Spring源码之循环依赖

https://www.cnblogs.com/longy2012/articles/12834762.html
https://www.bilibili.com/video/BV1iD4y1o7pM?p=7
https://www.jianshu.com/p/8bb67ca11831
https://cloud.tencent.com/developer/article/1497692
https://blog.nowcoder.net/n/2bb528b258b44c7eab1703a52170ef09

总结

1. 在DefaultSingletonBeanRegistry类getSingleton方法中将beanName（a）加入到singletonsCurrentlyInCreation set集合中
2. A进行实例化（未初始化）
3. A加入三级缓存singletonFactories中
4. 在AbstractAutowireCapableBeanFactory中populateBean方法中开始属性填充（field B）（调用AutowiredAnnotationBeanPostProcessor类中postProcessProperties方法）
5. 在DefaultSingletonBeanRegistry类getSingleton方法中将beanName（b）加入到singletonsCurrentlyInCreation set集合中
6. 将B加入singletonsCurrentlyInCreation，标志为正在创建中
7. B进行实例化（未初始化）
8. B加入三级缓存singletonFactories中
9. 开始对B类进行属性填充（A field）
10. 重新走到AbstractBeanFactory中doGetBean方法,调用DefaultSingletonBeanRegistry类getSingleton方法，将A put到二级缓存earlySingletonObjects中，并在三级缓存singletonFactories中移除
11. B进行初始化
12. getSingleton方法中调用DefaultSingletonBeanRegistry类afterSingletonCreation方法，将beanName（b）从singletonsCurrentlyInCreation移除
13. 走到DefaultSingletonBeanRegistry中getSingleton方法finally中，将B加入一级缓存，并从三级缓存中移除
14. 将B返回，A进行初始化
15. getSingleton方法中调用DefaultSingletonBeanRegistry类afterSingletonCreation方法，将beanName（a）从singletonsCurrentlyInCreation移除
16. 最后走到DefaultSingletonBeanRegistry中getSingleton方法finally中，将A加入一级缓存，并从二级缓存中移除

示例

1. @DependsOn

@DependsOn("b")
@Component
public class A {
}

@DependsOn("a")
@Component
public class B {
}

报错：

org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'b' defined

2. field属性注入循环依赖（不报错）

@Component
public class A {
    @Autowired
    private B b;
}

@Component
public class B {
    @Autowired
    private A a;
}

源码解析

三级缓存

DefaultSingletonBeanRegistry类中：

	/** Cache of singleton objects: bean name to bean instance. */
	//一级缓存
	private final Map<String, Object> singletonObjects = new ConcurrentHashMap<>(256);

	/** Cache of singleton factories: bean name to ObjectFactory. */
	//三级缓存
	private final Map<String, ObjectFactory<?>> singletonFactories = new HashMap<>(16);

	/** Cache of early singleton objects: bean name to bean instance. */
	//二级缓存
	private final Map<String, Object> earlySingletonObjects = new ConcurrentHashMap<>(16);

springboot中入口

调用链：
SpringApplication#run() --> SpringApplication#refreshContext() --> SpringApplication#refresh() --> ServletWebServerApplicationContext#refresh() --> AbstractApplicationContext#refresh() --> AbstractApplicationContext#finishBeanFactoryInitialization() --> DefaultListableBeanFactory#preInstantiateSingletons() --> AbstractBeanFactory#getBean() --> AbstractBeanFactory#doGetBean()

将beanName（a）加入到singletonsCurrentlyInCreation

AbstractBeanFactory#doGetBean()方法中：

if (!typeCheckOnly) {
	markBeanAsCreated(beanName);
}

markBeanAsCreated方法中：

this.alreadyCreated.add(beanName);

在getSingleton方法中，执行

beforeSingletonCreation(beanName);

this.singletonsCurrentlyInCreation.add(beanName)

其中singletonsCurrentlyInCreation为

/** Names of beans that are currently in creation. */
private final Set<String> singletonsCurrentlyInCreation =
		Collections.newSetFromMap(new ConcurrentHashMap<>(16));

A进行实例化（未初始化）

在在getSingleton方法执行到

singletonObject = singletonFactory.getObject();

调用匿名方法

() -> {
    	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;
    	}
    }

调用AbstractAutowireCapableBeanFactory#createBean() --> AbstractAutowireCapableBeanFactory#doCreateBean(),执行如下代码进行实例化：

if (instanceWrapper == null) {
	instanceWrapper = createBeanInstance(beanName, mbd, args);
}
Object bean = instanceWrapper.getWrappedInstance();

bean(A)具备地址

A加入三级缓存singletonFactories中

// Eagerly cache singletons to be able to resolve circular references
// even when triggered by lifecycle interfaces like BeanFactoryAware.
boolean earlySingletonExposure = (mbd.isSingleton() && this.allowCircularReferences &&
		isSingletonCurrentlyInCreation(beanName));
if (earlySingletonExposure) {
	if (logger.isTraceEnabled()) {
		logger.trace("Eagerly caching bean '" + beanName +
				"' to allow for resolving potential circular references");
	}
	addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
}

protected void addSingletonFactory(String beanName, ObjectFactory<?> singletonFactory) {
	Assert.notNull(singletonFactory, "Singleton factory must not be null");
	synchronized (this.singletonObjects) {
		if (!this.singletonObjects.containsKey(beanName)) {
			this.singletonFactories.put(beanName, singletonFactory);
			this.earlySingletonObjects.remove(beanName);
			this.registeredSingletons.add(beanName);
		}
	}
}

在populateBean方法中开始对A属性填充

populateBean(beanName, mbd, instanceWrapper);

对B重复上面步骤

调用链

AbstractApplicationContext#refresh() --> AbstractApplicationContext#finishBeanFactoryInitialization() --> DefaultListableBeanFactory#preInstantiateSingletons() --> AbstractBeanFactory#getBean() --> AbstractBeanFactory#doGetBean() --> AbstractAutowireCapableBeanFactory#createBean()
--> AbstractAutowireCapableBeanFactory#doCreateBean()

(上面为A，下面开始A属性B注入)
--> AbstractAutowireCapableBeanFactory#populateBean() --> AutowiredAnnotationBeanPostProcessor#postProcessProperties() --> InjectionMetadata#inject() --> AutowiredAnnotationBeanPostProcessor#inject() --> DefaultListableBeanFactory#resolveDependency() --> DefaultListableBeanFactory#doResolveDependency() --> DependencyDescriptor#resolveCandidate() --> AbstractBeanFactory#getBean() --> AbstractBeanFactory#doGetBean()

会和上面A同样做下面这些操作：

• 在DefaultSingletonBeanRegistry类getSingleton方法中将beanName（b）加入到singletonsCurrentlyInCreation set集合中
• 将B加入singletonsCurrentlyInCreation，标志为正在创建中
• B进行实例化（未初始化，具有B地址）
• B加入三级缓存singletonFactories中
• 开始对B类进行属性填充（A field）

将A put到二级缓存earlySingletonObjects中

重新对A执行doGetBean方法，执行getSingleton时，singletonFactory != null，故A放到二级缓存中，并从三级缓存中移除

Object sharedInstance = getSingleton(beanName);

protected Object getSingleton(String beanName, boolean allowEarlyReference) {
	// Quick check for existing instance without full singleton lock
	Object singletonObject = this.singletonObjects.get(beanName);
	if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
		singletonObject = this.earlySingletonObjects.get(beanName);
		if (singletonObject == null && allowEarlyReference) {
			synchronized (this.singletonObjects) {
				// Consistent creation of early reference within full singleton lock
				singletonObject = this.singletonObjects.get(beanName);
				if (singletonObject == null) {
					singletonObject = this.earlySingletonObjects.get(beanName);
					if (singletonObject == null) {
						ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName);
						if (singletonFactory != null) {
							singletonObject = singletonFactory.getObject();
							this.earlySingletonObjects.put(beanName, singletonObject);
							this.singletonFactories.remove(beanName);
						}
					}
				}
			}
		}
	}
	return singletonObject;
}

B进行初始化

此时B完成属性注入，A具有地址，开始初始化：

// Initialize the bean instance.
Object exposedObject = bean;
try {
	populateBean(beanName, mbd, instanceWrapper);
	exposedObject = initializeBean(beanName, exposedObject, mbd);
}

将beanName（b）从singletonsCurrentlyInCreation移除

B执行完AbstractBeanFactory类匿名方法中createBean(beanName, mbd, args)，接着getSingleton往下执行，在finally中：

finally {
	if (recordSuppressedExceptions) {
		this.suppressedExceptions = null;
	}
	afterSingletonCreation(beanName);
}

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

将B加入一级缓存，并从三级缓存中移除

在getSingleton方法执行addSingleton(beanName, singletonObject);从而将B从三级缓存中移除，并添加到一级缓存中

protected void addSingleton(String beanName, Object singletonObject) {
	synchronized (this.singletonObjects) {
		this.singletonObjects.put(beanName, singletonObject);
		this.singletonFactories.remove(beanName);
		this.earlySingletonObjects.remove(beanName);
		this.registeredSingletons.add(beanName);
	}
}

完成A中B属性注入，对A进行初始化

A开始执行下面代码：

exposedObject = initializeBean(beanName, exposedObject, mbd);

开始将beanName（a）从singletonsCurrentlyInCreation移除；并从二级缓存中移除，添加到一级缓存

A接着getSingleton往下执行，在finally中执行afterSingletonCreation将beanName（a）从singletonsCurrentlyInCreation移除；
最后在addSingleton(beanName, singletonObject)将A从二级缓存中移除，添加到一级缓存

面试问题

• 一级缓存是否够用？

不能。多线程情况下，会获取到实例化但没有初始化的对象，属性都为null

• 二级缓存是否够用？

如果创建是普通类，二级缓存满足

• 为什么需要三级缓存？（代理）

在动态代理中，返回是代理类。如果没有三级缓存，最开始放置是实例化好对象，然后缓存有了，后面进行代理处理，那原来的对象是否覆盖？？

	/**
	 * Obtain a reference for early access to the specified bean,
	 * typically for the purpose of resolving a circular reference.
	 * @param beanName the name of the bean (for error handling purposes)
	 * @param mbd the merged bean definition for the bean
	 * @param bean the raw bean instance
	 * @return the object to expose as bean reference
	 */
	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;
	}