1：前言

最近在项目中遇到了一次循环依赖报错的问题，虽然解决的很快，但是有些不明白的地方，特此记录。
在此我把 bean 的结构和 注入方式单独拎出来进行演示

1.1：报错提示

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1.2：错误日志

Exception encountered during context initialization - cancelling refresh attempt:org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'brokenComponent': Injection of resource dependencies failed; nested exception is org.springframework.beans.factory.UnsatisfiedDependencyException: Error creating bean with name 'tubunComponent' defined in class path resource [com/dev/config/sprngcache3/TwConfig.class]: Unsatisfied dependency expressed through method 'tubunComponent' parameter 0; nested exception is org.springframework.beans.factory.UnsatisfiedDependencyException: Error creating bean with name 'denpaComponent': Unsatisfied dependency expressed through field 'tolenComponent'; nested exception is org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'tolenComponent': Injection of resource dependencies failed; nested exception is org.springframework.beans.factory.BeanCurrentlyInCreationException: Error creating bean with name 'tubunComponent':Requested bean is currently in creation:Is there an unresolvable circular reference?

1.3：stack overflow问题描述

Requested bean is currently in creation: Is there an unresolvable circular reference?

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点击进入 stack overflow 问题描述

1.4：bean 依赖结构

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BrokenComponent：

@Componentpublic class BrokenComponent {    @Autowired    private TolenComponent tolenComponent;    @Resource    private TubunComponent tubunComponent;}

TolenComponent：

@Transactional@Componentpublic class TolenComponent {    @Resource    private TubunComponent tubunComponent;}

TubunComponent：

public class TubunComponent {    private DenpaComponent denpaComponent;    public TubunComponent(DenpaComponent denpaComponent) {        this.denpaComponent = denpaComponent;    }}

TwConfig：

@Configurationpublic class TwConfig {    @Bean    public TubunComponent tubunComponent(DenpaComponent denpaComponent) {        return new TubunComponent(denpaComponent);    }}

DenpaComponent：

@Componentpublic class DenpaComponent {    @Autowired    private TolenComponent tolenComponent;}

除了 tubunComponent 是构造器注入，其他的bean 都是set 注入。
分析bean依赖图可以发现确实是循环依赖的问题。

2：问题分析

由于本人对spring bean 的加载机制不是很清晰，这次特意花了几天时间做了梳理。

2.1：spring 的 bean 加载过程

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2.2：spring 的三级缓存

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解决了什么问题？

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推荐博客：https://cloud.tencent.com/developer/article/1497692

2.3：Spring 容器加载过程中比较重要的类/属性

public class DefaultSingletonBeanRegistry extends SimpleAliasRegistry implements SingletonBeanRegistry {		private final Map<String, Object> singletonObjects = new ConcurrentHashMap<>(256);		private final Map<String, ObjectFactory<?>> singletonFactories = new HashMap<>(16);		private final Map<String, Object> earlySingletonObjects = new HashMap<>(16);		private final Set<String> registeredSingletons = new linkedHashSet<>(256);		private final Set<String> singletonsCurrentlyInCreation = Collections.newSetFromMap(new ConcurrentHashMap<>(16));        ....}

2.4：注册后处理器->BeanPostProcessor

作用：对通过 BeanFactory 创建的 bean 进行属性填充。

这是AbstractAutowireCapableBeanFactory#populateBean 方法中的某一段代码：

for (BeanPostProcessor bp : getBeanPostProcessors()) {	if (bp instanceof InstantiationAwareBeanPostProcessor) {		InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp;		PropertyValues pvsToUse = ibp.postProcessProperties(pvs, bw.getWrappedInstance(), beanName);		if (pvsToUse == null) {			if (filteredPds == null) {				filteredPds = filterPropertyDescriptorsForDependencyCheck(bw, mbd.allowCaching);			}			pvsToUse = ibp.postProcessPropertyValues(pvs, filteredPds, bw.getWrappedInstance(), beanName);			if (pvsToUse == null) {				return;			}		}		pvs = pvsToUse;	}}

我打出了所有要进行轮询校验的 BeanPostProcessor

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brokenComponent 中有两个bean:

一个是通过@Resource 注解注入的，一个是通过@Autowire 进行注入的，奉劝各位同学千万不要两种混用啊！！！

2.4.1：CommonAnnotationBeanPostProcessor

public class CommonAnnotationBeanPostProcessor extends InitDestroyAnnotationBeanPostProcessor		implements InstantiationAwareBeanPostProcessor, BeanFactoryAware, Serializable {              ...	@Override	public PropertyValues postProcessProperties(PropertyValues pvs, Object bean, String beanName) {		Injectionmetadata metadata = findResourcemetadata(beanName, bean.getClass(), pvs);		try {			metadata.inject(bean, beanName, pvs);		}		catch (Throwable ex) {			throw new BeanCreationException(beanName, "Injection of resource dependencies failed", ex);		}		return pvs;	}  }

通过翻译及反复debug验证，这个类将对某个bean中所有被 @Resource 注解修饰的属性进行填充。

2.4.2：AutowiredAnnotationBeanPostProcessor

public class AutowiredAnnotationBeanPostProcessor extends InstantiationAwareBeanPostProcessorAdapter		implements MergedBeanDefinitionPostProcessor, PriorityOrdered, BeanFactoryAware {        ...	@Override	public PropertyValues postProcessProperties(PropertyValues pvs, Object bean, String beanName) {		Injectionmetadata metadata = findAutowiringmetadata(beanName, bean.getClass(), pvs);		try {			metadata.inject(bean, beanName, pvs);		}		catch (BeanCreationException ex) {			throw ex;		}		catch (Throwable ex) {			throw new BeanCreationException(beanName, "Injection of autowired dependencies failed", ex);		}		return pvs;	}}

这个类将对某个bean中所有被 @Autowire@Value 注解修饰的属性进行填充。

2.4.3：@Autowire VS @Resource

1：无论使用@Resource 还是 @Resource，都不影响bean的初始化顺序。

2：@Resource修饰的属性 要优先于 @Autowire修饰的属性 进行初始化。

2.5：报错流程

1: getBean("brokenComponent") -> getSingleton("brokenComponent") -> beforeSingletonCreation("brokenComponent") -> createBean("brokenComponent") -> addSingletonFactory("brokenComponent") ->  inject() ->2: getBean("tubunComponent",TubunComponent.class) -> getSingleton("tubunComponent") -> beforeSingletonCreation("tubunComponent") -> 3：getBean("twConfig") -> getSingleton("twConfig") -> beforeSingletonCreation("twconfig") -> addSingletonFactory("twConfig") -> afterSingletonCreation("twConfig")4：getBean("denpaComponent") -> getSingleton("denpaComponent") -> beforeSingletonCreation("denpaComponent") -> createBean("denpaComponent") ->addSingletonFactory("denpaComponent") -> inject() ->5：getBean("tolenComponent") -> getSingleton("tolenComponent") -> beforeSingletonCreation("tolenComponent") -> createBean("tolenComponent") ->addSingletonFactory("tolenComponent") -> inject() ->6：getBean("tubunComponent",TubunComponent.class) -> getSingleton("tubunComponent") -> beforeSingletonCreation("tubunComponent") -> 报错7：afterSingletonCreation("tolenComponent") -> afterSingletonCreation("denpaComponent") -> afterSingletonCreation("tubunComponent") -> afterSingletonCreation("brokenComponent")

可以看到 tubunComponent 进行了两次 getSingleton，经过循环依赖检查的时候报错了。

2.6：错误流程分析归纳

1：Spring 容器先加载 brokenComponent 这个 bean。

2：brokenComponent依赖了tubunComponent（@Resource修饰），因此优先填充 tubunComponent，因此去初始化tubunComponent 这个bean，并将tubunComponent 存放在 DefaultSingletonBeanRegistry.singletonsCurrentlyInCreation 集合中。

3：tubunComponent依赖了depenComponent，因此去初始化depenComponent 这个bean，并将depenComponent 存放在 DefaultSingletonBeanRegistry.singletonsCurrentlyInCreation 集合中。

4：depenComponent依赖了tolenComponent，因此去初始化depenComponent 这个bean，并将tolenComponent 存放在 DefaultSingletonBeanRegistry.singletonsCurrentlyInCreation 集合中。

5：tolenComponent依赖了tubunComponent，因此去初始化tubunComponent 这个bean，然而当将tubunComponent 存放在 DefaultSingletonBeanRegistry.singletonsCurrentlyInCreation 集合中时发现 tubunComponent 已经存在了，
从而判断这几个bean 产生了循环依赖并跑出异常。

beforeSingletonCreation(String beanName) 方法源码：

protected void beforeSingletonCreation(String beanName) {	if (!this.inCreationCheckExclusions.contains(beanName) && !this.singletonsCurrentlyInCreation.add(beanName)) {		throw new BeanCurrentlyInCreationException(beanName);	}}

2.7：解决方式

1：使用@Lazy 修饰。

2：这其实是一个操作不当造成的问题，因为spring的三级缓存已经解决了set注入导致的循环依赖，而这几个bean并不是全部都是使用构造器注入。

在 brokeComponent 中使用@Autowired修饰了tolenComponent，又用@Resource 修饰了tubunComponent，
导致tubunComponent要优先于tolenComponent优先加载，而tolenComponent 又依赖了tubunComponent，因此报错。
我们只需要让tolenComponent 优先于 tubunComponent 优先加载就可以了。

实验证明：

1：@Resource 修饰 tolenComponent + @Resource 修饰 tubunComponent 的组合不会报错。
2：@Resource 修饰 tolenComponent + @Autowired 修饰 tubunComponent 的组合不会报错。
3：@Autowired 修饰 tolenComponent + @Autowired 修饰 tubunComponent 的组合不会报错。

偏偏使用了会报错的一种组合......