spring如何解决循环依赖问题？

心已赠人 2022-11-04 01:56 183阅读 0赞

循环依赖其实就是循环引用，很多地方都说需要两个或则两个以上的bean互相持有对方最终形成闭环才是循环依赖，比如A依赖于B，B依赖于C，C又依赖于A。其实一个bean持有自己类型的属性也会产生循环依赖。

# setter singleton循环依赖 #

## 使用 ##

SingleSetterBeanA依赖SingleSetterBeanB，SingleSetterBeanB依赖SingleSetterBeanA。

@Data
    public class SingleSetterBeanA { 
    	@Autowired
    	private SingleSetterBeanB singleSetterBeanB;
    }

@Data
    public class SingleSetterBeanB { 
    	@Autowired
    	private SingleSetterBeanA singleSetterBeanA;
    }

## 源码分析 ##

spring是通过三级缓存来解决循环依赖的，那么三级缓存是怎么工作的呢？

三级缓存对应org.springframework.beans.factory.support.DefaultSingletonBeanRegistry类的三个属性：

/** Cache of singleton objects: bean name to bean instance. */
    private final Map<String, Object> singletonObjects = new ConcurrentHashMap<>(256); // 一级缓存
    
    /** Cache of singleton factories: bean name to ObjectFactory. */
    private final Map<String, ObjectFactory<?>> singletonFactories = new HashMap<>(16); // 二级缓存
    
    /** Cache of early singleton objects: bean name to bean instance. */
    private final Map<String, Object> earlySingletonObjects = new ConcurrentHashMap<>(16); // 三级缓存

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对于setter注入造成的依赖是通过Spring容器提前暴露刚完成实例化但未完成初始化的bean来完成的，而且只能解决单例作用域的bean循环依赖。通过提前暴露一个单例工厂方法，从而使其他bean能引用到该bean，关键源码如下所示：

org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory\#doCreateBean

// 处理循环依赖，实例化后放入三级缓存
    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实例化后放入三级缓存中：

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); // 三级缓存
    			this.earlySingletonObjects.remove(beanName);
    			this.registeredSingletons.add(beanName);
    		}
    	}
    }

放入三级缓存中的是ObjectFactory类型的lambda表达式：

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创建代理对象
    				exposedObject = ibp.getEarlyBeanReference(exposedObject, beanName);
    			}
    		}
    	}
    	return exposedObject;
    }

# 构造器参数循环依赖 #

通过构造器注入构成的循环依赖，此依赖是无法解决的，只能抛出BeanCurrentlyInCreationException异常表示循环依赖。

## 使用 ##

@Data
    public class SingleConstrutorBeanA { 
    	public SingleConstrutorBeanA(SingleConstrutorBeanB singleConstrutorBeanB) { 
    	}
    }

@Data
    public class SingleConstrutorBeanB { 
    	public SingleConstrutorBeanB(SingleConstrutorBeanA singleConstrutorBeanA) { 
    	}
    }

上面的代码运行时会抛出如下异常：

... ...
    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

## 源码分析 ##

Spring容器会将每一个正在创建的Bean标识符放在一个“当前创建Bean池”中，Bean标识符在创建过程中将一直保持在这个池中，因此如果在创建Bean过程中发现自己已经在“当前创建Bean池”里时将抛出BeanCurrentlyInCreationException异常表示循环依赖；而对于创建完毕的Bean将从“当前创建Bean池”中清除掉。

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

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## @Lazy打破循环依赖 ##

在上面的例子中只需要在SingleConstrutorBeanA或者SingleConstrutorBeanB的构造方法上面加上@Lazy注解，就会发现不会抛出异常了，这又是为什么呢？

下面假设在SingleConstrutorBeanA的构造方法上面加了@Lazy注解，在构造B时，发现参数A时被@Lazy注解修饰时，那么就不会调用getBean来获取对象，而是创建了一个代理对象，所以不会构成真正的循环依赖，不会抛出BeanCurrentlyInCreationException异常。

/** * 处理懒加载对象 * 懒加载返回的又是一个代理对象，不会真正的调用getBean，所以如果构造方法依赖中有循环依赖，那么不会报错 * @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) { 
    	// 调用beanFactory.getBean(beanName)从容器中获取依赖对象
    	result = doResolveDependency(descriptor, requestingBeanName, autowiredBeanNames, typeConverter);
    }
    return result;

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# setter prototype循环依赖 #

对于prototype作用域bean，Spring容器无法完成依赖注入，因为Spring容器不进行缓存"prototype"作用域的bean，因此无法提前暴露一个创建中的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); // 此时必须要获取Spring管理的实例，因为现在scope="prototype" 只有请求获取的时候才会实例化对象
    }

运行结果如下：

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

## 源码分析 ##

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org.springframework.beans.factory.support.AbstractBeanFactory\#doGetBean

... ...
    // 判断是否在当前创建Bean池中
    if (isPrototypeCurrentlyInCreation(beanName)) { 
    	throw new BeanCurrentlyInCreationException(beanName);
    }
    ... ...

异常就是在上面的代码中抛出来的，那么beanName是什么时候添加至当前创建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类型的bean的实例化
    	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容器不会对prototype类型的bean进行缓存，因此无法提前利用三级缓存暴露一个代理对象。

# 循环依赖开关 #

可以通过allowCircularReferences来禁止循环依赖，这样的话，singleton bean的setter循环依赖也会报错。

applicationContext.setAllowCircularReferences(false);

# 二级缓存可行？ #

<table> 
 <thead> 
  <tr> 
   <th>缓存</th> 
   <th>说明</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td>singletonObjects</td> 
   <td>第一级缓存，存放可用的成品Bean。</td> 
  </tr> 
  <tr> 
   <td>earlySingletonObjects</td> 
   <td>第二级缓存，存放半成品的Bean，半成品的Bean是已创建对象，但是未注入属性和初始化，用以解决循环依赖。</td> 
  </tr> 
  <tr> 
   <td>singletonFactories</td> 
   <td>第三级缓存，存的是Bean工厂对象，用来生成半成品的Bean并放入到二级缓存中，用以解决循环依赖。</td> 
  </tr> 
 </tbody> 
</table>

理论上二级缓存时可行的，只需要将三级缓存中BeanFactory创建的对象提前放入二级缓存中，这样三级缓存就可以移除了。

那么spring中为什么还要使用三级缓存呢？如果要使用二级缓存解决循环依赖，意味着所有Bean在实例化后就要完成AOP代理，这样违背了Spring设计的原则，Spring在设计之初就是通过AnnotationAwareAspectJAutoProxyCreator这个后置处理器来在Bean生命周期的最后一步来完成AOP代理，而不是在实例化后就立马进行AOP代理。

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