本文深入探讨了Spring框架中Bean的生命周期，从创建BeanDefinition开始，包括lookup-method和replace-method检测，InstantiationAwareBeanPostProcessor的触发，再到doCreateBean()方法的详细步骤，如创建Bean实例、MergedBeanDefinitionPostProcessor、早期引用对象的创建、属性解析、初始化、暴露早期引用对象以及注册销毁方法。同时，文章提供了Bean生命周期的图示，帮助读者更好地理解整个过程。
文章目录
一、Bean生命周期源码
BeanFactory获取Bean的时候，如果缓存中没有并且父Bean工厂也没有的时候会执行创建Bean的操作，主要逻辑是在AbstractAutowireCapableBeanFactory的createBean()方法中：
@Override
protected Object createBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args)
throws BeanCreationException {
if (logger.isTraceEnabled()) {
logger.trace("Creating instance of bean '" + beanName + "'");
}
// 1. 创建BeanDefinition
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.
//2. lookup-method和replace-method检测
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.
//3. InstantiationAwareBeanPostProcessor触发
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 {
//4. 调用doCreateBean()方法创建Bean
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);
}
}
1. 创建BeanDefinition
首先是调用resolveBeanClass()方法保证bean的类型已被加载。
Class<?> resolvedClass = resolveBeanClass(mbd, beanName);
if (resolvedClass != null && !mbd.hasBeanClass() && mbd.getBeanClassName() != null) {
mbdToUse = new RootBeanDefinition(mbd);
mbdToUse.setBeanClass(resolvedClass);
}
2. lookup-method和replace-method检测
mbdToUse.prepareMethodOverrides();
此部分用于检测lookup-method标签配置和replace-method配置的方法是否存在。如果不存在直接标记一下,省去了对方法参数的解析过程,直接调用即可。
3. InstantiationAwareBeanPostProcessor触发
这里会给后置处理器通过修改BeanDefinition偷换Bean的机会：
// Give BeanPostProcessors a chance to return a proxy instead of the target bean instance.
Object bean = resolveBeforeInstantiation(beanName, mbdToUse);
if (bean != null) {
return bean;
}
看一下resolveBeforeInstantiation()方法，在这里触发的是其postProcessBeforeInitialization和postProcessAfterInstantiation方法。：
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) {
bean = applyBeanPostProcessorsBeforeInstantiation(targetType, beanName);
if (bean != null) {
bean = applyBeanPostProcessorsAfterInitialization(bean, beanName);
}
}
}
mbd.beforeInstantiationResolved = (bean != null);
}
return bean;
}
从这里可以看出，如果InstantiationAwareBeanPostProcessor返回的不是空，那么将不会继续执行剩下的Spring初始化流程，此接口用于初始化自定义的bean，主要是在Spring内部使用。也就是说，如果这里bean已经被偷换了，那么直接返回，不用执行下面的doCreateBean()方法了。
4. 调用doCreateBean()方法创建Bean
如果前面的Bean没有被修改，那么执行真正的创建Bean的方法doCreateBean()：
Object beanInstance = doCreateBean(beanName, mbdToUse, args);
return beanInstance;
protected Object doCreateBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args)
throws BeanCreationException {
// Instantiate the bean.
BeanWrapper instanceWrapper = null;
if (mbd.isSingleton()) {
instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
}
if (instanceWrapper == null) {
instanceWrapper = createBeanInstance(beanName, mbd, args);
}
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;
}
}
// 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) {
if (logger.isTraceEnabled()) {
logger.trace("Eagerly caching bean '" + beanName +
"' to allow for resolving potential circular references");
}
addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
}
// Initialize the bean instance.
Object exposedObject = bean;
try {
populateBean(beanName, mbd, instanceWrapper);
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<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 " +
"'getBeanNamesForType' 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;
}
4.1 创建Bean实例
this.factoryBeanInstanceCache.remove(beanName) 先移除缓存中的同名。
factoryBeanInstanceCache是一个ConcurrentMap：ConcurrentMap<String, BeanWrapper> factoryBeanInstanceCache = new ConcurrentHashMap<>();
// Instantiate the bean.
BeanWrapper instanceWrapper = null;
if (mbd.isSingleton()) {
instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
}
if (instanceWrapper == null) {
instanceWrapper = createBeanInstance(beanName, mbd, args);
}
然后调用createBeanInstance(beanName, mbd, args)创建bean实例：
protected BeanWrapper createBeanInstance(String beanName, RootBeanDefinition mbd, @Nullable Object[] args) {
// Make sure bean class is actually resolved at this point.
Class<?> beanClass = resolveBeanClass(mbd, beanName);
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);
}
if (mbd.getFactoryMethodName() != null) {
return instantiateUsingFactoryMethod(beanName, mbd, args);
}
// Shortcut when re-creating the same bean...
boolean resolved = false;
boolean autowireNecessary = false;
if (args == null) {
synchronized (mbd.constructorArgumentLock) {
if (mbd.resolvedConstructorOrFactoryMethod != null) {
resolved = true;
autowireNecessary = mbd.constructorArgumentsResolved;
}
}
}
if (resolved) {
if (autowireNecessary) {
return autowireConstructor(beanName, mbd, null, null);
}
else {
return instantiateBean(beanName, mbd);
}
}
// Candidate constructors for autowiring?
Constructor<?>[] ctors = determineConstructorsFromBeanPostProcessors(beanClass, beanName);
if (ctors != null || mbd.getResolvedAutowireMode() == AUTOWIRE_CONSTRUCTOR ||
mbd.hasConstructorArgumentValues() || !ObjectUtils.isEmpty(args)) {
return autowireConstructor(beanName, mbd, ctors, args);
}
// Preferred constructors for default construction?
ctors = mbd.getPreferredConstructors();
if (ctors != null) {
return autowireConstructor(beanName, mbd, ctors, null);
}
// No special handling: simply use no-arg constructor.
return instantiateBean(beanName, mbd);
}
createBeanInstance的创建过程又分为以下几种情况:
（1）工厂bean:
调用instantiateUsingFactoryMethod方法:
protected BeanWrapper instantiateUsingFactoryMethod(
String beanName, RootBeanDefinition mbd, Object[] explicitArgs) {
return new ConstructorResolver(this).instantiateUsingFactoryMethod(beanName, mbd, explicitArgs);
}
（2）构造器自动装配
// Need to determine the constructor...
Constructor<?>[] ctors = determineConstructorsFromBeanPostProcessors(beanClass, beanName);
if (ctors != null ||
mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_CONSTRUCTOR ||
//配置了<constructor-arg>子元素
mbd.hasConstructorArgumentValues() || !ObjectUtils.isEmpty(args)) {
return autowireConstructor(beanName, mbd, ctors, args);
}
先根据determineConstructorsFromBeanPostProcessors()方法决定构造函数，determineConstructorsFromBeanPostProcessors()源码：
protected Constructor<?>[] determineConstructorsFromBeanPostProcessors(Class<?> beanClass, String beanName) {
if (beanClass != null && hasInstantiationAwareBeanPostProcessors()) {
for (BeanPostProcessor bp : getBeanPostProcessors()) {
if (bp instanceof SmartInstantiationAwareBeanPostProcessor) {
SmartInstantiationAwareBeanPostProcessor ibp =
(SmartInstantiationAwareBeanPostProcessor) bp;
Constructor<?>[] ctors = ibp.determineCandidateConstructors(beanClass, beanName);
if (ctors != null) {
return ctors;
}
}
}
}
return null;
}
可见是由SmartInstantiationAwareBeanPostProcessor决定的，默认是没有配置这种东西的。
之后就是判断bean的自动装配模式，可以通过如下方式配置:
<bean id="student" class="base.Student" primary="true" autowire="default" />
autowire共有以下几种选项:
- no: 默认的，不进行自动装配。在这种情况下，只能通过ref方式引用其它bean。
- byName: 根据bean里面属性的名字在BeanFactory中进行查找并装配。
- byType: 按类型。
- constructor: 以byType的方式查找bean的构造参数列表。
- default: 由父bean决定。
然后调用autowireConstructor()返回bean实例，autowireConstructor调用的是ConstructorResolver.autowireConstructor()，此方法主要做了两件事:
- 得到合适的构造器对象。
- 根据构造器参数的类型去BeanFactory查找相应的bean。
（3）默认的无参构造函数
一行代码，很简单:
// No special handling: simply use no-arg constructor.
return instantiateBean(beanName, mbd);
4.2 MergedBeanDefinitionPostProcessor
调用applyMergedBeanDefinitionPostProcessors() 方法合并BeanDefinition的后置处理器。
// 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;
}
}
4.3 创建早期引用对象
调用addSingletonFactory()方法，暴露bean的EarlyBeanReferenced，主要是为了循环引用，比如a中包含b，b中包含a，这样可以先把引用暴露出来，下面才会调用注入。这里添加一个ObjectFactory作为bean的生产方式。但是注意这里的bean其实并没有进行属性注入。
boolean earlySingletonExposure = (mbd.isSingleton() && this.allowCircularReferences &&
isSingletonCurrentlyInCreation(beanName));
if (earlySingletonExposure) {
if (logger.isTraceEnabled()) {
logger.trace("Eagerly caching bean '" + beanName +
"' to allow for resolving potential circular references");
}
addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
}
addSingletonFactory()方法：
protected void addSingletonFactory(String beanName, ObjectFactory<?> singletonFactory) {
Assert.notNull(singletonFactory, "Singleton factory must not be null");
synchronized (this.singletonObjects) {
if (!this.singletonObjects.containsKey(beanName)) {
this.singletonFactories.put(beanName, singletonFactory);
this.earlySingletonObjects.remove(beanName);
this.registeredSingletons.add(beanName);
}
}
}
getEarlyBeanReference()方法：
protected Object getEarlyBeanReference(String beanName, RootBeanDefinition mbd, Object bean) {
Object exposedObject = bean;
if (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {
for (SmartInstantiationAwareBeanPostProcessor bp : getBeanPostProcessorCache().smartInstantiationAware) {
exposedObject = bp.getEarlyBeanReference(exposedObject, beanName);
}
}
return exposedObject;
}
4.4 属性解析
调用populateBean()方法，填充bean中的引用内容，包括注解的字段，spel表达式等等。
protected void populateBean(String beanName, RootBeanDefinition mbd, BeanWrapper bw) {
//所有<property>的值
PropertyValues pvs = mbd.getPropertyValues();
if (mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_BY_NAME ||
mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_BY_TYPE) {
MutablePropertyValues newPvs = new MutablePropertyValues(pvs);
// Add property values based on autowire by name if applicable.
if (mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_BY_NAME) {
autowireByName(beanName, mbd, bw, newPvs);
}
// Add property values based on autowire by type if applicable.
if (mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_BY_TYPE) {
autowireByType(beanName, mbd, bw, newPvs);
}
pvs = newPvs;
}
//设值
applyPropertyValues(beanName, mbd, bw, pvs);
}
autowireByName()方法:
protected void autowireByName(
String beanName, AbstractBeanDefinition mbd, BeanWrapper bw, MutablePropertyValues pvs) {
//返回所有引用(ref="XXX")的bean名称
String[] propertyNames = unsatisfiedNonSimpleProperties(mbd, bw);
for (String propertyName : propertyNames) {
if (containsBean(propertyName)) {
//从BeanFactory获取
Object bean = getBean(propertyName);
pvs.add(propertyName, bean);
registerDependentBean(propertyName, beanName);
}
}
}
autowireByType也是同样的套路，所以可以得出结论: autowireByName和autowireByType方法只是先获取到引用的bean，真正的设值是在applyPropertyValues()中进行的。
Spring判断一个属性可不可以被设置(存不存在)是通过java bean的内省操作来完成的，也就是说，属性可以被设置的条件是此属性拥有public的setter方法，并且注入时的属性名应该是setter的名字。
4.5 初始化
此处的initializeBean()方法初始化指的是bean已经构造完成，执行诸如调用其init方法的操作，init-method，@post-process注解(JSR标准)，initializingBean接口，PostProcessor的前后bean初始化方法等等。
protected Object initializeBean(final String beanName, final Object bean, RootBeanDefinition mbd) {
if (System.getSecurityManager() != null) {
AccessController.doPrivileged(new PrivilegedAction<Object>() {
@Override
public Object run() {
invokeAwareMethods(beanName, bean); //（1）Aware方法触发
return null;
}
}, getAccessControlContext());
}
else {
invokeAwareMethods(beanName, bean);
}
Object wrappedBean = bean;
if (mbd == null || !mbd.isSynthetic()) {
//（2）初始化前置BeanPostProcessor触发
wrappedBean = applyBeanPostProcessorsBeforeInitialization(wrappedBean, beanName);
}
//（3）调用init方法:
invokeInitMethods(beanName, wrappedBean, mbd);
if (mbd == null || !mbd.isSynthetic()) {
//（4）初始化后置BeanPostProcessor触发
wrappedBean = applyBeanPostProcessorsAfterInitialization(wrappedBean, beanName);
}
return wrappedBean;
}
主要的操作步骤一目了然。
（1）Aware方法触发
调用inokeAwareMethods()方法，实现beanFactory相关的通知，也就是aware方法。BeanNameAware、BeanFactoryAware、BeanClassLoaderAware。
（2）初始化前置BeanPostProcessor触发
调用Bean处理器的初始化前处理方法applyBeanPostProcessorsBeforeInitialization()。PostConstruct注解就是在此处理：
（3）调用init方法
在XML配置中，bean可以有一个init-method属性来指定初始化时调用的方法。从原理来说，其实就是一个反射调用。不过注意这里有一个InitializingBean的概念。
此接口只有一个方法：
void afterPropertiesSet() throws Exception;
如果我们的bean实现了此接口，那么此方法会首先被调用。此接口的意义在于: 当此bean的所有属性都被设置(注入)后，给bean一个利用现有属性重新组织或是检查属性的机会。感觉和init方法有些冲突，不过此接口在Spring被广泛使用。
（4）初始化后置BeanPostProcessor触发
调用Bean处理器的初始化后的处理方法applyBeanPostProcessorsAfterInitialization() ，执行Bean处理器的初始化后处理。
4.6 暴露早期引用对象
如果允许提前暴露，调用getSingleTon()方法，将完全体的Bean注入到beanFactory中：
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<String> actualDependentBeans = new LinkedHashSet<>(dependentBeans.length);
for (String dependentBean : dependentBeans) {
if (!removeSingletonIfCreatedForTypeCheckOnly(dependentBean)) {
actualDependentBeans.add(dependentBean);
}
}
}
}
}
4.7 注册销毁方法
调用 registerDisposableBeanIfNecessary()方法，注册Bean的销毁方法。此处包括 destroy-method，还有JSR中的PreDestroy注解等。 然后返回Bean实例。
二、Bean生命周期图示
THE END.
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