本文章我们会解读一下Spring如何根据beanDefinition创建bean的；

代码入口：

AnnotationConfigApplicationContext applicationContext = new AnnotationConfigApplicationContext(AppConfig.class);applicationContext.refresh();

当spring执行refresh()这个方法进行上下文刷新时， 会进行非懒加载的bean的创建，创建非懒加载的bean的源码入口为:

finishBeanFactoryInitialization(beanFactory);

 该方法的具体实现：

/*** Finish the initialization of this context's bean factory,* initializing all remaining singleton beans.*/protected void finishBeanFactoryInitialization(ConfigurableListableBeanFactory beanFactory) {// Initialize conversion service for this context.// 如果BeanFactory中存在名字叫conversionService的Bean,则设置为BeanFactory的conversionService属性// ConversionService是用来进行类型转化的if (beanFactory.containsBean(CONVERSION_SERVICE_BEAN_NAME) &&beanFactory.isTypeMatch(CONVERSION_SERVICE_BEAN_NAME, ConversionService.class)) {beanFactory.setConversionService(beanFactory.getBean(CONVERSION_SERVICE_BEAN_NAME, ConversionService.class));}// Register a default embedded value resolver if no BeanFactoryPostProcessor// (such as a PropertySourcesPlaceholderConfigurer bean) registered any before:// at this point, primarily for resolution in annotation attribute values.// 设置默认的占位符解析器  ${xxx}  ---keyif (!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.// 实例化非懒加载的单例BeanbeanFactory.preInstantiateSingletons();}

上述方法中，进行实例化非懒加载的单例bean的代码是

		// 实例化非懒加载的单例BeanbeanFactory.preInstantiateSingletons();

点击进入

DefaultListableBeanFactory.java的public void preInstantiateSingletons() 的方法；

@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.List<String> beanNames = new ArrayList<>(this.beanDefinitionNames);// Trigger initialization of all non-lazy singleton beans...for (String beanName : beanNames) {// 获取合并后的BeanDefinitionRootBeanDefinition bd = getMergedLocalBeanDefinition(beanName);//bd.isAbstract() 是不是抽象的BeanDefinition  一般是xml定义的  抽象的beanDefinition不会被创建if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) {if (isFactoryBean(beanName)) {// 获取FactoryBean对象Object bean = getBean(FACTORY_BEAN_PREFIX + beanName);if (bean instanceof FactoryBean) {FactoryBean<?> factory = (FactoryBean<?>) bean;boolean isEagerInit;if (System.getSecurityManager() != null && factory instanceof SmartFactoryBean) {isEagerInit = AccessController.doPrivileged((PrivilegedAction<Boolean>) ((SmartFactoryBean<?>) factory)::isEagerInit,getAccessControlContext());}else {isEagerInit = (factory instanceof SmartFactoryBean &&((SmartFactoryBean<?>) factory).isEagerInit());}if (isEagerInit) {// 创建真正的Bean对象(getObject()返回的对象)getBean(beanName);}}}else {// 创建Bean对象getBean(beanName);}}}// 所有的非懒加载单例Bean都创建完了后// Trigger post-initialization callback for all applicable beans...for (String beanName : beanNames) {Object singletonInstance = getSingleton(beanName);if (singletonInstance instanceof SmartInitializingSingleton) {StartupStep smartInitialize = this.getApplicationStartup().start("spring.beans.smart-initialize").tag("beanName", beanName);SmartInitializingSingleton smartSingleton = (SmartInitializingSingleton) singletonInstance;if (System.getSecurityManager() != null) {AccessController.doPrivileged((PrivilegedAction<Object>) () -> {smartSingleton.afterSingletonsInstantiated();return null;}, getAccessControlContext());}else {smartSingleton.afterSingletonsInstantiated();}smartInitialize.end();}}}

此方法中 getBean(beanName);会创建bean;我们进入getBean（)这个方法

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

然后我们进入doGetBean(name, null, null, false)这个方法，主要判断该bean的类型然后根据具体情况进行调用protected Object createBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args) 创建bean的实例。

/*** 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 {//对beanName做处理// name有可能是 &xxx 或者 xxx，如果name是&xxx，那么beanName就是xxx// name有可能传入进来的是别名，那么beanName就是idString beanName = transformedBeanName(name);Object beanInstance;// Eagerly check singleton cache for manually registered singletons.//先在单例池中去获取beanObject sharedInstance = getSingleton(beanName);//如果获取到了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 + "'");}}// 如果获取到的bean(sharedInstance)是FactoryBean，需要对该bean进行进一步处理beanInstance = getObjectForBeanInstance(sharedInstance, name, beanName, null);}else {	//如果在单例池中没有获取到bean// Fail if we're already creating this bean instance:// We're assumably within a circular reference.if (isPrototypeCurrentlyInCreation(beanName)) {throw new BeanCurrentlyInCreationException(beanName);}// Check if bean definition exists in this factory.BeanFactory parentBeanFactory = getParentBeanFactory();//containsBeanDefinition(beanName) 检测spring容器中有没有这个名字的bean//当前容器中没有找到，去父工厂找if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {// Not found -> check parent.// &&&&xxx---->&xxxString 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);}//JFR代码运行机制StartupStep beanCreation = this.applicationStartup.start("spring.beans.instantiate").tag("beanName", name);try {if (requiredType != null) {beanCreation.tag("beanType", requiredType::toString);}//取出名字对应的合并后的beanDefinitionRootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);// 检查BeanDefinition是不是Abstract的,如果是Abstract  报错checkMergedBeanDefinition(mbd, beanName, args);// Guarantee initialization of beans that the current bean depends on.//创建当前bean之前，先需要创建@dependsOn里面的beanString[] dependsOn = mbd.getDependsOn();if (dependsOn != null) {// dependsOn表示当前beanName所依赖的，当前Bean创建之前dependsOn所依赖的Bean必须已经创建好了for (String dep : dependsOn) {// beanName是不是被dep依赖了，如果是则出现了循环依赖if (isDependent(beanName, dep)) {throw new BeanCreationException(mbd.getResourceDescription(), beanName,"Circular depends-on relationship between '" + beanName + "' and '" + dep + "'");}// dep被beanName依赖了，存入dependentBeanMap中，dep为key，beanName为valueregisterDependentBean(dep, beanName);// 创建所依赖的beantry {getBean(dep);}catch (NoSuchBeanDefinitionException ex) {throw new BeanCreationException(mbd.getResourceDescription(), beanName,"'" + beanName + "' depends on missing bean '" + dep + "'", ex);}}}// Create bean instance.if (mbd.isSingleton()) {// 创建单例bean的逻辑: 先从缓存中拿，如果没有的，再创建该bean,并且存入单例缓存中sharedInstance = getSingleton(beanName, () -> {try {return createBean(beanName, mbd, args);}catch (BeansException ex) {// Explicitly remove instance from singleton cache: It might have been put there// eagerly by the creation process, to allow for circular reference resolution.// Also remove any beans that received a temporary reference to the bean.destroySingleton(beanName);throw ex;}});beanInstance = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);}else if (mbd.isPrototype()) {//创建原型bean的逻辑:直接创建对象// It's a prototype -> create a new instance.Object prototypeInstance = null;try {beforePrototypeCreation(beanName);prototypeInstance = createBean(beanName, mbd, args);}finally {afterPrototypeCreation(beanName);}beanInstance = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);}else {//有作用域相关注解的bean  例如request、session等获取scope作用域注解信息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 {  // session.getAttriute(beaName)  setAttri//类似非懒加载单例bean的创建，先从缓存中取，取不到，创建bean且放入缓存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();}}// 检查通过name所获得到的beanInstance的类型是否是requiredTypereturn adaptBeanInstance(name, beanInstance, requiredType);}

进入protected Object createBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args) ，这里会进行bean的实例化。

/*** Central method of this class: creates a bean instance,* populates the bean instance, applies post-processors, etc.* @see #doCreateBean*/@Overrideprotected Object createBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args)throws BeanCreationException {if (logger.isTraceEnabled()) {logger.trace("Creating instance of bean '" + beanName + "'");}RootBeanDefinition mbdToUse = mbd;// Make sure bean class is actually resolved at this point, and// clone the bean definition in case of a dynamically resolved Class// which cannot be stored in the shared merged bean definition.// 马上就要实例化Bean了，确保beanClass被加载了，进行类加载Class<?> resolvedClass = resolveBeanClass(mbd, beanName);if (resolvedClass != null && !mbd.hasBeanClass() && mbd.getBeanClassName() != null) {mbdToUse = new RootBeanDefinition(mbd);mbdToUse.setBeanClass(resolvedClass);}// Prepare method overrides.try {mbdToUse.prepareMethodOverrides();}catch (BeanDefinitionValidationException ex) {throw new BeanDefinitionStoreException(mbdToUse.getResourceDescription(),beanName, "Validation of method overrides failed", ex);}try {// Give BeanPostProcessors a chance to return a proxy instead of the target bean instance.// 实例化前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);}}