BeanFactory体系
BeanFactory是Spring最核心的功能，它就是我们常说的Spring IoC容器。
BeanFactory体系下包含许多接口，它们分别代表Spring IoC容器的不同功能：
BeanFactory：提供最基础的获取bean信息的方法，如getBean()。HierarchicalBeanFactory：提供父子层级Spring容器的基础方法，如getParentBeanFactory()。AutowireCapableBeanFactory：提供实例化、自动装配等基础功能，如createBean()。ListableBeanFactory：提供枚举所有bean的功能，如getBeansOfType()。ConfigurableBeanFactory：提供配置Spring容器的基础功能，是BeanFactory接口的补充。如addBeanPostProcessor()。ConfigurableListableBeanFactory：提供获取和修改BeanDefinition、预实例化单例对象的功能。如getBeanDefinition()。ApplicationContext：应用层Spring容器的顶级接口。BeanDefinitionRegistry：提供注册BeanDefinition的功能，如registerBeanDefinition()。不是BeanFactory的子接口，但它是BeanFactory体系的核心组成部分。
虽然BeanFactory体系的接口众多，但是它们的核心实现类只有DefaultListableBeanFactory。
我们只需要按照DefaultListableBeanFactory的基本使用流程，掌握其中的关键性方法的源码，就能够很好的理解Spring IoC容器。
在对Spring IoC容器有了整体的认识后，再去针对性研究它提供的特性功能，就能够完全掌握Spring IoC容器。
2 DefaultListableBeanFactory
DefaultListableBeanFactory的成员变量很多，这里介绍其中最核心的：
beanDefinitionMap：BeanDefinition的缓存，key是beanName。mergedBeanDefinitions：合并后的BeanDefinition缓存。singletonFactories：单例bean的缓存，保存创建后且依赖注入前的单例对象。earlySingletonObjects：单例bean的缓存，保存依赖注入且回调完的单例对象。singletonObjects：单例bean的缓存，保存最终的单例对象。
DefaultListableBeanFactory中最核心的流程（方法）包括：
- 注册
BeanDefinition - 创建
bean - 获取
bean
2.1 注册BeanDefinition
注册BeanDefinition是BeanDefinitionRegistry接口提供的方法，DefaultListableBeanFactory对其进行了实现。
注册BeanDefinition的过程主要是将其保存到beanDefinitionMap缓存中，其中beanName作为key：
private final Map<String, BeanDefinition> beanDefinitionMap = new ConcurrentHashMap<>(256);
考虑到beanName可能被重复注册多次的情况。一方面，要对此做出限制，主要是通过allowBeanDefinitionOverriding属性；另一方面，在覆盖时需要将旧的beanDefinition衍生出的各种缓存清除，保证数据的一致性。
与beanDefinition相关的缓存包括（没有全部列出）：
mergedBeanDefinitions、mergedBeanDefinitionHolderssingletonObjects、singletonFactories、earlySingletonObjects、registeredSingletons、disposableBeans、dependentBeanMap
DefaultListableBeanFactory#registerBeanDefinition()：
public void registerBeanDefinition(String beanName, BeanDefinition beanDefinition)
throws BeanDefinitionStoreException {
// 校验beanDefinition信息的完整性
if (beanDefinition instanceof AbstractBeanDefinition) {
try {
((AbstractBeanDefinition) beanDefinition).validate();
}
catch (BeanDefinitionValidationException ex) {
throw new BeanDefinitionStoreException(beanDefinition.getResourceDescription(), beanName, "Validation of bean definition failed", ex);
}
}
// 获取缓存
BeanDefinition existingDefinition = this.beanDefinitionMap.get(beanName);
// 如果该beanDefinition已经注册
if (existingDefinition != null) {
// 如果beanFactory不允许beanDefinition重载，抛出异常
if (!isAllowBeanDefinitionOverriding()) {
throw new BeanDefinitionOverrideException(beanName, beanDefinition, existingDefinition);
}
// 如果beanFactory允许beanDefinition重载，进行覆盖（默认会进行覆盖）
this.beanDefinitionMap.put(beanName, beanDefinition);
}
// 如果该beanDefinition未注册
else {
// 如果beanFactory已经创建bean
if (hasBeanCreationStarted()) {
// 加锁，更新beanDefinitionMap缓存
synchronized (this.beanDefinitionMap) {
this.beanDefinitionMap.put(beanName, beanDefinition);
List<String> updatedDefinitions = new ArrayList<>(this.beanDefinitionNames.size() + 1);
updatedDefinitions.addAll(this.beanDefinitionNames);
updatedDefinitions.add(beanName);
this.beanDefinitionNames = updatedDefinitions;
removeManualSingletonName(beanName);
}
}
// 如果beanFactory还没有创建bean，仍处于注册bean阶段
else {
// 直接更新beanDefinitionMap缓存
this.beanDefinitionMap.put(beanName, beanDefinition);
this.beanDefinitionNames.add(beanName);
removeManualSingletonName(beanName);
}
this.frozenBeanDefinitionNames = null;
}
// 如果beanDefinition已存在，或单例bean已存在，需要清除相关缓存信息：mergedBeanDefinitions、singletonObjects等
if (existingDefinition != null || containsSingleton(beanName)) {
resetBeanDefinition(beanName);
}
// 如果beanDefinition不存在，并且单例bean不存在，并且beanFactory会缓存所有beanDefinition的元数据
else if (isConfigurationFrozen()) {
// 清除allBeanNamesByType和singletonBeanNamesByType
clearByTypeCache();
}
}
2.2 创建bean
创建bean的底层方法位于AbstractAutowireCapableBeanFactory#createBean()。
protected Object createBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args) throws BeanCreationException {
RootBeanDefinition mbdToUse = mbd;
// 解析bean对应的beanClass对象，用于后面的实例化
Class<?> resolvedClass = resolveBeanClass(mbd, beanName);
if (resolvedClass != null && !mbd.hasBeanClass() && mbd.getBeanClassName() != null) {
mbdToUse = new RootBeanDefinition(mbd);
mbdToUse.setBeanClass(resolvedClass);
}
// Prepare method overrides：校验lookup方法是否存在
try {
mbdToUse.prepareMethodOverrides();
}
catch (BeanDefinitionValidationException ex) {
throw new BeanDefinitionStoreException(mbdToUse.getResourceDescription(),
beanName, "Validation of method overrides failed", ex);
}
try {
// 触发InstantiationAwareBeanPostProcessor#postProcessBeforeInstantiation()和BeanPostProcessor#postProcessAfterInitialization()方法
Object bean = resolveBeforeInstantiation(beanName, mbdToUse);
// 如果通过InstantiationAwareBeanPostProcessor#postProcessBeforeInstantiation()方法实例化对象，就直接返回了
if (bean != null) {
return bean;
}
}
catch (Throwable ex) {
throw new BeanCreationException(mbdToUse.getResourceDescription(), beanName,
"BeanPostProcessor before instantiation of bean failed", ex);
}
try {
// 使用BeanFactory默认策略实例化对象：使用instanceSupplier、factoryMethodName或构造函数
Object beanInstance = doCreateBean(beanName, mbdToUse, args);
return beanInstance;
}
catch (BeanCreationException | ImplicitlyAppearedSingletonException ex) {
throw ex;
}
catch (Throwable ex) {
throw new BeanCreationException(
mbdToUse.getResourceDescription(), beanName, "Unexpected exception during bean creation", ex);
}
}
2.2.1 解析beanClass
为了创建bean，首先要知道它的beanClass是什么。
解析常规beanClass的底层方法位于AbstractBeanDefinition#resolveBeanClass()。简单来说，它会根据beanClassName加载对应的类对象，并且缓存起来：
public Class<?> resolveBeanClass(@Nullable ClassLoader classLoader) throws ClassNotFoundException {
String className = getBeanClassName();
if (className == null) {
return null;
}
Class<?> resolvedClass = ClassUtils.forName(className, classLoader);
this.beanClass = resolvedClass;
return resolvedClass;
}
2.2.2 实例化前的回调
在创建bean之前，会先触发InstantiationAwareBeanPostProcessor#postProcessBeforeInstantiation()和BeanPostProcessor#postProcessAfterInitialization()方法回调：
protected Object resolveBeforeInstantiation(String beanName, RootBeanDefinition mbd) {
Object bean = null;
if (!Boolean.FALSE.equals(mbd.beforeInstantiationResolved)) {
// Make sure bean class is actually resolved at this point.
if (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {
Class<?> targetType = determineTargetType(beanName, mbd);
if (targetType != null) {
// InstantiationAwareBeanPostProcessor#postProcessBeforeInstantiation()回调
bean = applyBeanPostProcessorsBeforeInstantiation(targetType, beanName);
if (bean != null) {
// BeanPostProcessor#postProcessAfterInitialization()
bean = applyBeanPostProcessorsAfterInitialization(bean, beanName);
}
}
}
mbd.beforeInstantiationResolved = (bean != null);
}
return bean;
}
在这个阶段，会触发常见的InstantiationAwareBeanPostProcessor实现类如下：
AbstractAutoProxyCreator：将符合条件的bean用AOP代理封装起来，并指定代理的拦截器。
如果在这个阶段创建了bean，那么会直接返回，不会继续执行后续创建bean操作。
2.2.3 创建bean
AbstractAutowireCapableBeanFactory#doCreateBean()会根据RootBeanDefinition的信息进行创建对象。
简单来说，包括以下步骤：
- 通过
instanceSupplier、工厂方法和构造函数等方式创建对象。‘ - 触发
MergedBeanDefinitionPostProcessor#postProcessMergedBeanDefinition()回调。 InstantiationAwareBeanPostProcessor#postProcessAfterInstantiation()回调。InstantiationAwareBeanPostProcessor#postProcessProperties()回调- 依赖注入。
- 触发
BeanNameAware、BeanClassLoaderAware和BeanFactoryAware回调。 - 触发
BeanPostProcessor#postProcessBeforeInitialization()回调。 - 触发
InitializingBean#afterPropertiesSet()回调。 - 触发
initMethod回调。 - 触发
BeanPostProcessor#postProcessAfterInitialization()回调。
AbstractAutowireCapableBeanFactory#doCreateBean()：
protected Object doCreateBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args) throws BeanCreationException {
BeanWrapper instanceWrapper = null;
if (mbd.isSingleton()) {
// 如果factoryBeanInstanceCache有，直接取
instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
}
if (instanceWrapper == null) {
/* 创建bean：
1、instanceSupplier#get()方法创建
2、factoryMethod工厂方法创建
3、有参构造函数创建
4、无参构造函数创建
*/
instanceWrapper = createBeanInstance(beanName, mbd, args);
}
Object bean = instanceWrapper.getWrappedInstance();
Class<?> beanType = instanceWrapper.getWrappedClass();
if (beanType != NullBean.class) {
mbd.resolvedTargetType = beanType;
}
// 触发MergedBeanDefinitionPostProcessor#postProcessMergedBeanDefinition()回调
synchronized (mbd.postProcessingLock) {
if (!mbd.postProcessed) {
try {
applyMergedBeanDefinitionPostProcessors(mbd, beanType, beanName);
}
catch (Throwable ex) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"Post-processing of merged bean definition failed", ex);
}
mbd.postProcessed = true;
}
}
// Eagerly cache singletons to be able to resolve circular references
// even when triggered by lifecycle interfaces like BeanFactoryAware.
boolean earlySingletonExposure = (mbd.isSingleton() && this.allowCircularReferences && isSingletonCurrentlyInCreation(beanName));
if (earlySingletonExposure) {
// 如果是提前暴露的单例bean：缓存单例对象：singletonFactories和registeredSingletons
addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
}
// 实例化bean
Object exposedObject = bean;
try {
/*
1、InstantiationAwareBeanPostProcessor#postProcessAfterInstantiation()回调
2、InstantiationAwareBeanPostProcessor#postProcessProperties()回调
3、依赖注入
*/
populateBean(beanName, mbd, instanceWrapper);
/*
1、触发BeanNameAware、BeanClassLoaderAware和BeanFactoryAware回调
2、触发BeanPostProcessor#postProcessBeforeInitialization()回调
3、触发InitializingBean#afterPropertiesSet()回调
4、触发initMethod回调
5、触发BeanPostProcessor#postProcessAfterInitialization()回调
*/
exposedObject = initializeBean(beanName, exposedObject, mbd);
}
catch (Throwable ex) {
}
// 如果是提前暴露的单例bean
if (earlySingletonExposure) {
// 从缓存中获取：singletonObjects、earlySingletonObjects、singletonFactories
Object earlySingletonReference = getSingleton(beanName, false);
if (earlySingletonReference != null) {
if (exposedObject == bean) {
exposedObject = earlySingletonReference;
}
else if (!this.allowRawInjectionDespiteWrapping && hasDependentBean(beanName)) {
// 清除依赖bean的缓存
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();
}
}
}
}
// Register bean as disposable.
try {
registerDisposableBeanIfNecessary(beanName, bean, mbd);
}
catch (BeanDefinitionValidationException ex) {
throw new BeanCreationException(
mbd.getResourceDescription(), beanName, "Invalid destruction signature", ex);
}
return exposedObject;
}
需要注意的是，由于存在循环依赖的情况：
- 在创建
bean后，依赖注入前，会将未完全实例化的bean信息缓存到singletonFactories。 - 在依赖注入并且执行完回调方法之后，会将完全实例化的
bean信息缓存到earlySingletonObjects中，并移除singletonFactories中的缓存。
2.3 获取bean
获取bean的底层方法位于AbstractBeanFactory#doGetBean()方法，主要包括以下步骤：
- 解析
beanName。 - 从三级缓存中获取
bean，如果存在则直接返回。 - 合并
BeanDefinition。 - 根据作用域创建
bean。
protected <T> T doGetBean(
String name, @Nullable Class<T> requiredType, @Nullable Object[] args, boolean typeCheckOnly)
throws BeansException {
// 获取最原始的beanName：别名处理、"&"前缀（factoryBean）
String beanName = transformedBeanName(name);
Object beanInstance;
// 依次从获取singletonObjects、earlySingletonObjects和singletonFactories三级缓存中获取（解决循环依赖）
Object sharedInstance = getSingleton(beanName);
if (sharedInstance != null && args == null) {
// 已存在，直接返回
beanInstance = getObjectForBeanInstance(sharedInstance, name, beanName, null);
}
else {
// Fail if we're already creating this bean instance:
// We're assumably within a circular reference.
if (isPrototypeCurrentlyInCreation(beanName)) {
throw new BeanCurrentlyInCreationException(beanName);
}
// 先从父容器中获取
BeanFactory parentBeanFactory = getParentBeanFactory();
if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {
// Not found -> check parent.
String 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);
}
}
// 标记已创建
if (!typeCheckOnly) {
markBeanAsCreated(beanName);
}
try {
// 合并BeanDefinition
RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
checkMergedBeanDefinition(mbd, beanName, args);
// Guarantee initialization of beans that the current bean depends on.
String[] dependsOn = mbd.getDependsOn();
if (dependsOn != null) {
for (String dep : dependsOn) {
if (isDependent(beanName, dep)) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"Circular depends-on relationship between '" + beanName + "' and '" + dep + "'");
}
// 注册依赖的bean
registerDependentBean(dep, beanName);
try {
// 创建依赖的bean
getBean(dep);
}
catch (NoSuchBeanDefinitionException ex) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"'" + beanName + "' depends on missing bean '" + dep + "'", ex);
}
}
}
// 创建单例bean，保存到singletonObjects、registeredSingletons缓存，从singletonFactories、earlySingletonObjects移除
if (mbd.isSingleton()) {
sharedInstance = getSingleton(beanName, () -> {
try {
return createBean(beanName, mbd, args);
}
catch (BeansException ex) {
destroySingleton(beanName);
throw ex;
}
});
beanInstance = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
}
// 创建prototype的bean
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);
}
// 创建自定义作用域的bean
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 {
// 根据自定义作用域的规则创建bean
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) {
cleanupAfterBeanCreationFailure(beanName);
throw ex;
}
finally {
beanCreation.end();
}
}
return adaptBeanInstance(name, beanInstance, requiredType);
}