1、什么是循环依赖？

循环依赖主要来次三个方面，第一种A相互依赖，第二种是 A依赖B，B依赖A，第三种是A依赖B，B依赖C，C依赖A。

总结一句话就是对象之间形成环形依赖。

代码如下：

第一种：

public class A {
    private A a;
}

第二种：

public class A {
    private B b;
}
class B {
    private A a;
}

第三种：

public class A {
    private B b;
}
class B {
    private C c;
}
class C {
    private A a;
}

2、Spring 三级缓存是什么？

• 第一级缓存：singletonObjects，用于缓存成品的bean，保证 Spring 的单例属性。
• 第二级缓存：earlySingletonObjects，用于保存实例化完成的半成品 bean 实例,包括AOP代理对象。
• 第三级缓存：singletonFactories，Map<String, ObjectFactory<?>>，Map 的 Value 是一个对象的代理工厂，用来存放对象的代理工厂用于保存 bean 创建工厂，以便后面有机会创建代理对象。为“打破循环”而生，存放的是生成半成品单例 Bean 的工厂方法。

要想说清楚三级缓存，我们先来看Spring bean 的生命周期，看bean的创建过程。

核心几个类我们从AnnotationConfigApplicationContext上下文为基础来讲解bean的生命周期，AnnotationConfigApplicationContext是基于注解的上下文，看下这核心构造器。

	/**
	 * Create a new AnnotationConfigApplicationContext, deriving bean definitions
	 * from the given component classes and automatically refreshing the context.
	 * @param componentClasses one or more component classes — for example,
	 * {@link Configuration @Configuration} classes
	 */
	public AnnotationConfigApplicationContext(Class<?>... componentClasses) {
		this();
		register(componentClasses);
		refresh();
	}

再看refresh()

public void refresh() throws BeansException, IllegalStateException {
		synchronized (this.startupShutdownMonitor) {
			StartupStep contextRefresh = this.applicationStartup.start("spring.context.refresh");

			// Prepare this context for refreshing.
			prepareRefresh();

			// Tell the subclass to refresh the internal bean factory.
			ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();

			// Prepare the bean factory for use in this context.
			prepareBeanFactory(beanFactory);

			try {
				// Allows post-processing of the bean factory in context subclasses.
				postProcessBeanFactory(beanFactory);

				StartupStep beanPostProcess = this.applicationStartup.start("spring.context.beans.post-process");
				// Invoke factory processors registered as beans in the context.
				invokeBeanFactoryPostProcessors(beanFactory);

				// Register bean processors that intercept bean creation.
				registerBeanPostProcessors(beanFactory);
				beanPostProcess.end();

				// Initialize message source for this context.
				initMessageSource();

				// Initialize event multicaster for this context.
				initApplicationEventMulticaster();

				// Initialize other special beans in specific context subclasses.
				onRefresh();

				// Check for listener beans and register them.
				registerListeners();

				// Instantiate all remaining (non-lazy-init) singletons.
				finishBeanFactoryInitialization(beanFactory);

				// Last step: publish corresponding event.
				finishRefresh();
			}

			catch (BeansException ex) {
				if (logger.isWarnEnabled()) {
					logger.warn("Exception encountered during context initialization - " +
							"cancelling refresh attempt: " + ex);
				}

				// Destroy already created singletons to avoid dangling resources.
				destroyBeans();

				// Reset 'active' flag.
				cancelRefresh(ex);

				// Propagate exception to caller.
				throw ex;
			}

			finally {
				// Reset common introspection caches in Spring's core, since we
				// might not ever need metadata for singleton beans anymore...
				resetCommonCaches();
				contextRefresh.end();
			}
		}
	}

核心主要是finishBeanFactoryInitialization()，其中beanFactory.preInstantiateSingletons()该方法中会生成所有非懒加载的单例bean。

	protected void finishBeanFactoryInitialization(ConfigurableListableBeanFactory beanFactory) {
		// Initialize conversion service for this context.
		if (beanFactory.containsBean(CONVERSION_SERVICE_BEAN_NAME) &&
				beanFactory.isTypeMatch(CONVERSION_SERVICE_BEAN_NAME, ConversionService.class)) {
			beanFactory.setConversionService(
					beanFactory.getBean(CONVERSION_SERVICE_BEAN_NAME, ConversionService.class));
		}

		// Register a default embedded value resolver if no BeanFactoryPostProcessor
		// (such as a PropertySourcesPlaceholderConfigurer bean) registered any before:
		// at this point, primarily for resolution in annotation attribute values.
		if (!beanFactory.hasEmbeddedValueResolver()) {
			beanFactory.addEmbeddedValueResolver(strVal -> getEnvironment().resolvePlaceholders(strVal));
		}

		// Initialize LoadTimeWeaverAware beans early to allow for registering their transformers early.
		String[] weaverAwareNames = beanFactory.getBeanNamesForType(LoadTimeWeaverAware.class, false, false);
		for (String weaverAwareName : weaverAwareNames) {
			getBean(weaverAwareName);
		}

		// Stop using the temporary ClassLoader for type matching.
		beanFactory.setTempClassLoader(null);

		// Allow for caching all bean definition metadata, not expecting further changes.
		beanFactory.freezeConfiguration();

		// Instantiate all remaining (non-lazy-init) singletons.
		beanFactory.preInstantiateSingletons();
	}

public void preInstantiateSingletons() throws BeansException {
    if (logger.isTraceEnabled()) {
    logger.trace("Pre-instantiating singletons in " + this);
}

// Iterate over a copy to allow for init methods which in turn register new bean definitions.
// While this may not be part of the regular factory bootstrap, it does otherwise work fine.
List<String> beanNames = new ArrayList<>(this.beanDefinitionNames);

// Trigger initialization of all non-lazy singleton beans...
for (String beanName : beanNames) {
    RootBeanDefinition bd = getMergedLocalBeanDefinition(beanName);
    if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) {
        if (isFactoryBean(beanName)) {
            Object bean = getBean(FACTORY_BEAN_PREFIX + beanName);
            if (bean instanceof FactoryBean) {
                FactoryBean<?> factory = (FactoryBean<?>) bean;
                boolean isEagerInit;
                if (System.getSecurityManager() != null && factory instanceof SmartFactoryBean) {
                    isEagerInit = AccessController.doPrivileged(
                        (PrivilegedAction<Boolean>) ((SmartFactoryBean<?>) factory)::isEagerInit,
                        getAccessControlContext());
                }
                else {
                    isEagerInit = (factory instanceof SmartFactoryBean &&
                                   ((SmartFactoryBean<?>) factory).isEagerInit());
                }
                if (isEagerInit) {
                    getBean(beanName);
                }
            }
        }
        else {
            getBean(beanName);
        }
    }
}

// Trigger post-initialization callback for all applicable beans...
for (String beanName : beanNames) {
    Object singletonInstance = getSingleton(beanName);
    if (singletonInstance instanceof SmartInitializingSingleton) {
        StartupStep smartInitialize = this.getApplicationStartup().start("spring.beans.smart-initialize")
            .tag("beanName", beanName);
        SmartInitializingSingleton smartSingleton = (SmartInitializingSingleton) singletonInstance;
        if (System.getSecurityManager() != null) {
            AccessController.doPrivileged((PrivilegedAction<Object>) () -> {
                smartSingleton.afterSingletonsInstantiated();
                return null;
            }, getAccessControlContext());
        }
        else {
            smartSingleton.afterSingletonsInstantiated();
        }
        smartInitialize.end();
    }
}
}

doGetBean方法中有getSingleton、getObjectForBeanInstance、createBean 比较核心，下面继续看下这代码。代码篇幅过长、简单理解下，在doGetBean方法中首先调用getSingleton方法检查单例缓存中是否有该bean，这也是我们分析三级缓存的关键，如果没有则判断当前bean的作用域是单例（singleton）还是原型（prototype），如果是单例的，再次调用getSingleton方法，不过这次的该方法是重载的一个；如果是原型的则调用createBean方法生成bean，上面几个步骤生成的beanInstance均要调用getObjectForBeanInstance方法获得bean对象。

protected <T> T doGetBean(
			String name, @Nullable Class<T> requiredType, @Nullable Object[] args, boolean typeCheckOnly)
			throws BeansException {

		String beanName = transformedBeanName(name);
		Object beanInstance;

		// 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 + "'");
				}
			}
			beanInstance = getObjectForBeanInstance(sharedInstance, name, beanName, null);
		}

		else {
			// Fail if we're already creating this bean instance:
			// We're assumably within a circular reference.
			if (isPrototypeCurrentlyInCreation(beanName)) {
				throw new BeanCurrentlyInCreationException(beanName);
			}

			// Check if bean definition exists in this factory.
			BeanFactory parentBeanFactory = getParentBeanFactory();
			if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {
				// Not found -> check parent.
				String nameToLookup = originalBeanName(name);
				if (parentBeanFactory instanceof AbstractBeanFactory) {
					return ((AbstractBeanFactory) parentBeanFactory).doGetBean(
							nameToLookup, requiredType, args, typeCheckOnly);
				}
				else if (args != null) {
					// Delegation to parent with explicit args.
					return (T) parentBeanFactory.getBean(nameToLookup, args);
				}
				else if (requiredType != null) {
					// No args -> delegate to standard getBean method.
					return parentBeanFactory.getBean(nameToLookup, requiredType);
				}
				else {
					return (T) parentBeanFactory.getBean(nameToLookup);
				}
			}

			if (!typeCheckOnly) {
				markBeanAsCreated(beanName);
			}

			StartupStep beanCreation = this.applicationStartup.start("spring.beans.instantiate")
					.tag("beanName", name);
			try {
				if (requiredType != null) {
					beanCreation.tag("beanType", requiredType::toString);
				}
				RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
				checkMergedBeanDefinition(mbd, beanName, args);

				// Guarantee initialization of beans that the current bean depends on.
				String[] dependsOn = mbd.getDependsOn();
				if (dependsOn != null) {
					for (String dep : dependsOn) {
						if (isDependent(beanName, dep)) {
							throw new BeanCreationException(mbd.getResourceDescription(), beanName,
									"Circular depends-on relationship between '" + beanName + "' and '" + dep + "'");
						}
						registerDependentBean(dep, beanName);
						try {
							getBean(dep);
						}
						catch (NoSuchBeanDefinitionException ex) {
							throw new BeanCreationException(mbd.getResourceDescription(), beanName,
									"'" + beanName + "' depends on missing bean '" + dep + "'", ex);
						}
					}
				}

				// Create bean instance.
				if (mbd.isSingleton()) {
					sharedInstance = getSingleton(beanName, () -> {
						try {
							return createBean(beanName, mbd, args);
						}
						catch (BeansException ex) {
							// Explicitly remove instance from singleton cache: It might have been put there
							// eagerly by the creation process, to allow for circular reference resolution.
							// Also remove any beans that received a temporary reference to the bean.
							destroySingleton(beanName);
							throw ex;
						}
					});
					beanInstance = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
				}

				else if (mbd.isPrototype()) {
					// It's a prototype -> create a new instance.
					Object prototypeInstance = null;
					try {
						beforePrototypeCreation(beanName);
						prototypeInstance = createBean(beanName, mbd, args);
					}
					finally {
						afterPrototypeCreation(beanName);
					}
					beanInstance = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);
				}

				else {
					String scopeName = mbd.getScope();
					if (!StringUtils.hasLength(scopeName)) {
						throw new IllegalStateException("No scope name defined for bean '" + beanName + "'");
					}
					Scope scope = this.scopes.get(scopeName);
					if (scope == null) {
						throw new IllegalStateException("No Scope registered for scope name '" + scopeName + "'");
					}
					try {
						Object scopedInstance = scope.get(beanName, () -> {
							beforePrototypeCreation(beanName);
							try {
								return createBean(beanName, mbd, args);
							}
							finally {
								afterPrototypeCreation(beanName);
							}
						});
						beanInstance = getObjectForBeanInstance(scopedInstance, name, beanName, mbd);
					}
					catch (IllegalStateException ex) {
						throw new ScopeNotActiveException(beanName, scopeName, ex);
					}
				}
			}
			catch (BeansException ex) {
				beanCreation.tag("exception", ex.getClass().toString());
				beanCreation.tag("message", String.valueOf(ex.getMessage()));
				cleanupAfterBeanCreationFailure(beanName);
				throw ex;
			}
			finally {
				beanCreation.end();
			}
		}

		return adaptBeanInstance(name, beanInstance, requiredType);
	}

我们知道了DefaultSingletonBeanRegistry类下的getSingleton是Spring bean 缓存的关键，那么看下代码，

	/**
	 * Return the (raw) singleton object registered under the given name.
	 * <p>Checks already instantiated singletons and also allows for an early
	 * reference to a currently created singleton (resolving a circular reference).
	 * @param beanName the name of the bean to look for
	 * @param allowEarlyReference whether early references should be created or not
	 * @return the registered singleton object, or {@code null} if none found
	 */
	@Nullable
	protected Object getSingleton(String beanName, boolean allowEarlyReference) {
		// Quick check for existing instance without full singleton lock
        //一级缓存
		Object singletonObject = this.singletonObjects.get(beanName);
		if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
            //二级缓存
			singletonObject = this.earlySingletonObjects.get(beanName);
			if (singletonObject == null && allowEarlyReference) {
				synchronized (this.singletonObjects) {
					// Consistent creation of early reference within full singleton lock
					singletonObject = this.singletonObjects.get(beanName);
					if (singletonObject == null) {
						singletonObject = this.earlySingletonObjects.get(beanName);
						if (singletonObject == null) {
                            // 三级缓存
							ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName);
							if (singletonFactory != null) {
								singletonObject = singletonFactory.getObject();
								this.earlySingletonObjects.put(beanName, singletonObject);
								this.singletonFactories.remove(beanName);
							}
						}
					}
				}
			}
		}
		return singletonObject;
	}

以上就是从源码上了解三级缓存，执行流程:

1.先从“第一级缓存”找对象，有就返回，没有就找“二级缓存”；

2.找“二级缓存”，有就返回，没有就找“三级缓存”；

3.找“三级缓存”，找到了，就获取对象，放到“二级缓存”，从“三级缓存”移除。

3、如何解决缓存依赖问题？

以上都是Spring Bean缓存的介绍，是如何解决循环依赖的问题呢？

我们再看一段代码

protected void addSingletonFactory(String beanName, ObjectFactory<?> singletonFactory) {
    Assert.notNull(singletonFactory, "Singleton factory must not be null");
    synchronized (this.singletonObjects) {
        // 判断一级缓存中不存在此对象
        if (!this.singletonObjects.containsKey(beanName)) { 
            // 直接从工厂中获取 Bean
            object o = singletonFactory.getObject(); 
            // 添加至二级缓存中
            this.earlySingletonObjects.put(beanName, o); 
            this.registeredSingletons.add(beanName);
        }
    }
}

总结来说，Spring 通过三级缓存实现循环依赖，来存放对象的代理工厂用于保存 bean 创建工厂。在遇到循环依赖时候优先通过二级缓存看是否存在依赖对象的半成品、没有则去三级缓存获取Bean Factory创建对象，创建成功之后放入到二级缓存中。

为什么多出二级缓存呢，只要三级缓存能不能实现？

二级缓存”的目的是为了避免因为 AOP 创建多个对象，其中存储的是半成品的 AOP 的单例 bean。

能不能只要二级缓存，二级缓存也可以解决循环依赖问题啊？

主要解决代理对象问题，假如不存在代理对象，二级缓存也是足够的。

Spring 只用二级缓存来解决循环依赖，那么所有 Bean 都需要在实例化完成之后就立马为其创建代理，而Spring 的设计原则是会在完成属性填充、并且执行完初始化之后再为其创建代理。所以，Spring 选择了三级缓存，，但是因为循环依赖的出现，导致了 Spring 不得不提前去创建代理，因为如果不提前创建代理对象，那么注入的就是原始对象，这样就会产生错误。