Spring核心知识点 - 补充

Spring 三级缓存解决循环依赖

• Spring只能解决field和setter注入导致的循环依赖，无法解决构造器注入导致的循环依赖。
• Spring只处理了单例Bean对象之间的循环依赖，没有处理原型对象之间的循环依赖

三层缓存

// 三层缓存
public class DefaultSingletonBeanRegistry extends SimpleAliasRegistry implements SingletonBeanRegistry {
	// 第一级缓存：初始化完成的完全单例对象 Map<beanname, beanInstance>
	private final Map<String, Object> singletonObjects = new ConcurrentHashMap<>(256);

	// 第二级缓存：提前暴露的单例对象引用(未完成依赖注入和初始化) Map<beanname, beanInstance>
	private final Map<String, Object> earlySingletonObjects = new HashMap<>(16);

	// 第三级缓存: 用于生成提前暴露的单例对象引用的工厂Bean对象 Map<beanname, objectFactory>
	// factoryBean在getObject()方法中，对原生Bean添加后置处理器，判断是否需要提前生成代理对象并对外暴露
	private final Map<String, ObjectFactory<?>> singletonFactories = new HashMap<>(16);
}

	// DI : 先从缓存中取Bean --- 三层缓存依次判断(为解决循环依赖问题)
	protected Object getSingleton(String beanName, boolean allowEarlyReference) {
		Object singletonObject = this.singletonObjects.get(beanName);
		if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
			synchronized (this.singletonObjects) {
				singletonObject = this.earlySingletonObjects.get(beanName);
				if (singletonObject == null && allowEarlyReference) {
					ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName);
					if (singletonFactory != null) {
						singletonObject = singletonFactory.getObject();
						this.earlySingletonObjects.put(beanName, singletonObject);
						this.singletonFactories.remove(beanName);
					}
				}
			}
		}
		return singletonObject;
	}

doGetBean阶段

// 核心代码 - AbstractBeanFactory#doGetBean()：
        ...
	// DI 3：这里使用了一个匿名内部类，创建Bean实例对象，并且存入正在创建的缓存池中
	sharedInstance = getSingleton(beanName, () -> {
		try {
			// DI 3: 创建一个指定Bean实例对象，如果有父级继承，则合并子类和父类的定义
			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.
			// 显式地从容器单例模式Bean缓存中清除实例对象
			destroySingleton(beanName);
			throw ex;
		}
	});

// 核心代码 - AbstractBeanFactory#getSingleton()：
	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) {
				if (this.singletonsCurrentlyInDestruction) {
					throw new BeanCreationNotAllowedException(beanName,
							"Singleton bean creation not allowed while singletons of this factory are in destruction " +
							"(Do not request a bean from a BeanFactory in a destroy method implementation!)");
				}
				if (logger.isDebugEnabled()) {
					logger.debug("Creating shared instance of singleton bean '" + beanName + "'");
				}
				// 此处 singletonsCurrentlyInCreation.add(beanName) 添加到正在创建缓存池
				beforeSingletonCreation(beanName);
				boolean newSingleton = false;
				boolean recordSuppressedExceptions = (this.suppressedExceptions == null);
				if (recordSuppressedExceptions) {
					this.suppressedExceptions = new LinkedHashSet<>();
				}
				try {
					singletonObject = singletonFactory.getObject();
					newSingleton = true;
				}
				catch (IllegalStateException ex) {
					// Has the singleton object implicitly appeared in the meantime ->
					// if yes, proceed with it since the exception indicates that state.
					singletonObject = this.singletonObjects.get(beanName);
					if (singletonObject == null) {
						throw ex;
					}
				}
				catch (BeanCreationException ex) {
					if (recordSuppressedExceptions) {
						for (Exception suppressedException : this.suppressedExceptions) {
							ex.addRelatedCause(suppressedException);
						}
					}
					throw ex;
				}
				finally {
					if (recordSuppressedExceptions) {
						this.suppressedExceptions = null;
					}
					// 此处，从正在创建缓存池移除
					afterSingletonCreation(beanName);
				}
				if (newSingleton) {
					// 创建完成，加入到第一级缓存
					addSingleton(beanName, singletonObject);
				}
			}
			return singletonObject;
		}
	}

doCreateBean阶段

// 核心代码 - AbstractAutowireCapableBeanFactory#doCreateBean()：
        ...
	// 向容器中缓存单例模式的Bean对象，以防循环引用
	boolean earlySingletonExposure = (mbd.isSingleton() && this.allowCircularReferences &&
			isSingletonCurrentlyInCreation(beanName));
	if (earlySingletonExposure) {
		if (logger.isDebugEnabled()) {
				logger.debug("Eagerly caching bean '" + beanName +
					"' to allow for resolving potential circular references");
		}
		// this.singletonFactories.put(beanName, singletonFactory);
		// 这里是一个匿名内部类(FactoryBean接口)，完成Bean引用(可能是原生引用，也可能是代理引用)的提前暴露，防止循环引用
		addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
	}

// 核心代码 - AbstractAutowireCapableBeanFactory#getEarlyBeanReference()：
	// 获取提前暴露的引用 - 三层缓存的原因:
        // factoryBean会调用getEarlyBeanReference方法，通过后置处理器尝试获取提前暴露的代理对象引用
	protected Object getEarlyBeanReference(String beanName, RootBeanDefinition mbd, Object bean) {
		Object exposedObject = bean;
		if (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {
			for (BeanPostProcessor bp : getBeanPostProcessors()) {
				if (bp instanceof SmartInstantiationAwareBeanPostProcessor) {
					SmartInstantiationAwareBeanPostProcessor ibp = (SmartInstantiationAwareBeanPostProcessor) bp;
					exposedObject = ibp.getEarlyBeanReference(exposedObject, beanName);
				}
			}
		}
		return exposedObject;
	}