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spring在起初是是通过xml文件进行bean的配置的，本文就从xml文件配置作为bean生命周期的一个入口讲起。
一、猜想bean的生命周期
先简单举例个xml的配置，通过如下方式即可配置我们的bean。
<bean id=? class=?>
<property name=? value=?>
<property name=? ref=?>
</bean>
<bean id=? class=?>
<constructor-arg name=? value=?>
<constructor-arg name=? ref=?>
</bean>
那么xml中的bean是如何加载到spring容器的呢？我们不妨做出如下的猜想：
后面章节我们也是大体按照猜想的过程，在源码中逐步的去阅读源码，找到bean真正的生命周期是什么样的。
二、几个问题
2.1 spring是以什么方式实例化对象的？
我们知道的实例化对象的常见方式应该有以下几种： 1）new() 2）反射 3）工厂模式
spring是以何种方式呢？ 众所周知，spring中的bean默认scope是单例的，所以new()的方式肯定不好，我们不妨假装默认spring使用反射的方式，去看看源码的实现是什么样的。
常见的反射方式：
1）Class clazz = Class.forName()
2）Class clazz = 类名.class
3）Class clazz = 对象名.getClass()
拿到clazz文件后，我们就可以通过以下方式去实例化对象，后面我们看看在源码中是否能找到以下代码？
Constructor ctor = clazz.getDeclaredConstructor();
Object object = ctor.newInstance();
2.2 实例化、初始化都做了哪些事情？这时候bean的状态是什么样的？
在我们学习jvm类加载机制时候就知道，类被加载的时候会有准备阶段和初始化的阶段，并且在准备阶段的时候，只是给静态文件在方法区分配内存，并赋默认值。
上面类加载阶段实际与spring的bean实例化，初始化阶段一样，在实例化阶段，会生成一个bean的对象，此时赋默认值，在初始化时，才会对bean进行一个赋值。
具体做了哪些操作我们通过下面的一幅图来简单概括：
三、跟着代码找流程
下面我们开始真正的跟随代码去找spring中bean的生命周期。Spring中，bean的整个生命周期其实是从AbstractApplicationContext中的refresh方法完成的。
public void refresh() throws BeansException, IllegalStateException {
synchronized (this.startupShutdownMonitor) {
// 准备，记录容器的启动时间startupDate, 标记容器为激活，初始化上下文环境如文件路径信息，验证必填属性是否填写
prepareRefresh();
// 获取新的beanFactory，销毁原有beanFactory、为每个bean生成BeanDefinition等
ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();
// 初始化beanFactory的各种属性
prepareBeanFactory(beanFactory);
try {
// 模板方法，此时，所有的beanDefinition已经加载，但是还没有实例化。
//允许在子类中对beanFactory进行扩展处理。比如添加ware相关接口自动装配设置，添加后置处理器等，是子类扩展prepareBeanFactory(beanFactory)的方法
postProcessBeanFactory(beanFactory);
// 实例化并调用所有注册的beanFactory后置处理器（实现接口BeanFactoryPostProcessor的bean，在beanFactory标准初始化之后执行）
invokeBeanFactoryPostProcessors(beanFactory);
//注册bean后置处理器
registerBeanPostProcessors(beanFactory);
// 初始化上下文的消息
initMessageSource();
// 初始化事件
initApplicationEventMulticaster();
// 模板方法，在容器刷新的时候可以自定义逻辑，不同的Spring容器做不同的事情。
onRefresh();
// 注册监听器，广播early application events
registerListeners();
// 实例化所有剩余的（非懒加载）单例
// 比如invokeBeanFactoryPostProcessors方法中根据各种注解解析出来的类，在这个时候都会被初始化。
// 实例化的过程各种BeanPostProcessor开始起作用。
finishBeanFactoryInitialization(beanFactory);
// refresh做完之后需要做的其他事情。
// 清除上下文资源缓存（如扫描中的ASM元数据）
// 初始化上下文的生命周期处理器，并刷新（找出Spring容器中实现了Lifecycle接口的bean并执行start()方法）。
// 发布ContextRefreshedEvent事件告知对应的ApplicationListener进行响应的操作
finishRefresh();
}
catch (BeansException ex) {
if (logger.isWarnEnabled()) {
logger.warn("Exception encountered during context initialization - " +
"cancelling refresh attempt: " + ex);
}
// 销毁已经生成的bean
destroyBeans();
// 重置激活状态.
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();
}
}
}
1、加载配置文件
我们知道，spring定义bean可以通过xml，或者注解的形式，那么是如何将定义好的配置文件加载到IOC容器中呢，spring提供了一个接口规范 BeanDefinitionReader ：
除了这个接口之外，还定义了针对注解的 AnnotatedBeanDefinitionReader 。
那么有了这两个组件后我们的整体流程会如下图所示：
将配置文件通过以上的接口和类加载到spring的ioc容器当中，处理成满足ioc容器的bean结构。
下面具体看下源码是如何实现的 ：
public void refresh() throws BeansException, IllegalStateException {
synchronized (this.startupShutdownMonitor) {
// 准备，记录容器的启动时间startupDate, 标记容器为激活，初始化上下文环境如文件路径信息，验证必填属性是否填写
prepareRefresh();
// 获取新的beanFactory，销毁原有beanFactory、为每个bean生成BeanDefinition等
ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();
在refresh方法中，目前主要关注obtainFreshBeanFactory()这个方法。
protected ConfigurableListableBeanFactory obtainFreshBeanFactory() {
//刷新
refreshBeanFactory();
return getBeanFactory();
}
继续跟踪refreshBeanFactory()。
protected final void refreshBeanFactory() throws BeansException {
//销毁原有的beanFactory
if (hasBeanFactory()) {
destroyBeans();
closeBeanFactory();
}
try {
//创建一个BeanFactory
DefaultListableBeanFactory beanFactory = createBeanFactory();
//给当前的bean设置id
beanFactory.setSerializationId(getId());
//设置是否循环引用和是否重名注册
customizeBeanFactory(beanFactory);
//从配置文件加载bean信息为beanDefinition
loadBeanDefinitions(beanFactory);
this.beanFactory = beanFactory;
}
catch (IOException ex) {
throw new ApplicationContextException("I/O error parsing bean definition source for " + getDisplayName(), ex);
}
}
主要关注loadBeanDefinitions(beanFactory)。
protected void loadBeanDefinitions(DefaultListableBeanFactory beanFactory) throws BeansException, IOException {
// 为beanFactory创建一个XML格式的bean定义读取器
XmlBeanDefinitionReader beanDefinitionReader = new XmlBeanDefinitionReader(beanFactory);
//给beanDefinitionReader设置环境变量
beanDefinitionReader.setEnvironment(getEnvironment());
//给beanDefinitionReader设置资源加载器
beanDefinitionReader.setResourceLoader(this);
//给beanDefinitionReader设置实体解析器
beanDefinitionReader.setEntityResolver(new ResourceEntityResolver(this));
//目前没有实内容
initBeanDefinitionReader(beanDefinitionReader);
//从配置文件获取bean的定义
loadBeanDefinitions(beanDefinitionReader);
}
进入loadBeanDefinitions(beanDefinitionReader)。
protected void loadBeanDefinitions(XmlBeanDefinitionReader reader) throws IOException {
//获取配置文件的地址，/WEB-INF/下的xml文件
String[] configLocations = getConfigLocations();
if (configLocations != null) {
//遍历
for (String configLocation : configLocations) {
//加载bean定义
reader.loadBeanDefinitions(configLocation);
}
}
}
里面的具体这里不再深究了，后面会通过递归的方式逐步解析xml文件的各个节点信息。
通过上面你的分析，我们发现通过 BeanDefinitionReader 的解析处理， xml中的bean信息最终变成了能被IOC容器接收的BeanDefinition，并且创建了一个BeanFactory ，自重的bean生命周期变成如下图所示：
2、做准备工作
上一小节中，通过加载配置文件，生成了BeanDefinition和Beanfactory，后面继续跟踪refresh代码。在代码后面的执行过程中，实际要做很多的准备工作，例如提供BeanFactoryPostProcessor，注册BeanFactoryPostProcessor，国际化，初始化事件多播器，注册监听器等
public void refresh() throws BeansException, IllegalStateException {
synchronized (this.startupShutdownMonitor) {
// 准备，记录容器的启动时间startupDate, 标记容器为激活，初始化上下文环境如文件路径信息，验证必填属性是否填写
prepareRefresh();
// 获取新的beanFactory，销毁原有beanFactory、为每个bean生成BeanDefinition等
ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();
// 初始化beanFactory的各种属性
prepareBeanFactory(beanFactory);
try {
// 空模板方法，此时，所有的beanDefinition已经加载，但是还没有实例化。
//允许在子类中对beanFactory进行扩展处理。比如添加ware相关接口自动装配设置，添加后置处理器等，是子类扩展prepareBeanFactory(beanFactory)的方法
postProcessBeanFactory(beanFactory);
// 实例化并调用所有注册的beanFactory后置处理器（实现接口BeanFactoryPostProcessor的bean，在beanFactory标准初始化之后执行）
invokeBeanFactoryPostProcessors(beanFactory);
//注册bean后置处理器
registerBeanPostProcessors(beanFactory);
// 初始化上下文的消息
initMessageSource();
// 初始化事件
initApplicationEventMulticaster();
// 模板方法，在容器刷新的时候可以自定义逻辑，不同的Spring容器做不同的事情。
onRefresh();
// 注册监听器，广播early application events
registerListeners();
这个过程中基本在做准备工作，除了BeanFactoryPostProcesser可以对已经定义的bean进行增强。经历过这些阶段后的流程如下。
3、实例化
当前面的准备工作做完后，真正开始bean的实例化：
// 实例化所有剩余的（非懒加载）单例
finishBeanFactoryInitialization(beanFactory);
跟踪到内部，发现，前面都在设置一些属性，直接看最后一句：
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 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.
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();
// 实例化所有剩下单例bean
beanFactory.preInstantiateSingletons();
}
跟踪 beanFactory.preInstantiateSingletons() 方法，忽略其他方法，直接到 getBean() 。
public void preInstantiateSingletons() throws BeansException {
if (logger.isTraceEnabled()) {
logger.trace("Pre-instantiating singletons in " + this);
}
// 获取bean名称
List<String> beanNames = new ArrayList<>(this.beanDefinitionNames);
// 遍历实例化所有非延时bean
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 {
//获取bean
getBean(beanName);
}
}
}
// Trigger post-initialization callback for all applicable beans...
for (String beanName : beanNames) {
Object singletonInstance = getSingleton(beanName);
if (singletonInstance instanceof SmartInitializingSingleton) {
SmartInitializingSingleton smartSingleton = (SmartInitializingSingleton) singletonInstance;
if (System.getSecurityManager() != null) {
AccessController.doPrivileged((PrivilegedAction<Object>) () -> {
smartSingleton.afterSingletonsInstantiated();
return null;
}, getAccessControlContext());
}
else {
smartSingleton.afterSingletonsInstantiated();
}
}
}
}
下面是真正实例化bean的方法：
@Override
public Object getBean(String name) throws BeansException {
//真正实例化bean的方法
return doGetBean(name, null, null, false);
}
首次启动时候，bean没有被创建过，我们直接跟踪到下面方法的 createBean(beanName, mbd, args) ：
protected <T> T doGetBean(
String name, @Nullable Class<T> requiredType, @Nullable Object[] args, boolean typeCheckOnly)
throws BeansException {
String beanName = transformedBeanName(name);
Object bean;
// Eagerly check singleton cache for manually registered singletons.
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 {
// Fail if we're already creating this bean instance:
// We're assumably within a circular reference.
if (isPrototypeCurrentlyInCreation(beanName)) {
throw new BeanCurrentlyInCreationException(beanName);
}
// Check if bean definition exists in this factory.
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 {
RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
checkMergedBeanDefinition(mbd, beanName, args);
// Guarantee initialization of beans that the current bean depends on.
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);
}
}
}
// 创建bean实例
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();
if (!StringUtils.hasLength(scopeName)) {
throw new IllegalStateException("No scope name defined for bean ´" + beanName + "'");
}
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 ScopeNotActiveException(beanName, scopeName, 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;
}
createBean(beanName, mbd, args)方法：
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.
Class<?> resolvedClass = resolveBeanClass(mbd, beanName);
if (resolvedClass != null && !mbd.hasBeanClass() && mbd.getBeanClassName() != null) {
mbdToUse = new RootBeanDefinition(mbd);
mbdToUse.setBeanClass(resolvedClass);
}
// Prepare method overrides.
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.
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 {
//真正创建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);
}
}
继续跟踪上面方法的 doCreateBean(beanName, mbdToUse, args) ，这里面逐渐接近 反射创建实例化对象 了：
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) {
//创建bean实例
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 {
//填充属性
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;
}
跟踪 createBeanInstance(beanName, mbd, args) ：
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);
}
// 使用无参构造实例化bean
return instantiateBean(beanName, mbd);
}
跟踪 instantiateBean(beanName, mbd) ：
protected BeanWrapper instantiateBean(String beanName, RootBeanDefinition mbd) {
try {
Object beanInstance;
if (System.getSecurityManager() != null) {
beanInstance = AccessController.doPrivileged(
(PrivilegedAction<Object>) () -> getInstantiationStrategy().instantiate(mbd, beanName, this),
getAccessControlContext());
}
else {
//获取实例化策略进行实例化
beanInstance = getInstantiationStrategy().instantiate(mbd, beanName, this);
}
BeanWrapper bw = new BeanWrapperImpl(beanInstance);
initBeanWrapper(bw);
return bw;
}
catch (Throwable ex) {
throw new BeanCreationException(
mbd.getResourceDescription(), beanName, "Instantiation of bean failed", ex);
}
}
跟踪 getInstantiationStrategy().instantiate(mbd, beanName, this) ，我们发现了很熟悉的一条代码 clazz.getDeclaredConstructor() ，刚好证明了前面的问题， spring是通过反射实现bean的实例化 的，并且通过最后一行代码 BeanUtils.instantiateClass(constructorToUse) ，在此方法内部执行了bean的实例化：
public Object instantiate(RootBeanDefinition bd, @Nullable String beanName, BeanFactory owner) {
// Don't override the class with CGLIB if no overrides.
if (!bd.hasMethodOverrides()) {
Constructor<?> constructorToUse;
synchronized (bd.constructorArgumentLock) {
constructorToUse = (Constructor<?>) bd.resolvedConstructorOrFactoryMethod;
if (constructorToUse == null) {
final Class<?> clazz = bd.getBeanClass();
if (clazz.isInterface()) {
throw new BeanInstantiationException(clazz, "Specified class is an interface");
}
try {
if (System.getSecurityManager() != null) {
constructorToUse = AccessController.doPrivileged(
(PrivilegedExceptionAction<Constructor<?>>) clazz::getDeclaredConstructor);
}
else {
//反射获取bean的构造
constructorToUse = clazz.getDeclaredConstructor();
}
bd.resolvedConstructorOrFactoryMethod = constructorToUse;
}
catch (Throwable ex) {
throw new BeanInstantiationException(clazz, "No default constructor found", ex);
}
}
}
return BeanUtils.instantiateClass(constructorToUse);
}
else {
// Must generate CGLIB subclass.
return instantiateWithMethodInjection(bd, beanName, owner);
}
}
通过上面很长一段代码跟踪，我们发现了bean的实例化是通过BeanFactory经过一系列的处理，最终调用初始化策略，使用反射的方式实例化的 ，那么最终的生命周期将变成下面样：
4、初始化
bean经过实例化后，其内部都是默认值，需要经过初始化进行赋值等操作，我们根据代码继续看它的实现过程。 在实例化过程中，有一个方法doCreateBean，在其执行过程中，创建完实例化对象后，对该对象进行了初始化，代码如下：
//填充属性
populateBean(beanName, mbd, instanceWrapper);
//初始化bean
exposedObject = initializeBean(beanName, exposedObject, mbd);
populateBean具体代码不做过多研究了，直接看initializeBean(beanName, exposedObject, mbd)：
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 {
//执行实现Aware接口的方法
invokeAwareMethods(beanName, bean);
}
Object wrappedBean = bean;
if (mbd == null || !mbd.isSynthetic()) {
//在初始化bean前执行bean的后置处理器
wrappedBean = applyBeanPostProcessorsBeforeInitialization(wrappedBean, beanName);
}
try {
//执行初始化方法（xml配置的init-method）
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()) {
//在初始化bean后执行bean的后置处理器
wrappedBean = applyBeanPostProcessorsAfterInitialization(wrappedBean, beanName);
}
return wrappedBean;
}
在上面的代码中有几个重要的过程： 1） 执行实现Aware接口的方法 。 2） 在初始化bean前执行bean的后置处理器 。 3） 执行初始化方法（xml配置的init-method） 。 4） 在初始化bean后执行bean的后置处理器 。 在经过上面的阶段后，则整个对象完成了其初始化的过程，成为了一个完整对象。 那么其生命周期就如下图所示：
