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Spring源码之循环依赖

程序员文章站 2022-05-04 08:44:06
https://www.cnblogs.com/longy2012/articles/12834762.htmlhttps://www.bilibili.com/video/BV1iD4y1o7pM?p=7https://www.jianshu.com/p/8bb67ca11831https://cloud.tencent.com/developer/article/1497692https://blog.nowcoder.net/n/2bb528b258b44c7eab1703a52170ef0....

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

Spring源码之循环依赖

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
  1. 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;
	}

本文地址:https://blog.csdn.net/weixin_43843104/article/details/109643586

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