Spring循环依赖

【大厂面试】三面三问Spring循环依赖，请一定要把这篇看完（建议收藏）_公号：一条coding-优快云博客真没想到这题这么高频https://blog.youkuaiyun.com/skylibiao/article/details/119222944spring循环依赖解决了A依赖B，B依赖A，通过三级缓存机制实现循环依赖。

三级缓存用于存放成品和半成品的bean以及工厂，位于DefaultSingletonBeanRegistry类中

public class DefaultSingletonBeanRegistry extends SimpleAliasRegistry implements SingletonBeanRegistry {

	/** 一级bean缓存，有完整初始化的bean */
	private final Map<String, Object> singletonObjects = new ConcurrentHashMap<>(256);

	/** 三级缓存，存放创建bean的beanFactory. */
	private final Map<String, ObjectFactory<?>> singletonFactories = new HashMap<>(16);

	/** 二级缓存，存放早期的bean，循环依赖的bean还没有被完全初始化 */
	private final Map<String, Object> earlySingletonObjects = new HashMap<>(16);

	/** 已经注册的bean，所有的bean信息都在这里面，bean的创建和销毁也是操作里面的数据 */
	private final Set<String> registeredSingletons = new LinkedHashSet<>(256);

/** 把bean放入一级缓存，同时在二级，三级缓存销毁*/
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);
		}
	}

/**  把beanFactory放入三级缓存 */
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);
				this.earlySingletonObjects.remove(beanName);
				this.registeredSingletons.add(beanName);
			}
		}
	}


    /**  从缓存中获得bean，只能从1，2，3级其中之一获得 */
    @Override
	@Nullable
	public Object getSingleton(String beanName) {
		return getSingleton(beanName, true);
	}

	
	@Nullable
	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;
	}


/**  获得所有注册的bean的名字  */
@Override
	public String[] getSingletonNames() {
		synchronized (this.singletonObjects) {
			return StringUtils.toStringArray(this.registeredSingletons);
		}
	}

/**  得到单例的bean数量 */
	@Override
	public int getSingletonCount() {
		synchronized (this.singletonObjects) {
			return this.registeredSingletons.size();
		}
	}
}

bean的获取过程及循环依赖的解决方案如下图

具体循环依赖解决方案可以见最上方链接