Spring IOC加载实例化bean流程概要
Spring启动流程主要是调用AbstractApplicationContext的refresh方法完成容器的一系列初始化操作，其中有两个子流程在加载实例化bean做了很多工作，
1.obtainFreshBeanFactory方法初始化BeanFactory并加载bean定义
2.finishBeanFactoryInitialization方法完成对非抽象的&&单例的&&非懒加载的bean进行实例化
public void refresh() throws BeansException, IllegalStateException {
synchronized (this.startupShutdownMonitor) {
// 容器刷新预处理,调用initPropertySources与validateRequiredProperties进行环境属性设置与校验
prepareRefresh();
// 初始化beanfactory,创建DefaultListableBeanFactory,加载bean定义
ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();
// beanfactory预处理工作,添加设置bean类加载器,bean后置处理器,,注册beanfactory相关依赖等
prepareBeanFactory(beanFactory);
try {
// beanfactory准备工作后置处理工作
postProcessBeanFactory(beanFactory);
// 激活beanfactory后置处理器,设置注解解析器
invokeBeanFactoryPostProcessors(beanFactory);
// 注册bean后置处理器
registerBeanPostProcessors(beanFactory);
// 初始化国际化资源处理器
initMessageSource();
// 创建事件多播器
initApplicationEventMulticaster();
// 刷新特定子类bean
onRefresh();
// 事件监听器注册到多播器
registerListeners();
// 实例化剩余的单例beanfactory,创建bean,从bean定义中判断出非抽象的&&单例的&&非懒加载的bean进行实例化
finishBeanFactoryInitialization(beanFactory);
// 完成刷新过程
finishRefresh();
}
catch (BeansException ex) {
if (logger.isWarnEnabled()) {
logger.warn("Exception encountered during context initialization - " +
"cancelling refresh attempt: " + ex);
}
// Destroy already created singletons to avoid dangling resources.
destroyBeans();
// Reset 'active' flag.
cancelRefresh(ex);
// Propagate exception to caller.
throw ex;
}
finally {
// Reset common introspection caches in Spring's core, since we
// might not ever need metadata for singleton beans anymore...
resetCommonCaches();
}
}
}
Bean定义的加载
obtainFreshBeanFactory方法入口可以找到refreshBeanFactory方法，loadBeanDefinition加载bean定义，如通过xml或者注解装载成BeanDefinition对象
protected final void refreshBeanFactory() throws BeansException {
if (hasBeanFactory()) {
destroyBeans();
closeBeanFactory();
}
try {
DefaultListableBeanFactory beanFactory = createBeanFactory();
beanFactory.setSerializationId(getId());
customizeBeanFactory(beanFactory);
//在此处进行bean定义的加载
loadBeanDefinitions(beanFactory);
synchronized (this.beanFactoryMonitor) {
this.beanFactory = beanFactory;
}
}
catch (IOException ex) {
throw new ApplicationContextException("I/O error parsing bean definition source for " + getDisplayName(), ex);
}
}
获取Bean实例
1.检查单例缓存池中是否有对应实例（实例可能未完全实例化完），有则通过工厂bean缓存池或对应工厂bean方法获取；
2.没有则递归向上寻找父工厂获取bean实例；
3.再没有获取bean实例则开始创建bean实例，首先先获取所有对应的依赖bean实例（没有则实例化）
4.调用createBean方法完成对自身的创建并返回自身bean实例
protected T doGetBean(final String name, @Nullable final Class requiredType,
@Nullable final Object[] args, boolean typeCheckOnly) throws BeansException {
//获取bean的名称
final String beanName = transformedBeanName(name);
Object bean;
//检查单例对象缓存池或实例工厂是否有对应的实例
//创建单例bean的时候依赖注入存在循环依赖问题
//Spring创建bean会将bean的ObjectFactory加入到缓存中,下一个创建依赖上一个bean则直接使用
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 + "'");
}
}
//通过FactoryBean中获取bean,使用指定方法返回实例,获取bean的过程做后置处理
bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
}
else {
//只有单例的情况下才会尝试
//原型模式下 存在下列情况
//A依赖B B依赖A 当依赖注入的时候 当A未创建完 对于B的创建再次返回创建A 形成循环依赖
if (isPrototypeCurrentlyInCreation(beanName)) {
throw new BeanCurrentlyInCreationException(beanName);
}
// Check if bean definition exists in this factory.
// 判断bean定义是否存在在当前工厂,没有则向父工厂递归寻找
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 {
// No args -> delegate to standard getBean method.
return parentBeanFactory.getBean(nameToLookup, requiredType);
}
}
if (!typeCheckOnly) {
markBeanAsCreated(beanName);
}
try {
final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
checkMergedBeanDefinition(mbd, beanName, args);
//对当前依赖的bean进行初始化
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bean依赖关系
registerDependentBean(dep, beanName);
try {
//对依赖bean
getBean(dep);
}
catch (NoSuchBeanDefinitionException ex) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"'" + beanName + "' depends on missing bean '" + dep + "'", ex);
}
}
}
// 实例化mbd所有依赖对象之后,开始实例化mbd
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;
}
});
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;
}
循环依赖
在bean实例化过程中，存在着循环依赖问题（即A->B B->A），Spring只对singleton模式下setter注入方式解决循环依赖问题，其他情况则抛出BeanCurrentlyInCreationException。
spring对于singleton模式下setter注入方式下循环依赖问题做了三层缓存：
singletonFactories一级缓存,存放ObjectFactory
earlySingletonObjects二级缓存,单例对象缓存池，存在初始化中间状态，用于检测循环依赖
singletonObjects三级缓存,单例对象缓存池,存放已经实例化的单例对象
singletonsCurrentlyInCreation保存正在创建实例化的单例bean名称
在单例对象没有完全初始化前，singletonsCurrentlyInCreation会一直保存bean名称，beforeSingletonCreation方法往singletonsCurrentlyInCreation添加bean名称。
protected Object getSingleton(String beanName, boolean allowEarlyReference) {
//解决单例循环依赖问题
//从单例对象缓存池中获取实例
Object singletonObject = this.singletonObjects.get(beanName);
if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
//如果单例对象缓存池中没有该实例且该对象正在被初始化
synchronized (this.singletonObjects) {
singletonObject = this.earlySingletonObjects.get(beanName);
if (singletonObject == null && allowEarlyReference) {
//该对象还没被创建
ObjectFactory singletonFactory = this.singletonFactories.get(beanName);
if (singletonFactory != null) {
singletonObject = singletonFactory.getObject();
//earlySingletonObjects与singletonFactories互斥
this.earlySingletonObjects.put(beanName, singletonObject);
this.singletonFactories.remove(beanName);
}
}
}
}
return singletonObject;
}
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 + "'");
}
//添加中间状态，singletonsCurrentlyInCreation保存bean名称
beforeSingletonCreation(beanName);
boolean newSingleton = false;
boolean recordSuppressedExceptions = (this.suppressedExceptions == null);
if (recordSuppressedExceptions) {
this.suppressedExceptions = new LinkedHashSet();
}
try {
//singletonFactory获取单例bean
singletonObject = singletonFactory.getObject();
newSingleton = true;
}
catch (IllegalStateException ex) {
// Has the singleton object implicitly appeared in the meantime ->
// if yes, proceed with it since the exception indicates that state.
//从单例缓存池获取bean
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;
}
//移除中间状态，singletonsCurrentlyInCreation移除bean名称
afterSingletonCreation(beanName);
}
if (newSingleton) {
addSingleton(beanName, singletonObject);
}
}
return (singletonObject != NULL_OBJECT ? singletonObject : null);
}
}
创建bean实例
1.createBeanInstance创建bean实例，底层调用反射机制或者cglib完成接口对象或类对象的创建，创建策略包括工厂方法、构造函数初始化、简单初始化；
2.检测是否存在单例循环依赖
3.populateBean方法完成属性值注入填充，进行依赖注入，底层调用autowireByName方法与autowireByType方法对注解@Autowire修饰的属性进行注入，最终调用applyPropertyValues方法匹配BeanWrapper的getPropertyDescriptors完成属性值设置；
4.initializeBean完成bean初始化，底层依次调用invokeAwareMethods方法（Aware子类实现方法，例如BeanNameAware、BeanFactoryAware）、applyBeanPostProcessorsBeforeInitialization方法（BeanPostProcessor后置处理器初始化前postProcessBeforeInstantiation方法）、invokeInitMethods方法（InitializingBean接口的afterPropertiesSet方法）、applyBeanPostProcessorsAfterInitialization方法（BeanPostProcessor后置处理器初始化后postProcessAfterInitialization方法）。
5.返回创建完的bean实例
protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final @Nullable Object[] args)
throws BeanCreationException {
// Instantiate the bean.
BeanWrapper instanceWrapper = null;
if (mbd.isSingleton()) {
instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
}
if (instanceWrapper == null) {
//创建bean实例
//工厂方法,构造函数初始化,简单初始化
instanceWrapper = createBeanInstance(beanName, mbd, args);
}
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 {
applyMergedBeanDefinitionPostProcessors(mbd, beanType, beanName);
}
catch (Throwable ex) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"Post-processing of merged bean definition failed", ex);
}
mbd.postProcessed = true;
}
}
//检测循环依赖
boolean earlySingletonExposure = (mbd.isSingleton() && this.allowCircularReferences &&
isSingletonCurrentlyInCreation(beanName));
if (earlySingletonExposure) {
if (logger.isDebugEnabled()) {
logger.debug("Eagerly caching bean '" + beanName +
"' to allow for resolving potential circular references");
}
//在bean初始化完成前,将ObjectFactory放进singletonFactories,避免循环依赖
addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
}
// Initialize the bean instance.
Object exposedObject = bean;
try {
//注入属性值
populateBean(beanName, mbd, instanceWrapper);
//bean初始化
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);
}
}
if (earlySingletonExposure) {
Object earlySingletonReference = getSingleton(beanName, false);
if (earlySingletonReference != null) {
if (exposedObject == bean) {
exposedObject = earlySingletonReference;
}
else if (!this.allowRawInjectionDespiteWrapping && hasDependentBean(beanName)) {
String[] dependentBeans = getDependentBeans(beanName);
Set actualDependentBeans = new LinkedHashSet<>(dependentBeans.length);
for (String dependentBean : dependentBeans) {
if (!removeSingletonIfCreatedForTypeCheckOnly(dependentBean)) {
actualDependentBeans.add(dependentBean);
}
}
//遍历获取所有依赖的bean
//actualDependentBeans不为空 表示当前bean创建后依赖的bean没有被全部创建完,存在循环依赖
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 {
registerDisposableBeanIfNecessary(beanName, bean, mbd);
}
catch (BeanDefinitionValidationException ex) {
throw new BeanCreationException(
mbd.getResourceDescription(), beanName, "Invalid destruction signature", ex);
}
return exposedObject;
}
流程总结
整个Spring IOC容器加载实例化bean的流程大概如图所示
Spring加载实例化bean流程图.png