ApplicationContext和Bean的初始化及销毁
对于BeanFactory，特别是ApplicationContext，容器自身也有一个初始化和销毁关闭的 过程。下面详细看看在这两个过程中，应用上下文完成了什么，可以让我们更多地理解应用 上下文的工作，容器初始化和关闭过程可以简要地通过下图来表现。 从图中可以看到，对ApplicationContext启动的过程是在AbstractApplicationContext中实现的。在使用应用上下文时需要做一些准备工作，这些准备工作prepareBeanFactory()方法中实现。在这个方法中，为容器配置了ClassLoader、PropertyEditor和BeanPostProcessor等，从而为容器的启动做好了必要的准备工作。
容器的初始化和关闭大致分为： （1）org.springframework.context.support.AbstractApplicationContext#prepareBeanFactory （2）org.springframework.beans.factory.config.ConfigurableListableBeanFactory.addPropertyEditorRegister （3）org.springframework.beans.factory.config.ConfigurableListableBeanFactory.addBeanPostProcessor （4）org.springframework.context.support.AbstractApplicationContext#doClose （5）org.springframework.context.support.AbstractApplicationContext#closeBeanFactory
org.springframework.context.support.AbstractApplicationContext#prepareBeanFactory的实现如下：
protected void prepareBeanFactory(ConfigurableListableBeanFactory beanFactory) {
// Tell the internal bean factory to use the context's class loader etc.
beanFactory.setBeanClassLoader(getClassLoader());
if (!shouldIgnoreSpel) {
beanFactory.setBeanExpressionResolver(new StandardBeanExpressionResolver(beanFactory.getBeanClassLoader()));
}
beanFactory.addPropertyEditorRegistrar(new ResourceEditorRegistrar(this, getEnvironment()));
// Configure the bean factory with context callbacks.
beanFactory.addBeanPostProcessor(new ApplicationContextAwareProcessor(this));
beanFactory.ignoreDependencyInterface(EnvironmentAware.class);
beanFactory.ignoreDependencyInterface(EmbeddedValueResolverAware.class);
beanFactory.ignoreDependencyInterface(ResourceLoaderAware.class);
beanFactory.ignoreDependencyInterface(ApplicationEventPublisherAware.class);
beanFactory.ignoreDependencyInterface(MessageSourceAware.class);
beanFactory.ignoreDependencyInterface(ApplicationContextAware.class);
beanFactory.ignoreDependencyInterface(ApplicationStartupAware.class);
// BeanFactory interface not registered as resolvable type in a plain factory.
// MessageSource registered (and found for autowiring) as a bean.
beanFactory.registerResolvableDependency(BeanFactory.class, beanFactory);
beanFactory.registerResolvableDependency(ResourceLoader.class, this);
beanFactory.registerResolvableDependency(ApplicationEventPublisher.class, this);
beanFactory.registerResolvableDependency(ApplicationContext.class, this);
// Register early post-processor for detecting inner beans as ApplicationListeners.
beanFactory.addBeanPostProcessor(new ApplicationListenerDetector(this));
// Detect a LoadTimeWeaver and prepare for weaving, if found.
if (!NativeDetector.inNativeImage() && beanFactory.containsBean(LOAD_TIME_WEAVER_BEAN_NAME)) {
beanFactory.addBeanPostProcessor(new LoadTimeWeaverAwareProcessor(beanFactory));
// Set a temporary ClassLoader for type matching.
beanFactory.setTempClassLoader(new ContextTypeMatchClassLoader(beanFactory.getBeanClassLoader()));
}
// Register default environment beans.
if (!beanFactory.containsLocalBean(ENVIRONMENT_BEAN_NAME)) {
beanFactory.registerSingleton(ENVIRONMENT_BEAN_NAME, getEnvironment());
}
if (!beanFactory.containsLocalBean(SYSTEM_PROPERTIES_BEAN_NAME)) {
beanFactory.registerSingleton(SYSTEM_PROPERTIES_BEAN_NAME, getEnvironment().getSystemProperties());
}
if (!beanFactory.containsLocalBean(SYSTEM_ENVIRONMENT_BEAN_NAME)) {
beanFactory.registerSingleton(SYSTEM_ENVIRONMENT_BEAN_NAME, getEnvironment().getSystemEnvironment());
}
if (!beanFactory.containsLocalBean(APPLICATION_STARTUP_BEAN_NAME)) {
beanFactory.registerSingleton(APPLICATION_STARTUP_BEAN_NAME, getApplicationStartup());
}
}
同样，在容器要关闭时，也需要完成一系列的工作，这些工作在doClose()方法中完成。 在这个方法中，先发出容器关闭的信号，然后将Bean逐个关闭，最后关闭容器自身。
org.springframework.context.support.AbstractApplicationContext#doClose实现如下：
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 {
// 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);
}
}
// 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 common introspection caches to avoid class reference leaks.
resetCommonCaches();
// 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);
}
}
容器的实现是通过IoC管理Bean的生命周期来实现的。SpringIoC容器在对Bean的生命 周期进行管理时提供了Bean生命周期各个时间点的回调。在分析Bean初始化和销毁过程的设计之前，简要介绍一下loC容器中的Bean生命周期。 （1）Bean实例的创建。 （2）为Bean实例设置属性。 （3）调用Bean的初始化方法。 （4）应用可以通过IoC容器使用Bean。 （5）当容器关闭时，调用Bean的销毁方法。 Bean的初始化方法调用是在以下的initializeBean方法中实现的：
org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory#initializeBean(java.lang.String, java.lang.Object, org.springframework.beans.factory.support.RootBeanDefinition)的实现如下：
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 {
invokeAwareMethods(beanName, bean);
}
Object wrappedBean = bean;
if (mbd == null || !mbd.isSynthetic()) {
wrappedBean = applyBeanPostProcessorsBeforeInitialization(wrappedBean, beanName);
}
try {
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()) {
wrappedBean = applyBeanPostProcessorsAfterInitialization(wrappedBean, beanName);
}
return wrappedBean;
}
在调用Bean的初始化方法之前，会调用一系列的aware接口实现，把相关的BeanName、BeanClassLoader，以及BeanFactoy注入到Bean中去。接着会看到对invokeInitMethods的调用，这时还会看到启动afterPropertiesSet的过程，当然，这需要Bean实现InitializingBean的接口，对应的初始化处理可以在InitializingBean接口的afterPropertiesSet方法中实现，这里同样是对Bean的一个回调。
org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory#invokeAwareMethods的实现如下：
private void invokeAwareMethods(String beanName, Object bean) {
if (bean instanceof Aware) {
if (bean instanceof BeanNameAware) {
((BeanNameAware) bean).setBeanName(beanName);
}
if (bean instanceof BeanClassLoaderAware) {
ClassLoader bcl = getBeanClassLoader();
if (bcl != null) {
((BeanClassLoaderAware) bean).setBeanClassLoader(bcl);
}
}
if (bean instanceof BeanFactoryAware) {
((BeanFactoryAware) bean).setBeanFactory(AbstractAutowireCapableBeanFactory.this);
}
}
}
org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory#applyBeanPostProcessorsBeforeInitialization的实现如下：
public Object applyBeanPostProcessorsBeforeInitialization(Object existingBean, String beanName)
throws BeansException {
Object result = existingBean;
for (BeanPostProcessor processor : getBeanPostProcessors()) {
Object current = processor.postProcessBeforeInitialization(result, beanName);
if (current == null) {
return result;
}
result = current;
}
return result;
}
org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory#invokeInitMethods实现如下：
protected void invokeInitMethods(String beanName, Object bean, @Nullable RootBeanDefinition mbd)
throws Throwable {
boolean isInitializingBean = (bean instanceof InitializingBean);
if (isInitializingBean && (mbd == null || !mbd.hasAnyExternallyManagedInitMethod("afterPropertiesSet"))) {
if (logger.isTraceEnabled()) {
logger.trace("Invoking afterPropertiesSet() on bean with name '" + beanName + "'");
}
if (System.getSecurityManager() != null) {
try {
AccessController.doPrivileged((PrivilegedExceptionAction<Object>) () -> {
((InitializingBean) bean).afterPropertiesSet();
return null;
}, getAccessControlContext());
}
catch (PrivilegedActionException pae) {
throw pae.getException();
}
}
else {
((InitializingBean) bean).afterPropertiesSet();
}
}
if (mbd != null && bean.getClass() != NullBean.class) {
String initMethodName = mbd.getInitMethodName();
if (StringUtils.hasLength(initMethodName) &&
!(isInitializingBean && "afterPropertiesSet".equals(initMethodName)) &&
!mbd.hasAnyExternallyManagedInitMethod(initMethodName)) {
// 执行自定义的初始化方法
invokeCustomInitMethod(beanName, bean, mbd);
}
}
}
protected void invokeCustomInitMethod(String beanName, Object bean, RootBeanDefinition mbd)
throws Throwable {
// 获取xml等形式配置的init初始化方法
String initMethodName = mbd.getInitMethodName();
Assert.state(initMethodName != null, "No init method set");
Method initMethod = (mbd.isNonPublicAccessAllowed() ?
BeanUtils.findMethod(bean.getClass(), initMethodName) :
ClassUtils.getMethodIfAvailable(bean.getClass(), initMethodName));
if (initMethod == null) {
if (mbd.isEnforceInitMethod()) {
throw new BeanDefinitionValidationException("Could not find an init method named '" +
initMethodName + "' on bean with name '" + beanName + "'");
}
else {
if (logger.isTraceEnabled()) {
logger.trace("No default init method named '" + initMethodName +
"' found on bean with name '" + beanName + "'");
}
// Ignore non-existent default lifecycle methods.
return;
}
}
if (logger.isTraceEnabled()) {
logger.trace("Invoking init method '" + initMethodName + "' on bean with name '" + beanName + "'");
}
Method methodToInvoke = ClassUtils.getInterfaceMethodIfPossible(initMethod, bean.getClass());
if (System.getSecurityManager() != null) {
AccessController.doPrivileged((PrivilegedAction<Object>) () -> {
ReflectionUtils.makeAccessible(methodToInvoke);
return null;
});
try {
AccessController.doPrivileged((PrivilegedExceptionAction<Object>)
() -> methodToInvoke.invoke(bean), getAccessControlContext());
}
catch (PrivilegedActionException pae) {
InvocationTargetException ex = (InvocationTargetException) pae.getException();
throw ex.getTargetException();
}
}
else {
try {
ReflectionUtils.makeAccessible(methodToInvoke);
methodToInvoke.invoke(bean);
}
catch (InvocationTargetException ex) {
throw ex.getTargetException();
}
}
}
org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory#applyBeanPostProcessorsAfterInitialization的实现如下：
public Object applyBeanPostProcessorsAfterInitialization(Object existingBean, String beanName)
throws BeansException {
Object result = existingBean;
for (BeanPostProcessor processor : getBeanPostProcessors()) {
Object current = processor.postProcessAfterInitialization(result, beanName);
if (current == null) {
return result;
}
result = current;
}
return result;
}
与Bean初始化类似，当容器关闭时，可以看到对Bean销毁方法的调用。Bean销毁过程是这样的：
org.springframework.context.support.AbstractApplicationContext#doClose的实现如下：
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 {
// 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);
}
}
// 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 common introspection caches to avoid class reference leaks.
resetCommonCaches();
// 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);
}
}
对Bean进行销毁处理的方法为org.springframework.beans.factory.support.DisposableBeanAdapter#destroy，实现如下：
public void destroy() {
if (!CollectionUtils.isEmpty(this.beanPostProcessors)) {
// 执行DestructionAwareBeanPostProcessor接口的方法
for (DestructionAwareBeanPostProcessor processor : this.beanPostProcessors) {
processor.postProcessBeforeDestruction(this.bean, this.beanName);
}
}
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) {
if (logger.isWarnEnabled()) {
String msg = "Invocation of destroy method failed on bean with name '" + this.beanName + "'";
if (logger.isDebugEnabled()) {
// Log at warn level like below but add the exception stacktrace only with debug level
logger.warn(msg, ex);
}
else {
logger.warn(msg + ": " + ex);
}
}
}
}
if (this.invokeAutoCloseable) {
if (logger.isTraceEnabled()) {
logger.trace("Invoking close() on bean with name '" + this.beanName + "'");
}
try {
if (System.getSecurityManager() != null) {
AccessController.doPrivileged((PrivilegedExceptionAction<Object>) () -> {
((AutoCloseable) this.bean).close();
return null;
}, this.acc);
}
else {
((AutoCloseable) this.bean).close();
}
}
catch (Throwable ex) {
if (logger.isWarnEnabled()) {
String msg = "Invocation of close method failed on bean with name '" + this.beanName + "'";
if (logger.isDebugEnabled()) {
// Log at warn level like below but add the exception stacktrace only with debug level
logger.warn(msg, ex);
}
else {
logger.warn(msg + ": " + ex);
}
}
}
}
// 执行自定义的destry方法
else if (this.destroyMethod != null) {
invokeCustomDestroyMethod(this.destroyMethod);
}
else if (this.destroyMethodName != null) {
Method destroyMethod = determineDestroyMethod(this.destroyMethodName);
if (destroyMethod != null) {
invokeCustomDestroyMethod(ClassUtils.getInterfaceMethodIfPossible(destroyMethod, this.bean.getClass()));
}
}
}
调用到close的调用堆栈如下：
destroy:195, DisposableBeanAdapter (org.springframework.beans.factory.support)
destroyBean:587, DefaultSingletonBeanRegistry (org.springframework.beans.factory.support)
destroySingleton:559, DefaultSingletonBeanRegistry (org.springframework.beans.factory.support)
destroySingleton:1163, DefaultListableBeanFactory (org.springframework.beans.factory.support)
destroySingletons:520, DefaultSingletonBeanRegistry (org.springframework.beans.factory.support)
destroySingletons:1156, DefaultListableBeanFactory (org.springframework.beans.factory.support)
destroyBeans:1109, AbstractApplicationContext (org.springframework.context.support)
doClose:1078, AbstractApplicationContext (org.springframework.context.support)
close:1024, AbstractApplicationContext (org.springframework.context.support)
lazy-init属性和预实例化
正如前面所述，在IoC容器的初始化过程中，主要的工作是对BeanDefinition的定位、载入、解析和注册。此时依赖注入并没有发生，依赖注入发生在应用第一次向容器索要Bean时。向容器索要Bean是通过getBean的调用来完成的，该getBean是容器提供Bean服务的最基本的接口。在前面的分析中也提到，对于容器的初始化，也有一种例外情况，就是用户可以通过设置Bean的lazy-init属性来控制预实例化的过程。这个预实例化在初始化容器时完成Bean的依赖注入。毫无疑问，这种容器的便用方式会对容器初始化的性能有一些影响，但却能够提高应用第一次取得Bean的性能。因为应用在第一次取得Bean时，依赖注入已经结束了，应用可以取得已有的Bean。
下面将从lazy-init属性配置实现的角度进行分析。对这个属性的处理也是容器refresh的一部分。在finishBeanFactoryInitialization的方法中，封装了对lazy-init属性的处理，实际的处理是在DefaultListableBeanFactory这个基本容器的preInstantiateSingletons方法中完成的。 该方法对单件Bean完成预实例化，这个预实例化的完成巧妙地委托给容器来实现。如果需要预实例化，那么就直接在这里采用getBean去触发依赖注入，与正常依赖注入的触发相比，只有触发的时间和场合不同。在这里，依赖注入发生在容器执行refresh的过程中，也就是发生在IoC容器初始化的过程中，而不像一般的依赖注入一样发生在IoC容器初始化完成以后，第一次向容器执行getBean时。
public void refresh() throws BeansException, IllegalStateException {
synchronized (this.startupShutdownMonitor) {
StartupStep contextRefresh = this.applicationStartup.start("spring.context.refresh");
// Prepare this context for refreshing.
prepareRefresh();
// Tell the subclass to refresh the internal bean factory.
ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();
// Prepare the bean factory for use in this context.
prepareBeanFactory(beanFactory);
try {
// Allows post-processing of the bean factory in context subclasses.
postProcessBeanFactory(beanFactory);
StartupStep beanPostProcess = this.applicationStartup.start("spring.context.beans.post-process");
// Invoke factory processors registered as beans in the context.
invokeBeanFactoryPostProcessors(beanFactory);
// Register bean processors that intercept bean creation.
registerBeanPostProcessors(beanFactory);
beanPostProcess.end();
// Initialize message source for this context.
initMessageSource();
// Initialize event multicaster for this context.
initApplicationEventMulticaster();
// Initialize other special beans in specific context subclasses.
onRefresh();
// Check for listener beans and register them.
registerListeners();
// Instantiate all remaining (non-lazy-init) singletons.对lazy-init属性进行处理的地方
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();
}
}
}
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();
// Instantiate all remaining (non-lazy-init) singletons.
beanFactory.preInstantiateSingletons();
}
org.springframework.beans.factory.support.DefaultListableBeanFactory#preInstantiateSingletons的实现如下：
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);
// Trigger initialization of all non-lazy singleton beans...
// 在这里就开始getBean，也就是去触发Bean的依赖注入
// 这个getBean和前面分析的触发依赖注入的过程是一样的，只是发生的地方不同。如果设置
lazy-init属性，那么这个依赖注入是发生在容器初始化结束以后，第一次向容器发出getBean时，
如果不设置1azy-init属性，那么依赖注入发生在容器初始化的过程中，会对beanDefinitionMap中所有的Bean进行依赖注入，这样在初始化过程结束以后，容器执行getBean得到的就是已经准备好的Bean，不需要进行依始注入
for (String beanName : beanNames) {
RootBeanDefinition bd = getMergedLocalBeanDefinition(beanName);
if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) {
if (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 {
getBean(beanName);
}
}
}
// Trigger post-initialization callback for all applicable beans...
for (String beanName : beanNames) {
Object singletonInstance = getSingleton(beanName);
if (singletonInstance instanceof SmartInitializingSingleton) {
StartupStep smartInitialize = 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();
}
}
}
FactoryBean的实现
getObjectForBeanInstance做了哪些处理？在getObjectForBeanInstance的实现方法中可以看到在FactoryBean中常见的getObject方法的接口，详细的实现过程如下所示。
org.springframework.beans.factory.support.AbstractBeanFactory#getObjectForBeanInstance实现如下：
protected Object getObjectForBeanInstance(
Object beanInstance, String name, String beanName, @Nullable RootBeanDefinition mbd) {
// Don't let calling code try to dereference the factory if the bean isn't a factory.
// 如果这里不是对FactoryBean的调用，那么结束处理
if (BeanFactoryUtils.isFactoryDereference(name)) {
if (beanInstance instanceof NullBean) {
return beanInstance;
}
if (!(beanInstance instanceof FactoryBean)) {
throw new BeanIsNotAFactoryException(beanName, beanInstance.getClass());
}
if (mbd != null) {
mbd.isFactoryBean = true;
}
return beanInstance;
}
// Now we have the bean instance, which may be a normal bean or a FactoryBean.
// If it's a FactoryBean, we use it to create a bean instance, unless the
// caller actually wants a reference to the factory.
if (!(beanInstance instanceof FactoryBean)) {
return beanInstance;
}
Object object = null;
if (mbd != null) {
mbd.isFactoryBean = true;
}
else {
object = getCachedObjectForFactoryBean(beanName);
}
if (object == null) {
// Return bean instance from factory.
FactoryBean<?> factory = (FactoryBean<?>) beanInstance;
// Caches object obtained from FactoryBean if it is a singleton.
if (mbd == null && containsBeanDefinition(beanName)) {
mbd = getMergedLocalBeanDefinition(beanName);
}
boolean synthetic = (mbd != null && mbd.isSynthetic());
object = getObjectFromFactoryBean(factory, beanName, !synthetic);
}
return object;
}
// 从FactoryBean中得到bean
protected Object getObjectFromFactoryBean(FactoryBean<?> factory, String beanName, boolean shouldPostProcess) {
if (factory.isSingleton() && containsSingleton(beanName)) {
synchronized (getSingletonMutex()) {
Object object = this.factoryBeanObjectCache.get(beanName);
if (object == null) {
object = doGetObjectFromFactoryBean(factory, beanName);
// Only post-process and store if not put there already during getObject() call above
// (e.g. because of circular reference processing triggered by custom getBean calls)
Object alreadyThere = this.factoryBeanObjectCache.get(beanName);
if (alreadyThere != null) {
object = alreadyThere;
}
else {
if (shouldPostProcess) {
if (isSingletonCurrentlyInCreation(beanName)) {
// Temporarily return non-post-processed object, not storing it yet..
return object;
}
beforeSingletonCreation(beanName);
try {
object = postProcessObjectFromFactoryBean(object, beanName);
}
catch (Throwable ex) {
throw new BeanCreationException(beanName,
"Post-processing of FactoryBean's singleton object failed", ex);
}
finally {
afterSingletonCreation(beanName);
}
}
if (containsSingleton(beanName)) {
this.factoryBeanObjectCache.put(beanName, object);
}
}
}
return object;
}
}
else {
Object object = doGetObjectFromFactoryBean(factory, beanName);
if (shouldPostProcess) {
try {
object = postProcessObjectFromFactoryBean(object, beanName);
}
catch (Throwable ex) {
throw new BeanCreationException(beanName, "Post-processing of FactoryBean's object failed", ex);
}
}
return object;
}
}
private Object doGetObjectFromFactoryBean(FactoryBean<?> factory, String beanName) throws BeanCreationException {
Object object;
try {
if (System.getSecurityManager() != null) {
AccessControlContext acc = getAccessControlContext();
try {
object = AccessController.doPrivileged((PrivilegedExceptionAction<Object>) factory::getObject, acc);
}
catch (PrivilegedActionException pae) {
throw pae.getException();
}
}
else {
// 这里调用factory的getObject方法从FactoryBean中得到Bean
object = factory.getObject();
}
}
catch (FactoryBeanNotInitializedException ex) {
throw new BeanCurrentlyInCreationException(beanName, ex.toString());
}
catch (Throwable ex) {
throw new BeanCreationException(beanName, "FactoryBean threw exception on object creation", ex);
}
// Do not accept a null value for a FactoryBean that's not fully
// initialized yet: Many FactoryBeans just return null then.
if (object == null) {
if (isSingletonCurrentlyInCreation(beanName)) {
throw new BeanCurrentlyInCreationException(
beanName, "FactoryBean which is currently in creation returned null from getObject");
}
object = new NullBean();
}
return object;
}
这里返回的已经是作为工厂的FactoryBean生产的产品，而不是FactoryBean本身。这种FactoryBean的机制可以为我们提供一个很好的封装机制，比如封装Proxy、RMI、JNDI等。通过对FactoryBean实现过程的原理进行分析，相信读者会对getObject方法有很深刻的印象，这个方法就是主要的FactoryBean的接口，需要实现特定的工厂的生产过程，至于这个生产过程是怎样和IoC容器整合的，就是在上面分析的内容。
对比两者的实现，可以看到FactoryBean类似于AbstractFactory抽象工厂getObjectForBeanInstance()方法类似于createProductA()这样的生产接口，而具体的FactoryBean实现，如TransactionProxyFactoryBean，就是具体的工厂实现，其生成出的 TransactionProxy就是抽象工厂模式中对应的ConcreteProduct。有了抽象工厂设计模式 的参考和对比，对FactoryBean的设计和实现就更容易理解一些了。
BeanPostProcessor的实现
BeanPostProcessor是使用IoC容器时经常会遇到的一个特性，这个Bean的后置处理器是一个监听器，它可以监听容器触发的事件。将它向IoC容器注册后，容器中管理的Bean具备了接收IoC容器事件回调的能力。BeanPostProcessor的使用非常简单，只需要通过设计一个具体的后置处理器来实现。同时，这个具体的后置处理器需要实现接口类BeanPostProcessor，然后设置到XML的Bean配置文件中。这个BeanPostProcessor是一个接口类，它有两个接口方法，一个是postProcessBeforeInitialization，在Bean的初始化前提供回调人口；一个是postProcessAfterInitialization，在Bean的初始化后提供回调入口，这两个回调的触发都是和容器管理Bean的生命周期相关的。这两个回调方法的参数都是一样的，分别是Bean的实例化对象和 Bean的名字。BeanPostProcessor为具体的处理提供基本的回调输入，如下代码所示。
public interface BeanPostProcessor {
@Nullable
default Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException {
return bean;
}
@Nullable
default Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException {
return bean;
}
}
autowiring的实现
在前面对IoC容器实现原理的分析中，一直是通过BeanDefinition的属性值和构造函数以显式的方式对Bean的依赖关系进行管理的。在Spring中，相对这种显式的依赖管理方式：IoC容器还提供了自动依赖装配的方式，为应用使用容器提供更大的方便。在自动装配中，不需要对Bean属性做显式的依赖关系声明，只需要配置好autowiring属性，IoC容器会根据这个属性的配置，使用反射自动查找属性的类型或者名字，然后基于属性的类型或名字来自动匹配loC容器中的Bean，从而自动地完成依赖注入。
org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory#populateBean中的实现如下：
int resolvedAutowireMode = mbd.getResolvedAutowireMode();
if (resolvedAutowireMode == AUTOWIRE_BY_NAME || resolvedAutowireMode == AUTOWIRE_BY_TYPE) {
MutablePropertyValues newPvs = new MutablePropertyValues(pvs);
// Add property values based on autowire by name if applicable.
if (resolvedAutowireMode == AUTOWIRE_BY_NAME) {
autowireByName(beanName, mbd, bw, newPvs);
}
// Add property values based on autowire by type if applicable.
if (resolvedAutowireMode == AUTOWIRE_BY_TYPE) {
autowireByType(beanName, mbd, bw, newPvs);
}
pvs = newPvs;
}
在对autowiring类型做了一些简单的逻辑判断以后，通过调用autowireByName和autowireByType来完成自动依赖装配。以autowireByName为例子来看看容器的自动依赖装配功能是怎样实现的。对autowireByName来说，它首先需要得到当前Bean的属性名，这些属性名已经在BeanWrapper和BeanDefinition中封装好了，然后是对这一系列属性名进行匹配的过程。在匹配的过程中，因为已经有了属性的名字，所以可以直接使用属性名作为Bean名字向容器索取Bean，这个getBean会触发当前Bean的依赖Bean的依赖注入，从而得到属性对应的依赖Bean。在执行完这个getBean后，把这个依赖Bean注入到当前Bean的属性中去，这样就完成了通过这个依赖属性名自动完成依赖注入的过程。
org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory#autowireByName的实现如下：
protected void autowireByName(
String beanName, AbstractBeanDefinition mbd, BeanWrapper bw, MutablePropertyValues pvs) {
String[] propertyNames = unsatisfiedNonSimpleProperties(mbd, bw);
for (String propertyName : propertyNames) {
if (containsBean(propertyName)) {
Object bean = getBean(propertyName);
pvs.add(propertyName, bean);
registerDependentBean(propertyName, beanName);
if (logger.isTraceEnabled()) {
logger.trace("Added autowiring by name from bean name '" + beanName +
"' via property '" + propertyName + "' to bean named '" + propertyName + "'");
}
}
else {
if (logger.isTraceEnabled()) {
logger.trace("Not autowiring property '" + propertyName + "' of bean '" + beanName +
"' by name: no matching bean found");
}
}
}
}
Bean对IOC容器的感知
容器管理的Bean一般不需要了解容器的状态和直接使用容器，但在某些情况下，是需要在Bean中直接对IoC容器进行操作的，这时候，就需要在Bean中设定对容器的感知。Spring IoC容器也提供了该功能，它是通过特定的aware接口来完成的。aware接口有以下这些： （1）BeanNameAware，可以在Bean中得到它在IoC容器中的Bean实例名称。 （2）BeanFactoryAware，可以在Bean中得到Bean所在的IoC容器，从而直接在Bean中使用 IoC容器的服务。 （3）ApplicationContextAware，可以在Bean中得到Bean所在的应用上下文，从而直接在 Bean中使用应用上下文的服务。 （4）MessageSourceAware，在Bean中可以得到消息源。 （5）ApplicationEventPublisherAware，在Bean中可以得到应用上下文的事件发布器，从而 可以在Bean中发布应用上下文的事件。 （6）ResourceLoaderAware，在Bean中可以得到ResourceLoader，从而在Bean中使用 ResourceLoader加载外部对应的Resource资源。
在设置Bean的属性之后，调用初始化回调方法之前，Spring会调用aware接口中的setter 方法。以ApplicationContextAware为例，分析对应的设计和实现。这个接口定义得很简单。
public interface ApplicationContextAware extends Aware {
void setApplicationContext(ApplicationContext applicationContext) throws BeansException;
}
这个对setApplicationContext方法的回调是由容器自动完成的。可以看到，一个ApplicationContextAwareProcessor作为BeanPostProcessor的实现，对一系列的aware回调进行了调用，比如对ResourceLoaderAware接口的调用，对ApplicationEventPublisherAware接口的调用，以及对MessageSourceAware和ApplicationContextAware的接口调用等。
class ApplicationContextAwareProcessor implements BeanPostProcessor {
private final ConfigurableApplicationContext applicationContext;
private final StringValueResolver embeddedValueResolver;
public ApplicationContextAwareProcessor(ConfigurableApplicationContext applicationContext) {
this.applicationContext = applicationContext;
this.embeddedValueResolver = new EmbeddedValueResolver(applicationContext.getBeanFactory());
}
@Override
@Nullable
public Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException {
if (!(bean instanceof EnvironmentAware || bean instanceof EmbeddedValueResolverAware ||
bean instanceof ResourceLoaderAware || bean instanceof ApplicationEventPublisherAware ||
bean instanceof MessageSourceAware || bean instanceof ApplicationContextAware ||
bean instanceof ApplicationStartupAware)) {
return bean;
}
AccessControlContext acc = null;
if (System.getSecurityManager() != null) {
acc = this.applicationContext.getBeanFactory().getAccessControlContext();
}
if (acc != null) {
AccessController.doPrivileged((PrivilegedAction<Object>) () -> {
invokeAwareInterfaces(bean);
return null;
}, acc);
}
else {
invokeAwareInterfaces(bean);
}
return bean;
}
private void invokeAwareInterfaces(Object bean) {
if (bean instanceof EnvironmentAware) {
((EnvironmentAware) bean).setEnvironment(this.applicationContext.getEnvironment());
}
if (bean instanceof EmbeddedValueResolverAware) {
((EmbeddedValueResolverAware) bean).setEmbeddedValueResolver(this.embeddedValueResolver);
}
if (bean instanceof ResourceLoaderAware) {
((ResourceLoaderAware) bean).setResourceLoader(this.applicationContext);
}
if (bean instanceof ApplicationEventPublisherAware) {
((ApplicationEventPublisherAware) bean).setApplicationEventPublisher(this.applicationContext);
}
if (bean instanceof MessageSourceAware) {
((MessageSourceAware) bean).setMessageSource(this.applicationContext);
}
if (bean instanceof ApplicationStartupAware) {
((ApplicationStartupAware) bean).setApplicationStartup(this.applicationContext.getApplicationStartup());
}
if (bean instanceof ApplicationContextAware) {
((ApplicationContextAware) bean).setApplicationContext(this.applicationContext);
}
}
}
而作为依赖注入的一部分，postProcessBeforelnitialization会在initializeBean的实现过程 中被调用，从而实现对aware接口的相关注入。
