准备阶段
A类和B类循环依赖
@Component
public class A {
@Autowiredprivate B b;public A() {
}public B getB() {
return b;}public void setB(B b) {
this.b = b;}
}
@Component
public class B {
@Autowiredprivate A a;public B(){
}public A getA() {
return a;}public void setA(A a) {
this.a = a;}
}
@ComponentScan("spring.bean")
public class Main {
public static void main(String[] args) {
// 创建Spring容器AnnotationConfigApplicationContext applicationContext = new AnnotationConfigApplicationContext(Main.class);A springBean = (A) applicationContext.getBean("a");if (springBean == null) {
return;}System.out.println(springBean);// Spring容器销毁applicationContext.close();}
}
调试阶段
1、创建Spring容器
// 创建Spring容器
AnnotationConfigApplicationContext applicationContext = new AnnotationConfigApplicationContext(Main.class);
2、AnnotationConfigApplicationContext类的构造器刷新上下文
public AnnotationConfigApplicationContext(Class<?>... componentClasses) {
this();register(componentClasses);refresh();
}
AnnotationConfigApplicationContext的父类GenericApplicationContext构造器中
创建了DefaultListableBeanFactory实例
public GenericApplicationContext() {this.beanFactory = new DefaultListableBeanFactory();
}
3、AbstractApplicationContext类的refresh()方法
public void refresh() throws BeansException, IllegalStateException {
synchronized (this.startupShutdownMonitor) {
StartupStep contextRefresh = this.applicationStartup.start("spring.context.refresh");// 准备此上下文以进行刷新、设置其启动日期和活动标志以及执行任何属性源的初始化// Prepare this context for refreshing.prepareRefresh();// 告诉子类刷新内部 bean 工厂。// Tell the subclass to refresh the internal bean factory.ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();// 准备 bean 工厂以在此上下文中使用。// 配置工厂的标准上下文特征，例如上下文的 ClassLoader 和后处理器。 // Prepare the bean factory for use in this context.prepareBeanFactory(beanFactory);try {
// 允许在上下文子类中对 bean 工厂进行后处理// 在此方法可以做一些回调操作，可以当作一个扩展功能// Allows post-processing of the bean factory in context subclasses.postProcessBeanFactory(beanFactory);StartupStep beanPostProcess = this.applicationStartup.start("spring.context.beans.post-process");// 调用在上下文中注册为 bean 的工厂处理器。// 执行BeanFactoryPostProcessor的postProcessBeanFactory方法 // Invoke factory processors registered as beans in the context.invokeBeanFactoryPostProcessors(beanFactory);// 注册拦截 bean 创建的 bean 处理器(BeanPostProcessor)// Register bean processors that intercept bean creation.registerBeanPostProcessors(beanFactory);beanPostProcess.end();// 初始化MessageSource（国际化）// Initialize message source for this context.initMessageSource();// 为此上下文初始化事件多播器。// Initialize event multicaster for this context.initApplicationEventMulticaster();// Initialize other special beans in specific context subclasses.onRefresh();// 注册监听器 // Check for listener beans and register them.registerListeners();// 实例化所有剩余的（非惰性初始化）单例。（创建bean的重要方法） // Instantiate all remaining (non-lazy-init) singletons.finishBeanFactoryInitialization(beanFactory);// Last step: publish corresponding event.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();contextRefresh.end();}}
}
4、finishBeanFactoryInitialization(beanFactory)方法初始化需要Spring管理的单例bean
refresh()中调用finishBeanFactoryInitialization(beanFactory)方法创建需要Spring管理的单例bean并放入Spring容器。
此时A类和B类未在容器里面。
protected void finishBeanFactoryInitialization(ConfigurableListableBeanFactory beanFactory) {
// Initialize conversion service for this context.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 BeanFactoryPostProcessor// (such as a PropertySourcesPlaceholderConfigurer 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.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);// Allow for caching all bean definition metadata, not expecting further changes.beanFactory.freezeConfiguration();// DefaultListableBeanFactory调用preInstantiateSingletons方法// 实例化所有剩余的（非惰性初始化）单例。// Instantiate all remaining (non-lazy-init) singletons.beanFactory.preInstantiateSingletons();
}
5、DefaultListableBeanFactory调用preInstantiateSingletons()方法实例化单例bean
public void preInstantiateSingletons() throws BeansException {
if (logger.isTraceEnabled()) {
logger.trace("Pre-instantiating singletons in " + this);}// 获取所有bean名字 // 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);// 触发所有非惰性单例 bean 的初始化...// Trigger initialization of all non-lazy singleton beans...for (String beanName : beanNames) {
// 获取RootBeanDefinitionRootBeanDefinition bd = getMergedLocalBeanDefinition(beanName);// 判断不是抽象类，是单例，不需要懒加载if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) {
// 判断是否是FactoryBeanif (isFactoryBean(beanName)) {
Object bean = getBean(FACTORY_BEAN_PREFIX + beanName);if (bean instanceof FactoryBean) {
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 {
// 普通Bean走这边// 获取bean getBean(beanName);}}}// 为所有适用的 bean 触发初始化后回调... (实现了SmartInitializingSingleton接口的bean)// Trigger post-initialization callback for all applicable beans...for (String beanName : beanNames) {
Object singletonInstance = getSingleton(beanName);if (singletonInstance instanceof SmartInitializingSingleton) {
StartupStep smartInitialize = this.getApplicationStartup().start("spring.beans.smart-initialize").tag("beanName", beanName);SmartInitializingSingleton smartSingleton = (SmartInitializingSingleton) singletonInstance;if (System.getSecurityManager() != null) {
AccessController.doPrivileged((PrivilegedAction<Object>) () -> {
smartSingleton.afterSingletonsInstantiated();return null;}, getAccessControlContext());}else {
smartSingleton.afterSingletonsInstantiated();}smartInitialize.end();}}
}
6、AbstractBeanFactory的getBean(beanName)方法
do开头的方法才是真正干活的
public Object getBean(String name) throws BeansException {
return doGetBean(name, null, null, false);
}
7、doGetBean方法返回指定 bean 的一个实例
protected <T> T doGetBean(String name, @Nullable Class<T> requiredType, @Nullable Object[] args, boolean typeCheckOnly)throws BeansException {
............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 + "'");}}beanInstance = getObjectForBeanInstance(sharedInstance, name, beanName, null);}else {
............// Create bean instance.if (mbd.isSingleton()) {
// 获取单例bean // 返回以给定名称注册的（原始）单例对象，如果尚未注册，则创建并注册一个新对象。参数： beanName - bean 的名称 singletonFactory // - 懒惰地创建单例的 ObjectFactory，如果需要返回：注册的单例对象 sharedInstance = getSingleton(beanName, () -> {
try {
// 创建bean对象// getSingleton方法中会调用ObjectFactory的getObject方法// 及调用当前代码块实现，调用createBean创建bean 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;}});beanInstance = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);}............}catch (BeansException ex) {
beanCreation.tag("exception", ex.getClass().toString());beanCreation.tag("message", String.valueOf(ex.getMessage()));cleanupAfterBeanCreationFailure(beanName);throw ex;}finally {
beanCreation.end();}}return adaptBeanInstance(name, beanInstance, requiredType);
}
8、createBean方法创建bean
AbstractAutowireCapableBeanFactory类的createBean方法
此类的中心方法：创建 bean 实例、填充 bean 实例、应用后处理器等。
protected Object createBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args)throws BeanCreationException {
............try {
// 创建bean实例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);}
}
9、doCreateBean方法实际创建指定的bean
do开头的才是干活的方法
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) {
// 使用适当的实例化策略为指定的 bean 创建一个新实例：工厂方法、构造函数自动装配或简单实例化。// 返回bean的包装器 instanceWrapper = createBeanInstance(beanName, mbd, args);}// 从包装器中获取实例bean 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;}}// 即使被 BeanFactoryAware 等生命周期接口触发，也急切地缓存单例以解析循环引用。// isSingletonCurrentlyInCreation判断是否在创建中 // 是否需要提前暴露bean的引用，解决bean循环依赖问题、AOP代理等 // 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");}// 如果需要提前暴露，将Lamda表达式（ObjectFactory）加入到singletonFactories中// getEarlyBeanReference 获取对指定 bean 的早期访问的引用，通常用于解析循环引用。addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));}// Initialize the bean instance.Object exposedObject = bean;try {
// 对bean实例进行填充属性// 例如@Autowired指定的依赖populateBean(beanName, mbd, instanceWrapper);// 初始化给定的 bean 实例，应用工厂回调以及 init 方法和 bean 后处理器。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;
}
10、Bean属性填充populateBean方法
使用 bean 定义中的属性值填充给定 BeanWrapper 中的 bean 实例。
解决bean的依赖注入问题，如果出现循环依赖那么
populateBean——>getBean——>doGetBean——>createBean——>doCreateBean——>populateBean…
进行递归创建bean实例
11、Bean的初始化initializeBean方法
protected Object initializeBean(String beanName, Object bean, @Nullable RootBeanDefinition mbd) {
if (System.getSecurityManager() != null) {
AccessController.doPrivileged((PrivilegedAction<Object>) () -> {
invokeAwareMethods(beanName, bean);return null;}, getAccessControlContext());}else {
// 执行Aware接口的方法// 这是一个private方法// 只对实现了BeanNameAware BeanClassLoaderAware BeanFactoryAware接口的bean调用对应的setter方法赋值invokeAwareMethods(beanName, bean);}Object wrappedBean = bean;if (mbd == null || !mbd.isSynthetic()) {
// 调用BeanPostProcessor的postProcessBeforeInitialization前置通知方法wrappedBean = applyBeanPostProcessorsBeforeInitialization(wrappedBean, beanName);}try {
// 调用bean实现InitializingBean接口的afterPropertiesSet方法// 以及在给定的 bean 上调用指定的自定义 init 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BeanPostProcessor的postProcessAfterInitialization后置通知方法 wrappedBean = applyBeanPostProcessorsAfterInitialization(wrappedBean, beanName);}return wrappedBean;
}
12、执行applicationContext.close()容器销毁
关闭此应用程序上下文，销毁其 bean 工厂中的所有 bean。
public void close() {
synchronized (this.startupShutdownMonitor) {
doClose();// If we registered a JVM shutdown hook, we don't need it anymore now:// We've already explicitly closed the context.if (this.shutdownHook != null) {
try {
Runtime.getRuntime().removeShutdownHook(this.shutdownHook);}catch (IllegalStateException ex) {
// ignore - VM is already shutting down}}}
}
13、doClose()执行上下文关闭
实际执行上下文关闭：发布一个 ContextClosedEvent 并销毁此应用程序上下文的 bean 工厂中的单例。由 close() 和 JVM 关闭挂钩（如果有）调用。
protected void doClose() {
// Check whether an actual close attempt is necessary...if (this.active.get() && this.closed.compareAndSet(false, true)) {
if (logger.isDebugEnabled()) {
logger.debug("Closing " + this);}if (!NativeDetector.inNativeImage()) {
LiveBeansView.unregisterApplicationContext(this);}try {
// 发布关闭事件// Publish shutdown event.publishEvent(new ContextClosedEvent(this));}catch (Throwable ex) {
logger.warn("Exception thrown from ApplicationListener handling ContextClosedEvent", ex);}// Stop all Lifecycle beans, to avoid delays during individual destruction.if (this.lifecycleProcessor != null) {
try {
this.lifecycleProcessor.onClose();}catch (Throwable ex) {
logger.warn("Exception thrown from LifecycleProcessor on context close", ex);}}// 销毁上下文的 BeanFactory 中所有缓存的单例。// 执行bean实现DisposableBean接口的destroy方法// 以及在给定的 bean 上调用指定的自定义 destroy 方法// Destroy all cached singletons in the context's BeanFactory.destroyBeans();// Close the state of this context itself.closeBeanFactory();// Let subclasses do some final clean-up if they wish...onClose();// Reset local application listeners to pre-refresh state.if (this.earlyApplicationListeners != null) {
this.applicationListeners.clear();this.applicationListeners.addAll(this.earlyApplicationListeners);}// Switch to inactive.this.active.set(false);}
}
总结
Bean的创建过程（生命周期）
1、Bean的实例化
通过反射调用构造方法进行实例化或工厂方法进行实例化
2、Bean的初始化
1、调用bean实现Aware接口的setter方法（回调）
2、调用BeanPostProcessor的前置通知方法postProcessBeforeInitialization
3、调用Bean实现InitializingBean接口的afterPropertiesSet方法
4、调用Bean指定的自定义 init 方法
5、调用BeanPostProcessor的后置通知方法postProcessAfterInitialization
3、Bean的使用
通过spring容器进行getBean获取使用
4、Bean的销毁
1、调用bean实现DisposableBean接口的destroy方法
2、 调用 bean 上调用指定的自定义 destroy 方法
其中还有很多细节的地方没有深究