Spring源码分析のBean创建流程（上）

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前言

  原生Spring在refresh方法中，会在finishBeanFactoryInitialization：preInstantiateSingletons方法中直接创建所有非懒加载的单例Bean。

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一、preInstantiateSingletons

  preInstantiateSingletons方法的主要逻辑：

1. 遍历bean名称的集合。
2. 合并bean的定义 得到合并后的BeanDefinition。
3. 判断bean是否应该在此处被实例化。
4. 判断bean是否属于FactoryBean/SmartFactoryBean。
5. 进行实例化。
6. 实例化完成后，再次判断bean是否实现了SmartInitializingSingleton接口，如果实现了，则回调其中的afterSingletonsInstantiated方法。

	@Overridepublic 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.//存放所有bean的名称List<String> beanNames = new ArrayList<>(this.beanDefinitionNames);// Trigger initialization of all non-lazy singleton beans...//遍历bean名称的集合for (String beanName : beanNames) {//1.1 getMergedLocalBeanDefinition 合并bean的定义 得到合并后的BeanDefinitionRootBeanDefinition bd = getMergedLocalBeanDefinition(beanName);//进入该分支的条件//1、Bean不是抽象的//2、bean是单例//3、bean不是懒加载if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) {//1.2 isFactoryBean 判断bean是否实现了FactoryBean接口if (isFactoryBean(beanName)) {//如果实现了 就获取实现了FactoryBean接口的bean Object bean = getBean(FACTORY_BEAN_PREFIX + beanName);if (bean instanceof FactoryBean) {//进行强转 将bean转换为接口FactoryBean的类型FactoryBean<?> factory = (FactoryBean<?>) bean;boolean isEagerInit;//这一块是判断bean实现的是否是SmartFactoryBean  SmartFactoryBean 也是继承自 FactoryBean//只要满足条件，无论是if还是else都会回调SmartFactoryBean 的 isEagerInit 方法的逻辑 这里if分支只是额外加入了安全判断if (System.getSecuritbeanyManager() != 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);}}}//在创建/获取bean之后 再次进行判断 如果有的bean实现了SmartInitializingSingleton 接口， 同样会去回调其中的afterSingletonsInstantiated方法。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();}}}

1.1、getMergedLocalBeanDefinition

  getMergedLocalBeanDefinition方法的作用是，得到合并后的bean定义。这里牵涉到Spring父子容器的概念：

public class Demo1 {public static void main(String[] args) {AnnotationConfigApplicationContext contextParent = new AnnotationConfigApplicationContext(AppConfigParent.class);AnnotationConfigApplicationContext context = new AnnotationConfigApplicationContext();context.register(AppConfigParent.class);context.setParent(contextParent);context.refresh();}
}

  以上是在配置文件中关于父子容器的定义，假如在context中获取某个bean，而context中没有，就会去它的父容器中找。
  而在Spring中，合并父子bean定义，主要的逻辑:

• 如果当前bean没有父类，则直接克隆当前的BeanDefinition，并使用它作为最终的合并结果。
• 如果有父类，则递归调用getMergedLocalBeanDefinition去进行合并。

	protected RootBeanDefinition getMergedLocalBeanDefinition(String beanName) throws BeansException {// Quick check on the concurrent map first, with minimal locking.//去合并后的bean定义缓存中找RootBeanDefinition mbd = this.mergedBeanDefinitions.get(beanName);//如果能找到就直接返回if (mbd != null && !mbd.stale) {return mbd;}//否则进行合并return getMergedBeanDefinition(beanName, getBeanDefinition(beanName));}/***	参数一 当前的bean名称 参数二 根据当前bean名称找到的对应的bean定义*/protected RootBeanDefinition getMergedBeanDefinition(String beanName, BeanDefinition bd, @Nullable BeanDefinition containingBd)throws BeanDefinitionStoreException {//加锁 防止重复合并synchronized (this.mergedBeanDefinitions) {RootBeanDefinition mbd = null;RootBeanDefinition previous = null;// Check with full lock now in order to enforce the same merged instance.if (containingBd == null) {//再次尝试通过beanName 从 	mergedBeanDefinitions 中获取RootBeanDefinition mbd = this.mergedBeanDefinitions.get(beanName);}//获取不到 就走合并的逻辑if (mbd == null || mbd.stale) {previous = mbd;//如果当前bean名称找到的对应的bean定义 的 父beanName属性为null//说明可能两种情况 ：1、当前bean定义就是RootBeanDefinition 2、当前bean定义不需要合并if (bd.getParentName() == null) {// Use copy of given root bean definition.if (bd instanceof RootBeanDefinition) {mbd = ((RootBeanDefinition) bd).cloneBeanDefinition();}else {mbd = new RootBeanDefinition(bd);}}//否则需要合并else {// Child bean definition: needs to be merged with parent.BeanDefinition pbd;try {//对当前bean的父bean进行名称转换String parentBeanName = transformedBeanName(bd.getParentName());//当前bean名称和父bean名称不相同if (!beanName.equals(parentBeanName)) {//递归合并 因为当前bean 的 父bean有可能还有父beanpbd = getMergedBeanDefinition(parentBeanName);}//当前bean名称和父bean名称相同else {//获取父 BeanFactoryBeanFactory parent = getParentBeanFactory();if (parent instanceof ConfigurableBeanFactory) {// 从父工厂中获取合并后的父类 BeanDefinitionpbd = ((ConfigurableBeanFactory) parent).getMergedBeanDefinition(parentBeanName);}else {throw new NoSuchBeanDefinitionException(parentBeanName,"Parent name '" + parentBeanName + "' is equal to bean name '" + beanName +"': cannot be resolved without a ConfigurableBeanFactory parent");}}}catch (NoSuchBeanDefinitionException ex) {throw new BeanDefinitionStoreException(bd.getResourceDescription(), beanName,"Could not resolve parent bean definition '" + bd.getParentName() + "'", ex);}// Deep copy with overridden values.//创建一个新的 RootBeanDefinition，基于父类 BeanDefinitionmbd = new RootBeanDefinition(pbd);//当前的bd覆盖父类的属性mbd.overrideFrom(bd);}// Set default singleton scope, if not configured before.//如果没有设置作用域 默认是单例的if (!StringUtils.hasLength(mbd.getScope())) {mbd.setScope(SCOPE_SINGLETON);}// A bean contained in a non-singleton bean cannot be a singleton itself.// Let's correct this on the fly here, since this might be the result of// parent-child merging for the outer bean, in which case the original inner bean// definition will not have inherited the merged outer bean's singleton status.if (containingBd != null && !containingBd.isSingleton() && mbd.isSingleton()) {mbd.setScope(containingBd.getScope());}// Cache the merged bean definition for the time being// (it might still get re-merged later on in order to pick up metadata changes)//进行缓存if (containingBd == null && isCacheBeanMetadata()) {this.mergedBeanDefinitions.put(beanName, mbd);}}if (previous != null) {copyRelevantMergedBeanDefinitionCaches(previous, mbd);}//返回合并后的定义return mbd;}}

1.2、isFactoryBean

  isFactoryBean主要用于判断当前的bean是否实现了FactoryBean接口，如果某个类实现了FactoryBean接口，则会产生两个实例，第一个是实现了FactoryBean接口的bean的本身，第二个是其中getObject()方法返回的实例。默认的getBean()，得到的是getObject()方法返回的实例。如果要获取实现了FactoryBean接口的bean的本身，则需要getBean()时在名称前加上"&"符号。

	@Overridepublic boolean isFactoryBean(String name) throws NoSuchBeanDefinitionException {//这里获取到的是实现了`FactoryBean`接口的bean的本身String beanName = transformedBeanName(name);//尝试从单例池中获取Object beanInstance = getSingleton(beanName, false);//如果获取到了，就判断是否实现了FactoryBeanif (beanInstance != null) {return (beanInstance instanceof FactoryBean);}// 判断是否要从父容器中获取if (!containsBeanDefinition(beanName) && getParentBeanFactory() instanceof ConfigurableBeanFactory) {// No bean definition found in this factory -> delegate to parent.return ((ConfigurableBeanFactory) getParentBeanFactory()).isFactoryBean(name);}//这里也会走bean定义合并return isFactoryBean(beanName, getMergedLocalBeanDefinition(beanName));}/***	这里的isFactoryBean属性作为一个缓存的效果，后续直接取该类bean定义的该属性的值即可。**/protected boolean isFactoryBean(String beanName, RootBeanDefinition mbd) {//尝试从合并后的bean定义中 获取isFactoryBean属性Boolean result = mbd.isFactoryBean;//获取不到if (result == null) {//判断当前bean是否实现了FactoryBean接口Class<?> beanType = predictBeanType(beanName, mbd, FactoryBean.class);result = (beanType != null && FactoryBean.class.isAssignableFrom(beanType));//并且将isFactoryBean属性设置值。mbd.isFactoryBean = result;}return result;}

二、getBean

  getBean是Spring创建所有非懒加载单例bean的方法，总体的逻辑，如果容器中已经有了目标bean，就直接返回，否则就去创建。
  该方法有以下的情况：

1. 单例池中能获取到，走实例化 bean 的逻辑。
2. 单例池中没有，Bean 父工厂不为空并且在 Bean 定义中不存在该 BeanName，委托给父工厂。父工厂会尝试获取该 Bean，如果父工厂也没有，则继续查找。
3. 单例池中没有，Bean 父工厂为空并且在 Bean 定义中存在该 BeanName，则进行 Bean 定义合并。合并时，父 Bean 定义会被合并到当前 Bean 定义中，最终生成 Bean 实例。
4. 根据 Bean 的作用域执行不同操作：
   • 单例 Bean：创建后加入单例缓存池，后续请求直接返回缓存中的实例。
   • 原型 Bean：每次请求创建新的实例，不缓存。
   • 其他作用域：根据配置的作用域（如 request、session 等）执行对应的实例化和管理逻辑。
5. 循环依赖的处理：Spring 会通过提前暴露实例来解决 单例 Bean 的循环依赖问题，原型 Bean 则无法解决循环依赖。
6. 初始化和销毁：Spring 会在容器启动时初始化单例 Bean，并处理 Bean 的初始化和销毁方法。

	protected <T> T doGetBean(String name, @Nullable Class<T> requiredType, @Nullable Object[] args, boolean typeCheckOnly)throws BeansException {//对bean的名称进行转换 这里会去除beanName中的&String beanName = transformedBeanName(name);Object beanInstance;// Eagerly check singleton cache for manually registered singletons.//尝试从单例池中获取bean Object sharedInstance = getSingleton(beanName);//单例池中能获取到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 + "'");}}//实例化beanbeanInstance = getObjectForBeanInstance(sharedInstance, name, beanName, null);}//单例池中没有 说明是第一次获取else {// Fail if we're already creating this bean instance:// We're assumably within a circular reference.//判断是否存在循环依赖的问题if (isPrototypeCurrentlyInCreation(beanName)) {throw new BeanCurrentlyInCreationException(beanName);}// Check if bean definition exists in this factory.//得到bean工厂BeanFactory parentBeanFactory = getParentBeanFactory();//bean工厂不为空 并且在bean定义中不存在该beanNameif (parentBeanFactory != null && !containsBeanDefinition(beanName)) {// Not found -> check parent.//如果传入的name中，有&字符串，则在此处重新加上&字符串String nameToLookup = originalBeanName(name);// 如果父工厂是 AbstractBeanFactory 类型if (parentBeanFactory instanceof AbstractBeanFactory) {// 委托给父工厂执行 doGetBean 方法，查找并返回 Bean 实例return ((AbstractBeanFactory) parentBeanFactory).doGetBean(nameToLookup, requiredType, args, typeCheckOnly);}// 如果传入了构造参数 argselse if (args != null) {// 委托给父工厂并传递构造参数来获取 Bean 实例return (T) parentBeanFactory.getBean(nameToLookup, args);}// 如果指定了 requiredType 类型else if (requiredType != null) {// 委托给父工厂的标准 getBean 方法return parentBeanFactory.getBean(nameToLookup, requiredType);}else {// 委托给父工厂的标准 getBean 方法，返回该 Beanreturn (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);}//进行bean定义合并RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);//如果bean定义是抽象的，则抛出异常checkMergedBeanDefinition(mbd, beanName, args);// Guarantee initialization of beans that the current bean depends on.//找到该bean定义的DependsOn属性。//@DependsOn注解，用于标记在创建该bean时，应该先创建该bean依赖的某个beanString[] dependsOn = mbd.getDependsOn();//如果存在依赖的beanif (dependsOn != null) {for (String dep : dependsOn) {//这里是对于循环依赖的判断 即当前bean所依赖的bean，又依赖了当前bean//这种情况等价于 构造方法的循环依赖 无解 所以直接抛出异常if (isDependent(beanName, dep)) {throw new BeanCreationException(mbd.getResourceDescription(), beanName,"Circular depends-on relationship between '" + beanName + "' and '" + dep + "'");}//注册bean的依赖关系registerDependentBean(dep, beanName);try {//先去创建当前bean所依赖的beangetBean(dep);}catch (NoSuchBeanDefinitionException ex) {throw new BeanCreationException(mbd.getResourceDescription(), beanName,"'" + beanName + "' depends on missing bean '" + dep + "'", ex);}}}// 下面这一段的逻辑 根据bean不同的作用域 执行不同的操作 单例bean就直接创建 并加入单例池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);}//原型bean会创建 不会被缓存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);}//其他的作用域，走自己的初始化逻辑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);}

  其中bean的依赖关系，是存放在DefaultSingletonBeanRegistry的成员变量中：
dependentBeanMap： 当前bean被哪些bean依赖;dependenciesForBeanMap：当前bean依赖了哪些bean
  然后会调用createBean方法完成bean的创建。

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