概述
循环依赖就是依赖关系形成环，比如最简单的循环依赖：A对象依赖B，B对象依赖A
属性注入与循环依赖
- 如果是构造器注入，如果循环依赖对象没法构建，因为还未实例化
- 如果是属性注入但是作用域是prototype，spring不会缓存其对象实例，也不能处理循环依赖的情况
- 如果是属性注入singleton的，其bean的实例化过程与属性注入过程是分开的，并且spring提供了三个map（就是大家说三级缓存）来实现。
spring属性注入处理循环依赖的方式
通过以下xml方式配置一个循环依赖的示例：
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<bean id="person1" class="com.example.leetcode.spring.bean.Person"> <property name="parent" ref="person2"></property> <property name="name" value="tom"></property></bean><bean id="person2" class="com.example.leetcode.spring.bean.Person"> <property name="parent" ref="person1"></property> <property name="name" value="jack"></property></bean> |
spring循环依赖处理几个关键位置：
获取bean对象
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protected <T> T doGetBean(final String name, @Nullable final Class<T> requiredType, @Nullable final Object[] args, boolean typeCheckOnly) throws BeansException { final String beanName = transformedBeanName(name); Object bean; // 这里会检查单例bean是否已经在注册表，并返回。 // Eagerly check singleton cache for manually registered singletons. Object sharedInstance = getSingleton(beanName); if (sharedInstance != null && args == null) { if (logger.isTraceEnabled()) { if (isSingletonCurrentlyInCreation(beanName)) { logger.trace("Returning eagerly cached instance of singleton bean '" + beanName + "' that is not fully initialized yet - a consequence of a circular reference"); } else { logger.trace("Returning cached instance of singleton bean '" + beanName + "'"); } } bean = getObjectForBeanInstance(sharedInstance, name, beanName, null); } ...} |
DefaultSingletonBeanRegistry（单例对象注册表）的几个关键属性。
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// 用来存储已经创建好的单例对象/** Cache of singleton objects: bean name to bean instance. */private final Map<String, Object> singletonObjects = new ConcurrentHashMap<>(256);// 用来存储单例beanname到ObjectFactory的映射/** Cache of singleton factories: bean name to ObjectFactory. */private final Map<String, ObjectFactory<?>> singletonFactories = new HashMap<>(16);// 用来提前存储还未初始化好的单例对象/** Cache of early singleton objects: bean name to bean instance. */private final Map<String, Object> earlySingletonObjects = new HashMap<>(16); |
DefaultSingletonBeanRegistry.getSingleton()的实现.
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protected Object getSingleton(String beanName, boolean allowEarlyReference) { Object singletonObject = this.singletonObjects.get(beanName); if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) { synchronized (this.singletonObjects) { singletonObject = this.earlySingletonObjects.get(beanName); if (singletonObject == null && allowEarlyReference) { ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName); if (singletonFactory != null) { singletonObject = singletonFactory.getObject(); this.earlySingletonObjects.put(beanName, singletonObject); this.singletonFactories.remove(beanName); } } } } return singletonObject;} |
AbstractAutowireCapableBeanFactory.doCreateBean创建对象与注入属性
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protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final @Nullable Object[] args) throws BeanCreationException { ... instanceWrapper = createBeanInstance(beanName, mbd, args); ... // 检查是否提前将单例bean存入缓存 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"); } // 这里将beanname与工厂映射放入缓存注册表中(也就是上面的singletonFactories) addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean)); } ... // 注入依赖属性 populateBean(beanName, mbd, instanceWrapper); ... } |
假设我们从beanfactory获取person1对象， 循环依赖处理流程如下：
1.通过AbstractBeanFactory.doGetBean("persion1")获取对象
2.因为一开始通过DefaultSingletonBeanRegistry.getSingleton()什么都没有，进入AbstractAutowireCapableBeanFactory.doCreateBean()进行创建
3.AutowireCapableBeanFactory.doCreateBean()里面执行完创建逻辑，因为是singleton将beanname与工厂的映射加入到addSingletonFactory()到缓存
4.开始处理person1对象的属性依赖populateBean()
5.当发现person1的parent属性是一个引用时，通过beanfactory.getBean("person2")获取依赖对象（org.springframework.beans.factory.support.BeanDefinitionValueResolver#resolveReference）
6.此时进入person2的创建流程， person2也没有缓存，开始实例化并加入到addSingletonFactory()到缓存
7.person2在通过populateBean()注入属性依赖发现依赖person1, 此时通过beanfactory.getBean("person1")获取依赖对象
8.此时AbstractBeanFactory.doGetBean("persion1")获取对象执行到getSingleton("person1")进行以下判断：
- 从singletonObjects.get(beanName)获取到null
- 进入if条件，对singletonObjects同步
- 从earlySingletonObjects.get(beanName);获取也为null
- 进入内层if，通过singletonFactories.get(beanName);获取到最开始bean实例化之后的beanname与工厂缓存信息
- 获取到仅实例化完成的bean，并earlySingletonObjects.put(beanName, singletonObject);
- 然后删除singletonFactories.remove(beanName);
9.此时从getSingleton("person1")返回了一个仅实例化尚未注入的bean引用
10.person2在第7步获取到person1仅实例化未注入的对象引用。
11.person2完成属性注入并返回。
12.person2被addSingleton(beanName, singletonObject);中singletonObjects.put(beanName, singletonObject)缓存，并删除singletonFactories.remove(beanName);earlySingletonObjects.remove(beanName);
13.person1在5步获取到person2的对象并完成属性注入
14.person1对象返回（因为一开始person2获取的是person1的引用，此时person1完成注入是能看到注入后的对象）
15.person1被addSingleton(beanName, singletonObject);中singletonObjects.put(beanName, singletonObject)缓存，并删除singletonFactories.remove(beanName);earlySingletonObjects.remove(beanName);
16.返回最终的person1对象
关于三个map（三级缓存）
在出现循环依赖时，三个map之间的流程如下：
先从singletonFactories获取工厂，并通过getObject获取对象并移除缓存，将对象缓存到earlySingletonObjects
通过earlySingletonObjects获取提前曝光的对象
对象创建并初始化完成之后，对象信息保留在singletonObjects并移除过earlySingletonObjects中的缓存
earlySingletonObjects二级缓存是鸡肋吗？
earlySingletonObjects缓存的目的是，通过三级缓存在获取对象会执行一些列的后置处理器，通过earlySingletonObjects来缓存提升性能。
以上就是spring解决循环依赖的详细内容，更多关于sping 循环依赖的资料请关注服务器之家其它相关文章！
原文链接：https://juejin.im/post/6877734926121664526?utm_source=tuicool&utm_medium=referral
