<BeanDefinition> findCandidateComponents(String basePackage) {if (this.componentsIndex != null && indexSupportsIncludeFilters()) {return addCandidateComponentsFromIndex(this.componentsIndex, basePackage);}else {return scanCandidateComponents(basePackage); // 大部分情况是这一种}
}

其中大部分情况是下面一种。上面这种的componentsIndex是为了提高扫描效率

当配置了spring.components文件（文件内容不为空），才会走上面的componentsIndex逻辑。

spring.components文件内容示例：（相当于告诉Spring要去扫描哪些Bean）

com.gax.service.UserService=org.springframework.stereotype.Component
#com.gax.service.OrderService=org.springframework.stereotype.Component

注意：以上配置的类上面仍然需要添加@Component，不添加会报Bean找不到的异常

实例化非懒加载的单例Bean源码：

// 遍历、合并、创建非懒加载单例Bean
@Override
public void preInstantiateSingletons() throws BeansException {if (logger.isTraceEnabled()) {logger.trace("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<String> beanNames = new ArrayList<>(this.beanDefinitionNames);// Trigger initialization of all non-lazy singleton beans...for (String beanName : beanNames) {// 获取合并后的BeanDefinitionRootBeanDefinition bd = getMergedLocalBeanDefinition(beanName);// 非懒加载的单例Bean，注意这里是抽象的BeanDefinition，和抽象类区分开// <bean id='user' class='' abstract='true' scope='prototype'> 抽象的BeanDefinition// 抽象的BeanDefinition不能生成Bean，但是可以给其他Bean当父BeanDefinition// <bean id='userService' class='' parent='user'>if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) {if (isFactoryBean(beanName)) {// 获取FactoryBean对象Object bean = getBean(FACTORY_BEAN_PREFIX + beanName);if (bean instanceof FactoryBean) {FactoryBean<?> factory = (FactoryBean<?>) bean;boolean isEagerInit;if (System.getSecurityManager() != null && factory instanceof SmartFactoryBean) {isEagerInit = AccessController.doPrivileged((PrivilegedAction<Boolean>) ((SmartFactoryBean<?>) factory)::isEagerInit,getAccessControlContext());}else {isEagerInit = (factory instanceof SmartFactoryBean &&((SmartFactoryBean<?>) factory).isEagerInit());}if (isEagerInit) {// 创建真正的Bean对象(getObject()返回的对象)getBean(beanName);}}}else {// 创建Bean对象getBean(beanName);}}}// 所有的非懒加载单例Bean都创建完了后// Trigger post-initialization callback for all applicable beans...for (String beanName : beanNames) {Object singletonInstance = getSingleton(beanName);if (singletonInstance instanceof SmartInitializingSingleton) {StartupStep smartInitialize = this.getApplicationStartup().start("spring.beans.smart-initialize").tag("beanName", beanName);SmartInitializingSingleton smartSingleton = (SmartInitializingSingleton) singletonInstance;if (System.getSecurityManager() != null) {AccessController.doPrivileged((PrivilegedAction<Object>) () -> {smartSingleton.afterSingletonsInstantiated();return null;}, getAccessControlContext());}else {smartSingleton.afterSingletonsInstantiated();}smartInitialize.end();}}
}

注意：抽象的BeanDefinition和抽象类的区别。思考抽象的BeanDefinition有什么用？

getMergedLocalBeanDefinition合并BeanDefinition是什么意思？

这个方法非常重要，合并之后的BeanDefinition。合并属性等信息：自己定义了用自己的，自己没定义用父亲的

对于父子BeanDefinition，合并是生成一个新的BeanDefinition，原来的父子BeanDefinition不变，把合并后的属性添加给新的BeanDefinition。子BeanDefinition的属性覆盖父BeanDefinition的属性，这就是合并

合并后的BeanDefinition放在mergedBeanDefinitions里面，很重要

afterSingletonsInstantiated方法什么时候调用的？

所有的非懒加载单例Bean创建完之后，才会调用每个单例Bean的afterSingletonsInstantiated方法

isFactoryBean(beanName)方法，也很重要，很多地方有用到

@Override
public boolean isFactoryBean(String name) throws NoSuchBeanDefinitionException {String beanName = transformedBeanName(name);Object beanInstance = getSingleton(beanName, false);if (beanInstance != null) {return (beanInstance instanceof FactoryBean);}// No singleton instance found -> check bean definition.if (!containsBeanDefinition(beanName) && getParentBeanFactory() instanceof ConfigurableBeanFactory) {// No bean definition found in this factory -> delegate to parent.return ((ConfigurableBeanFactory) getParentBeanFactory()).isFactoryBean(name);}return isFactoryBean(beanName, getMergedLocalBeanDefinition(beanName));
}/*** Check whether the given bean is defined as a {@link FactoryBean}.* @param beanName the name of the bean* @param mbd the corresponding bean definition*/
protected boolean isFactoryBean(String beanName, RootBeanDefinition mbd) {Boolean result = mbd.isFactoryBean;if (result == null) {// 根据BeanDefinition推测Bean类型（获取BeanDefinition的beanClass属性）Class<?> beanType = predictBeanType(beanName, mbd, FactoryBean.class);// 判断是不是实现了FactoryBean接口result = (beanType != null && FactoryBean.class.isAssignableFrom(beanType));mbd.isFactoryBean = result;}return result;
}