很多文章认为Bean 的生命周期分为四步
- 实例化 Instantiation
- 属性赋值 Populate
- 初始化 Initialization
- 销毁 Destruction
但是从我个人认为，这个分法是不完整的。少了一个Metadata 的构建过程，任何一个bean 不是天生就有的，而且在BeanFactory 里获得它的metadata BeanDefinition（里面包含了各种bean的基本信息，比如name， 依赖等等）， 没有这一步，无从谈起下一步。所以我更喜欢下面一张图。
image.png
一个app 的bean 的结构关系，可以用一个DAG 来描述。在这个DAG 中，有显示的依赖关系，有隐式的依赖关系。依赖意味着顺序， 因此在描述所有的Bean的时候，实际上还要描述Bean的构造顺序。 比如@AutoConfigureOrder annotation 或者Ordered 之类的接口存在，来描述他们之间的先后关系。
保证所有的Bean 能够按照顺序来进行执行是非常重要的，否则在spring 升级，或者系统升级后出现各种各样的问题。在单个bean 的生命周期上，从实例化是没错的。但是如果放在整个app，忽略metadata 的构建的顺序和过程是不可以的，尤其是在spring 的基础上再次开发，提供平台的用户来说。这是必须要掌握的内容。
这是BeanFactory的创建和销毁bean的流程。
image.png
创建时机
我们可以在Spring应用上下文初始化时或者我们自己创建BeanFactory去实例化bean。
Spring应用上下文在invokeBeanFactoryPostProcessors、finishBeanFactoryInitialization等阶段会实例化bean。
事实上当通过BeanFactory的getBean()方法来请求对象实例时， 才有可能触发Bean实例化阶段的活动。
一些概念：
RequiredAnnotationBeanPostProcessor 是处理@Required注解的逻辑
AutowiredAnnotationBeanPostProcessor 是处理@Autowired和@Value和@javax.inject.Inject
CommonAnnotationBeanPostProcessor jsr250注解如javax.annotation.Resource、@PreDestroy、@PostConstruct的处理
销毁时机
在beanFactory创建bean时，最后会调用registerDisposableBeanIfNecessary方法。
单例bean 会在Spring应用正常结束时被shutdownhook或手动调用beanFactory的destroySingletons方法去销毁。
自定义Scope的bean 需要自实现registerDestructionCallback方法，将DisposableBeanAdapter按照自己的逻辑自行销毁。
prototype bean spring没有提供销毁逻辑。我们可以通过自实现一个BeanPostProcessor去实现（即通过单例的BeanPostProcessor会在初始化阶段收集prototype bean，销毁则跟随destroySingletons方法触发自实现销毁）。
拓展点
InstantiationAwareBeanPostProcessor 感知Bean实例化的处理器
MergedBeanDefinitionPostProcessor 对merged beanDefinition进行处理
BeanNameAware、BeanClassLoaderAware、BeanFactoryAware bean可实现这些接口获取对应资源
BeanPostProcessor 可以新bean实例化前后做一些自定义操作。
InitializingBean、DisposableBean、initMethod、destroyMethod 提供给bean的初始化和销毁的感知处理
BeanDefinitionRegistryPostProcessor 可用于增加额外beanDefinition，在Spring上下文refresh的invokeBeanFactoryPostProcessors时调用。
SmartInitializingSingleton接口 在所有单例bean都被初始化完成后回调执行， 以便在常规实例化后执行一些初始化，避免意外的早期初始化带来的副作用（例如，来自ListableBeanFactory.getBeansOfType调用），懒加载的bean不能触发执行
源码分析
/**
* 完成bean工厂的初始化
* 实例化所有的单例（非懒惰）bean
*/
protected void finishBeanFactoryInitialization(ConfigurableListableBeanFactory beanFactory) {
// Initialize conversion service for this context.
// 1.初始化conversionService
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));
}
// 2.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(strVal -> getEnvironment().resolvePlaceholders(strVal));
}
// Initialize LoadTimeWeaverAware beans early to allow for registering their transformers early.
// 3.初始化LoadTimeWeaverAware，使得尽早进行注册transformer(运行期织入)
String[] weaverAwareNames = beanFactory.getBeanNamesForType(LoadTimeWeaverAware.class, false, false);
for (String weaverAwareName : weaverAwareNames) {
getBean(weaverAwareName);
}
// 4.Stop using the temporary ClassLoader for type matching.
beanFactory.setTempClassLoader(null);
// 5.Allow for caching all bean definition metadata, not expecting further changes.
beanFactory.freezeConfiguration();
// Instantiate all remaining (non-lazy-init) singletons.
// 6.初始化所有非懒惰的bean
beanFactory.preInstantiateSingletons();
}
public 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);
// 3.1Trigger initialization of all non-lazy singleton beans...
for (String beanName : beanNames) {
// 3.2获取bean的合并的beandefinition
RootBeanDefinition bd = getMergedLocalBeanDefinition(beanName);
if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) {
//3.3判断是否工厂bean
if (isFactoryBean(beanName)) {
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());
}
if (isEagerInit) {
getBean(beanName);
}
}
}
else {
//3.4getBean
getBean(beanName);
}
}
}
// Trigger post-initialization callback for all applicable beans...
// 3.5为实现了SmartInitializingSingleton的bean执行afterSingletonsInstantiated方法
for (String beanName : beanNames) {
Object singletonInstance = getSingleton(beanName);
if (singletonInstance instanceof SmartInitializingSingleton) {
final SmartInitializingSingleton smartSingleton = (SmartInitializingSingleton) singletonInstance;
if (System.getSecurityManager() != null) {
Access Controller.doPrivileged((PrivilegedAction<Object>) () -> {
smartSingleton.afterSingletonsInstantiated();
return null;
}, getAccessControlContext());
}
else {
//3.6 smartSingleton.afterSingletonsInstantiated();
}
}
}
}
我们重点看3.4步getBean方法， 事实上当通过BeanFactory的getBean()方法来请求对象实例时， 才有可能触发Bean实例化阶段的活动。
注：在前面应用上下文的启动阶段，某些bean（如BeanFactoryPostProcessor）已经通过调用调用该方法实例化过了。
//AbstractBeanFactory
@Override
public Object getBean(String name) throws BeansException {
return doGetBean(name, null, null, false);
}
//AbstractBeanFactory.doGetBean
protected <T> T doGetBean(final String name, @Nullable final Class<T> requiredType,
@Nullable final Object[] args, boolean typeCheckOnly) throws BeansException {
//3.4.1转换成准确的bean name，去除beanfactory bean的&前缀，获取别名map中真名。
final String beanName = transformedBeanName(name);
Object bean;
// Eagerly check singleton cache for manually registered singletons.
// 3.4.2查找那些已经通过手动注册了的bean
Object sharedInstance = getSingleton(beanName);
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 + "'");
}
}
bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
}
else {
//3.4.3 Fail if we're already creating this bean instance:
// We're assumably within a circular reference.
if (isPrototypeCurrentlyInCreation(beanName)) {
throw new BeanCurrentlyInCreationException(beanName);
}
//3.4.4 查看父beanFactory是否含有该bean的bean definition
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);
}
}
if (!typeCheckOnly) {
markBeanAsCreated(beanName);
}
try {
final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
checkMergedBeanDefinition(mbd, beanName, args);
// Guarantee initialization of beans that the current bean depends on.
// 3.4.5保证当前beanname声明的@DependsOn或xml的depends-on类的初始化
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 + "'");
}
registerDependentBean(dep, beanName);
try {
getBean(dep);
}
catch (NoSuchBeanDefinitionException ex) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"'" + beanName + "' depends on missing bean '" + dep + "'", ex);
}
}
}
// Create bean instance.
// 3.4.6 创建单例bean
if (mbd.isSingleton()) {
// 3.4.6 创建单例bean
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.isTraceEnabled()) {
logger.trace("Failed to convert bean '" + name + "' to required type '" +
ClassUtils.getQualifiedName(requiredType) + "'", ex);
}
throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
}
}
return (T) bean;
}
注意3.4.6的代码，会创建一个bean对象
/**
* Central method of this class: creates a bean instance,
* populates the bean instance, applies post-processors, etc.
* @see #doCreateBean
*/
//核心方法，创建bean实例，填充bean实例，执行各种后置处理器
@Override
protected 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.
//3.4.6.2.1
Class<?> resolvedClass = resolveBeanClass(mbd, beanName);
if (resolvedClass != null && !mbd.hasBeanClass() && mbd.getBeanClassName() != null) {
mbdToUse = new RootBeanDefinition(mbd);
mbdToUse.setBeanClass(resolvedClass);
}
// Prepare method overrides.
//3.4.6.2.2更新overloaded标识值
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.
// 3.4.6.2.3执行InstantiationAwareBeanPostProcessor的postProcessBeforeInstantiation
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 {
//3.4.6.2.4实际上创建bean的方法
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);
}
}
接着我们来看下AbstractAutowireCapableBeanFactory的doCreateBean方法。
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);
}
//3.4.6.2.4.1
if (instanceWrapper == null) {
// 通过工厂方法或构造器注入或通过无参构造方法方法创建bean实例
//若 bean 的配置信息中配置了 lookup-method 和 replace-method，则会使用CGLIB增强bean实例
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 {
//3.4.6.2.4.2 执行MergedBeanDefinitionPostProcessor，
//例如对merged beanDefinition做些缓存工作，也可以做属性修改，
//例如对@ProConstruct和@PreDestroy注解的读取处理在此处
applyMergedBeanDefinitionPostProcessors(mbd, beanType, beanName);
}
catch (Throwable ex) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"Post-processing of merged bean definition failed", ex);
}
mbd.postProcessed = true;
}
}
// 饥渴缓存单例bean，以便即使在诸如BeanFactoryAware之类的生命周期接口触发时也能够解析循环引用。
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");
}
//3.4.6.2.4.3
addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
}
// Initialize the bean instance.
Object exposedObject = bean;
try {
//3.4.6.2.4.4设置属性
//1)执行InstantiationAwareBeanPostProcessor的postProcessAfterInstantiation方法
//2)如果beanDefinition设置了AUTOWIRE_BY_NAME或AUTOWIRE_BY_TYPE，通过名称或类型填充autowired属性
//3)执行InstantiationAwareBeanPostProcessor的postProcessProperties和postProcessPropertyValues填充属性。
// 执行RequiredAnnotationBeanPostProcessor（@Required注解处理）、AutowiredAnnotationBeanPostProcessor（通过名称或类型填充@Autowired和@Value和@javax.inject.Inject标注的属性）、CommonAnnotationBeanPostProcessor（jsr250注解如javax.annotation.Resource的处理）
populateBean(beanName, mbd, instanceWrapper);
//3.4.6.2.4.5执行后置处理器
//1)invokeAwareMethods，判断bean是否为BeanNameAware、BeanClassLoaderAware、BeanFactoryAware并执行相关实现方法
//2)执行BeanPostProcessor的postProcessBeforeInitialization方法
//3)调用初始化方法 1.是InitializingBean则调用afterPropertiesSet()方法 2.检查并调用beanDefinition的用户自定义的initMethodName（就是@Bean配置的initMethod或xml配置的init-method）
//4)执行BeanPostProcessor的postProcessAfterInitialization方法
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);
}
}
//3.4.6.2.4.6
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 " +
"'getBeanNamesOfType' with the 'allowEagerInit' flag turned off, for example.");
}
}
}
}
// Register bean as disposable.
//3.4.6.2.4.7
//调用registerDisposableBean方法，将单例实例bean包裹到DisposableBeanAdapter并放到disposable bean集合中，供容器close时执行销毁操作
//自定义scope会将实例bean包裹到DisposableBeanAdapter，作为参数调用scope.registerDestructionCallback方法
try {
registerDisposableBeanIfNecessary(beanName, bean, mbd);
}
catch (BeanDefinitionValidationException ex) {
throw new BeanCreationException(
mbd.getResourceDescription(), beanName, "Invalid destruction signature", ex);
}
return exposedObject;
}
分析DisposableBeanAdapter的destroy方法。
public void destroy() {
if (!CollectionUtils.isEmpty(this.beanPostProcessors)) {
//调用DestructionAwareBeanPostProcessor的postProcessBeforeDestruction
//@PreDestroy声明的方法在此时执行
for (DestructionAwareBeanPostProcessor processor : this.beanPostProcessors) {
processor.postProcessBeforeDestruction(this.bean, this.beanName);
}
}
//如果是DisposableBean调用bean的destroy方法
if (this.invokeDisposableBean) {
if (logger.isTraceEnabled()) {
logger.trace("Invoking destroy() on bean with name '" + this.beanName + "'");
}
try {
if (System.getSecurityManager() != null) {
AccessController.doPrivileged((PrivilegedExceptionAction<Object>) () -> {
((DisposableBean) this.bean).destroy();
return null;
}, this.acc);
}
else {
((DisposableBean) this.bean).destroy();
}
}
catch (Throwable ex) {
String msg = "Invocation of destroy method failed on bean with name '" + this.beanName + "'";
if (logger.isDebugEnabled()) {
logger.info(msg, ex);
}
else {
logger.info(msg + ": " + ex);
}
}
}
//调用自定义的destroy方法（就是@Bean的destroy-method或XML配置的destroy-method）
if (this.destroyMethod != null) {
invokeCustomDestroyMethod(this.destroyMethod);
}
else if (this.destroyMethodName != null) {
Method methodToCall = determineDestroyMethod(this.destroyMethodName);
if (methodToCall != null) {
invokeCustomDestroyMethod(methodToCall);
}
}
}
