终于把Spring三级缓存彻底看明白了

从简单到复杂应用开发中，难免会遇到spring bean循环依赖问题，即Bean A依赖Bean B, 同时Bean B依赖Bean A。Spring大神提出了三级缓存帮助我们解决循环依赖的问题。

singletonFactories ：一级缓存，存放类工厂(ObjectFactory)，实例Bean通过ObjectFactory创建。
earlySingletonObjects ：二级缓存，存放创建不完全的Bean实例。
singletonObjects ：三级缓存，存放实例化完成的可用Bean。

AbstractBeanFactory的doGetBean方法创建Bean的入口

protected  T doGetBean(final String name, @Nullable final Class requiredType,
      @Nullable final Object[] args, boolean typeCheckOnly) throws BeansException {

   final String beanName = transformedBeanName(name);
   Object bean;

   // 检查Bean是否已经存在
   Object sharedInstance = getSingleton(beanName);
   if (sharedInstance != null && args == null) {
      // 省略........
   } else {
      // 省略........
      if (!typeCheckOnly) {
         //标记Bean已经被创建
         markBeanAsCreated(beanName);
      }
      try {
         // 省略........
         // 创建Bean
         if (mbd.isSingleton()) {
            // 调用父类DefaultSingletonBeanRegistry方法
            sharedInstance = getSingleton(beanName, () -> {
               try {
                  return createBean(beanName, mbd, args);
               }
               catch (BeansException ex) {
                  // 省略........
               }
            });
            bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
         }
         // 省略........
      }
      catch (BeansException ex) {
         // 省略........
      }
   }
   // 省略........
   return (T) bean;
}

DefaultSingletonBeanRegistry类Bean的注册类，三级缓存所在类

public Object getSingleton(String beanName, ObjectFactory singletonFactory) {
   Assert.notNull(beanName, "Bean name must not be null");
   synchronized (this.singletonObjects) {
      Object singletonObject = this.singletonObjects.get(beanName);
      if (singletonObject == null) {
         // 标记Bean正在被创建
         beforeSingletonCreation(beanName);
         // 省略.....
         try {
            // 创建实例
            singletonObject = singletonFactory.getObject();
            newSingleton = true;
         }
         catch (IllegalStateException ex) {
            // 省略.....
         }
         catch (BeanCreationException ex) {
            // 省略.....
         }
         finally {
            // 省略.....
         }
         if (newSingleton) {
            // Bean放入singletonObjects，并从singletonFactories， earlySingletonObjects中移除
            addSingleton(beanName, singletonObject);
         }
      }
      return singletonObject;
   }
}

AbstractAutowireCapableBeanFactory

@Override
protected Object createBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args)
      throws BeanCreationException {
   // 省略....... 
   try {
       // 创建Bean
      Object beanInstance = doCreateBean(beanName, mbdToUse, args);
      if (logger.isTraceEnabled()) {
         logger.trace("Finished creating instance of bean '" + beanName + "'");
      }
      return beanInstance;
   }
   catch (BeanCreationException | ImplicitlyAppearedSingletonException ex) {
      // 省略....... 
   }
   catch (Throwable ex) {
      // 省略....... 
   }
}

protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final @Nullable Object[] args)
      throws BeanCreationException {

   // 创建实例
   BeanWrapper instanceWrapper = null;
   if (mbd.isSingleton()) {
      instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
   }
   if (instanceWrapper == null) {
      instanceWrapper = createBeanInstance(beanName, mbd, args);
   }
   
   // 初始化Bean
   Object exposedObject = bean;
   try {
      // 属性注入
      populateBean(beanName, mbd, instanceWrapper);
      // 调用实现InitializeBean接口的初始化方法
      exposedObject = initializeBean(beanName, exposedObject, mbd);
   }
   catch (Throwable ex) {
      //省略.......
   }
   //省略.......
   return exposedObject;
}

protected void populateBean(String beanName, RootBeanDefinition mbd, @Nullable BeanWrapper bw) {
   // 省略.......
   if (resolvedAutowireMode == AUTOWIRE_BY_NAME || resolvedAutowireMode == AUTOWIRE_BY_TYPE) {
      MutablePropertyValues newPvs = new MutablePropertyValues(pvs);
      // bean名称注入
      if (resolvedAutowireMode == AUTOWIRE_BY_NAME) {
         autowireByName(beanName, mbd, bw, newPvs);
      }
      // bean类型注入
      if (resolvedAutowireMode == AUTOWIRE_BY_TYPE) {
         autowireByType(beanName, mbd, bw, newPvs);
      }
      pvs = newPvs;
   }
   // 省略.....
}

protected void autowireByName(
      String beanName, AbstractBeanDefinition mbd, BeanWrapper bw, MutablePropertyValues pvs) {

   // 省略......
   for (String propertyName : propertyNames) {
      if (containsBean(propertyName)) {
         // 获取依赖Bean实例
         Object bean = getBean(propertyName);
         pvs.add(propertyName, bean);
         // 注入依赖
         registerDependentBean(propertyName, beanName);
         // 省略.....
      }
      else {
         // 省略.....
      }
   }
}

DefaultSingletonBeanRegistry

protected Object getSingleton(String beanName, boolean allowEarlyReference) {
   Object singletonObject = this.singletonObjects.get(beanName);
   // 主动注入Bean这里直接返回，依赖注入Bean才会执行里面逻辑
   if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
      synchronized (this.singletonObjects) {
         // singletonObjects没有找到，从earlySingletonObjects查找
         singletonObject = this.earlySingletonObjects.get(beanName);
         if (singletonObject == null && allowEarlyReference) {
            ObjectFactory singletonFactory = this.singletonFactories.get(beanName);
            if (singletonFactory != null) {
                // earlySingletonObjects没有找到则创建实例
               singletonObject = singletonFactory.getObject();
               // 创建的实例放入earlySingletonObjects，此时实例还没有进行属性注入
               this.earlySingletonObjects.put(beanName, singletonObject);
               this.singletonFactories.remove(beanName);
            }
         }
      }
   }
   return singletonObject;
}