首先，我们想要知道一个接口有哪些功能，就必须要看这个接口的源代码，在idea中，选中这个接口Ctrl+F12，来查看这个接口里面有哪些方法：

表面上来看，功能其实很少，查看源码及其方法、功能

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package org.springframework.beans.factory;
import org.springframework.beans.BeansException;
import org.springframework.core.ResolvableType;
import org.springframework.lang.Nullable;
public interface BeanFactory {
    // factoryBean 的转义标识符。
	String FACTORY_BEAN_PREFIX = "&";
    // 根据 name 从容器中拿对应的 bean。
	Object getBean(String name) throws BeansException;
    // 根据 name 和 type 从容器中拿对应的 bean，要对 bean 的类型做校验。
	<T> T getBean(String name, Class<T> requiredType) throws BeansException;
    // 在容器中能否找到与 name 匹配的 bean 或者 beanDefinition。
	boolean containsBean(String name);
	// 判断 name 对对应的 bean 是不是 单例。
	boolean isSingleton(String name) throws NoSuchBeanDefinitionException;
	boolean isPrototype(String name) throws NoSuchBeanDefinitionException;
	// 判断 name 对应的 bean 与指定的类型是否匹配。
	boolean isTypeMatch(String name, ResolvableType typeToMatch) throws 
     NoSuchBeanDefinitionException;
	boolean isTypeMatch(String name, @Nullable Class<?> typeToMatch) throws 
       NoSuchBeanDefinitionException;
	//根据 name 获取对应的 bean 的类型。
	@Nullable
	Class<?> getType(String name) throws NoSuchBeanDefinitionException;
    // 根据 name 获取对应 bean 的 别名。
	String[] getAliases(String name);
}

BeanFactory表面上来看只有 getBean有点用,实际上我们不能只光看它接口，还要看它的实现类，实际上控制反转、基本的依赖注入、直至 Bean 的生命周期的各种功能，都由它的实现类提供

• HierarchicalBeanFactory：提供父容器的访问功能
• ListableBeanFactory：提供了批量获取Bean的方法
• AutowireCapableBeanFactory：在BeanFactory基础上实现对已存在实例的管理
• ConfigurableBeanFactory：主要单例bean的注册，生成实例，以及统计单例bean
• ConfigurableListableBeanFactory：继承了上述的所有接口，增加了其他功能：比如类加载器,类型转化,属性编辑器,BeanPostProcessor,作用域,bean定义,处理bean依赖关系, bean如何销毁…
• 实现类DefaultListableBeanFactory：实现了ConfigurableListableBeanFactory，注册BeanDefinition，实现上述BeanFactory所有功能

来看一下DefaultListableBeanFactory的继承关系图：

可以看到，BeanFactory只是它实现的很少一部分，除了BeanFactory提供的getBean，还有其他方法，所以我们不能光看一个接口，还要看它的具体实现类

在这里我们就只看它的DefaultSingletonBeanRegistry接口中的单例对象，这个为大家比较熟悉的，来看源码：

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public class DefaultSingletonBeanRegistry extends SimpleAliasRegistry implements SingletonBeanRegistry {
	/** Maximum number of suppressed exceptions to preserve. */
	/**
	 * 抑制异常数量最大值
	 */
	private static final int SUPPRESSED_EXCEPTIONS_LIMIT =;
	/** Cache of singleton objects: bean name to bean instance. */
	/**
	 * 一级缓存 这个就是我们大名鼎鼎的单例缓存池 用于保存我们所有的单实例bean
	 */
	private final Map<String, Object> singletonObjects = new ConcurrentHashMap<>();
	/** Cache of singleton factories: bean name to ObjectFactory. */
	/**
	 * 三级缓存 该map用户缓存 key为 beanName  value 为ObjectFactory(包装为早期对象)
	 */
	private final Map<String, ObjectFactory<?>> singletonFactories = new HashMap<>();
	/** Cache of early singleton objects: bean name to bean instance. */
	/**
	 * 二级缓存 ，用户缓存我们的key为beanName value是我们的早期对象(对象属性还没有来得及进行赋值)
	 */
	private final Map<String, Object> earlySingletonObjects = new ConcurrentHashMap<>();
	/** Set of registered singletons, containing the bean names in registration order. */
	/**
	 * 已注册的单例名称set
	 */
	private final Set<String> registeredSingletons = new LinkedHashSet<>();
	/** Names of beans that are currently in creation. */
	/**
	 * 该集合用于缓存当前正在创建bean的名称
	 */
	private final Set<String> singletonsCurrentlyInCreation =
			Collections.newSetFromMap(new ConcurrentHashMap<>());
	/** Names of beans currently excluded from in creation checks. */
	/**
	 * 排除当前创建检查的
	 */
	private final Set<String> inCreationCheckExclusions =
			Collections.newSetFromMap(new ConcurrentHashMap<>());
	/**
	 * Collection of suppressed Exceptions, available for associating related causes.
	 */
	@Nullable
	/**抑制异常的集合，可用于关联相关原因*/
	private Set<Exception> suppressedExceptions;
	/** Flag that indicates whether we're currently within destroySingletons. */
	/**
	 * 指示我们当前是否在 destroySingletons 中的标志。
	 */
	private boolean singletonsCurrentlyInDestruction = false;
	/** Disposable bean instances: bean name to disposable instance. */
	/**
	 * 用于缓存记录实现了DisposableBean 接口的实例
	 */
	private final Map<String, Object> disposableBeans = new LinkedHashMap<>();
	/** Map between containing bean names: bean name to Set of bean names that the bean contains. */
	/**
	 * 缓存bean的属性关系的映射<service,<aDao,bDa>>
	 */
	private final Map<String, Set<String>> containedBeanMap = new ConcurrentHashMap<>();
	/** Map between dependent bean names: bean name to Set of dependent bean names. */
	/**
	 * 保存的是依赖 beanName 之间的映射关系：beanName - > 依赖 beanName 的集合
	 */
	private final Map<String, Set<String>> dependentBeanMap = new ConcurrentHashMap<>();
	/** Map between depending bean names: bean name to Set of bean names for the bean's dependencies. */
	/**
	 * 保存的是依赖 beanName 之间的映射关系：依赖 beanName - > beanName 的集合
	 */
	private final Map<String, Set<String>> dependenciesForBeanMap = new ConcurrentHashMap<>();
	/**
	 * 注册单例Bean
	 *
	 * @param beanName        the name of the bean
	 * @param singletonObject the existing singleton object
	 * @throws IllegalStateException
	 */
	@Override
	public void registerSingleton(String beanName, Object singletonObject) throws IllegalStateException {
		//断言beanName是否为空
		Assert.notNull(beanName, "Bean name must not be null");
		//断言singletonObject是否为空
		Assert.notNull(singletonObject, "Singleton object must not be null");
		synchronized (this.singletonObjects) {
			//从一级缓存中通过beanName拿取Bean
			Object oldObject = this.singletonObjects.get(beanName);
			//一级缓存中存在了，抛出IllegalStateException
			if (oldObject != null) {
				throw new IllegalStateException("Could not register object [" + singletonObject +
						"] under bean name '" + beanName + "': there is already object [" + oldObject + "] bound");
			}
			//如果不存在，将singletonObject添加到一级缓存
			addSingleton(beanName, singletonObject);
		}
	}
	/**
	 * Add the given singleton object to the singleton cache of this factory.
	 * <p>To be called for eager registration of singletons.
	 * 把对象加入到单例缓存池中(所谓的一级缓存 并且考虑循环依赖和正常情况下,移除二三级缓存)
	 *
	 * @param beanName        the name of the bean
	 * @param singletonObject the singleton object
	 */
	protected void addSingleton(String beanName, Object singletonObject) {
		synchronized (this.singletonObjects) {
			//将singletonObject添加到一级缓存中，同时移除二级、三级缓存、并标记当前Bean已注册
			this.singletonObjects.put(beanName, singletonObject);
			//移除三级缓存
			this.singletonFactories.remove(beanName);
			//移除二级缓存
			this.earlySingletonObjects.remove(beanName);
			//标记当前Bean已被注册
			this.registeredSingletons.add(beanName);
		}
	}
	/**
	 * Add the given singleton factory for building the specified singleton
	 * if necessary.
	 * <p>To be called for eager registration of singletons, e.g. to be able to
	 * resolve circular references.
	 * 该方法用于把早期对象包装成一个ObjectFactory 暴露到三级缓存中 用于将解决循环依赖...
	 *
	 * @param beanName         the name of the bean
	 * @param singletonFactory the factory for the singleton object
	 */
	protected void addSingletonFactory(String beanName, ObjectFactory<?> singletonFactory) {
		//断言singletonFactory不为空
		Assert.notNull(singletonFactory, "Singleton factory must not be null");
		//同步加锁
		synchronized (this.singletonObjects) {
			//单例缓存池中没有包含当前的bean
			if (!this.singletonObjects.containsKey(beanName)) {
				//加入到三级缓存中，，，，，暴露早期对象用于解决循环依赖
				this.singletonFactories.put(beanName, singletonFactory);
				//从二级缓存中移除
				this.earlySingletonObjects.remove(beanName);
				//标记当前Bean已经被注册过
				this.registeredSingletons.add(beanName);
			}
		}
	}
	/**
	 * 该方法是一个空壳方法
	 *
	 * @param beanName the name of the bean to look for
	 * @return 缓存中的对象(有可能是一个单例完整对象, 也有可能是一个早期对象 ( 用于解决循环依赖))
	 */
	@Override
	@Nullable
	public Object getSingleton(String beanName) {
		//在这里 系统一般是允许早期对象引用的 allowEarlyReference通过这个参数可以控制解决循环依赖
		return getSingleton(beanName, true);
	}
	/**
	 * 在网上很多很多写源码的大佬或者是<spring源码深度解析>一书上,也没有说清楚为啥要使用三级缓存(二级缓存可不可以能够
	 * 解决) 答案是：可以, 但是没有很好的扩展性为啥这么说.......
	 * 原因: 获取三级缓存-----getEarlyBeanReference()经过一系列的后置处理来给我们早期对象进行特殊化处理
	 * //从三级缓存中获取包装对象的时候 ，他会经过一次后置处理器的处理对我们早期对象的bean进行
	 * 特殊化处理，但是spring的原生后置处理器没有经过处理，而是留给了我们程序员进行扩展
	 * singletonObject = singletonFactory.getObject();
	 * 把三级缓存移植到二级缓存中
	 * this.earlySingletonObjects.put(beanName, singletonObject);
	 * //删除三级缓存中的之
	 * this.singletonFactories.remove(beanName);
	 *
	 * @param beanName            bean的名称
	 * @param allowEarlyReference 是否允许暴露早期对象  通过该参数可以控制是否能够解决循环依赖的.
	 * @return 这里可能返回一个null（IOC容器加载单实例bean的时候,第一次进来是返回null）
	 * 也有可能返回一个单例对象(IOC容器加载了单实例了,第二次来获取当前的Bean)
	 * 也可能返回一个早期对象(用于解决循环依赖问题)
	 */
	@Nullable
	protected Object getSingleton(String beanName, boolean allowEarlyReference) {
		// Quick check for existing instance without full singleton lock
		/**
		 * 第一步:我们尝试去一级缓存(单例缓存池中去获取对象,一般情况从该map中获取的对象是直接可以使用的)
		 * IOC容器初始化加载单实例bean的时候第一次进来的时候 该map中一般返回空
		 */
		Object singletonObject = this.singletonObjects.get(beanName);
		//如果一级缓存为空，并且标记正在创建
		if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
			/**
			 * 尝试去二级缓存中获取对象(二级缓存中的对象是一个早期对象)
			 * 何为早期对象:就是bean刚刚调用了构造方法，还来不及给bean的属性进行赋值的对象(纯净态)
			 * 就是早期对象
			 */
			singletonObject = this.earlySingletonObjects.get(beanName);
			/**
			 * 二级缓存中也没有获取到对象,allowEarlyReference为true(参数是有上一个方法传递进来的true)
			 */
			if (singletonObject == null && allowEarlyReference) {
				synchronized (this.singletonObjects) {
					// Consistent creation of early reference within full singleton lock
					/**
					 * 再次尝试从一级缓存中去拿，如果还是没拿到则尝试去二级缓存中拿
					 */
					singletonObject = this.singletonObjects.get(beanName);
					//一级缓存中没拿到
					if (singletonObject == null) {
						//尝试从二级缓存中去拿
						singletonObject = this.earlySingletonObjects.get(beanName);
						//二级缓存还是空
						if (singletonObject == null) {
							/**
							 * 直接从三级缓存中获取 ObjectFactory对象 这个对接就是用来解决循环依赖的关键所在
							 * 在ioc后期的过程中,当bean调用了构造方法的时候,把早期对象包裹成一个ObjectFactory
							 * 暴露到三级缓存中
							 */
							ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName);
							//三级缓存中获取的对象不为空
							if (singletonFactory != null) {
								/**
								 * 在这里通过暴露的ObjectFactory 包装对象中,通过调用他的getObject()来获取我们的早期对象
								 * 在这个环节中会调用到 getEarlyBeanReference()来进行后置处理
								 */
								singletonObject = singletonFactory.getObject();
								//把早期对象放置在二级缓存,
								this.earlySingletonObjects.put(beanName, singletonObject);
								//ObjectFactory 包装对象从三级缓存中删除掉
								this.singletonFactories.remove(beanName);
							}
						}
					}
				}
			}
		}
		//返回这个Bean
		return singletonObject;
	}
	/**
	 * Return the (raw) singleton object registered under the given name,
	 * creating and registering a new one if none registered yet.
	 * 获取单例对象(该流程用于触发构建bean)
	 *
	 * @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) {
		//断言beanName不为空
		Assert.notNull(beanName, "Bean name must not be null");
		//同步加锁
		synchronized (this.singletonObjects) {
			//尝试从一级缓存池中获取对象
			Object singletonObject = this.singletonObjects.get(beanName);
			//从一级缓存中没拿到
			if (singletonObject == null) {
				//当前是否是正在销毁，是的话抛出BeanCreationNotAllowedException异常
				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!)");
				}
				//判断是否已启用Debug调试模式
				if (logger.isDebugEnabled()) {
					logger.debug("Creating shared instance of singleton bean '" + beanName + "'");
				}
				/**
				 * 标记当前的bean马上就要被创建了
				 * singletonsCurrentlyInCreation 在这里会把beanName加入进来，若第二次循环依赖（构造器注入会抛出异常）
				 */
				beforeSingletonCreation(beanName);
				//标记是否为新创建的单例Bean
				boolean newSingleton = false;
				//标记是否记录抑制异常
				boolean recordSuppressedExceptions = (this.suppressedExceptions == null);
				//如果为空，创建抑制异常集合
				if (recordSuppressedExceptions) {
					this.suppressedExceptions = new LinkedHashSet<>();
				}
				try {
					// 初始化 bean
					// 这个过程其实是调用 createBean() 方法
					singletonObject = singletonFactory.getObject();
					//标记这个Bean是新创建的
					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.
					//
					//在此期间是否隐式创建了单例对象 -> 如果是，则继续处理它，因为异常指该状态。
					singletonObject = this.singletonObjects.get(beanName);
					//一级缓存中没有，抛出异常
					if (singletonObject == null) {
						throw ex;
					}
				} catch (BeanCreationException ex) {
					//记录抑制异常
					if (recordSuppressedExceptions) {
						//遍历抑制异常集合，添加相关原因
						for (Exception suppressedException : this.suppressedExceptions) {
							ex.addRelatedCause(suppressedException);
						}
					}
					throw ex;
				} finally {
					//记录抑制异常集合置空，复用
					if (recordSuppressedExceptions) {
						this.suppressedExceptions = null;
					}
					//后置处理
					//主要做的事情就是把singletonsCurrentlyInCreation标记正在创建的bean从集合中移除
					afterSingletonCreation(beanName);
				}
				//是新建的单例Bean，添加到一级缓存中去
				if (newSingleton) {
					addSingleton(beanName, singletonObject);
				}
			}
			//返回单例Bean
			return singletonObject;
		}
	}
	/**
	 * Register an exception that happened to get suppressed during the creation of a
	 * singleton bean instance, e.g. a temporary circular reference resolution problem.
	 * <p>The default implementation preserves any given exception in this registry's
	 * collection of suppressed exceptions, up to a limit of exceptions, adding
	 * them as related causes to an eventual top-level {@link BeanCreationException}.
	 * 注册在创建单例 bean 实例期间碰巧被抑制的异常，例如一个临时的循环引用解析问题。
	 *
	 * @param ex the Exception to register
	 * @see BeanCreationException#getRelatedCauses()
	 */
	protected void onSuppressedException(Exception ex) {
		synchronized (this.singletonObjects) {
			//抑制异常集合不为空，且小于SUPPRESSED_EXCEPTIONS_LIMIT最大限制
			if (this.suppressedExceptions != null && this.suppressedExceptions.size() < SUPPRESSED_EXCEPTIONS_LIMIT) {
				//向抑制集合中添加异常
				this.suppressedExceptions.add(ex);
			}
		}
	}
	/**
	 * Remove the bean with the given name from the singleton cache of this factory,
	 * to be able to clean up eager registration of a singleton if creation failed.
	 * 从该工厂的单例缓存中删除bean ，以便能够在创建失败时清除单例的急切注册。
	 *
	 * @param beanName the name of the bean
	 * @see #getSingletonMutex()
	 */
	protected void removeSingleton(String beanName) {
		//同步加锁
		synchronized (this.singletonObjects) {
			//从一级缓存中移除
			this.singletonObjects.remove(beanName);
			//从三级缓存中移除
			this.singletonFactories.remove(beanName);
			//从二级缓存中移除
			this.earlySingletonObjects.remove(beanName);
			//从Bean注册标记集合中移除
			this.registeredSingletons.remove(beanName);
		}
	}
	/**
	 * 一级缓存中是否存在该Bean
	 *
	 * @param beanName the name of the bean to look for
	 * @return
	 */
	@Override
	public boolean containsSingleton(String beanName) {
		//判断一级缓存中是否存在该Bean
		return this.singletonObjects.containsKey(beanName);
	}
	/**
	 * 获取已注册的单例Bean名字的集合
	 *
	 * @return
	 */
	@Override
	public String[] getSingletonNames() {
		synchronized (this.singletonObjects) {
			//获取已注册的单例Bean名字的集合
			return StringUtils.toStringArray(this.registeredSingletons);
		}
	}
	/**
	 * 获取已注册单例Bean实例的个数
	 *
	 * @return
	 */
	@Override
	public int getSingletonCount() {
		synchronized (this.singletonObjects) {
			//获取已注册单例Bean实例的个数
			return this.registeredSingletons.size();
		}
	}
	/**
	 * 标记当前Bean正在创建，主要解决循环依赖
	 *
	 * @param beanName   Bean名字
	 * @param inCreation 是否已标记
	 */
	public void setCurrentlyInCreation(String beanName, boolean inCreation) {
		//断言Bean不为空
		Assert.notNull(beanName, "Bean name must not be null");
		//如果未标记，将beanName加到inCreationCheckExclusions集合中，已标记则移除
		if (!inCreation) {
			this.inCreationCheckExclusions.add(beanName);
		} else {
			this.inCreationCheckExclusions.remove(beanName);
		}
	}
	/**
	 * 返回当前Bean是否是正在创建
	 *
	 * @param beanName
	 * @return
	 */
	public boolean isCurrentlyInCreation(String beanName) {
		Assert.notNull(beanName, "Bean name must not be null");
		return (!this.inCreationCheckExclusions.contains(beanName) && isActuallyInCreation(beanName));
	}
	/**
	 * 返回当前Bean实际上是否在创建中
	 *
	 * @param beanName
	 * @return
	 */
	protected boolean isActuallyInCreation(String beanName) {
		return isSingletonCurrentlyInCreation(beanName);
	}
	/**
	 * Return whether the specified singleton bean is currently in creation
	 * (within the entire factory).
	 * 返回指定的单例 bean 当前是否正在创建中
	 *
	 * @param beanName the name of the bean
	 */
	public boolean isSingletonCurrentlyInCreation(String beanName) {
		return this.singletonsCurrentlyInCreation.contains(beanName);
	}
	/**
	 * Callback before singleton creation.
	 * <p>The default implementation register the singleton as currently in creation.
	 * 单例Bean创建前回调方法，默认实现将单例注册为当前正在创建中
	 *
	 * @param beanName the name of the singleton about to be created
	 * @see #isSingletonCurrentlyInCreation
	 */
	protected void beforeSingletonCreation(String beanName) {
		if (!this.inCreationCheckExclusions.contains(beanName) && !this.singletonsCurrentlyInCreation.add(beanName)) {
			throw new BeanCurrentlyInCreationException(beanName);
		}
	}
	/**
	 * Callback after singleton creation.
	 * <p>The default implementation marks the singleton as not in creation anymore.
	 * 创建单例后回调。 默认实现将单例标记为不再创建。
	 *
	 * @param beanName the name of the singleton that has been created
	 * @see #isSingletonCurrentlyInCreation
	 */
	protected void afterSingletonCreation(String beanName) {
		if (!this.inCreationCheckExclusions.contains(beanName) && !this.singletonsCurrentlyInCreation.remove(beanName)) {
			throw new IllegalStateException("Singleton '" + beanName + "' isn't currently in creation");
		}
	}
	/**
	 * Add the given bean to the list of disposable beans in this registry.
	 * <p>Disposable beans usually correspond to registered singletons,
	 * matching the bean name but potentially being a different instance
	 * (for example, a DisposableBean adapter for a singleton that does not
	 * naturally implement Spring's DisposableBean interface).
	 * 将给定的 bean 添加到此注册表中的一次性 bean 列表中。 一次性 bean 通常对应于已注册的单例，
	 * 与 bean 名称匹配，但可能是不同的实例（例如，单例的 DisposableBean 适配器不自然实现 Spring 的 DisposableBean 接口）。
	 *
	 * @param beanName the name of the bean
	 * @param bean     the bean instance
	 */
	public void registerDisposableBean(String beanName, DisposableBean bean) {
		synchronized (this.disposableBeans) {
			this.disposableBeans.put(beanName, bean);
		}
	}
	/**
	 * Register a containment relationship between two beans,
	 * e.g. between an inner bean and its containing outer bean.
	 * <p>Also registers the containing bean as dependent on the contained bean
	 * in terms of destruction order.
	 * 注册两个 bean 之间的包含关系，例如在内部 bean 和包含它的外部 bean 之间。还根据销毁顺序将包含的 bean 注册为依赖于所包含的 bean。
	 *
	 * @param containedBeanName  the name of the contained (inner) bean
	 * @param containingBeanName the name of the containing (outer) bean
	 * @see #registerDependentBean
	 */
	public void registerContainedBean(String containedBeanName, String containingBeanName) {
		synchronized (this.containedBeanMap) {
			Set<String> containedBeans =
					this.containedBeanMap.computeIfAbsent(containingBeanName, k -> new LinkedHashSet<>());
			if (!containedBeans.add(containedBeanName)) {
				return;
			}
		}
		registerDependentBean(containedBeanName, containingBeanName);
	}
	/**
	 * Register a dependent bean for the given bean,
	 * to be destroyed before the given bean is destroyed.
	 *
	 * @param beanName          the name of the bean
	 * @param dependentBeanName the name of the dependent bean
	 */
	public void registerDependentBean(String beanName, String dependentBeanName) {
		//获取原始的beanName
		String canonicalName = canonicalName(beanName);
		// 添加 <canonicalName, <dependentBeanName>> 到 dependentBeanMap 中
		synchronized (this.dependentBeanMap) {
			Set<String> dependentBeans =
					this.dependentBeanMap.computeIfAbsent(canonicalName, k -> new LinkedHashSet<>());
			if (!dependentBeans.add(dependentBeanName)) {
				return;
			}
		}
		// 添加 <dependentBeanName, <canonicalName>> 到 dependenciesForBeanMap 中
		synchronized (this.dependenciesForBeanMap) {
			Set<String> dependenciesForBean =
					this.dependenciesForBeanMap.computeIfAbsent(dependentBeanName, k -> new LinkedHashSet<>());
			dependenciesForBean.add(canonicalName);
		}
	}
	/**
	 * Determine whether the specified dependent bean has been registered as
	 * dependent on the given bean or on any of its transitive dependencies.
	 * 判断指定的 bean 是否依赖于 dependentBeanName 。
	 *
	 * @param beanName          the name of the bean to check
	 * @param dependentBeanName the name of the dependent bean
	 * @since.0
	 */
	//判断指定的 bean 是否依赖于 dependentBeanName
	protected boolean isDependent(String beanName, String dependentBeanName) {
		synchronized (this.dependentBeanMap) {
			return isDependent(beanName, dependentBeanName, null);
		}
	}
	//判断指定的 bean 是否依赖于 dependentBeanName
	private boolean isDependent(String beanName, String dependentBeanName, @Nullable Set<String> alreadySeen) {
		// alreadySeen 已经检测的依赖 bean
		if (alreadySeen != null && alreadySeen.contains(beanName)) {
			return false;
		}
		// 获取原始 beanName
		String canonicalName = canonicalName(beanName);
		//获取创建当前bean 所依赖的bean的名称集合
		Set<String> dependentBeans = this.dependentBeanMap.get(canonicalName);
		//不依赖任何前置Bean 直接返回
		if (dependentBeans == null) {
			return false;
		}
		// 存在，则证明存在已经注册的依赖
		if (dependentBeans.contains(dependentBeanName)) {
			return true;
		}
		// 递归检测依赖
		for (String transitiveDependency : dependentBeans) {
			if (alreadySeen == null) {
				alreadySeen = new HashSet<>();
			}
			// 添加到 alreadySeen 中
			alreadySeen.add(beanName);
			//递归检查依赖
			if (isDependent(transitiveDependency, dependentBeanName, alreadySeen)) {
				return true;
			}
		}
		return false;
	}
	/**
	 * Determine whether a dependent bean has been registered for the given name.
	 *
	 * @param beanName the name of the bean to check
	 */
	//判断beanName是否注册为依赖Bean
	protected boolean hasDependentBean(String beanName) {
		return this.dependentBeanMap.containsKey(beanName);
	}
	/**
	 * Return the names of all beans which depend on the specified bean, if any.
	 * 返回Bean所依赖的所有Bean集合
	 *
	 * @param beanName the name of the bean
	 * @return the array of dependent bean names, or an empty array if none
	 */
	public String[] getDependentBeans(String beanName) {
		//Bean依赖集合
		Set<String> dependentBeans = this.dependentBeanMap.get(beanName);
		if (dependentBeans == null) {
			return new String[];
		}
		synchronized (this.dependentBeanMap) {
			return StringUtils.toStringArray(dependentBeans);
		}
	}
	/**
	 * Return the names of all beans that the specified bean depends on, if any.
	 * 返回Bean所依赖的所有Bean集合
	 *
	 * @param beanName the name of the bean
	 * @return the array of names of beans which the bean depends on,
	 * or an empty array if none
	 */
	public String[] getDependenciesForBean(String beanName) {
		Set<String> dependenciesForBean = this.dependenciesForBeanMap.get(beanName);
		if (dependenciesForBean == null) {
			return new String[];
		}
		synchronized (this.dependenciesForBeanMap) {
			return StringUtils.toStringArray(dependenciesForBean);
		}
	}
	/**
	 * 销毁所有bean的所有信息
	 */
	public void destroySingletons() {
		if (logger.isTraceEnabled()) {
			logger.trace("Destroying singletons in " + this);
		}
		//标记为正在销毁
		synchronized (this.singletonObjects) {
			this.singletonsCurrentlyInDestruction = true;
		}
		String[] disposableBeanNames;
		//获取需要销毁的Bean集合
		synchronized (this.disposableBeans) {
			disposableBeanNames = StringUtils.toStringArray(this.disposableBeans.keySet());
		}
		//循环校徽单例Bean
		for (int i = disposableBeanNames.length -; i >= 0; i--) {
			destroySingleton(disposableBeanNames[i]);
		}
		//	清空依赖和映射关系缓存
		this.containedBeanMap.clear();
		this.dependentBeanMap.clear();
		this.dependenciesForBeanMap.clear();
		// 清理Bean的一级二级三级缓存
		clearSingletonCache();
	}
	/**
	 * Clear all cached singleton instances in this registry.
	 * 清除所有缓存的单例实例。
	 *
	 * @since.3.15
	 */
	protected void clearSingletonCache() {
		synchronized (this.singletonObjects) {
			this.singletonObjects.clear();
			this.singletonFactories.clear();
			this.earlySingletonObjects.clear();
			this.registeredSingletons.clear();
			this.singletonsCurrentlyInDestruction = false;
		}
	}
	/**
	 * Destroy the given bean. Delegates to {@code destroyBean}
	 * if a corresponding disposable bean instance is found.
	 *
	 * @param beanName the name of the bean
	 * @see #destroyBean
	 */
	public void destroySingleton(String beanName) {
		// Remove a registered singleton of the given name, if any.
		//从缓存中移除当前bean的相关信息,由于不知道在哪里发生异常，所以我们把跟当前bean的所有缓存记录都清除
		removeSingleton(beanName);
		// Destroy the corresponding DisposableBean instance.
		//创建一个变量用于接受 实现了DisposableBean接口的对象变量
		DisposableBean disposableBean;
		synchronized (this.disposableBeans) {
			disposableBean = (DisposableBean) this.disposableBeans.remove(beanName);
		}
		//进行bean的销毁
		destroyBean(beanName, disposableBean);
	}
	/**
	 * Destroy the given bean. Must destroy beans that depend on the given
	 * bean before the bean itself. Should not throw any exceptions.
	 * 销毁bean的依赖关系
	 *
	 * @param beanName the name of the bean
	 * @param bean     the bean instance to destroy
	 */
	protected void destroyBean(String beanName, @Nullable DisposableBean bean) {
		// Trigger destruction of dependent beans first...
		// 销毁dependentBeanMap中保存的是当前bean和依赖bean之间的映射
		Set<String> dependencies;
		synchronized (this.dependentBeanMap) {
			// Within full synchronization in order to guarantee a disconnected Set
			//把当前创建dependon 依赖的bean从缓存中移除并且返回处理
			dependencies = this.dependentBeanMap.remove(beanName);
		}
		//如果bean依赖不为空
		if (dependencies != null) {
			if (logger.isTraceEnabled()) {
				logger.trace("Retrieved dependent beans for bean '" + beanName + "': " + dependencies);
			}
			//递归销毁bean
			for (String dependentBeanName : dependencies) {
				destroySingleton(dependentBeanName);
			}
		}
		// Actually destroy the bean now...
		//真正的调用bean的destory()方法
		if (bean != null) {
			try {
				bean.destroy();
			} catch (Throwable ex) {
				if (logger.isWarnEnabled()) {
					logger.warn("Destruction of bean with name '" + beanName + "' threw an exception", ex);
				}
			}
		}
		// 删除bean的属性关系的映射
		Set<String> containedBeans;
		synchronized (this.containedBeanMap) {
			// Within full synchronization in order to guarantee a disconnected Set
			containedBeans = this.containedBeanMap.remove(beanName);
		}
		if (containedBeans != null) {
			for (String containedBeanName : containedBeans) {
				destroySingleton(containedBeanName);
			}
		}
		// Remove destroyed bean from other beans' dependencies.
		//销毁dependentBeanMap 中 Bean的依赖
		synchronized (this.dependentBeanMap) {
			for (Iterator<Map.Entry<String, Set<String>>> it = this.dependentBeanMap.entrySet().iterator(); it.hasNext(); ) {
				Map.Entry<String, Set<String>> entry = it.next();
				Set<String> dependenciesToClean = entry.getValue();
				dependenciesToClean.remove(beanName);
				if (dependenciesToClean.isEmpty()) {
					it.remove();
				}
			}
		}
		// Remove destroyed bean's prepared dependency information.
		//从dependenciesForBeanMap集合移除
		this.dependenciesForBeanMap.remove(beanName);
	}
	/**
	 * Exposes the singleton mutex to subclasses and external collaborators.
	 * <p>Subclasses should synchronize on the given Object if they perform
	 * any sort of extended singleton creation phase. In particular, subclasses
	 * should <i>not</i> have their own mutexes involved in singleton creation,
	 * to avoid the potential for deadlocks in lazy-init situations.
	 */
	/**
	 * 将单例互斥体暴露给子类和外部合作者。 如果子类执行任何类型的扩展单例创建阶段，
	 * 它们应该在给定的对象上同步。特别是子类不应该在单例创建中使用它们自己的互斥锁，
	 * 以避免在惰性初始化情况下潜在的死锁。
	 */
	@Override
	public final Object getSingletonMutex() {
		return this.singletonObjects;
	}
}

它的方法大多为私有的，可以通过debug和反射，在这里我们通过反射来获取私有的成员变量：

复制
  // DefaultSingletonBeanRegistry类管理所有的单例对象
     //获取所有的私有成员变量
        Field singletonObjects = 
       DefaultSingletonBeanRegistry.class.getDeclaredField("singletonObjects");
      //允许可以访问私有成员变量
        singletonObjects.setAccessible(true);
        //通过反射获取
       //获取beanFactory
        ConfigurableListableBeanFactory beanFactory = context.getBeanFactory();
      //反射调用,获取beanFactory的属性
        Map<String, Object> map = (Map<String, Object>) singletonObjects.get(beanFactory);
       //过滤,获取component相关的
        map.entrySet().stream().filter(e -> e.getKey().startsWith("component"))
                .forEach(e -> {
                    System.out.println(e.getKey() + "=" + e.getValue());
                });

总结：

BeanFactory 能干点啥？

• 表面上只有 getBean
• 实际上控制反转、基本的依赖注入、直至 Bean 的生命周期的各种功能，都由它的实现类提供
• 例子中通过反射查看了它的成员变量 singletonObjects，内部包含了所有的单例 bean