Spring IoC源码解析之getBean
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一、实例化所有的非懒加载的单实例Bean
从org.springframework.context.support.AbstractApplicationContext#refresh方法开发，进入到实例化所有的非懒加载的单实例Bean的finishBeanFactoryInitialization(beanFactory)的方法：
protected void finishBeanFactoryInitialization(ConfigurableListableBeanFactory beanFactory) {
// 为Bean工厂设置类型转化器
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(new StringValueResolver() {
@Override
public String resolveStringValue(String strVal) {
return 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);
//冻结所有的Bean定义 ， 至此注册的Bean定义将不被修改或任何进一步的处理
beanFactory.freezeConfiguration();
//实例化剩下的单实例Bean
beanFactory.preInstantiateSingletons();
}
进入实例化剩下的单实例Bean的beanFactory.preInstantiateSingletons()的方法：
public void preInstantiateSingletons() throws BeansException {
if (logger.isDebugEnabled()) {
logger.debug("Pre-instantiating singletons in " + this);
}
//获取我们容器中所有Bean定义的名称
List<String> beanNames = new ArrayList<>(this.beanDefinitionNames);
//循环我们所有的bean定义名称
for (String beanName : beanNames) {
//合并我们的bean定义
RootBeanDefinition bd = getMergedLocalBeanDefinition(beanName);
//根据bean定义判断是不是抽象的 && 不是单例的 && 不是懒加载的
if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) {
//是否FactoryBean
if (isFactoryBean(beanName)) {
//是 给beanName+前缀 & 符号
Object bean = getBean(FACTORY_BEAN_PREFIX + beanName);
if (bean instanceof FactoryBean) {
final 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());
}
//调用真正的getBean
if (isEagerInit) {
getBean(beanName);
}
}
}
else {//非工厂Bean就是普通的bean
getBean(beanName);
}
}
}
//获取所有的bean的名称 至此所有的单实例的bean已经加入到单实例Bean的缓存池中，所谓的单实例缓存池实际上就是一个ConcurrentHashMap
for (String beanName : beanNames) {
//从单例缓存池中获取所有的对象
Object singletonInstance = getSingleton(beanName);
//判断当前的bean是否实现了SmartInitializingSingleton接口
if (singletonInstance instanceof SmartInitializingSingleton) {
final SmartInitializingSingleton smartSingleton = (SmartInitializingSingleton) singletonInstance;
if (System.getSecurityManager() != null) {
AccessController.doPrivileged((PrivilegedAction<Object>) () -> {
smartSingleton.afterSingletonsInstantiated();
return null;
}, getAccessControlContext());
}
else {
//触发实例化之后的方法afterSingletonsInstantiated
smartSingleton.afterSingletonsInstantiated();
}
}
}
}
前面多个地方涉及到getBean，接下来就分析下getBean（很重要）
二、getBean流程
进入getBean(beanName)的方法：
@Override
public Object getBean(String name) throws BeansException {
//真正的获取Bean的逻辑
return doGetBean(name, null, null, false);
}
该方法啥没干，又交给了干活的doGetBean(name, null, null, false)方法：
protected <T> T doGetBean(final String name, @Nullable final Class<T> requiredType,
@Nullable final Object[] args, boolean typeCheckOnly) throws BeansException {
//在这里传入进来的name可能是别名、也有可能是工厂beanName,所以在这里需要转换
final String beanName = transformedBeanName(name);
Object bean;
//先尝试去缓存中获取对象
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 + "'");
}
}
/**
* 如果sharedInstance是普通的单例bean，下面的方法会直接返回。但如果
* sharedInstance是FactoryBean类型的，则需调用getObject工厂方法获取真正的
* bean实例。如果用户想获取 FactoryBean 本身，这里也不会做特别的处理，直接返回
* 即可。毕竟 FactoryBean 的实现类本身也是一种 bean，只不过具有一点特殊的功能而已。
*/
bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
}
else {
//Spring只能解决单例对象的setter注入的循环依赖,不能解决构造器注入，也不能解决多实例的循环依赖
if (isPrototypeCurrentlyInCreation(beanName)) {
throw new BeanCurrentlyInCreationException(beanName);
}
//判断是否有父工厂
BeanFactory parentBeanFactory = getParentBeanFactory();
//若存在父工厂,切当前的bean工厂不存在当前的bean定义,那么bean定义是存在于父beanFactory中
if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {
//获取bean的原始名称
String nameToLookup = originalBeanName(name);
//若为AbstractBeanFactory类型，委托父类处理
if (parentBeanFactory instanceof AbstractBeanFactory) {
return ((AbstractBeanFactory) parentBeanFactory).doGetBean(
nameToLookup, requiredType, args, typeCheckOnly);
}
else if (args != null) {
// 委托给构造函数getBean()处理
return (T) parentBeanFactory.getBean(nameToLookup, args);
}
else {
// 没有args，委托给标准的getBean()处理
return parentBeanFactory.getBean(nameToLookup, requiredType);
}
}
/**
* 方法参数typeCheckOnly ，是用来判断调用getBean(...) 方法时，表示是否为仅仅进行类型检查获取Bean对象
* 如果不是仅仅做类型检查，而是创建Bean对象，则需要调用markBeanAsCreated(String beanName) 方法，进行记录
*/
if (!typeCheckOnly) {
markBeanAsCreated(beanName);
}
try {
//从容器中获取beanName相应的GenericBeanDefinition对象，并将其转换为RootBeanDefinition对象
final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
//检查当前创建的bean定义是不是抽象的bean定义
checkMergedBeanDefinition(mbd, beanName, args);
//处理dependsOn的依赖(这个不是我们所谓的循环依赖 而是bean创建前后的依赖)
//依赖bean的名称
String[] dependsOn = mbd.getDependsOn();
if (dependsOn != null) {
for (String dep : dependsOn) {
//beanName是当前正在创建的bean,dep是正在创建的bean的依赖的bean的名称
if (isDependent(beanName, dep)) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"Circular depends-on relationship between '" + beanName + "' and '" + dep + "'");
}
//保存的是依赖beanName之间的映射关系：依赖beanName -> beanName的集合
registerDependentBean(dep, beanName);
try {
//获取dependsOn的bean
getBean(dep);
}
catch (NoSuchBeanDefinitionException ex) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"'" + beanName + "' depends on missing bean '" + dep + "'", ex);
}
}
}
//是单例则创建单例Bean
if (mbd.isSingleton()) {
//把beanName和一个singletonFactory匿名内部类传入用于回调
sharedInstance = getSingleton(beanName, () -> {
try {
//创建bean的逻辑
return createBean(beanName, mbd, args);
}
catch (BeansException ex) {
//创建bean的过程中发生异常,需要销毁关于当前bean的所有信息
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();
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, () -> {
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.isDebugEnabled()) {
logger.debug("Failed to convert bean '" + name + "' to required type '" +
ClassUtils.getQualifiedName(requiredType) + "'", ex);
}
throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
}
}
return (T) bean;
}
第一步：先尝试去缓存中获取对象
Object sharedInstance = getSingleton(beanName)，由于第一次肯定为空:
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);
}
进入到getSingleton(beanName, true)方法，这里涉及到Spring的三级缓存，用它来解决循环依赖：
protected Object getSingleton(String beanName, boolean allowEarlyReference) {
/**
* 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)
* Spring 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);
/**
* 若在第一级缓存中没有获取到对象,并且singletonsCurrentlyInCreation正在创建的单实例的list包含该beanName
* Spring IoC容器初始化加载单实例bean的时候第一次进来的时候 该list中一般返回空,但是循环依赖的时候可以满足该条件
*/
if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
synchronized (this.singletonObjects) {
/**
* 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，还没给bean的属性进行赋值的对象就是早期对象
*/
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) {
/**
* 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getBean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);
//从三级缓存中获取到对象不为空
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);
}
}
}
}
return singletonObject;
}
第二步：走else逻辑
Spring只能解决单例对象的setter注入的循环依赖,不能解决构造器注入，也不能解决多实例的循环依赖，所以会抛Bean当前正在创建的异常，接着判断是否有父工厂，有就调用父工厂的getBean，如果不是仅仅做类型检查，而是创建Bean对象，则需要调用markBeanAsCreated方法进行标识
合并Bean定义信息
① 接着调用getMergedLocalBeanDefinition(beanName)进行Bean定义的合并方法如下：
protected RootBeanDefinition getMergedLocalBeanDefinition(String beanName) throws BeansException {
// 快速从缓存中获取，如果不为空，则直接返回
RootBeanDefinition mbd = this.mergedBeanDefinitions.get(beanName);
if (mbd != null) {
return mbd;
}
//获取 RootBeanDefinition 对象。若获取的 BeanDefinition 为子 BeanDefinition，则需要合并父类的相关属性.
return getMergedBeanDefinition(beanName, getBeanDefinition(beanName));
}
② 进入到getMergedBeanDefinition(beanName, getBeanDefinition(beanName))方法：
protected RootBeanDefinition getMergedBeanDefinition(String beanName, BeanDefinition bd)
throws BeanDefinitionStoreException {
//调用重载的方法
return getMergedBeanDefinition(beanName, bd, null);
}
③ 进入到getMergedBeanDefinition(beanName, bd, null)的方法：
protected RootBeanDefinition getMergedBeanDefinition(
String beanName, BeanDefinition bd, @Nullable BeanDefinition containingBd)
throws BeanDefinitionStoreException {
//加锁
synchronized (this.mergedBeanDefinitions) {
RootBeanDefinition mbd = null;
// Check with full lock now in order to enforce the same merged instance.
if (containingBd == null) {
mbd = this.mergedBeanDefinitions.get(beanName);
}
if (mbd == null) {
//bd.getParentName() == null，表明无父配置，这时直接将当前的BeanDefinition升级为RootBeanDefinition
if (bd.getParentName() == null) {
//直接把原始的bean定义升级为RootBeanDefinition
if (bd instanceof RootBeanDefinition) {
mbd = ((RootBeanDefinition) bd).cloneBeanDefinition();
}
else {
//包裹为RootBeanDefinition
mbd = new RootBeanDefinition(bd);
}
}
//有父定义
else {
BeanDefinition pbd;
try {
/*
* 判断父类beanName与子类beanName名称是否相同。若相同，则父类bean一定
* 在父容器中。原因也很简单，容器底层是用Map缓存<beanName, bean> 键值对
* 的。同一个容器下，使用同一个 beanName 映射两个bean实例显然是不合适的
*/
String parentBeanName = transformedBeanName(bd.getParentName());
if (!beanName.equals(parentBeanName)) {
pbd = getMergedBeanDefinition(parentBeanName);
}
else {
/*
* 这里再次调用getMergedBeanDefinition，只不过参数值变为了
* parentBeanName，用于合并父BeanDefinition 和爷爷辈的
* BeanDefinition。如果爷爷辈的BeanDefinition仍有父
* BeanDefinition，则继续合并
*/
BeanFactory parent = getParentBeanFactory();
if (parent instanceof ConfigurableBeanFactory) {
pbd = ((ConfigurableBeanFactory) parent).getMergedBeanDefinition(parentBeanName);
}
else {
throw new NoSuchBeanDefinitionException(parentBeanName,
"Parent name '" + parentBeanName + "' is equal to bean name '" + beanName +
"': cannot be resolved without an AbstractBeanFactory parent");
}
}
}
catch (NoSuchBeanDefinitionException ex) {
throw new BeanDefinitionStoreException(bd.getResourceDescription(), beanName,
"Could not resolve parent bean definition '" + bd.getParentName() + "'", ex);
}
//以父BeanDefinition的配置信息为基本创建RootBeanDefinition
mbd = new RootBeanDefinition(pbd);
//用子BeanDefinition中的属性覆盖父BeanDefinition中的属性
mbd.overrideFrom(bd);
}
// 如果用户未配置scope属性，则默认将该属性配置为singleton
if (!StringUtils.hasLength(mbd.getScope())) {
mbd.setScope(RootBeanDefinition.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());
}
//缓存合并后的BeanDefinition
if (containingBd == null && isCacheBeanMetadata()) {
this.mergedBeanDefinitions.put(beanName, mbd);
}
}
return mbd;
}
}
④ 然后检查当前创建的bean定义是不是抽象的bean定义，checkMergedBeanDefinition(mbd, beanName, args)方法：
protected void checkMergedBeanDefinition(RootBeanDefinition mbd, String beanName, @Nullable Object[] args)
throws BeanDefinitionStoreException {
//抽象的bean定义是不能够被实例化的
if (mbd.isAbstract()) {
throw new BeanIsAbstractException(beanName);
}
}
处理dependsOn的依赖(这个不是我们所谓的循环依赖 而是bean创建前后的依赖)
① 进入到isDependent(beanName, dependentBeanName, null)的方法：
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;
}
② 进入到保存的是依赖beanName之间的映射关系：依赖beanName -> beanName的集合registerDependentBean(dep, beanName)的方法：
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<>(8));
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<>(8));
dependenciesForBean.add(canonicalName);
}
}
③ 然后到获取dependsOn的Bean的getBean(dep)，就获取到了dependsOn的Bean了；
处理单实例Bean
① 把beanName和一个singletonFactory匿名内部类传入用于回调的getSingleton方法：
public Object getSingleton(String beanName, ObjectFactory<?> singletonFactory) {
Assert.notNull(beanName, "Bean name must not be null");
//加锁
synchronized (this.singletonObjects) {
//尝试从单例缓存池中获取对象
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 + "'");
}
/**
* 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);
boolean newSingleton = false;
boolean recordSuppressedExceptions = (this.suppressedExceptions == null);
if (recordSuppressedExceptions) {
this.suppressedExceptions = new LinkedHashSet<>();
}
try {
//创建bean这个过程其实是调用 createBean() 方法
singletonObject = singletonFactory.getObject();
newSingleton = true;
}
catch (IllegalStateException ex) {
//回调我们singletonObjects的get方法,进行正在的创建bean的逻辑
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;
}
//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);
}
if (newSingleton) {
//加入缓存中
addSingleton(beanName, singletonObject);
}
}
return singletonObject;
}
}
第一：先到单例缓存池中获取对象获取，有就返回没有就继续往下
第二：调用标记当前的bean马上就要被创建了beforeSingletonCreation(beanName)方法：
protected void beforeSingletonCreation(String beanName) {
//若singletonsCurrentlyInCreation没有 则添加成功
if (!this.inCreationCheckExclusions.contains(beanName) && !this.singletonsCurrentlyInCreation.add(beanName)) {
throw new BeanCurrentlyInCreationException(beanName);
}
第三：创建Bean的singletonFactory.getObject()其实是调用createBean()方法：
protected Object createBean(String beanName, RootBeanDefinition mbd, Object[] args) throws BeanCreationException {
if (logger.isDebugEnabled()) {
logger.debug("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.
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.
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);
}
Object beanInstance = doCreateBean(beanName, mbdToUse, args);
if (logger.isDebugEnabled()) {
logger.debug("Finished creating instance of bean '" + beanName + "'");
}
return beanInstance;
}
真正的创建我们的bean的实例对象的doCreateBean(beanName, mbdToUse, args)的方法：
protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final @Nullable Object[] args)
throws BeanCreationException {
//BeanWrapper是对Bean的包装，其接口中所定义的功能很简单包括设置获取被包装的对象，获取被包装bean的属性描述器
BeanWrapper instanceWrapper = null;
if (mbd.isSingleton()) {
//从没有完成的FactoryBean中移除
instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
}
if (instanceWrapper == null) {
//使用合适的实例化策略来创建新的实例：工厂方法、构造函数自动注入、简单初始化 比较复杂也很重要
instanceWrapper = createBeanInstance(beanName, mbd, args);
}
//从beanWrapper中获取我们的早期对象
final Object bean = instanceWrapper.getWrappedInstance();
Class<?> beanType = instanceWrapper.getWrappedClass();
if (beanType != NullBean.class) {
mbd.resolvedTargetType = beanType;
}
// Allow post-processors to modify the merged bean definition.
synchronized (mbd.postProcessingLock) {
if (!mbd.postProcessed) {
try {
//进行后置处理@AutoWired的注解的预解析
applyMergedBeanDefinitionPostProcessors(mbd, beanType, beanName);
}
catch (Throwable ex) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"Post-processing of merged bean definition failed", ex);
}
mbd.postProcessed = true;
}
}
/**
* 该对象进行判断是否能够暴露早期对象的条件
* 单实例 this.allowCircularReferences 默认为true
* isSingletonCurrentlyInCreation(表示当前的bean对象正在创建singletonsCurrentlyInCreation包含当前正在创建的bean)
*/
boolean earlySingletonExposure = (mbd.isSingleton() && this.allowCircularReferences &&
isSingletonCurrentlyInCreation(beanName));
//上述条件满足，允许中期暴露对象
if (earlySingletonExposure) {
if (logger.isDebugEnabled()) {
logger.debug("Eagerly caching bean '" + beanName +
"' to allow for resolving potential circular references");
}
//把我们的早期对象包装成一个singletonFactory对象 该对象提供了一个getObject方法,该方法内部调用getEarlyBeanReference方法
addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
}
// Initialize the bean instance.
Object exposedObject = bean;
try {
//给我们的属性进行赋值(调用set方法进行赋值)
populateBean(beanName, mbd, instanceWrapper);
//进行对象初始化操作(在这里可能生成代理对象)
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);
}
}
//允许早期对象的引用
if (earlySingletonExposure) {
/**
* 去缓存中获取到我们的对象 由于传递的allowEarlyReference 是false 要求只能在一级二级缓存中去获取
* 正常普通的bean(不存在循环依赖的bean) 创建的过程中，压根不会把三级缓存提升到二级缓存中
*/
Object earlySingletonReference = getSingleton(beanName, false);
//能够获取到
if (earlySingletonReference != null) {
//经过后置处理的bean和早期的bean引用还相等的话(表示当前的bean没有被代理过)
if (exposedObject == bean) {
exposedObject = earlySingletonReference;
}
//处理依赖的bean
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 " +
"'getBeanNamesOfType' with the 'allowEagerInit' flag turned off, for example.");
}
}
}
}
// Register bean as disposable.
try {
//注册销毁的bean的销毁接口
registerDisposableBeanIfNecessary(beanName, bean, mbd);
}
catch (BeanDefinitionValidationException ex) {
throw new BeanCreationException(
mbd.getResourceDescription(), beanName, "Invalid destruction signature", ex);
}
return exposedObject;
}
① 实例化：使用合适的实例化策略来创建新的实例：工厂方法、构造函数自动注入、简单初始化的createBeanInstance方法：
protected BeanWrapper createBeanInstance(String beanName, RootBeanDefinition mbd, @Nullable Object[] args) {
//从bean定义中解析出当前bean的class对象
Class<?> beanClass = resolveBeanClass(mbd, beanName);
//检测类的访问权限。默认情况下，对于非 public 的类，是允许访问的。若禁止访问，这里会抛出异常
if (beanClass != null && !Modifier.isPublic(beanClass.getModifiers()) && !mbd.isNonPublicAccessAllowed()) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"Bean class isn't public, and non-public access not allowed: " + beanClass.getName());
}
Supplier<?> instanceSupplier = mbd.getInstanceSupplier();
if (instanceSupplier != null) {
return obtainFromSupplier(instanceSupplier, beanName);
}
//工厂方法,我们通过配置类来进行配置的话 采用的就是工厂方法
if (mbd.getFactoryMethodName() != null) {
return instantiateUsingFactoryMethod(beanName, mbd, args);
}
//判断当前构造函数是否被解析过
boolean resolved = false;
//有没有必须进行依赖注入
boolean autowireNecessary = false;
/**
* 通过getBean传入进来的构造函数是否来指定需要推断构造函数
* 若传递进来的args不为空，那么就可以直接选出对应的构造函数
*/
if (args == null) {
synchronized (mbd.constructorArgumentLock) {
//判断我们的bean定义信息中的resolvedConstructorOrFactoryMethod(用来缓存我们的已经解析的构造函数或者工厂方法)
if (mbd.resolvedConstructorOrFactoryMethod != null) {
//修改已经解析过的构造函数的标志
resolved = true;
//修改标记为ture 标识构造函数或者工厂方法已经解析过
autowireNecessary = mbd.constructorArgumentsResolved;
}
}
}
//若被解析过
if (resolved) {
if (autowireNecessary) {
//通过有参的构造函数进行反射调用
return autowireConstructor(beanName, mbd, null, null);
}
else {
//调用无参数的构造函数进行创建对象
return instantiateBean(beanName, mbd);
}
}
//通过bean的后置处理器进行选举出合适的构造函数对象
Constructor<?>[] ctors = determineConstructorsFromBeanPostProcessors(beanClass, beanName);
//通过后置处理器解析出构造器对象不为null或获取bean定义中的注入模式是构造器注入或bean定义信息ConstructorArgumentValues或获取通过getBean的方式传入的构造器函数参数类型不为null
if (ctors != null || mbd.getResolvedAutowireMode() == AUTOWIRE_CONSTRUCTOR ||
mbd.hasConstructorArgumentValues() || !ObjectUtils.isEmpty(args)) {
//通过构造函数创建对象
return autowireConstructor(beanName, mbd, ctors, args);
}
//使用无参数的构造函数调用创建对象
return instantiateBean(beanName, mbd);
}
判断是否暴露早期对象条件满足就暴露早期对象，把我们的早期对象包装成一个singletonFactory对象 该对象提供了一个getObject方法,该方法内部调用getEarlyBeanReference方法：
protected Object getEarlyBeanReference(String beanName, RootBeanDefinition mbd, Object bean) {
Object exposedObject = bean;
//判读我们容器中是否有InstantiationAwareBeanPostProcessors类型的后置处理器
if (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {
//获取我们所有的后置处理器
for (BeanPostProcessor bp : getBeanPostProcessors()) {
//判断我们的后置处理器是不是实现了SmartInstantiationAwareBeanPostProcessor接口
if (bp instanceof SmartInstantiationAwareBeanPostProcessor) {
//进行强制转换
SmartInstantiationAwareBeanPostProcessor ibp = (SmartInstantiationAwareBeanPostProcessor) bp;
//挨个调用SmartInstantiationAwareBeanPostProcessor的getEarlyBeanReference
exposedObject = ibp.getEarlyBeanReference(exposedObject, beanName);
}
}
}
return exposedObject;
}
addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean))方法：
/**
* 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) {
Assert.notNull(singletonFactory, "Singleton factory must not be null");
//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);
this.earlySingletonObjects.remove(beanName);
this.registeredSingletons.add(beanName);
}
}
}
②属性赋值：给我们的属性进行赋值(调用set方法进行赋值)populateBean(beanName, mbd, instanceWrapper)方法：
/**
*给我们的对象BeanWrapper属性赋值
* @param beanName bean的名称
* @param mbd bean的定义
* @param bw bean实例包装对象
*/
protected void populateBean(String beanName, RootBeanDefinition mbd, @Nullable BeanWrapper bw) {
//若bw为null的话,则说明对象没有实例化
if (bw == null) {
//进入if说明对象有属性，bw为空,不能为他设置属性，那就在下面就执行抛出异常
if (mbd.hasPropertyValues()) {
throw new BeanCreationException(
mbd.getResourceDescription(), beanName, "Cannot apply property values to null instance");
}
else {
// Skip property population phase for null instance.
return;
}
}
/**
* 在属性被填充前，给 InstantiationAwareBeanPostProcessor 类型的后置处理器一个修改
* bean 状态的机会。官方的解释是：让用户可以自定义属性注入。比如用户实现一
* 个 InstantiationAwareBeanPostProcessor 类型的后置处理器，并通过
* postProcessAfterInstantiation 方法向 bean 的成员变量注入自定义的信息。
*当时我们发现系统中的的InstantiationAwareBeanPostProcessor.postProcessAfterInstantiationM没有进行任何处理，
*若我们自己实现了这个接口 可以自定义处理.....spring 留给我们自己扩展接口的
*特殊需求，直接使用配置中的信息注入即可。
*/
boolean continueWithPropertyPopulation = true;
//是否持有 InstantiationAwareBeanPostProcessor
if (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {
//获取容器中的所有的BeanPostProcessor
for (BeanPostProcessor bp : getBeanPostProcessors()) {
//判断我们的后置处理器是不是InstantiationAwareBeanPostProcessor
if (bp instanceof InstantiationAwareBeanPostProcessor) {
//进行强制转化
InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp;
//若存在后置处理器给我们属性赋值了，那么返回false 可以来修改我们的开关变量，就不会走下面的逻辑了
if (!ibp.postProcessAfterInstantiation(bw.getWrappedInstance(), beanName)) {
// 返回值为是否继续填充 bean
// postProcessAfterInstantiation：如果应该在 bean上面设置属性则返回 true，否则返回 false
// 一般情况下，应该是返回true 。
// 返回 false 的话，将会阻止在此 Bean 实例上调用任何后续的 InstantiationAwareBeanPostProcessor 实
continueWithPropertyPopulation = false;
break;
}
}
}
}
// 如果后续处理器发出停止填充命令，则终止后续操作
if (!continueWithPropertyPopulation) {
return;
}
//获取bean定义的属性
PropertyValues pvs = (mbd.hasPropertyValues() ? mbd.getPropertyValues() : null);
/**
* 判断我们的bean的属性注入模型
* AUTOWIRE_BY_NAME 根据名称注入
* AUTOWIRE_BY_TYPE 根据类型注入
*/
if (mbd.getResolvedAutowireMode() == AUTOWIRE_BY_NAME || mbd.getResolvedAutowireMode() == AUTOWIRE_BY_TYPE) {
//把PropertyValues封装成为MutablePropertyValues
MutablePropertyValues newPvs = new MutablePropertyValues(pvs);
//根据bean的属性名称注入
if (mbd.getResolvedAutowireMode() == AUTOWIRE_BY_NAME) {
autowireByName(beanName, mbd, bw, newPvs);
}
//根据bean的类型进行注入
if (mbd.getResolvedAutowireMode() == AUTOWIRE_BY_TYPE) {
autowireByType(beanName, mbd, bw, newPvs);
}
//把处理过的 属性覆盖原来的
pvs = newPvs;
}
/**
* 这里又是一种后置处理，用于在 Spring 填充属性到 bean 对象前，对属性的值进行相应的处理，
* 比如可以修改某些属性的值。这时注入到 bean 中的值就不是配置文件中的内容了，
* 而是经过后置处理器修改后的内容
*/
boolean hasInstAwareBpps = hasInstantiationAwareBeanPostProcessors();
//判断是否需要检查依赖
boolean needsDepCheck = (mbd.getDependencyCheck() != AbstractBeanDefinition.DEPENDENCY_CHECK_NONE);
if (hasInstAwareBpps || needsDepCheck) {
if (pvs == null) {
pvs = mbd.getPropertyValues();
}
//提出当前正在创建的beanWrapper 依赖的对象
PropertyDescriptor[] filteredPds = filterPropertyDescriptorsForDependencyCheck(bw, mbd.allowCaching);
if (hasInstAwareBpps) {
//获取所有的后置处理器
for (BeanPostProcessor bp : getBeanPostProcessors()) {
if (bp instanceof InstantiationAwareBeanPostProcessor) {
InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp;
//对依赖对象进行后置处理
pvs = ibp.postProcessPropertyValues(pvs, filteredPds, bw.getWrappedInstance(), beanName);
if (pvs == null) {
return;
}
}
}
}
//判断是否检查依赖
if (needsDepCheck) {
checkDependencies(beanName, mbd, filteredPds, pvs);
}
}
/**
* 其实，上面只是完成了所有注入属性的获取，将获取的属性封装在 PropertyValues 的实例对象 pvs 中，
* 并没有应用到已经实例化的 bean 中。而 #applyPropertyValues(String beanName, BeanDefinition mbd, BeanWrapper bw, PropertyValues pvs) 方法，
* 则是完成这一步骤的
*/
if (pvs != null) {
applyPropertyValues(beanName, mbd, bw, pvs);
}
}
③初始化：进行对象初始化操作调用initializeBean，这里会进行Aware接口进行方法的回调，然后调用Bean的后置处理器的Before方法（postProcessorsBeforeInitialization），然后Bean的初始化方法，最后调用Bean的后置处理器的After方法（PostProcessorsAfterInitialization）
protected Object initializeBean(final String beanName, final Object bean, @Nullable RootBeanDefinition mbd) {
if (System.getSecurityManager() != null) {
AccessController.doPrivileged((PrivilegedAction<Object>) () -> {
invokeAwareMethods(beanName, bean);
return null;
}, getAccessControlContext());
}
else {
//若我们的Bean实现了Aware接口进行方法的回调
invokeAwareMethods(beanName, bean);
}
Object wrappedBean = bean;
if (mbd == null || !mbd.isSynthetic()) {
//调用我们的bean的后置处理器的postProcessorsBeforeInitialization方法
wrappedBean = applyBeanPostProcessorsBeforeInitialization(wrappedBean, beanName);
}
try {
//调用初始化方法
invokeInitMethods(beanName, wrappedBean, mbd);
}
catch (Throwable ex) {
throw new BeanCreationException(
(mbd != null ? mbd.getResourceDescription() : null),
beanName, "Invocation of init method failed", ex);
}
if (mbd == null || !mbd.isSynthetic()) {
//调用我们bean的后置处理器的PostProcessorsAfterInitialization方法 动态代理就是在这里实现的
wrappedBean = applyBeanPostProcessorsAfterInitialization(wrappedBean, beanName);
}
return wrappedBean;
}
进入调用初始化方法invokeInitMethods(beanName, wrappedBean, mbd)方法：
protected void invokeInitMethods(String beanName, final Object bean, @Nullable RootBeanDefinition mbd)
throws Throwable {
//判断我们的容器中是否实现了InitializingBean接口
boolean isInitializingBean = (bean instanceof InitializingBean);
if (isInitializingBean && (mbd == null || !mbd.isExternallyManagedInitMethod("afterPropertiesSet"))) {
if (logger.isDebugEnabled()) {
logger.debug("Invoking afterPropertiesSet() on bean with name '" + beanName + "'");
}
if (System.getSecurityManager() != null) {
try {
AccessController.doPrivileged((PrivilegedExceptionAction<Object>) () -> {
((InitializingBean) bean).afterPropertiesSet();
return null;
}, getAccessControlContext());
}
catch (PrivilegedActionException pae) {
throw pae.getException();
}
}
else {
//回调InitializingBean的afterPropertiesSet()方法
((InitializingBean) bean).afterPropertiesSet();
}
}
if (mbd != null && bean.getClass() != NullBean.class) {
//我们beanClass中看是否有自己定义的init方法
String initMethodName = mbd.getInitMethodName();
//判断自定义的init方法名称不叫afterPropertiesSet
if (StringUtils.hasLength(initMethodName) &&
!(isInitializingBean && "afterPropertiesSet".equals(initMethodName)) &&
!mbd.isExternallyManagedInitMethod(initMethodName)) {
//调用我们自己的初始化方法
invokeCustomInitMethod(beanName, bean, mbd);
}
}
}
至此doCreateBean创建完成然后返回
接下来bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd)，为什么不直接等于sharedInstance，原因可能刚刚创建的Bean是FactoryBean类型的Bean，如果是就要调用getObject方法来获取真正的Bean，运用场景就是那些创建Bean的逻辑比较复杂的情况下可以用这个，比如Spring整合Mybatis的SqlSessionFactoryBean
getObjectForBeanInstance(sharedInstance, name, beanName, mbd)方法：
protected Object getObjectForBeanInstance(
Object beanInstance, String name, String beanName, @Nullable RootBeanDefinition mbd) {
// 如果name以&开头，但beanInstance却不是FactoryBean，则认为有问题。
if (BeanFactoryUtils.isFactoryDereference(name)) {
if (beanInstance instanceof NullBean) {
return beanInstance;
}
if (!(beanInstance instanceof FactoryBean)) {
throw new BeanIsNotAFactoryException(beanName, beanInstance.getClass());
}
}
/**
* 如果上面的判断通过了，表明 beanInstance 可能是一个普通的 bean，也可能是一个
* FactoryBean。如果是一个普通的 bean，这里直接返回 beanInstance 即可。如果是
* FactoryBean，则要调用工厂方法生成一个 bean 实例。
*/
if (!(beanInstance instanceof FactoryBean) || BeanFactoryUtils.isFactoryDereference(name)) {
return beanInstance;
}
Object object = null;
if (mbd == null) {
/**
* 如果 mbd 为空，则从缓存中加载 bean。FactoryBean 生成的单例 bean 会被缓存
* 在 factoryBeanObjectCache 集合中，不用每次都创建
*/
object = getCachedObjectForFactoryBean(beanName);
}
if (object == null) {
// 经过前面的判断，到这里可以保证beanInstance是 FactoryBean类型的，所以可以进行类型转换
FactoryBean<?> factory = (FactoryBean<?>) beanInstance;
// 如果 mbd 为空，则判断是否存在名字为 beanName 的 BeanDefinition
if (mbd == null && containsBeanDefinition(beanName)) {
mbd = getMergedLocalBeanDefinition(beanName);
}
//synthetic 字面意思是"合成的"。通过全局查找，我发现在 AOP 相关的类中会将该属性设为 true。
//所以我觉得该字段可能表示某个 bean 是不是被 AOP 增强过，也就是 AOP 基于原始类合成了一个新的代理类。
//不过目前只是猜测，没有深究
boolean synthetic = (mbd != null && mbd.isSynthetic());
//调用 getObjectFromFactoryBean 方法继续获取实例
object = getObjectFromFactoryBean(factory, beanName, !synthetic);
}
return object;
}
接着调用object = getObjectFromFactoryBean(factory, beanName, !synthetic)方法：
protected Object getObjectFromFactoryBean(FactoryBean<?> factory, String beanName, boolean shouldPostProcess) {
/**
* FactoryBean 也有单例和非单例之分，针对不同类型的 FactoryBean，这里有两种处理方式：
* 1. 单例 FactoryBean 生成的 bean 实例也认为是单例类型。需放入缓存中，供后续重复使用
* 2. 非单例 FactoryBean 生成的 bean 实例则不会被放入缓存中，每次都会创建新的实例
**/
if (factory.isSingleton() && containsSingleton(beanName)) {
synchronized (getSingletonMutex()) {
//从缓存中取bean实例，避免多次创建bean实例
Object object = this.factoryBeanObjectCache.get(beanName);
if (object == null) {
//使用工厂对象中创建实例
object = doGetObjectFromFactoryBean(factory, beanName);
// Only post-process and store if not put there already during getObject() call above
// (e.g. because of circular reference processing triggered by custom getBean calls)
Object alreadyThere = this.factoryBeanObjectCache.get(beanName);
if (alreadyThere != null) {
object = alreadyThere;
}
else {
if (shouldPostProcess) {
//判断当地的bean是否正在创建
if (isSingletonCurrentlyInCreation(beanName)) {
// Temporarily return non-post-processed object, not storing it yet..
return object;
}
beforeSingletonCreation(beanName);
try {
object = postProcessObjectFromFactoryBean(object, beanName);
}
catch (Throwable ex) {
throw new BeanCreationException(beanName,
"Post-processing of FactoryBean's singleton object failed", ex);
}
finally {
afterSingletonCreation(beanName);
}
}
// 这里的beanName对应于FactoryBean的实现类，FactoryBean的实现类也会被实例化，并被缓存在singletonObjects中
if (containsSingleton(beanName)) {
// 这里的beanName对应于FactoryBean的实现类，FactoryBean的实现类也会被实例化，并被缓存在singletonObjects中
this.factoryBeanObjectCache.put(beanName, object);
}
}
}
return object;
}
}
else {
Object object = doGetObjectFromFactoryBean(factory, beanName);
if (shouldPostProcess) {
try {
object = postProcessObjectFromFactoryBean(object, beanName);
}
catch (Throwable ex) {
throw new BeanCreationException(beanName, "Post-processing of FactoryBean's object failed", ex);
}
}
return object;
}
}
使用工厂对象中创建实例object = doGetObjectFromFactoryBean(factory, beanName)方法：
private Object doGetObjectFromFactoryBean(final FactoryBean<?> factory, final String beanName)
throws BeanCreationException {
Object object;
try {
if (System.getSecurityManager() != null) {
AccessControlContext acc = getAccessControlContext();
try {
object = AccessController.doPrivileged((PrivilegedExceptionAction<Object>) factory::getObject, acc);
}
catch (PrivilegedActionException pae) {
throw pae.getException();
}
}
else {
//真正的调用工厂bean的getObject()方法
object = factory.getObject();
}
}
catch (FactoryBeanNotInitializedException ex) {
throw new BeanCurrentlyInCreationException(beanName, ex.toString());
}
catch (Throwable ex) {
throw new BeanCreationException(beanName, "FactoryBean threw exception on object creation", ex);
}
// Do not accept a null value for a FactoryBean that's not fully
// initialized yet: Many FactoryBeans just return null then.
if (object == null) {
if (isSingletonCurrentlyInCreation(beanName)) {
throw new BeanCurrentlyInCreationException(
beanName, "FactoryBean which is currently in creation returned null from getObject");
}
object = new NullBean();
}
return object;
}
处理多实例Bean
mbd.isPrototype()，每次使用的时候再创建，因为不会加入到单实例缓冲池中，也就无法解决循环依赖问题。
至此doGetBean创建完成，返回，getBean创建完成
最后获取所有的bean的名称（至此所有的单实例的bean已经加入到单实例Bean的缓存池中，所谓的单实例缓存池实际上就是一个ConcurrentHashMap），遍历所有的bean名称，根据beanName从单例缓存池中获取所有的对象，然后判断是否是SmartInitializingSingleton类型，是再触发实例化之后的方法afterSingletonsInstantiated；
完整的Spring IoC源码解析见：Spring系列（三）：Spring IoC源码解析
三、getBean流程图
① getBean流程图：
② @AutoWired注入属性和set方法注入流程图：
总结：通过对Spring IoC的getBean流程分析，了解了Bean的创建过程，先到单实例缓存池(ConcurrentHashMap)中获取，取不到就调用createBean创建，创建成功后加入到单实例缓存池(ConcurrentHashMap)中，下次获取的时候直接从JVM级别的缓存中获取，Bean的生命周期，以及如何用三级缓存解决循环依赖，还有就是多实例为什么不能解决循环依赖问题。
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