【Spring源码分析】7、Spring 利用三级缓存解决循环依赖问题分析（面试热点）

悠悠 2022-12-02 13:42 241阅读 0赞

### 文章目录 ###

*  1、提出循环依赖问题
 *   *   *  （1）问题描述
 *  2、循环依赖源码分析
 *   *   *  （1）spring循环依赖逻辑分析
         *  （2）spring循环依赖解决过程
         *  
 *  3、更多细节学习
 *  微信公众号

# 1、提出循环依赖问题 #

> Spring怎么解决循环依赖问题？这是spring一个非常典型的面试题，堪比spring生命周期一样常问

> 知识点：循环引用指单例之间的引用

### （1）问题描述 ###

下面通过一个简单的互相注入的案例，引入循环依赖问题。首先我们准备四个类，配置类，IndexService，UserService，测试类，然后使用IndexService注入UserService，UserService注入IndexService，这种使用场景，在项目中是一定会使用的，面试官通常会问Spring怎么解决循环依赖问题？下面我们就一步步分析解决过程，相关测试类没有什么代码，就是互相注入，主要是分析spring源码  
配置类

package cn.tellsea.loop;
    
    import org.springframework.context.annotation.ComponentScan;
    
    /** * @author Tellsea * @date 2020-9-2 */
    @ComponentScan("cn.tellsea.loop")
    public class AppConfig { 
    }

IndexService，其中注入UserService

package cn.tellsea.loop;
    
    import org.springframework.beans.factory.annotation.Autowired;
    import org.springframework.stereotype.Component;
    
    /** * @author Tellsea * @date 2020-9-2 */
    @Component
    public class IndexService { 
    
    	@Autowired
    	private UserService userService;
    
    	public IndexService() { 
    		System.out.println("IndexService Constructor ...");
    	}
    }

UserService，注入IndexService，两个反过来

package cn.tellsea.loop;
    
    import org.springframework.beans.factory.annotation.Autowired;
    import org.springframework.stereotype.Component;
    
    /** * @author Tellsea * @date 2020-9-2 */
    @Component
    public class UserService { 
    
    	@Autowired
    	private IndexService indexService;
    }

测试类，实际上没有什么代码，主要是看spring源码的流程

package cn.tellsea.loop;
    
    import org.springframework.context.annotation.AnnotationConfigApplicationContext;
    
    /** * @author Tellsea * @date 2020-9-2 */
    public class AppTest { 
    
    	public static void main(String[] args) { 
    		AnnotationConfigApplicationContext applicationContext = new AnnotationConfigApplicationContext(AppConfig.class);
    	}
    }

测试结果：IndexService默认在UserService上面加载，后面我们就先分析IndexService的加载

IndexService Constructor ...
    org.springframework.context.annotation.internalConfigurationAnnotationProcessor
    org.springframework.context.annotation.internalAutowiredAnnotationProcessor
    org.springframework.context.annotation.internalCommonAnnotationProcessor
    org.springframework.context.event.internalEventListenerProcessor
    org.springframework.context.event.internalEventListenerFactory
    appConfig
    indexService
    userService

# 2、循环依赖源码分析 #

分析spring循环依赖问题之前， 必须要知道[【spring bean的生命周期】][spring bean]，因为循环依赖解决的过程，就是伴随着生命周期完成的

### （1）spring循环依赖逻辑分析 ###

我们知道bean的生命周期过程中， 都是一步步按照顺序执行的。上面我们说过，有IndexService和UserService两个组件，当我们程序执行到IndexService时，发现需要注入UserService属性，但是spring中才刚执行到IndexService的生命周期时，根本就没有UserService的bean对象，也就是说，当前的IndexService是正在创建中，而且UserService还是一个普通的java对象，不是spring bean对象，这里就是循环依赖的核心问题，下面逐级分析

### （2）spring循环依赖解决过程 ###

这里可以通过运行测试方法，打印出默认执行的bean的顺序，IndexService在UserService前面，所以我们先走IndexService的生命周期。首先通过 AnnotationConfigApplicationContext 创建IOC容器

AnnotationConfigApplicationContext applicationContext = new AnnotationConfigApplicationContext(AppConfig.class);

然后在AnnotationConfigApplicationContext的构造方法中找到 `refresh()` 方法

/** * 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 * 创建一个新的AnnotationConfigApplicationContext，并自动刷新上下文 */
    	public AnnotationConfigApplicationContext(Class<?>... componentClasses) { 
    		this();
    		register(componentClasses);
    		// 9个大步骤
    		refresh();
    	}

AbstractApplicationContext类中的核心方法是`refresh()`，所有过程都是在这里执行的

@Override
    	public void refresh() throws BeansException, IllegalStateException { 
    		synchronized (this.startupShutdownMonitor) { 
    			// 准备此上下文以进行刷新。
    			prepareRefresh();
    
    			// 告诉子类刷新内部bean工厂。
    			ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();
    
    			// 准备在这种情况下使用的bean工厂。
    			prepareBeanFactory(beanFactory);
    
    			try { 
    				// 1、允许在上下文子类中对beanFactory进行后处理。
    				postProcessBeanFactory(beanFactory);
    
    				// 2、调用在上下文中注册为bean的工厂处理器。
    				invokeBeanFactoryPostProcessors(beanFactory);
    
    				// 3、注册拦截Bean创建的Bean处理器。
    				registerBeanPostProcessors(beanFactory);
    
    				// 4、为此上下文初始化消息源。
    				initMessageSource();
    
    				// 5、为此上下文初始化事件多播器。
    				initApplicationEventMulticaster();
    
    				// 6、在特定上下文子类中初始化其他特殊bean。
    				onRefresh();
    
    				// 7、检查侦听器bean并注册它们。
    				registerListeners();
    
    				// 8、实例化所有剩余的（非延迟初始化）单例。
    				finishBeanFactoryInitialization(beanFactory);
    
    				// 9、最后一步：发布相应的事件。
    				finishRefresh();
    			}
    
    			catch (BeansException ex) { 
    				if (logger.isWarnEnabled()) { 
    					logger.warn("Exception encountered during context initialization - " +
    							"cancelling refresh attempt: " + ex);
    				}
    
    				// 销毁已创建的单例以避免资源悬空。
    				destroyBeans();
    
    				// 重置“活动”标志。
    				cancelRefresh(ex);
    
    				// 将异常传播给呼叫者。
    				throw ex;
    			}
    
    			finally { 
    				//在Spring的核心中重置常见的自省缓存，因为我们
    				//可能不再需要单例bean的元数据...
    				resetCommonCaches();
    			}
    		}
    	}

进入第8点， 实例化所有剩余的（非延迟初始化）单例，实际上到这里，IndexService的构造方法都还没有调用，断点进入方法`finishBeanFactoryInitialization(beanFactory);`，这个方法断点一路执行会发现，只有走到最后一步的时候，IndexService才会打印调用了构造方法， 我们继续断点进入方法`beanFactory.preInstantiateSingletons();`，相关源码

/** * Finish the initialization of this context's bean factory, * initializing all remaining singleton beans. * 完成这个上下文的bean工厂的初始化， * 初始化所有剩余的单例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 bean post-processor
    		// (such as a PropertyPlaceholderConfigurer 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.
    		// 实例化所有单例，非lazy加载
    		beanFactory.preInstantiateSingletons();
    	}

进入之后，发现`preInstantiateSingletons()`是ConfigurableListableBeanFactory接口的方法，idea点击Ctrl + alt + B进入该方法的实现`DefaultListableBeanFactory.preInstantiateSingletons()`，打个断点继续进入实现类

我们发现会遍历所有BeanDefinition的name值，并验证名字合法性，这里我们使用idea的debug的小技巧，增加条件，beanName.equals(“indexService”)，因为spring默认还有好几个类需要加载，否则要等断点，然后点击Done，就直接到beanName = "indexService"的循环了  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzM4NzYyMjM3_size_16_color_FFFFFF_t_70_pic_center]  
当IndexService执行到下面的判断时，进入了else语句，

if (isFactoryBean(beanName)) { 
    					...
    				}
    				else { 
    					getBean(beanName);
    				}

进入getBean(String name)方法，在AbstractBeanFactory类中

@Override
    	public Object getBean(String name) throws BeansException { 
    		return doGetBean(name, null, null, false);
    	}

然后进入doGetBean()方法，do开头的方法才是做事的方法，doGetBean()方法源码比较长，我们挑重点分析，进入方法之后，设置断点，并增加条件name.equals(“indexService”)，看一下日志，发现执行到这里IndexService的构造方法依旧没有执行

/** * Return an instance, which may be shared or independent, of the specified bean. * @param name the name of the bean to retrieve * @param requiredType the required type of the bean to retrieve * @param args arguments to use when creating a bean instance using explicit arguments * (only applied when creating a new instance as opposed to retrieving an existing one) * @param typeCheckOnly whether the instance is obtained for a type check, * not for actual use * @return an instance of the bean * @throws BeansException if the bean could not be created */
    	@SuppressWarnings("unchecked")
    	protected <T> T doGetBean(
    			String name, @Nullable Class<T> requiredType, @Nullable Object[] args, boolean typeCheckOnly)
    			throws BeansException { 
    
    			String beanName = transformedBeanName(name);
    		Object bean;
    
    		// Eagerly check singleton cache for manually registered singletons.
    		// 单例池中拿
    		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);
    		}
    
    		else { 
    			// Fail if we're already creating this bean instance:
    			// We're assumably within a circular reference.
    			//如果我们已经创建了这个bean实例，则失败:
    			//我们可以假定是在循环引用中。
    			if (isPrototypeCurrentlyInCreation(beanName)) { 
    				throw new BeanCurrentlyInCreationException(beanName);
    			}
    
    			// 检查此工厂中是否存在bean定义
    			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);
    			}
    
    			try { 
    				RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
    				checkMergedBeanDefinition(mbd, beanName, args);
    
    				// 保证初始化当前bean所依赖的bean。
    				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 { 
    							// 再走一遍创建bean的流程
    							// 循环依赖中，依赖第二个bean的时候，找不到，然后创建一个
    							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;
    						}
    					});
    					bean = 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);
    					}
    					bean = 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);
    							}
    						});
    						bean = getObjectForBeanInstance(scopedInstance, name, beanName, mbd);
    					}
    					catch (IllegalStateException ex) { 
    						throw new BeanCreationException(beanName,
    								"Scope '" + scopeName + "' is not active for the current thread; consider " +
    								"defining a scoped proxy for this bean if you intend to refer to it from a singleton",
    								ex);
    					}
    				}
    			}
    			catch (BeansException ex) { 
    				cleanupAfterBeanCreationFailure(beanName);
    				throw ex;
    			}
    		}
    
    		// Check if required type matches the type of the actual bean instance.
    		if (requiredType != null && !requiredType.isInstance(bean)) { 
    			try { 
    				T convertedBean = getTypeConverter().convertIfNecessary(bean, requiredType);
    				if (convertedBean == null) { 
    					throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
    				}
    				return convertedBean;
    			}
    			catch (TypeMismatchException ex) { 
    				if (logger.isTraceEnabled()) { 
    					logger.trace("Failed to convert bean '" + name + "' to required type '" +
    							ClassUtils.getQualifiedName(requiredType) + "'", ex);
    				}
    				throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
    			}
    		}
    		return (T) bean;
    	}

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzM4NzYyMjM3_size_16_color_FFFFFF_t_70_pic_center 1]  
首先，发现调用了getSingleton(beanName)方法

Object sharedInstance = getSingleton(beanName);

我们继续进入`getSingleton(beanName)`方法，发现是空方法， 然后直接调用了`getSingleton(beanName, true)`方法，具体源码如下，在DefaultSingletonBeanRegistry类中

/** * 返回在给定名称下注册的(raw)单例对象。 * 检查已经实例化的单例，并且允许早期 * 对当前创建的单例对象的引用(解决循环引用)。 * 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) { 
    		Object singletonObject = this.singletonObjects.get(beanName);
    		if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) { 
    			synchronized (this.singletonObjects) { 
    				// 先从早期单例对象的缓存拿（二级缓存）
    				singletonObject = this.earlySingletonObjects.get(beanName);
    				if (singletonObject == null && allowEarlyReference) { 
    					// 上面拿不到，再从单例工厂的缓存拿，拿到的是ObjectFactory对象工厂并得到返回对象（三级缓存）
    					ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName);
    					if (singletonFactory != null) { 
    						singletonObject = singletonFactory.getObject();
    						// 放到早期单例对象的缓存
    						this.earlySingletonObjects.put(beanName, singletonObject);
    						// 清除单例工厂的缓存
    						this.singletonFactories.remove(beanName);
    					}
    				}
    			}
    		}
    		return singletonObject;
    	}

这里重点分析一下`getSingleton(beanName, true)`方法，因为循环依赖解决的核心就在这里处理的。我到这里取消了其他的所有断点，只打了一个断点在getSingleton()方法中，并设置了debug条件beanName.equals(“indexService”)，然后直接debug重启项目，走到这里，就是当前的IndexService执行了

断点到这里，你会发现，debug直接走不了第一个判断，我们发现，根据beanName从单例池中获取的获取的单例对象为空，然后直接返回了

Object singletonObject = this.singletonObjects.get(beanName);

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzM4NzYyMjM3_size_16_color_FFFFFF_t_70_pic_center 2]  
到这里需要注意，因为有很多bean，断点非常容易乱，我到这里是整理了断点，并将断点设置在了doGetBean()方法的的下一句判断

> 当前位置：AbstractBeanFactory.doGetBean()

我们这里从单例池拿到的IndexService实例为空，继续执行下面的代码，这里对于循环依赖不是很核心，以下代码就简单说明一下作用，直接跳过了：

*  第一段：我们可以假定是在循环引用中

if (isPrototypeCurrentlyInCreation(beanName)) { 
    				throw new BeanCurrentlyInCreationException(beanName);
    			}

*  第二段：检查此工厂中是否存在bean定义（这里肯定不能进if，因是空的，并没有当前bean）

// 检查此工厂中是否存在bean定义
    			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);
    				}
    			}

*  第三段：将指定的bean标记为已经创建(或即将创建)，我们断点到这里，typeCheckOnly = false，所以会调用方法markBeanAsCreated(beanName)

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

*  第四段：检查给定的合并bean定义，保证初始化当前bean所依赖的bean（这里dependsOn == null）

try { 
    				RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
    				// 检查给定的合并bean定义
    				checkMergedBeanDefinition(mbd, beanName, args);
    
    				// 保证初始化当前bean所依赖的bean。
    				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);
    						}
    					}
    				}

*  第五段：接下来，**循环依赖的核心逻辑处理**  
    我们先整理一下逻辑，现在是在创建IndexService的bean，一步步执行到这个方法，mbd.isSingleton() == true，进入方法，这里的方法getSingleton() 是函数式编程的方法，和上面的方法不同，虽然方法名相同，但是参数不同，这里不要搞混淆了，在DefaultSingletonBeanRegistry类中，两个方法大致相同

/** * 返回在给定名称下注册的(raw)单例对象。 * 检查已经实例化的单例，并且允许早期 * 对当前创建的单例对象的引用(解决循环引用)。 */
    	protected Object getSingleton(String beanName, boolean allowEarlyReference) { 
    	}
    	
    	/** * 返回注册在给定名称下的(raw)单例对象， * 如果还没有注册，创建并注册一个新的。 */
    	public Object getSingleton(String beanName, ObjectFactory<?> singletonFactory) { 
    	}

以下，是doGetBean()方法调用的第五段核心代码

// 创建bean实例
    				if (mbd.isSingleton()) { 
    					sharedInstance = getSingleton(beanName, () -> { 
    						try { 
    							// 再走一遍创建bean的流程
    							// 循环依赖中，依赖第二个bean的时候，找不到，然后创建一个
    							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;
    						}
    					});
    					bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
    				}

然后断点进入DefaultSingletonBeanRegistry.getSingleton()方法

/** * 返回注册在给定名称下的(raw)单例对象， * 如果还没有注册，创建并注册一个新的。 * @param beanName the name of the bean * @param singletonFactory the ObjectFactory to lazily create the singleton * with, if necessary * @return the registered singleton object */
    	public Object getSingleton(String beanName, ObjectFactory<?> singletonFactory) { 
    		Assert.notNull(beanName, "Bean name must not be null");
    		synchronized (this.singletonObjects) { 
    			Object singletonObject = this.singletonObjects.get(beanName);
    			if (singletonObject == null) { 
    				if (this.singletonsCurrentlyInDestruction) { 
    					throw new BeanCreationNotAllowedException(beanName,
    							"Singleton bean creation not allowed while singletons of this factory are in destruction " +
    							"(Do not request a bean from a BeanFactory in a destroy method implementation!)");
    				}
    				if (logger.isDebugEnabled()) { 
    					logger.debug("Creating shared instance of singleton bean '" + beanName + "'");
    				}
    				// 将当前对象设置为创建中
    				beforeSingletonCreation(beanName);
    				boolean newSingleton = false;
    				boolean recordSuppressedExceptions = (this.suppressedExceptions == null);
    				if (recordSuppressedExceptions) { 
    					this.suppressedExceptions = new LinkedHashSet<>();
    				}
    				try { 
    					singletonObject = singletonFactory.getObject();
    					newSingleton = true;
    				}
    				catch (IllegalStateException ex) { 
    					// Has the singleton object implicitly appeared in the meantime ->
    					// if yes, proceed with it since the exception indicates that state.
    					singletonObject = this.singletonObjects.get(beanName);
    					if (singletonObject == null) { 
    						throw ex;
    					}
    				}
    				catch (BeanCreationException ex) { 
    					if (recordSuppressedExceptions) { 
    						for (Exception suppressedException : this.suppressedExceptions) { 
    							ex.addRelatedCause(suppressedException);
    						}
    					}
    					throw ex;
    				}
    				finally { 
    					if (recordSuppressedExceptions) { 
    						this.suppressedExceptions = null;
    					}
    					afterSingletonCreation(beanName);
    				}
    				if (newSingleton) { 
    					addSingleton(beanName, singletonObject);
    				}
    			}
    			return singletonObject;
    		}
    	}

一样的，首先，单例池中拿，当然，我们还是拿不到IndexService

Object singletonObject = this.singletonObjects.get(beanName);

然后，**通过`beforeSingletonCreation(beanName);`将当前对象设置为创建中**，还调用了单例创建后的回调，`afterSingletonCreation(beanName);`，后面的代码就调用了这两个方法，debug就返回到了doGetBean()中

if (singletonObject == null) { 
    				if (this.singletonsCurrentlyInDestruction) { 
    					throw new BeanCreationNotAllowedException(beanName,
    							"Singleton bean creation not allowed while singletons of this factory are in destruction " +
    							"(Do not request a bean from a BeanFactory in a destroy method implementation!)");
    				}
    				if (logger.isDebugEnabled()) { 
    					logger.debug("Creating shared instance of singleton bean '" + beanName + "'");
    				}
    				// 将当前对象设置为创建中
    				beforeSingletonCreation(beanName);
    				boolean newSingleton = false;
    				boolean recordSuppressedExceptions = (this.suppressedExceptions == null);
    				if (recordSuppressedExceptions) { 
    					this.suppressedExceptions = new LinkedHashSet<>();
    				}
    				try { 
    					singletonObject = singletonFactory.getObject();
    					newSingleton = true;
    				}
    				catch (IllegalStateException ex) { 
    					// Has the singleton object implicitly appeared in the meantime ->
    					// if yes, proceed with it since the exception indicates that state.
    					singletonObject = this.singletonObjects.get(beanName);
    					if (singletonObject == null) { 
    						throw ex;
    					}
    				}
    				catch (BeanCreationException ex) { 
    					if (recordSuppressedExceptions) { 
    						for (Exception suppressedException : this.suppressedExceptions) { 
    							ex.addRelatedCause(suppressedException);
    						}
    					}
    					throw ex;
    				}
    				finally { 
    					if (recordSuppressedExceptions) { 
    						this.suppressedExceptions = null;
    					}
    					// 单例创建后的回调
    					afterSingletonCreation(beanName);
    				}
    				if (newSingleton) { 
    					addSingleton(beanName, singletonObject);
    				}
    			}
    			return singletonObject;

到此为止，第五段中调用的`getSingleton(String beanName, ObjectFactory<?> singletonFactory)`方法已经完成，函数式方法中有一个回调方法，`createBean(beanName, mbd, args)`方法，直接断点到回调，继续看内部执行的方法， 因为我们的属性注入都还没有操作，也就是没有解决循环依赖。在AbstractAutowireCapableBeanFactory类中

/** *此类的中央方法：创建一个bean实例， *填充bean实例，应用后处理器等。 * @see #doCreateBean */
    	@Override
    	protected Object createBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args)
    			throws BeanCreationException { 
    
    		if (logger.isTraceEnabled()) { 
    			logger.trace("Creating instance of bean '" + beanName + "'");
    		}
    		RootBeanDefinition mbdToUse = mbd;
    
    		//确保此时确实解析了bean类，并且
    		//在动态解析的类的情况下克隆bean定义
    		//无法存储在共享的合并bean定义中。
    		Class<?> resolvedClass = resolveBeanClass(mbd, beanName);
    		if (resolvedClass != null && !mbd.hasBeanClass() && mbd.getBeanClassName() != null) { 
    			mbdToUse = new RootBeanDefinition(mbd);
    			mbdToUse.setBeanClass(resolvedClass);
    		}
    
    		// 准备方法替代。
    		try { 
    			mbdToUse.prepareMethodOverrides();
    		}
    		catch (BeanDefinitionValidationException ex) { 
    			throw new BeanDefinitionStoreException(mbdToUse.getResourceDescription(),
    					beanName, "Validation of method overrides failed", ex);
    		}
    
    		try { 
    			// 给BeanPostProcessors一个返回代理而不是目标bean实例的机会。
    			Object bean = resolveBeforeInstantiation(beanName, mbdToUse);
    			if (bean != null) { 
    				return bean;
    			}
    		}
    		catch (Throwable ex) { 
    			throw new BeanCreationException(mbdToUse.getResourceDescription(), beanName,
    					"BeanPostProcessor before instantiation of bean failed", ex);
    		}
    
    		try { 
    			Object beanInstance = doCreateBean(beanName, mbdToUse, args);
    			if (logger.isTraceEnabled()) { 
    				logger.trace("Finished creating instance of bean '" + beanName + "'");
    			}
    			return beanInstance;
    		}
    		catch (BeanCreationException | ImplicitlyAppearedSingletonException ex) { 
    			// A previously detected exception with proper bean creation context already,
    			// or illegal singleton state to be communicated up to DefaultSingletonBeanRegistry.
    			throw ex;
    		}
    		catch (Throwable ex) { 
    			throw new BeanCreationException(
    					mbdToUse.getResourceDescription(), beanName, "Unexpected exception during bean creation", ex);
    		}
    	}

这个方法除了做了简单的操作外，核心还是调用`doCreateBean(beanName, mbdToUse, args)`方法创建bean，并返回bean对象

try { 
    			Object beanInstance = doCreateBean(beanName, mbdToUse, args);
    			if (logger.isTraceEnabled()) { 
    				logger.trace("Finished creating instance of bean '" + beanName + "'");
    			}
    			return beanInstance;
    		}

然后我们进入方法`doCreateBean(beanName, mbdToUse, args)`方法

/** * 实际创建指定的bean。预创建处理已经发生 * 此时，例如检查{@code postProcessBeforeInstantiation}回调。 * <p>区分默认的bean实例化和使用 * 工厂方法，并自动装配构造函数。 * * @param beanName bean的名称 * @param mbd 合并bean定义 * @param args 用于构造函数或工厂方法调用的显式参数 * @return Bean的新实例 * @throws BeanCreationException 如果无法创建该bean * @see #instantiateBean * @see #instantiateUsingFactoryMethod * @see #autowireConstructor */
    	protected Object doCreateBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args)
    			throws BeanCreationException { 
    
    		// 实例化bean。
    		BeanWrapper instanceWrapper = null;
    		if (mbd.isSingleton()) { 
    			instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
    		}
    		if (instanceWrapper == null) { 
    			// 实例化对象
    			// 第二次调用后置处理器
    			instanceWrapper = createBeanInstance(beanName, mbd, args);
    		}
    		// 循环依赖第二次这里这里拿到的是对象（半成品）
    		Object bean = instanceWrapper.getWrappedInstance();
    		Class<?> beanType = instanceWrapper.getWrappedClass();
    		if (beanType != NullBean.class) { 
    			mbd.resolvedTargetType = beanType;
    		}
    
    		// 允许后处理器修改合并的bean定义。
    		synchronized (mbd.postProcessingLock) { 
    			if (!mbd.postProcessed) { 
    				try { 
    					// 应用合并BeanDefinition后置处理器
    					// 第三次调用后置处理器
    					applyMergedBeanDefinitionPostProcessors(mbd, beanType, beanName);
    				}
    				catch (Throwable ex) { 
    					throw new BeanCreationException(mbd.getResourceDescription(), beanName,
    							"Post-processing of merged bean definition failed", ex);
    				}
    				mbd.postProcessed = true;
    			}
    		}
    
    		//判断是否允许循环依赖
    		// isSingletonCurrentlyInCreation(beanName) 放入缓存中
    		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");
    			}
    			// 第四次调用后置处理器
    			// 判断是否需要aop
    			// 放入缓存中，然后继续执行生命周期
    			addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
    		}
    
    		// 初始化bean实例。
    		Object exposedObject = bean;
    		try { 
    			//属性赋值
    			populateBean(beanName, mbd, instanceWrapper);
    			// 又会调用getBean方法，
    			//初始化bean
    			exposedObject = initializeBean(beanName, exposedObject, mbd);
    		}
    		catch (Throwable ex) { 
    			if (ex instanceof BeanCreationException && beanName.equals(((BeanCreationException) ex).getBeanName())) { 
    				throw (BeanCreationException) ex;
    			}
    			else { 
    				throw new BeanCreationException(
    						mbd.getResourceDescription(), beanName, "Initialization of bean failed", ex);
    			}
    		}
    
    		// 处理循环依赖
    		if (earlySingletonExposure) { 
    			Object earlySingletonReference = getSingleton(beanName, false);
    			if (earlySingletonReference != null) { 
    				if (exposedObject == bean) { 
    					exposedObject = earlySingletonReference;
    				}
    				else if (!this.allowRawInjectionDespiteWrapping && hasDependentBean(beanName)) { 
    					String[] dependentBeans = getDependentBeans(beanName);
    					Set<String> actualDependentBeans = new LinkedHashSet<>(dependentBeans.length);
    					for (String dependentBean : dependentBeans) { 
    						if (!removeSingletonIfCreatedForTypeCheckOnly(dependentBean)) { 
    							actualDependentBeans.add(dependentBean);
    						}
    					}
    					if (!actualDependentBeans.isEmpty()) { 
    						throw new BeanCurrentlyInCreationException(beanName,
    								"Bean with name '" + beanName + "' has been injected into other beans [" +
    								StringUtils.collectionToCommaDelimitedString(actualDependentBeans) +
    								"] in its raw version as part of a circular reference, but has eventually been " +
    								"wrapped. This means that said other beans do not use the final version of the " +
    								"bean. This is often the result of over-eager type matching - consider using " +
    								"'getBeanNamesForType' with the 'allowEagerInit' flag turned off, for example.");
    					}
    				}
    			}
    		}
    
    		// 必要时注册一次性Bean
    		try { 
    			registerDisposableBeanIfNecessary(beanName, bean, mbd);
    		}
    		catch (BeanDefinitionValidationException ex) { 
    			throw new BeanCreationException(
    					mbd.getResourceDescription(), beanName, "Invalid destruction signature", ex);
    		}
    
    		return exposedObject;
    	}

我们分析一下当前的doCreateBean()中做了些什么事，首先通过createBeanInstance()方法创建了IndexService的实例对象，这里可以看见控制台输出了

IndexService Constructor ...

然后通过applyMergedBeanDefinitionPostProcessors()调用相关后置处理器，继续往下走，有一个判断，判断是否允许循环依赖，如果允许循环依赖（spring默认是允许的），则调用addSingletonFactory()方法，将当前的IndexService放入singletonObjects单例池（一级缓存），然后从earlySingletonObjects早期单例对象缓存（二级缓存）中移出，相关源码在DefaultSingletonBeanRegistry类中

/** *添加给定的单例工厂以构建指定的单例 *如有必要。 * <p>To be called for eager registration of singletons, e.g. to be able to * resolve circular references. * @param beanName the name of the bean * @param singletonFactory the factory for the singleton object */
    	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);
    			}
    		}
    	}

继续debug回到doCreateBean()中， 往下走，就是熟悉的代码了，调用了populateBean()方法，源码我就不粘贴了，在AbstractAutowireCapableBeanFactory类中

populateBean(beanName, mbd, instanceWrapper);

这不就是spring bean生命周期的第二阶段嘛，属性赋值，因为我们马上要给IndexService中注入UserService了。populateBean()方法中，先调用相关后置处理器一顿操作结束后，注意，我这里是用debug条件beanName.equals(“indexService”)进入的，源中通过getBeanPostProcessors()拿到所有BeanPostProcessors处理器，然后遍历调用，我们断点找到AutowiredAnnotationBeanPostProcessor处理器的循环，spring中处理@Autowired注入的是AutowiredAnnotationBeanPostProcessor处理器

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzM4NzYyMjM3_size_16_color_FFFFFF_t_70_pic_center 3]  
spring底层通过AutowiredAnnotationBeanPostProcessor处理器发现IndexService中，需要注入UserService，然后会调用AbstractBeanFactory.doGetBean()方法， 我提前在doGetBean()中打了断点，发现确实调用了getBean(userService)

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzM4NzYyMjM3_size_16_color_FFFFFF_t_70_pic_center 4]  
然后就是常规操作了，接着走IndexService走过的路线，下面我们继续走UserService

第五段最后调用

bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);

最终拿到IndexService的Bean，到此，实际上分析的是bean的创建过程，但是循环依赖就是在bean创建过程中解决的

###  ###

炸了

若果判断从单例池中获取的实例不为空，则将当前的IndexService设置为正在创建中，调用的相关方法为`isSingletonCurrentlyInCreation(beanName)`，这里就不详细分析这个方法了，就是将beanName放入DefaultSingletonBeanRegistry的`Set<String> singletonsCurrentlyInCreation`中，

创建完成后，发现需要UserService，又会调用getBean(UserService)方法，这里我就不粘贴代码了，和第一次IndexService实例化一样的，首先调用 getSingleton(beanName)方法，拿不到，然后检查此工厂中是否存在UserService对象的定义，又将指定的UserService对象标记为已经创建(或即将创建)，到这里，实际上UserService还是一个普通的对象，不是bean，代码继续往下走，又到了getSingleton()方法， getSingleton()算是解决循环依赖的核心方法了

继续分析UserService执行到getSingleton()方法，先从单例池拿，拿不到，然后将UserService对象设置为创建中，这个时候，当前正在创建的bean的名称有两个，indexService和userService

DefaultSingletonBeanRegistry类中
    
    	/** 当前正在创建的bean的名称。 */
    	private final Set<String> singletonsCurrentlyInCreation =
    			Collections.newSetFromMap(new ConcurrentHashMap<>(16));

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzM4NzYyMjM3_size_16_color_FFFFFF_t_70_pic_center 5]  
调用完getSingleton()之后，继续调用createBean，我们这里创建的是UserService，然后和IndexService走一样的流程，我们继续走UserService的生命周期，走到populateBean()时，调用到AutowiredAnnotationBeanPostProcessor处理器时，发现UserService需要注入IndexService，然后spring底层又调用getBean(indexService)，到这了有些人懵逼了，逻辑图走一下

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzM4NzYyMjM3_size_16_color_FFFFFF_t_70_pic_center 6]  
总的大纲，我们现在是在处理IndexService的生命周期，发现需要UserService的bean，spring中又没有UserService的bean，然后开始创建UserService对象，然后走UserService的生命周期，又发现需要注入IndexService的bean

debug在doGetBean()中打一个断点，F8直接到断点，看截图，没骗你吧，UserService又来获取IndexService了

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzM4NzYyMjM3_size_16_color_FFFFFF_t_70_pic_center 7]  
接着往下走，进入getSingleton()方法，现在是UserService需要获取IndexService的Bean并进行注入，getSingleton()方法核心代码

1.  先从单例池singletonObjects拿，debug可以看见，是空的
2.  然后从早期单例对象的缓存拿（二级缓存）拿，还是空的
3.  再从单例工厂缓存拿，拿到的是ObjectFactory对象工厂并得到返回对象（三级缓存），拿到了并返回
4.  最后，放到早期单例对象缓存earlySingletonObjects，清除单例工厂缓存singletonFactories

到这里，UserService就拿到了IndexService的半成品，然后走完了UserService的生命周期，并且调用的方法一路返回，返回，… 之后，IndexService就拿到了UserService，所以IndexService也是一个成品的Bean，至此，spring循环依赖已经解决完成

> 最终，UserService填充好了IndexService，然后调用方法返回后，IndexService也填充好了UserService

# 3、更多细节学习 #

*  [Tellsea全系列文章目录][Tellsea]

# 微信公众号 #

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[spring bean]: https://blog.csdn.net/qq_38762237/article/details/107983186
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[Tellsea]: https://blog.csdn.net/qq_38762237/article/details/107228946
[Get]: /images/20221123/a3006ab8ac864bc29e42507f451db50f.png