ApplicationContext applicationContext = new ClassPathXmlApplicationContext("bean.xml");
User user = (User) applicationContext.getBean("user");

spring的bean加载是从getBean方法开始的。

一、从缓存中获取bean

Object sharedInstance = getSingleton(beanName);

protected Object getSingleton(String beanName, boolean allowEarlyReference)；

该方法首先尝试从singletonObjects里面获取实例，如果获取不到再从earlySingletonObjects里获取，如果还获取不到，再尝试从sigletonFactories里面获取beanName对应的ObjectFactory,然后调用这个ObjectFactory的getObject来创建bean,并放到earlySingletonObjects，并将sigletonFactories的对应bean缓存删除掉。

存储bean的map解释：

/** Cache of singleton objects: bean name to bean instance. */
private final Map<String, Object> singletonObjects = new ConcurrentHashMap<>(256);

singletonObjects 用于保存BeanName和创建bean实例之间的关系。

/** Cache of early singleton objects: bean name to bean instance. */
private final Map<String, Object> earlySingletonObjects = new ConcurrentHashMap<>(16);

earlySingletonObjects 提前曝光的单例对象缓存，用于解决循环依赖

/** Cache of singleton factories: bean name to ObjectFactory. */
private final Map<String, ObjectFactory<?>> singletonFactories = new HashMap<>(16);

singletonFactories 用于保存BeanName和创建bean的工厂之间的关系。

内部方法解释：

public boolean isSingletonCurrentlyInCreation(String beanName);

判断当前bean是否处于创建中，即正在初始化，但是尚未完成初始化。

allowEarlyReference参数：是否允许从singletonFactories缓存中通过getObject方法拿到bean对象。

二、从bean实例中获取对象

beanInstance = getObjectForBeanInstance(sharedInstance, name, beanName, null);
protected Object getObjectForBeanInstance(
			Object beanInstance, String name, String beanName, @Nullable RootBeanDefinition mbd)； 

if (!(beanInstance instanceof FactoryBean)) {
			return beanInstance;
   }

object = getObjectFromFactoryBean(factory, beanName, !synthetic);

在getBean方法中获取到的bean，只是原始状态的bean，不一定是我们想要的。需要调用getObjectForBeanInstance进行处理。getObjectForBeanInstance对非factoryBean不做处理，直接返回，将解析bean的工作委托给getObjectFromFactoryBean。

if (factory.isSingleton() && containsSingleton(beanName)) 

Object object = doGetObjectFromFactoryBean(factory, beanName);

getObjectFromFactoryBean方法主要内容是如果是单例则从缓存中获取,缓存没有则从factoryBean中获取。如果不是单例，则直接去从factoryBean中获取bean.获取bean的操作又委托给了doGetObjectFromFactoryBean,最终调用 object = factory.getObject(); 获取bean.

三、获取bean流程

从spring容器中获取单例时有两种情况：缓存中存在和缓存中不存在。在上面缓存中不存在单例bean时，是通过getSingleton的重载方法来实现bean的加载的。

sharedInstance = getSingleton(beanName, () -> {
   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 received a temporary reference to the bean.
      destroySingleton(beanName);
      throw ex;
   }
});

public Object getSingleton(String beanName, ObjectFactory<?> singletonFactory)；

主要过程为：

1，再次尝试从缓存中取出bean

Object singletonObject = this.singletonObjects.get(beanName);

2, 若没有加载，则记录beanName的正在加载状态

beforeSingletonCreation(beanName);

protected void beforeSingletonCreation(String beanName) {
		if (!this.inCreationCheckExclusions.contains(beanName) && !this.singletonsCurrentlyInCreation.add(beanName)) {
			throw new BeanCurrentlyInCreationException(beanName);
		}
	}
	
/** Names of beans that are currently in creation. */
	private final Set<String> singletonsCurrentlyInCreation =
			Collections.newSetFromMap(new ConcurrentHashMap<>(16));	
	

3, 通过传入ObjectFactory创建bean

singletonObject = singletonFactory.getObject();

4,当bean加载结束后需要移除缓存中正在加载状态

protected void afterSingletonCreation(String beanName) {
   if (!this.inCreationCheckExclusions.contains(beanName) && !this.singletonsCurrentlyInCreation.remove(beanName)) {
      throw new IllegalStateException("Singleton '" + beanName + "' isn't currently in creation");
   }
}

5,将结果记录至缓存并删除加载bean所记录的各种辅助状态

addSingleton(beanName, singletonObject);

this.singletonObjects.put(beanName, singletonObject);
this.singletonFactories.remove(beanName);
this.earlySingletonObjects.remove(beanName);
this.registeredSingletons.add(beanName);

四、创建bean

return createBean(beanName, mbd, args);

太长，另起一篇。