《深入理解Spring系列之四：BeanDefinition装载前奏曲》中提到，对于非延迟单例bean的初始化在finishBeanFactoryInitialization(beanFactory)中完成。进入这个方法，代码如下。

protected void finishBeanFactoryInitialization(ConfigurableListableBeanFactory beanFactory) {// Initialize conversion service for this context.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));}// Initialize LoadTimeWeaverAware beans early to allow for registering their transformers early.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.beanFactory.freezeConfiguration();// Instantiate all remaining (non-lazy-init) singletons.beanFactory.preInstantiateSingletons();
}复制代码

关注最后一行代码，

beanFactory.preInstantiateSingletons()完成初始化所有非延迟的单例bean，进入这个方法的具体实现，代码如下。
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 singletonbeans...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() {@Overridepublic 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() {@Overridepublic Object run() {smartSingleton.afterSingletonsInstantiated();return null;}}, getAccessControlContext());}else {
smartSingleton.afterSingletonsInstantiated();}}}
}复制代码

从上面的代码中看到，只会对非延迟单例bean进行初始化，scope为其它值的bean会在使用到的时候进行初始化，如prototype。这里关注getBean方法，这个方法看着很眼熟，其实就是《深入理解Spring系列之一：开篇》示例代码中用到的getBean，Spring对这个方法做了重复使用。getBean方法的具体实现在doGetBean方法中，这个方法的代码很长就不贴代码了。在doGetBean中，首先会初始化其依赖的bean，然后进行自身的初始化，这个方法里关注如下的代码段。

 // Create bean instance.if (mbd.isSingleton()) {sharedInstance = getSingleton(beanName, new ObjectFactory() {@Overridepublic Object getObject() throws BeansException {try {return createBean(beanName, mbd, args);}catch (BeansException ex) {// Explicitly remove instance from singleton cache: It might have been put there// eagerly by the creation process, to allow for circular reference resolution.// Also remove any beans that receiveda temporary reference to the bean.destroySingleton(beanName);throw ex;}}});bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);}复制代码

这段代码完成了单例bean的初始化，追踪代码进入doCreateBean方法中，在这个方法中进行bean实例的创建、属性填充、将bean实例加入单例bean实例的缓存中。doCreateBean方法中有如下代码段。

if (instanceWrapper == null) {instanceWrapper = createBeanInstance(beanName, mbd, args);
}复制代码

createBeanInstance方法里完成bean实例的创建，具体过程可继续追踪代码查看，其实就是使用反射进行实例对象的创建。