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关于 Spring 中 getBean 的全流程源码解析

时间：2021-04-19 11:40:20   收藏：0   阅读：9

技术分享图片

作者：小傅哥
博客：https://bugstack.cn

沉淀、分享、成长，让自己和他人都能有所收获！??

一、前言

你提出问题，就要给出解决方案！

最近有粉丝小伙伴反馈，与自己的上级沟通总是遇到障碍，感觉不被理解。大部分时候他提出来的事情都可能会被领导说：“我没get到你的点”、“你想做的这个项目没有业务价值”、“你提出问题，就要给出解决方案”，等等诸如此类的回答。

鉴于具体情况要具体分析，可能我们并不一定能判断出是谁的问题，导致在每次的交谈中出现的分歧。可能是leader有leader的苦衷和视角，也可能是员工有员工的理解和想法，所以最终没有达成一致。

但就带团队来讲，有效沟通很重要。就像：如果你说的都对，那我为什么和你争吵呢？与其压制遇到的矛盾点，不如都摊开了聊，谁的视角和心胸更大，谁就多有一些同理心。

如果尖锐的批评完全消失，温和的批评将会变得刺耳。

如果温和的批评也不被允许，沉默将被认为居心叵测。

如果沉默也不再允许，赞扬不够卖力将是一种罪行。

如果只允许一种声音存在，那么，唯一存在的那个声音就是谎言。

二、面试题

谢飞机，小记！，总感觉 Spring 也没啥看的，怎么面试官一问就能问出花？

面试官：Spring 的 getBean 中，transformedBeanName 的作用是什么？

谢飞机：不知道呀，看单词意思好像是改变Bean名称。

面试官：那这么说，你的 Bean 如果有 alias 别名，Spring 在获取 Bean 时候要怎么处理？

谢飞机：这！

面试官：那如果用了 depends-on 呢？

谢飞机：啊， 我没用过 depends-on 我不知道！

面试官：那你调试代码时候，看见过BeanName前面有 & 的情况吗，为啥会出现？

谢飞机：我不配知道！再见！

三、Bean 的获取过程

对于刚接触看 Spring 源码的伙伴来说，可能很疑惑于怎么就获取一个 Bean 就这么多流程呢？

所以，它为了适应各类的需求，变得越来越复杂了。而这部分知识的深入学习绝对不只是为了应付八股文，更多的是考虑到在日常的 Spring 使用中遇到复杂问题时有没有一个大致知晓的流程，可以快速定位问题，以及此类需求的技术实现方案是否能在以后的应用开发中起到一定的指导作用，因为它是一种设计方案的具体实现。

1. getBean 核心流程图

技术分享图片

接下来，我们就依次的把关于获取 Bean 实例的重点代码列举出来做分析，读者伙伴也可以结合流程图一起看，这样会更方便理解。

2. getBean 从哪开始读源码

@Test
public void test_getBean() {
    BeanFactory beanFactory = new ClassPathXmlApplicationContext("spring-config.xml");
    UserDao userDao = beanFactory.getBean("userDao", UserDao.class);
    logger.info("获取 Bean：{}", userDao);
}

3. getBean 源码全局预览

源码位置：AbstractBeanFactory -> getBean() -> doGetBean()

@Override
public <T> T getBean(String name, Class<T> requiredType) throws BeansException { 
    // getBean 就像你的领导其实没做啥，都在 doGetBean 里
	return doGetBean(name, requiredType, null, false);
}

protected <T> T doGetBean(final String name, final Class<T> requiredType, final Object[] args, boolean typeCheckOnly)
		throws BeansException {     
    
    // 处理别名BeanName、处理带&符的工厂BeanName
	final String beanName = transformedBeanName(name);
	Object bean;  

	// 先尝试从缓存中获取Bean实例，这个位置就是三级缓存解决循环依赖的方法
	Object sharedInstance = getSingleton(beanName);   

	if (sharedInstance != null && args == null) {
		if (logger.isDebugEnabled()) {
			if (isSingletonCurrentlyInCreation(beanName)) {
				logger.debug("Returning eagerly cached instance of singleton bean ‘" + beanName +
						"‘ that is not fully initialized yet - a consequence of a circular reference");
			}
			else {
				logger.debug("Returning cached instance of singleton bean ‘" + beanName + "‘");
			}
		}        
        
        // 1. 如果 sharedInstance 是普通的 Bean 实例，则下面的方法会直接返回
        // 2. 如果 sharedInstance 是工厂Bean类型，则需要获取 getObject 方法，可以参考关于 FactoryBean 的实现类 
		bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
	}
	else {
		
        // 循环依赖有三种，setter注入、多实例和构造函数，Spring 只能解决 setter 注入，所以这里是 Prototype 则会抛出异常
		if (isPrototypeCurrentlyInCreation(beanName)) {
			throw new BeanCurrentlyInCreationException(beanName);
		}    

		// 1. 父 bean 工厂存在
        // 2. 当前 bean 不存在于当前bean工厂，则到父工厂查找 bean 实例
		BeanFactory parentBeanFactory = getParentBeanFactory();
		if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {
			// 获取 name 对应的 beanName，如果 name 是以 & 开头，则返回 & + beanName
			String nameToLookup = originalBeanName(name);         
            
            // 根据 args 参数是否为空，调用不同的父容器方法获取 bean 实例
			if (args != null) {
				return (T) parentBeanFactory.getBean(nameToLookup, args);
			}
			else {
				return parentBeanFactory.getBean(nameToLookup, requiredType);
			}
		}       

        // 1. typeCheckOnly，用于判断调用 getBean 方法时，是否仅是做类型检查
        // 2. 如果不是只做类型检查，就会调用 markBeanAsCreated 进行记录
		if (!typeCheckOnly) {
			markBeanAsCreated(beanName);
		}
		try {    
    
            // 从容器 getMergedLocalBeanDefinition 获取 beanName 对应的 GenericBeanDefinition，转换为 RootBeanDefinition
			final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName); 
            // 检查当前创建的 bean 定义是否为抽象 bean 定义
			checkMergedBeanDefinition(mbd, beanName, args);
			
            // 处理使用了 depends-on 注解的依赖创建 bean 实例
			String[] dependsOn = mbd.getDependsOn();
			if (dependsOn != null) {
				for (String dep : dependsOn) {   
                    // 监测是否存在 depends-on 循环依赖，若存在则会抛出异常
					if (isDependent(beanName, dep)) {
						throw new BeanCreationException(mbd.getResourceDescription(), beanName,
								"Circular depends-on relationship between ‘" + beanName + "‘ and ‘" + dep + "‘");
					}       
                    
                    // 注册依赖记录
					registerDependentBean(dep, beanName);
					try {    
					    // 加载 depends-on 依赖（dep 是 depends-on 缩写）
						getBean(dep);
					}
					catch (NoSuchBeanDefinitionException ex) {
						throw new BeanCreationException(mbd.getResourceDescription(), beanName,
								"‘" + beanName + "‘ depends on missing bean ‘" + dep + "‘", ex);
					}
				}
			}  

			// 创建单例 bean 实例
			if (mbd.isSingleton()) {    

			    // 把 beanName 和 new ObjectFactory 匿名内部类传入回调
				sharedInstance = getSingleton(beanName, new ObjectFactory<Object>() {
					@Override
					public Object getObject() throws BeansException {
						try {    
                            // 创建 bean
							return createBean(beanName, mbd, args);
						}
						catch (BeansException ex) {
							// 创建失败则销毁
							destroySingleton(beanName);
							throw ex;
						}
					}
				});
				bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
			}      
            // 创建其他类型的 bean 实例
			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();
				final 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, new ObjectFactory<Object>() {
						@Override
						public Object getObject() throws BeansException {
							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;
		}
	}
	// 如果需要类型转换，这里会进行操作
	if (requiredType != null && bean != null && !requiredType.isInstance(bean)) {
		try {
			return getTypeConverter().convertIfNecessary(bean, requiredType);
		}
		catch (TypeMismatchException ex) {
			if (logger.isDebugEnabled()) {
				logger.debug("Failed to convert bean ‘" + name + "‘ to required type ‘" +
						ClassUtils.getQualifiedName(requiredType) + "‘", ex);
			}
			throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
		}
	}    

    // 返回 Bean
	return (T) bean;
}

综上基本就是 getBean 过程涉及到的核心处理方法，基本包括；

4. beanName 转换操作

处理 & 符：transformedBeanName() -> BeanFactoryUtils.transformedBeanName(name)

public static String transformedBeanName(String name) {
	Assert.notNull(name, "‘name‘ must not be null");
	String beanName = name;
	while (beanName.startsWith(BeanFactory.FACTORY_BEAN_PREFIX)) {
		beanName = beanName.substring(BeanFactory.FACTORY_BEAN_PREFIX.length());
	}
	return beanName;
}

别名转换：transformedBeanName() -> canonicalName

public String canonicalName(String name) {
	String canonicalName = name;
	// Handle aliasing...
	String resolvedName;
	do {
		resolvedName = this.aliasMap.get(canonicalName);
		if (resolvedName != null) {
			canonicalName = resolvedName;
		}
	}
	while (resolvedName != null);
	return canonicalName;
}

<bean id="userService" class="org.itstack.interview.UserService"/>
<alias name="userService" alias="userService-alias01"/>
<alias name="userService-alias01" alias="userService-alias02"/>

5. depends-on 依赖 Bean

AbstractBeanFactory -> isDependent(beanName, dep) -> DefaultSingletonBeanRegistry

protected boolean isDependent(String beanName, String dependentBeanName) {
	synchronized (this.dependentBeanMap) {
		return isDependent(beanName, dependentBeanName, null);
	}

<bean id="userService" class="org.itstack.interview.UserService" depends-on="userDao"/>

<bean id="userDao" class="org.itstack.interview.UserDao"/>

private boolean isDependent(String beanName, String dependentBeanName, Set<String> alread
	if (alreadySeen != null && alreadySeen.contains(beanName)) {
		return false;
	}
	String canonicalName = canonicalName(beanName);
	Set<String> dependentBeans = this.dependentBeanMap.get(canonicalName);
	if (dependentBeans == null) {
		return false;
	}
	if (dependentBeans.contains(dependentBeanName)) {
		return true;
	}
	for (String transitiveDependency : dependentBeans) {
		if (alreadySeen == null) {
			alreadySeen = new HashSet<String>();
		}
		alreadySeen.add(beanName);
		if (isDependent(transitiveDependency, dependentBeanName, alreadySeen)) {
			return true;
		}
	}
	return false;
}

AbstractBeanFactory -> registerDependentBean(dep, beanName) -> DefaultSingletonBeanRegistry

public void registerDependentBean(String beanName, String dependentBeanName) {
	String canonicalName = canonicalName(beanName);  

	synchronized (this.dependentBeanMap) {
		Set<String> dependentBeans = this.dependentBeanMap.get(canonicalName);
		if (dependentBeans == null) {
			dependentBeans = new LinkedHashSet<String>(8);
			this.dependentBeanMap.put(canonicalName, dependentBeans);
		}
		dependentBeans.add(dependentBeanName);
	}   

	synchronized (this.dependenciesForBeanMap) {
		Set<String> dependenciesForBean = this.dependenciesForBeanMap.get(dependentBeanName
		if (dependenciesForBean == null) {
			dependenciesForBean = new LinkedHashSet<String>(8);
			this.dependenciesForBeanMap.put(dependentBeanName, dependenciesForBean);
		}
		dependenciesForBean.add(canonicalName);
	}
}

最后：getBean(dep)，就可以获取到 depends-on 依赖的 Bean 了

6. 处理单实例 Bean

AbstractBeanFactory -> mbd.isSingleton()

if (mbd.isSingleton()) {
	sharedInstance = getSingleton(beanName, new ObjectFactory<Object>() {
		@Override
		public Object getObject() throws BeansException {
			try {
				return createBean(beanName, mbd, args);
			}
			catch (BeansException ex) {
				destroySingleton(beanName);
				throw ex;
			}
		}
	});
	bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
}

public Object getSingleton(String beanName, ObjectFactory<?> singletonFactory) {
	Assert.notNull(beanName, "‘beanName‘ 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 + "‘");
			}
			beforeSingletonCreation(beanName);
			boolean newSingleton = false;
			boolean recordSuppressedExceptions = (this.suppressedExceptions == null);
			if (recordSuppressedExceptions) {
				this.suppressedExceptions = new LinkedHashSet<Exception>();
			}
			try {
				singletonObject = singletonFactory.getObject();
				newSingleton = true;
			}
			catch (IllegalStateException ex) {
				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 != NULL_OBJECT ? singletonObject : null);
	}
}

7. 从缓存中获取 bean 实例

doCreateBean -> if (earlySingletonExposure) -> getSingleton(beanName, false)

protected Object getSingleton(String beanName, boolean allowEarlyReference) {
    // 从 singletonObjects 获取实例，singletonObjects 中缓存的实例都是完全实例化好的 bean，可以直接使用
	Object singletonObject = this.singletonObjects.get(beanName);
    // 如果 singletonObject 为空，则没有创建或创建中
	if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
        // 加锁
		synchronized (this.singletonObjects) {
            // 单例缓存池中，没有当前beanName
			singletonObject = this.earlySingletonObjects.get(beanName);
			if (singletonObject == null && allowEarlyReference) {
				ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName);
				if (singletonFactory != null) {
                    // 加入到三级缓存，暴漏早期对象用于解决三级缓存
					singletonObject = singletonFactory.getObject();  
					this.earlySingletonObjects.put(beanName, singletonObject);
					this.singletonFactories.remove(beanName);
				}
			}
		}
	}
	return (singletonObject != NULL_OBJECT ? singletonObject : null);
}

8. FactoryBean 中获取 bean 实例

AbstractBeanFactory -> getObjectForBeanInstance(sharedInstance, name, beanName, null)

protected Object getObjectForBeanInstance(Object beanInstance, String name, String beanName, RootBeanDefinition mbd) {
    
    // 如果 beanName 以 & 开头，但 beanInstance 却不是 FactoryBean，则会抛出异常
	if (BeanFactoryUtils.isFactoryDereference(name) && !(beanInstance instanceof FactoryBean)) {
		throw new BeanIsNotAFactoryException(transformedBeanName(name), beanInstance.getClass());
	}

    // 这里判断就是这个 bean 是不是 FactoryBean，不是就直接返回了
	if (!(beanInstance instanceof FactoryBean) || BeanFactoryUtils.isFactoryDereference(name)) {
		return beanInstance;
	}   

	Object object = null;
	if (mbd == null) {    
        // 如果 mbd 为空，则从缓存加载 bean（FactoryBean 生成的单例 bean 实例会缓存到 factoryBeanObjectCache 集合中，方便使用）
		object = getCachedObjectForFactoryBean(beanName);
	}   

	if (object == null) {
		// 到这，beanInstance 是 FactoryBean 类型，所以就强转了
		FactoryBean<?> factory = (FactoryBean<?>) beanInstance;
		// mbd 为空且判断 containsBeanDefinition 是否包含 beanName
		if (mbd == null && containsBeanDefinition(beanName)) {    
            // 合并 BeanDefinition
			mbd = getMergedLocalBeanDefinition(beanName);
		}    
		boolean synthetic = (mbd != null && mbd.isSynthetic());   
        // 调用 getObjectFromFactoryBean 获取实例 
		object = getObjectFromFactoryBean(factory, beanName, !synthetic);
	}
	return object;
}

四、测试案例

1. 别名

<bean id="userService" class="org.itstack.interview.UserService"/>
<!-- 起个别名 -->
<alias name="userService" alias="userService-alias01"/>
<!-- 别名的别名 -->
<alias name="userService-alias01" alias="userService-alias02"/>

@Test
public void test_alias() {
    BeanFactory beanFactory = new ClassPathXmlApplicationContext("spring-config-alias.xml");
    UserService userService = beanFactory.getBean("userService-alias02", UserService.class);
    logger.info("获取 Bean 通过别名：{}", userService);
}

技术分享图片

2. 依赖

<bean id="userService" class="org.itstack.interview.UserService" depends-on="userDao"/>
<bean id="userDao" class="org.itstack.interview.UserDao"/>

@Test
public void test_depends_on() {
    BeanFactory beanFactory = new ClassPathXmlApplicationContext("spring-config-depends-on.xml");
    UserService userService = beanFactory.getBean(UserService.class, "userService");
    logger.info("获取 Bean：{}", userService.getUserDao());
}

技术分享图片

3. BeanFactory

<bean id="userDao" class="org.itstack.interview.MyFactoryBean"/>

@Test
public void test_factory_bean() {
    BeanFactory beanFactory = new ClassPathXmlApplicationContext("spring-config-factory-bean.xml");
    UserDao userDao = beanFactory.getBean("userDao", UserDao.class);
    logger.info("获取 Bean：{}", userDao);
}

技术分享图片

五、总结

六、系列推荐

原文：https://www.cnblogs.com/xiaofuge/p/14675550.html

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