spring如何解決循環(huán)依賴問(wèn)題詳解
循環(huán)依賴其實(shí)就是循環(huán)引用，很多地方都說(shuō)需要兩個(gè)或則兩個(gè)以上的bean互相持有對(duì)方最終形成閉環(huán)才是循環(huán)依賴，比如A依賴于B，B依賴于C，C又依賴于A。其實(shí)一個(gè)bean持有自己類型的屬性也會(huì)產(chǎn)生循環(huán)依賴。
setter singleton循環(huán)依賴
使用
SingleSetterBeanA依賴SingleSetterBeanB，SingleSetterBeanB依賴SingleSetterBeanA。
@Data
public class SingleSetterBeanA {
@Autowired
private SingleSetterBeanB singleSetterBeanB;
}
@Data
public class SingleSetterBeanB {
@Autowired
private SingleSetterBeanA singleSetterBeanA;
}
源碼分析
spring是通過(guò)三級(jí)緩存來(lái)解決循環(huán)依賴的，那么三級(jí)緩存是怎么工作的呢？
三級(jí)緩存對(duì)應(yīng)org.springframework.beans.factory.support.DefaultSingletonBeanRegistry類的三個(gè)屬性：
/** Cache of singleton objects: bean name to bean instance. */ private final Map<String, Object> singletonObjects = new ConcurrentHashMap<>(256); // 一級(jí)緩存 /** Cache of singleton factories: bean name to ObjectFactory. */ private final Map<String, ObjectFactory<?>> singletonFactories = new HashMap<>(16); // 二級(jí)緩存 /** Cache of early singleton objects: bean name to bean instance. */ private final Map<String, Object> earlySingletonObjects = new ConcurrentHashMap<>(16); // 三級(jí)緩存
對(duì)于setter注入造成的依賴是通過(guò)Spring容器提前暴露剛完成實(shí)例化但未完成初始化的bean來(lái)完成的，而且只能解決單例作用域的bean循環(huán)依賴。通過(guò)提前暴露一個(gè)單例工廠方法，從而使其他bean能引用到該bean，關(guān)鍵源碼如下所示：
org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory#doCreateBean
// 處理循環(huán)依賴，實(shí)例化后放入三級(jí)緩存
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));
}
bean實(shí)例化后放入三級(jí)緩存中：
org.springframework.beans.factory.support.DefaultSingletonBeanRegistry#addSingletonFactory
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); // 三級(jí)緩存
this.earlySingletonObjects.remove(beanName);
this.registeredSingletons.add(beanName);
}
}
}
放入三級(jí)緩存中的是ObjectFactory類型的lambda表達(dá)式：
org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory#getEarlyBeanReference
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;
/**
* @see org.springframework.aop.framework.autoproxy.AbstractAutoProxyCreator#getEarlyBeanReference(java.lang.Object, java.lang.String)
*/
// 使用AbstractAutoProxyCreator#getEarlyBeanReference創(chuàng)建代理對(duì)象
exposedObject = ibp.getEarlyBeanReference(exposedObject, beanName);
}
}
}
return exposedObject;
}
構(gòu)造器參數(shù)循環(huán)依賴
通過(guò)構(gòu)造器注入構(gòu)成的循環(huán)依賴，此依賴是無(wú)法解決的，只能拋出BeanCurrentlyInCreationException異常表示循環(huán)依賴。
使用
@Data
public class SingleConstrutorBeanA {
public SingleConstrutorBeanA(SingleConstrutorBeanB singleConstrutorBeanB) {
}
}
@Data
public class SingleConstrutorBeanB {
public SingleConstrutorBeanB(SingleConstrutorBeanA singleConstrutorBeanA) {
}
}
上面的代碼運(yùn)行時(shí)會(huì)拋出如下異常：
... ... Caused by: org.springframework.beans.factory.UnsatisfiedDependencyException: Error creating bean with name 'singleConstrutorBeanB': Unsatisfied dependency expressed through constructor parameter 0; nested exception is org.springframework.beans.factory.BeanCurrentlyInCreationException: Error creating bean with name 'singleConstrutorBeanA': Requested bean is currently in creation: Is there an unresolvable circular reference? at org.springframework.beans.factory.support.ConstructorResolver.createArgumentArray(ConstructorResolver.java:805) at org.springframework.beans.factory.support.ConstructorResolver.autowireConstructor(ConstructorResolver.java:228) at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.autowireConstructor(AbstractAutowireCapableBeanFactory.java:1403) at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBeanInstance(AbstractAutowireCapableBeanFactory.java:1245) at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:579) at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:538) at org.springframework.beans.factory.support.AbstractBeanFactory.lambda$doGetBean$0(AbstractBeanFactory.java:329) at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:234) at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:323) at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:202) at org.springframework.beans.factory.config.DependencyDescriptor.resolveCandidate(DependencyDescriptor.java:276) at org.springframework.beans.factory.support.DefaultListableBeanFactory.doResolveDependency(DefaultListableBeanFactory.java:1321) at org.springframework.beans.factory.support.DefaultListableBeanFactory.resolveDependency(DefaultListableBeanFactory.java:1240) at org.springframework.beans.factory.support.ConstructorResolver.resolveAutowiredArgument(ConstructorResolver.java:892) at org.springframework.beans.factory.support.ConstructorResolver.createArgumentArray(ConstructorResolver.java:796) ... 76 more Caused by: org.springframework.beans.factory.BeanCurrentlyInCreationException: Error creating bean with name 'singleConstrutorBeanA': Requested bean is currently in creation: Is there an unresolvable circular reference? at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.beforeSingletonCreation(DefaultSingletonBeanRegistry.java:355) at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:227) at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:323) at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:202) at org.springframework.beans.factory.config.DependencyDescriptor.resolveCandidate(DependencyDescriptor.java:276) at org.springframework.beans.factory.support.DefaultListableBeanFactory.doResolveDependency(DefaultListableBeanFactory.java:1321) at org.springframework.beans.factory.support.DefaultListableBeanFactory.resolveDependency(DefaultListableBeanFactory.java:1240) at org.springframework.beans.factory.support.ConstructorResolver.resolveAutowiredArgument(ConstructorResolver.java:892) at org.springframework.beans.factory.support.ConstructorResolver.createArgumentArray(ConstructorResolver.java:796) ... 90 more
源碼分析
Spring容器會(huì)將每一個(gè)正在創(chuàng)建的Bean標(biāo)識(shí)符放在一個(gè)“當(dāng)前創(chuàng)建Bean池”中，Bean標(biāo)識(shí)符在創(chuàng)建過(guò)程中將一直保持在這個(gè)池中，因此如果在創(chuàng)建Bean過(guò)程中發(fā)現(xiàn)自己已經(jīng)在“當(dāng)前創(chuàng)建Bean池”里時(shí)將拋出BeanCurrentlyInCreationException異常表示循環(huán)依賴；而對(duì)于創(chuàng)建完畢的Bean將從“當(dāng)前創(chuàng)建Bean池”中清除掉。
protected void beforeSingletonCreation(String beanName) {
if (!this.inCreationCheckExclusions.contains(beanName) && !this.singletonsCurrentlyInCreation.add(beanName)) {
throw new BeanCurrentlyInCreationException(beanName);
}
}
@Lazy打破循環(huán)依賴
在上面的例子中只需要在SingleConstrutorBeanA或者SingleConstrutorBeanB的構(gòu)造方法上面加上@Lazy注解，就會(huì)發(fā)現(xiàn)不會(huì)拋出異常了，這又是為什么呢？
下面假設(shè)在SingleConstrutorBeanA的構(gòu)造方法上面加了@Lazy注解，在構(gòu)造B時(shí)，發(fā)現(xiàn)參數(shù)A時(shí)被@Lazy注解修飾時(shí)，那么就不會(huì)調(diào)用getBean來(lái)獲取對(duì)象，而是創(chuàng)建了一個(gè)代理對(duì)象，所以不會(huì)構(gòu)成真正的循環(huán)依賴，不會(huì)拋出BeanCurrentlyInCreationException異常。
/**
* 處理懶加載對(duì)象
* 懶加載返回的又是一個(gè)代理對(duì)象，不會(huì)真正的調(diào)用getBean，所以如果構(gòu)造方法依賴中有循環(huán)依賴，那么不會(huì)報(bào)錯(cuò)
* @see org.springframework.context.annotation.ContextAnnotationAutowireCandidateResolver#getLazyResolutionProxyIfNecessary(org.springframework.beans.factory.config.DependencyDescriptor, java.lang.String)
*/
Object result = getAutowireCandidateResolver().getLazyResolutionProxyIfNecessary(
descriptor, requestingBeanName);
if (result == null) {
// 調(diào)用beanFactory.getBean(beanName)從容器中獲取依賴對(duì)象
result = doResolveDependency(descriptor, requestingBeanName, autowiredBeanNames, typeConverter);
}
return result;
setter prototype循環(huán)依賴
對(duì)于prototype作用域bean，Spring容器無(wú)法完成依賴注入，因?yàn)镾pring容器不進(jìn)行緩存"prototype"作用域的bean，因此無(wú)法提前暴露一個(gè)創(chuàng)建中的bean。
使用
@Data
@Component
@Scope(ConfigurableBeanFactory.SCOPE_PROTOTYPE)
public class PrototypeBeanA {
@Autowired
private PrototypeBeanB prototypeBeanB;
}
@Data
@Component
@Scope(ConfigurableBeanFactory.SCOPE_PROTOTYPE)
public class PrototypeBeanB {
@Autowired
private PrototypeBeanA prototypeBeanA;
}
@Test
public void test3() {
AnnotationConfigApplicationContext applicationContext = new AnnotationConfigApplicationContext();
applicationContext.register(PrototypeBeanA.class);
applicationContext.register(PrototypeBeanB.class);
applicationContext.refresh();
applicationContext.getBean(PrototypeBeanA.class); // 此時(shí)必須要獲取Spring管理的實(shí)例，因?yàn)楝F(xiàn)在scope="prototype" 只有請(qǐng)求獲取的時(shí)候才會(huì)實(shí)例化對(duì)象
}
運(yùn)行結(jié)果如下：
Caused by: org.springframework.beans.factory.BeanCurrentlyInCreationException: Error creating bean with name 'prototypeBeanA': Requested bean is currently in creation: Is there an unresolvable circular reference? at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:269) at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:202) at org.springframework.beans.factory.config.DependencyDescriptor.resolveCandidate(DependencyDescriptor.java:276) at org.springframework.beans.factory.support.DefaultListableBeanFactory.doResolveDependency(DefaultListableBeanFactory.java:1322) at org.springframework.beans.factory.support.DefaultListableBeanFactory.resolveDependency(DefaultListableBeanFactory.java:1240) at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor$AutowiredFieldElement.resolveFieldValue(AutowiredAnnotationBeanPostProcessor.java:668) ... 89 more
源碼分析
org.springframework.beans.factory.support.AbstractBeanFactory#doGetBean
... ...
// 判斷是否在當(dāng)前創(chuàng)建Bean池中
if (isPrototypeCurrentlyInCreation(beanName)) {
throw new BeanCurrentlyInCreationException(beanName);
}
... ...
異常就是在上面的代碼中拋出來(lái)的，那么beanName是什么時(shí)候添加至當(dāng)前創(chuàng)建Bean池中的呢？
org.springframework.beans.factory.support.AbstractBeanFactory#doGetBean
else if (mbd.isPrototype()) {
// It's a prototype -> create a new instance.
// prototype類型的bean的實(shí)例化
Object prototypeInstance = null;
try {
beforePrototypeCreation(beanName);
prototypeInstance = createBean(beanName, mbd, args);
}
finally {
afterPrototypeCreation(beanName);
}
bean = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);
}
org.springframework.beans.factory.support.AbstractBeanFactory#beforePrototypeCreation
protected void beforePrototypeCreation(String beanName) {
// ThreadLocal
Object curVal = this.prototypesCurrentlyInCreation.get();
if (curVal == null) {
this.prototypesCurrentlyInCreation.set(beanName);
}
else if (curVal instanceof String) {
Set<String> beanNameSet = new HashSet<>(2);
beanNameSet.add((String) curVal);
beanNameSet.add(beanName);
this.prototypesCurrentlyInCreation.set(beanNameSet);
}
else {
Set<String> beanNameSet = (Set<String>) curVal;
beanNameSet.add(beanName);
}
}
其根本原因就是Spring容器不會(huì)對(duì)prototype類型的bean進(jìn)行緩存，因此無(wú)法提前利用三級(jí)緩存暴露一個(gè)代理對(duì)象。
循環(huán)依賴開關(guān)
可以通過(guò)allowCircularReferences來(lái)禁止循環(huán)依賴，這樣的話，singleton bean的setter循環(huán)依賴也會(huì)報(bào)錯(cuò)。
二級(jí)緩存可行？
| 緩存 | 說(shuō)明 |
|---|---|
| singletonObjects | 第一級(jí)緩存，存放可用的成品Bean。 |
| earlySingletonObjects | 第二級(jí)緩存，存放半成品的Bean，半成品的Bean是已創(chuàng)建對(duì)象，但是未注入屬性和初始化，用以解決循環(huán)依賴。 |
| singletonFactories | 第三級(jí)緩存，存的是Bean工廠對(duì)象，用來(lái)生成半成品的Bean并放入到二級(jí)緩存中，用以解決循環(huán)依賴。 |
理論上二級(jí)緩存時(shí)可行的，只需要將三級(jí)緩存中BeanFactory創(chuàng)建的對(duì)象提前放入二級(jí)緩存中，這樣三級(jí)緩存就可以移除了。
那么spring中為什么還要使用三級(jí)緩存呢？如果要使用二級(jí)緩存解決循環(huán)依賴，意味著所有Bean在實(shí)例化后就要完成AOP代理，這樣違背了Spring設(shè)計(jì)的原則，Spring在設(shè)計(jì)之初就是通過(guò)AnnotationAwareAspectJAutoProxyCreator這個(gè)后置處理器來(lái)在Bean生命周期的最后一步來(lái)完成AOP代理，而不是在實(shí)例化后就立馬進(jìn)行AOP代理。
前言
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