1：前言

最近在項目中遇到了一次循環(huán)依賴報錯的問題，雖然解決的很快，但是有些不明白的地方，特此記錄。在此我把 bean 的結構和 注入方式單獨拎出來進行演示

1.1：報錯提示

1.2：錯誤日志

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問題描述

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

點擊進入 stack overflow 問題描述

1.4：bean 依賴結構

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 是構造器注入，其他的bean 都是set 注入。分析bean依賴圖可以發(fā)現(xiàn)確實是循環(huán)依賴的問題。

2：問題分析

由于本人對spring bean 的加載機制不是很清晰，這次特意花了幾天時間做了梳理。

2.1：spring 的 bean 加載過程

2.2：spring 的三級緩存

解決了什么問題？

推薦博客：https://cloud.tencent.com/developer/article/1497692

2.3：Spring 容器加載過程中比較重要的類/屬性

public class DefaultSingletonBeanRegistry extends SimpleAliasRegistry implements SingletonBeanRegistry {/** Cache of singleton objects: bean name to bean instance.（一級緩存，存放已初始化完畢的bean） */private final Map singletonObjects = new ConcurrentHashMap(256);/** Cache of singleton factories: bean name to ObjectFactory.（三級緩存，存放未進行過初始化的beanFactory） */private final Map singletonFactories = new HashMap(16);/** Cache of early singleton objects: bean name to bean instance. （二級緩存，存放已實例化，但未進行裝配過屬性的bean）*/private final Map earlySingletonObjects = new HashMap(16);/** Set of registered singletons, containing the bean names in registration order.（存放已注冊了的單例集合 = singletonObjects.size + earlySingletonObjects）*/private final Set registeredSingletons = new LinkedHashSet(256);/** Names of beans that are currently in creation.（存放正在被創(chuàng)建的實例，用于檢查是否有循環(huán)依賴） */private final Set singletonsCurrentlyInCreation = Collections.newSetFromMap(new ConcurrentHashMap(16)); ….}

2.4：注冊后處理器->BeanPostProcessor

作用：對通過 BeanFactory 創(chuàng)建的 bean 進行屬性填充。

這是AbstractAutowireCapableBeanFactory#populateBean 方法中的某一段代碼：

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;}}

我打出了所有要進行輪詢校驗的 BeanPostProcessor

brokenComponent 中有兩個bean:

一個是通過@Resource 注解注入的，一個是通過@Autowire 進行注入的，奉勸各位同學千萬不要兩種混用?。。?！

2.4.1：CommonAnnotationBeanPostProcessor

/** * … *

The central element is the {@link javax.annotation.Resource} annotation * for annotation-driven injection of named beans, by default from the containing * Spring BeanFactory, with only {@code mappedName} references resolved in JNDI. * The {@link #setAlwaysUseJndiLookup “alwaysUseJndiLookup” flag} enforces JNDI lookups * equivalent to standard Java EE 5 resource injection for {@code name} references * and default names as well. The target beans can be simple POJOs, with no special * requirements other than the type having to match. * *

The JAX-WS {@link javax.xml.ws.WebServiceRef} annotation is supported too, * analogous to {@link javax.annotation.Resource} but with the capability of creating * specific JAX-WS service endpoints. This may either point to an explicitly defined * resource by name or operate on a locally specified JAX-WS service class. Finally, * this post-processor also supports the EJB 3 {@link javax.ejb.EJB} annotation, * analogous to {@link javax.annotation.Resource} as well, with the capability to * specify both a local bean name and a global JNDI name for fallback retrieval. * The target beans can be plain POJOs as well as EJB 3 Session Beans in this case. * … */public class CommonAnnotationBeanPostProcessor extends InitDestroyAnnotationBeanPostProcessorimplements InstantiationAwareBeanPostProcessor, BeanFactoryAware, Serializable { …@Overridepublic 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;} }

通過翻譯及反復debug驗證，這個類將對某個bean中所有被 @Resource 注解修飾的屬性進行填充。

2.4.2：AutowiredAnnotationBeanPostProcessor

/** * {@link org.springframework.beans.factory.config.BeanPostProcessor} implementation * that autowires annotated fields, setter methods and arbitrary config methods. * Such members to be injected are detected through a Java 5 annotation: by default, * Spring’s {@link Autowired @Autowired} and {@link Value @Value} annotations. * *

Also supports JSR-330’s {@link javax.inject.Inject @Inject} annotation, * if available, as a direct alternative to Spring’s own {@code @Autowired}. * … * @see Autowired * @see Value */public class AutowiredAnnotationBeanPostProcessor extends InstantiationAwareBeanPostProcessorAdapterimplements MergedBeanDefinitionPostProcessor, PriorityOrdered, BeanFactoryAware { …@Overridepublic 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;}}

這個類將對某個bean中所有被 @Autowire@Value 注解修飾的屬性進行填充。

2.4.3：@Autowire VS @Resource

1：無論使用@Resource 還是 @Resource，都不影響bean的初始化順序。

2：@Resource修飾的屬性 要優(yōu)先于 @Autowire修飾的屬性 進行初始化。

2.5：報錯流程

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”) -> 報錯7：afterSingletonCreation(“tolenComponent”) -> afterSingletonCreation(“denpaComponent”) -> afterSingletonCreation(“tubunComponent”) -> afterSingletonCreation(“brokenComponent”)

可以看到 tubunComponent 進行了兩次 getSingleton，經過循環(huán)依賴檢查的時候報錯了。

2.6：錯誤流程分析歸納

1：Spring 容器先加載 brokenComponent 這個 bean。

2：brokenComponent依賴了tubunComponent（@Resource修飾），因此優(yōu)先填充 tubunComponent，因此去初始化tubunComponent 這個bean，并將tubunComponent 存放在 DefaultSingletonBeanRegistry.singletonsCurrentlyInCreation 集合中。

3：tubunComponent依賴了depenComponent，因此去初始化depenComponent 這個bean，并將depenComponent 存放在 DefaultSingletonBeanRegistry.singletonsCurrentlyInCreation 集合中。

4：depenComponent依賴了tolenComponent，因此去初始化depenComponent 這個bean，并將tolenComponent 存放在 DefaultSingletonBeanRegistry.singletonsCurrentlyInCreation 集合中。

5：tolenComponent依賴了tubunComponent，因此去初始化tubunComponent 這個bean，然而當將tubunComponent 存放在 DefaultSingletonBeanRegistry.singletonsCurrentlyInCreation 集合中時發(fā)現(xiàn) tubunComponent 已經存在了，從而判斷這幾個bean 產生了循環(huán)依賴并跑出異常。

beforeSingletonCreation(String beanName) 方法源碼：

/** * Callback before singleton creation. *

The default implementation register the singleton as currently in creation. * @param beanName the name of the singleton about to be created * @see #isSingletonCurrentlyInCreation */protected void beforeSingletonCreation(String beanName) {if (!this.inCreationCheckExclusions.contains(beanName) && !this.singletonsCurrentlyInCreation.add(beanName)) {throw new BeanCurrentlyInCreationException(beanName);}}

2.7：解決方式

1：使用@Lazy 修飾。

2：這其實是一個操作不當造成的問題，因為spring的三級緩存已經解決了set注入導致的循環(huán)依賴，而這幾個bean并不是全部都是使用構造器注入。

在 brokeComponent 中使用@Autowired修飾了tolenComponent，又用@Resource 修飾了tubunComponent，導致tubunComponent要優(yōu)先于tolenComponent優(yōu)先加載，而tolenComponent 又依賴了tubunComponent，因此報錯。我們只需要讓tolenComponent 優(yōu)先于 tubunComponent 優(yōu)先加載就可以了。

實驗證明：

1：@Resource 修飾 tolenComponent + @Resource 修飾 tubunComponent 的組合不會報錯。2：@Resource 修飾 tolenComponent + @Autowired 修飾 tubunComponent 的組合不會報錯。3：@Autowired 修飾 tolenComponent + @Autowired 修飾 tubunComponent 的組合不會報錯。

偏偏使用了會報錯的一種組合……