Spring解决循环依赖的方法(三级缓存)

    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);

@Nullable
    protected Object getSingleton(String beanName, boolean allowEarlyReference) {
        Object singletonObject = this.singletonObjects.get(beanName);　　//首先通过beanName从一级缓存获取bean
        if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {　　//如果一级缓存中没有，并且beanName映射的bean正在创建中
            synchronized (this.singletonObjects) {
                singletonObject = this.earlySingletonObjects.get(beanName);　　//从二级缓存中获取
                if (singletonObject == null && allowEarlyReference) {　　//二级缓存也没有
                    ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName);　　//从三级缓存获取
                    if (singletonFactory != null) {
                        singletonObject = singletonFactory.getObject();　　//获取到bean
                        this.earlySingletonObjects.put(beanName, singletonObject);　　//将获取的bean提升至二级缓存
                        this.singletonFactories.remove(beanName);　　//从三级缓存删除
                    }
                }
            }
        }
        return singletonObject;
    }

ClassPathResource resource = new ClassPathResource("bean.xml");
 DefaultListableBeanFactory factory = new DefaultListableBeanFactory();
 XmlBeanDefinitionReader reader = new XmlBeanDefinitionReader(factory);
 reader.loadBeanDefinitions(resource);
 UserBean user = (UserBean) factory.getBean("user");

// Eagerly cache singletons to be able to resolve circular references
// even when triggered by lifecycle interfaces like BeanFactoryAware.
boolean earlySingletonExposure = (mbd.isSingleton() && this.allowCircularReferences &&
                isSingletonCurrentlyInCreation(beanName)); //通过条件判断该bean是否允许提前曝露
   if (earlySingletonExposure) {
      if (logger.isTraceEnabled()) {
                logger.trace("Eagerly caching bean '" + beanName +
                        "' to allow for resolving potential circular references");
     }
     addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));　//允许提前暴露的bean 添加到三级缓存
}

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);　　//添加到三级缓存
                this.earlySingletonObjects.remove(beanName);
                this.reGISteredSingletons.add(beanName);
            }
        }
    }

protected void addSingleton(String beanName, Object singletonObject) {
　　synchronized (this.singletonObjects) {
   　　 this.singletonObjects.put(beanName, singletonObject);
    　　this.singletonFactories.remove(beanName);
   　　 this.earlySingletonObjects.remove(beanName);
   　　 this.registeredSingletons.add(beanName);
　　}
}