七千字長文深刻解讀，Spirng中是如何初始化單例Bean的，和面試中最常問的Spring是如何解決循環依賴？
今天解讀Spring核心方法refresh()中最最重要的一個方法finishBeanFactoryInitialization()方法，該方法負責初始化所有的單例bean。
finishBeanFactoryInitialization()方法位於refresh()中下標為8的位置。
到目前為止，應該說是是 BeanFactory 已經創建完成，並且所有的實現了 BeanFactoryPostProcessor 接口的 Bean 都已經初始化並且其中的 postProcessBeanFactory(factory) 方法已經得到回調執行了。而且 Spring 已經「手動」註冊了一些特殊的 Bean，如 environment、systemProperties 等。
剩下的就是初始化 singleton beans 了，大多數我們的業務中都是單例bean，就像我們寫的@Controller、@Service的類（沒有設置懶加載的）都是在這個地方初始化，以供我們使用，如果沒有設置懶加載，那麼 Spring 會在接下來初始化所有的 singleton beans。
我們先看一下refresh()的源碼，大概看下finishBeanFactoryInitialization(beanFactory)所處的位置。
@Override
public void refresh() throws BeansException, IllegalStateException {
synchronized (this.startupShutdownMonitor) {
// Prepare this context for refreshing.
//1、刷新前的準備
prepareRefresh();
// Tell the subclass to refresh the internal bean factory.
//2、將會初始化 BeanFactory、加載 Bean、註冊 Bean
ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();
// Prepare the bean factory for use in this context.
//3、設置 BeanFactory 的類加載器，添加幾個 BeanPostProcessor，手動註冊幾個特殊的 bean
prepareBeanFactory(beanFactory);
try {
//4、模板方法
// Allows post-processing of the bean factory in context subclasses.
postProcessBeanFactory(beanFactory);
// Invoke factory processors registered as beans in the context.
//執行BeanFactory後置處理器
invokeBeanFactoryPostProcessors(beanFactory);
// 5、Register bean processors that intercept bean creation.
//註冊bean後置處理器
registerBeanPostProcessors(beanFactory);
// Initialize message source for this context.
//國際化
initMessageSource();
// Initialize event multicaster for this context.
initApplicationEventMulticaster();
// Initialize other special beans in specific context subclasses.
//6、模板方法--springboot實現了這個方法
onRefresh();
// Check for listener beans and register them.
//7、註冊監聽器
registerListeners();
// Instantiate all remaining (non-lazy-init) singletons.
//8、完成bean工廠的初始化**方法重要**********************************************
finishBeanFactoryInitialization(beanFactory);
//9、 Last step: publish corresponding event.
finishRefresh();
}
1.2.3.4.5.6.7.8.9.10.11.12.13.14.15.16.17.18.19.20.21.22.23.24.25.26.27.28.29.30.31.32.33.34.35.36.37.38.39.40.41.42.43.44.45.46.47.48.49.50.我們深入finishBeanFactoryInitialization(beanFactory)中，裡面的調用線路錯綜複雜，還望讀者可以做好心理準備。
/**
* 負責單例bean的初始化
* Finish the initialization of this context's bean factory,
* initializing all remaining singleton beans.
*/
protected void finishBeanFactoryInitialization(ConfigurableListableBeanFactory beanFactory) {
// Initialize conversion service for this context.
//最先初始化名字為 conversionService的類，conversionService類 它用來將前端傳過來的參數和後端的 controller 方法上的參數進行綁定的時候用
//尤其是用於非基礎類型的轉換
if (beanFactory.containsBean(CONVERSION_SERVICE_BEAN_NAME) &&
beanFactory.isTypeMatch(CONVERSION_SERVICE_BEAN_NAME, ConversionService.class)) {
beanFactory.setConversionService(
//初始化在getBean()方法中實現
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.
// 先初始化 LoadTimeWeaverAware 類型的 Bean aop相關注：大概有個印象，以後解析aop會和它串起來。
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.
//freeze的單詞意思是凍結，這個時候已經開始預初始化， bean 定義解析、加載、註冊先停止
beanFactory.freezeConfiguration();
// Instantiate all remaining (non-lazy-init) singletons.
//開始初始化
beanFactory.preInstantiateSingletons();
}
1.2.3.4.5.6.7.8.9.10.11.12.13.14.15.16.17.18.19.20.21.22.23.24.25.26.27.28.29.30.31.32.33.34.35.36.37.38.39.40.41.該方法是判斷bean的一系列是不是屬於某個類型的bean，如果是就調用getBean()方法，如果不是，就調用beanFactory.preInstantiateSingletons()進行初始化，我們先把getBean()放一放，重點看一看beanFactory.preInstantiateSingletons()方法。
@Override
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.
// this.beanDefinitionNames 保存了所有的 beanNames
List<String> beanNames = new ArrayList<>(this.beanDefinitionNames);
// Trigger initialization of all non-lazy singleton beans...
//// 下面這個循環，觸發所有的非懶加載的 singleton beans 的初始化操作
for (String beanName : beanNames) {
RootBeanDefinition bd = getMergedLocalBeanDefinition(beanName);
// 非抽象、非懶加載的 singletons。如果配置了 'abstract = true'，那是不需要初始化的
if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) {
// 處理 FactoryBean （負責初始化工廠的bean）
if (isFactoryBean(beanName)) {
// FactoryBean 的話，在 beanName 前面加上 『&』 符號
//此處調用getBean()方法
Object bean = getBean(FACTORY_BEAN_PREFIX + beanName);
if (bean instanceof FactoryBean) {
FactoryBean<?> factory = (FactoryBean<?>) bean;
// 判斷當前 FactoryBean 是否是 SmartFactoryBean 的實現
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 {
// 對於普通的 Bean，只要調用 getBean(beanName) 這個方法就可以進行初始化了
getBean(beanName);
}
}
}
// Trigger post-initialization callback for all applicable beans...
// 到這裡說明所有的非懶加載的 singleton beans 已經完成了初始化
// 如果我們定義的 bean 是實現了 SmartInitializingSingleton 接口的，那麼在這裡得到回調
//如果你想在單例bean初始化後做一些事 那就實現該接口
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();
}
}
}
}
1.2.3.4.5.6.7.8.9.10.11.12.13.14.15.16.17.18.19.20.21.22.23.24.25.26.27.28.29.30.31.32.33.34.35.36.37.38.39.40.41.42.43.44.45.46.47.48.49.50.51.52.53.54.55.56.57.58.59.60.61.62.63.64.65.66.67.68.preInstantiateSingletons()方法的主要任務是進行初始化的，在初始化前同樣是一系列判斷，如，是否是懶加載的，是否是一個factorybean（一個特別的bean，負責工廠創建的bean），最後調用getBean()方法。
其中有個插曲是否實現了SmartInitializingSingleton接口，將接口讓你可以在bean初始化後做一些事，我們寫一個簡單的實例測試一下。
其他地方讀者看注釋了解一下即可，我們開始繼續深入getBean()方法。
getBean()方法內部調用了doGetBean()我們直接看doGetBean方法。
// 我們在剖析初始化 Bean 的過程，但是 getBean 方法我們經常是用來從容器中獲取 Bean 用的，注意切換思路，
// 已經初始化過了就從容器中直接返回，否則就先初始化再返回
protected <T> T doGetBean(
String name, @Nullable Class<T> requiredType, @Nullable Object[] args, boolean typeCheckOnly)
throws BeansException {
// 獲取一個 「正統的」 beanName，處理兩種情況，一個是前面說的 FactoryBean(前面帶 『&』)，
// 一個是別名問題，因為這個方法是 getBean，獲取 Bean 用的，你要是傳一個別名進來，是完全可以的
String beanName = transformedBeanName(name);
// 返回值
Object bean;
// Eagerly check singleton cache for manually registered singletons.
// 檢查下是不是已經創建過了
Object sharedInstance = getSingleton(beanName);
// 這裡說下 args ，雖然看上去一點不重要。前面我們一路進來的時候都是 getBean(beanName)，
// 所以 args 傳參其實是 null 的，但是如果 args 不為空的時候，那麼意味著調用方不是希望獲取 Bean，而是創建 Bean
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 + "'");
}
}
// 下面這個方法：如果是普通 Bean 的話，直接返回 sharedInstance，
// 如果是 FactoryBean 的話，返回它創建的那個實例對象
bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
}
else {
// Fail if we're already creating this bean instance:
// We're assumably within a circular reference.
// 創建過了此 beanName 的 prototype 類型的 bean，那麼拋異常，
// 往往是因為陷入了循環引用 哦，原來之前的循環依賴都是在這拋的異常，再有問題就不是無頭蒼蠅了
if (isPrototypeCurrentlyInCreation(beanName)) {
throw new BeanCurrentlyInCreationException(beanName);
}
// Check if bean definition exists in this factory.
// 檢查一下這個 BeanDefinition 在容器中是否存在 BeanDefinition既是包含了bean的一系列信息
BeanFactory parentBeanFactory = getParentBeanFactory();
if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {
// Not found -> check parent.
// 如果當前容器不存在這個 BeanDefinition，試試父容器中有沒有
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.
// 返回父容器的查詢結果
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) {
// typeCheckOnly 為 false，將當前 beanName 放入一個 alreadyCreated 的 Set 集合中。
markBeanAsCreated(beanName);
}
/*
* 稍稍總結一下：
* 到這裡的話，要準備創建 Bean 了，對於 singleton 的 Bean 來說，容器中還沒創建過此 Bean；
* 對於 prototype 的 Bean 來說，本來就是要創建一個新的 Bean。
*/
try {
RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
checkMergedBeanDefinition(mbd, beanName, args);
// Guarantee initialization of beans that the current bean depends on.
// 先初始化依賴的所有 Bean，這個很好理解。
// 注意，這裡的依賴指的是 depends-on 中定義的依賴
String[] dependsOn = mbd.getDependsOn();
if (dependsOn != null) {
for (String dep : dependsOn) {
// 檢查是不是有循環依賴，這裡的循環依賴和我們前面說的循環依賴又不一樣
if (isDependent(beanName, dep)) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"Circular depends-on relationship between '" + beanName + "' and '" + dep + "'");
}
// 註冊一下依賴關係
registerDependentBean(dep, beanName);
try {
// 先初始化被依賴項
getBean(dep);
}
catch (NoSuchBeanDefinitionException ex) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"'" + beanName + "' depends on missing bean '" + dep + "'", ex);
}
}
}
// Create bean instance.
// 如果是 singleton scope 的，創建 singleton 的實例
if (mbd.isSingleton()) {
sharedInstance = getSingleton(beanName, () -> {
try {
// 執行創建 Bean，詳情繼續深入
// 第三個參數 args 數組代表創建實例需要的參數，不就是給構造方法用的參數，或者是工廠 Bean 的參數嘛，不過要注意，在我們的初始化階段，args 是 null。
// 這回我們要到一個新的類了 AbstractAutowireCapableBeanFactory，看類名，AutowireCapable？類名是不是也說明了點問題了。
// 主要是為了以下場景，採用 @Autowired 註解注入屬性值：
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);
}
// 如果是 prototype scope 的，創建 prototype 的實例
else if (mbd.isPrototype()) {
// It's a prototype -> create a new instance.
Object prototypeInstance = null;
try {
beforePrototypeCreation(beanName);
// 執行創建 Bean
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 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.
// 最後，檢查一下類型對不對，不對的話就拋異常，對的話就返回了
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;
}
1.2.3.4.5.6.7.8.9.10.11.12.13.14.15.16.17.18.19.20.21.22.23.24.25.26.27.28.29.30.31.32.33.34.35.36.37.38.39.40.41.42.43.44.45.46.47.48.49.50.51.52.53.54.55.56.57.58.59.60.61.62.63.64.65.66.67.68.69.70.71.72.73.74.75.76.77.78.79.80.81.82.83.84.85.86.87.88.89.90.91.92.93.94.95.96.97.98.99.100.101.102.103.104.105.106.107.108.109.110.111.112.113.114.115.116.117.118.119.120.121.122.123.124.125.126.127.128.129.130.131.132.133.134.135.136.137.138.139.140.141.142.143.144.145.146.147.148.149.150.151.152.153.154.155.156.157.158.159.160.161.162.163.164.165.166.167.168.169.170.171.172.173.174.175.176.177.178.179.180.181.182.183.184.185.186.187.188.189.190.191.192.193.194.195.196.具體的實例化過程在createBean()方法中，我們繼續深入createBean()方法。
@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.
// 確保 BeanDefinition 中的 Class 被加載
Class<?> resolvedClass = resolveBeanClass(mbd, beanName);
if (resolvedClass != null && !mbd.hasBeanClass() && mbd.getBeanClassName() != null) {
mbdToUse = new RootBeanDefinition(mbd);
mbdToUse.setBeanClass(resolvedClass);
}
// Prepare method overrides.
// 準備方法覆寫，這裡又涉及到一個概念：MethodOverrides，它來自於 bean 定義中的 <lookup-method />
// 和 <replaced-method />，如果讀者感興趣，回到 bean 解析的地方看看對這兩個標籤的解析。
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.
// 讓 InstantiationAwareBeanPostProcessor 在這一步有機會返回代理，
// 在 《Spring AOP 源碼分析》那篇文章中有解釋，這裡先跳過
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 {
// 重頭戲，創建 bean
Object beanInstance = doCreateBean(beanName, mbdToUse, args);
if (logger.isTraceEnabled()) {
logger.trace("Finished creating instance of bean '" + beanName + "'");
}
return beanInstance;
}
catch (BeanCreationException | ImplicitlyAppearedSingletonException ex) {
// A previously detected exception with proper bean creation context already,
// or illegal singleton state to be communicated up to DefaultSingletonBeanRegistry.
throw ex;
}
catch (Throwable ex) {
throw new BeanCreationException(
mbdToUse.getResourceDescription(), beanName, "Unexpected exception during bean creation", ex);
}
}
1.2.3.4.5.6.7.8.9.10.11.12.13.14.15.16.17.18.19.20.21.22.23.24.25.26.27.28.29.30.31.32.33.34.35.36.37.38.39.40.41.42.43.44.45.46.47.48.49.50.51.52.53.54.55.56.57.58.59.60.61.62.我們繼續往裡看 doCreateBean 這個方法,這個調用過程是真的深。
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) {
// 說明不是 FactoryBean，這裡實例化 Bean，這裡非常關鍵，細節之後再說**********
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.
// 下面這塊代碼是為了解決循環依賴的問題，這是個重頭戲，解決循環依賴問題
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 {
// 這一步也是非常關鍵的，這一步負責屬性裝配，因為前面的實例只是實例化了，並沒有設值，這裡就是設值***************
populateBean(beanName, mbd, instanceWrapper);
// 還記得 init-method 嗎？還有 InitializingBean 接口？還有 BeanPostProcessor 接口？
// 這裡就是處理 bean 初始化完成後的各種回調**************
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);
}
}
// 下面這塊代碼是為了解決循環依賴的問題，這是個重頭戲，解決循環依賴問題
if (earlySingletonExposure) {
//循環依賴的核心方法調用
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;
}
1.2.3.4.5.6.7.8.9.10.11.12.13.14.15.16.17.18.19.20.21.22.23.24.25.26.27.28.29.30.31.32.33.34.35.36.37.38.39.40.41.42.43.44.45.46.47.48.49.50.51.52.53.54.55.56.57.58.59.60.61.62.63.64.65.66.67.68.69.70.71.72.73.74.75.76.77.78.79.80.81.82.83.84.85.86.87.88.89.90.91.92.93.94.95.96.97.98.99.100.101.102.103.到這裡，我們已經分析完了 doCreateBean 方法，總的來說，我們已經說完了整個初始化流程。
在實例化bean後有一個特別重要的知識點，也是面試中最常問的，Spring怎麼解決循環依賴問題？核心代碼就在這個方法裡面。
循環依賴其實就是循環引用，也就是兩個或則兩個以上的bean互相持有對方，最終形成閉環。比如A依賴於B，B依賴於C，C又依賴於A。如下圖：
doCreateBean 方法有三個核心流程。
（1）createBeanInstance：實例化，其實也就是調用對象的構造方法實例化對象
（2）populateBean：填充屬性，這一步主要是多bean的依賴屬性進行填充
（3）initializeBean：調用spring xml中的init 方法。
從上面講述的單例bean初始化步驟我們可以知道，循環依賴主要發生在第一、第二步。也就是構造器循環依賴和field循環依賴。
那麼我們要解決循環引用也應該從初始化過程著手，對於單例來說，在Spring容器整個生命周期內，有且只有一個對象，所以很容易想到這個對象應該存在Cache中，Spring為了解決單例的循環依賴問題，使用了三級緩存。
我們看一下getSingleton方法。
該方法還依賴於三個map，這三個map就是三級緩存。
/** Cache of singleton objects: bean name to bean instance. */
//單例對象的cache
private final Map<String, Object> singletonObjects = new ConcurrentHashMap<>(256);
/** Cache of singleton factories: bean name to ObjectFactory. */
// 單例對象工廠的cache
private final Map<String, ObjectFactory<?>> singletonFactories = new HashMap<>(16);
/** Cache of early singleton objects: bean name to bean instance. */
//提前曝光的單例對象的Cache
private final Map<String, Object> earlySingletonObjects = new ConcurrentHashMap<>(16);
1.2.3.4.5.6.7.8.9.10.11.@Nullable
protected Object getSingleton(String beanName, boolean allowEarlyReference) {
// Quick check for existing instance without full singleton lock
Object singletonObject = this.singletonObjects.get(beanName);
//判斷當前單例bean是否正在創建中，也就是沒有初始化完成(比如A的構造器依賴了B對象所以得先去創建B對象
// 或則在A的populateBean過程中依賴了B對象，得先去創建B對象，這時的A就是處於創建中的狀態。
if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
singletonObject = this.earlySingletonObjects.get(beanName);
// 是否允許從singletonFactories中通過getObject拿到對象
if (singletonObject == null && allowEarlyReference) {
synchronized (this.singletonObjects) {
// Consistent creation of early reference within full singleton lock
singletonObject = this.singletonObjects.get(beanName);
if (singletonObject == null) {
singletonObject = this.earlySingletonObjects.get(beanName);
if (singletonObject == null) {
ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName);
if (singletonFactory != null) {
singletonObject = singletonFactory.getObject();
this.earlySingletonObjects.put(beanName, singletonObject);
this.singletonFactories.remove(beanName);
}
}
}
}
}
}
return singletonObject;
}
1.2.3.4.5.6.7.8.9.10.11.12.13.14.15.16.17.18.19.20.21.22.23.24.25.26.27.28.29.30.31.分析getSingleton()的整個過程，Spring首先從一級緩存singletonObjects中獲取。如果獲取不到，並且對象正在創建中，就再從二級緩存earlySingletonObjects中獲取。
如果還是獲取不到且允許singletonFactories通過getObject()獲取，就從三級緩存singletonFactory.getObject()(三級緩存)獲取，如果獲取到了則：
this.earlySingletonObjects.put(beanName, singletonObject);
this.singletonFactories.remove(beanName);
1.2.3.從singletonFactories中移除，並放入earlySingletonObjects中。其實也就是從三級緩存移動到了二級緩存。
從上面三級緩存的分析，我們可以知道，Spring解決循環依賴的訣竅就在於singletonFactories這個三級cache。
里就是解決循環依賴的關鍵，這段代碼發生在createBeanInstance之後，也就是說單例對象此時已經被創建出來(調用了構造器)。這個對象已經被生產出來了，雖然還不完美（還沒有進行初始化的第二步和第三步），但是已經能被人認出來了（根據對象引用能定位到堆中的對象），所以Spring此時將這個對象提前曝光出來讓大家認識，讓大家使用。
這樣做有什麼好處呢？
讓我們來分析一下「A的某個field或者setter依賴了B的實例對象，同時B的某個field或者setter依賴了A的實例對象」這種循環依賴的情況。
A首先完成了初始化的第一步，並且將自己提前曝光到singletonFactories中，此時進行初始化的第二步，發現自己依賴對象B，此時就嘗試去get(B)，發現B還沒有被create，所以走create流程，B在初始化第一步的時候發現自己依賴了對象A，於是嘗試get(A)，嘗試一級緩存singletonObjects(肯定沒有，因為A還沒初始化完全)，嘗試二級緩存earlySingletonObjects（也沒有），嘗試三級緩存singletonFactories，由於A通過ObjectFactory將自己提前曝光了，所以B能夠通過ObjectFactory.getObject拿到A對象(雖然A還沒有初始化完全，但是總比沒有好呀)，B拿到A對象後順利完成了初始化階段1、2、3，完全初始化之後將自己放入到一級緩存singletonObjects中。
此時返回A中，A此時能拿到B的對象順利完成自己的初始化階段2、3，最終A也完成了初始化，進去了一級緩存singletonObjects中，而且更加幸運的是，由於B拿到了A的對象引用，所以B現在hold住的A對象完成了初始化。
知道了這個原理時候，肯定就知道為啥Spring不能解決「A的構造方法中依賴了B的實例對象，同時B的構造方法中依賴了A的實例對象」這類問題了！因為加入singletonFactories三級緩存的前提是執行了構造器，所以構造器的循環依賴沒法解決。
接下來我們挑 doCreateBean 中的三個細節出來說說。一個是創建 Bean 實例的 createBeanInstance 方法，一個是依賴注入的 populateBean 方法，還有就是回調方法 initializeBean。
這三個方法也是極其複雜的，讀者有興趣可以繼續的深入進去。
1、 createBeanInstance 方法
protected BeanWrapper createBeanInstance(String beanName, RootBeanDefinition mbd, @Nullable Object[] args) {
// Make sure bean class is actually resolved at this point.
// 確保已經加載了此 class
Class<?> beanClass = resolveBeanClass(mbd, beanName);
// 校驗一下這個類的訪問權限
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) {
// 採用工廠方法實例化，不熟悉這個概念的讀者請看附錄，注意，不是 FactoryBean
return instantiateUsingFactoryMethod(beanName, mbd, args);
}
// Shortcut when re-creating the same bean...
// 如果不是第一次創建，比如第二次創建 prototype bean。
// 這種情況下，我們可以從第一次創建知道，採用無參構造函數，還是構造函數依賴注入 來完成實例化
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) {
// 構造函數依賴注入
return autowireConstructor(beanName, mbd, null, null);
}
else {
// 無參構造函數
return instantiateBean(beanName, mbd);
}
}
// Candidate constructors for autowiring?
// 判斷是否採用有參構造函數
Constructor<?>[] ctors = determineConstructorsFromBeanPostProcessors(beanClass, beanName);
if (ctors != null || mbd.getResolvedAutowireMode() == AUTOWIRE_CONSTRUCTOR ||
mbd.hasConstructorArgumentValues() || !ObjectUtils.isEmpty(args)) {
// 構造函數依賴注入
return autowireConstructor(beanName, mbd, ctors, args);
}
// Preferred constructors for default construction?
ctors = mbd.getPreferredConstructors();
if (ctors != null) {
// 構造函數依賴注入
return autowireConstructor(beanName, mbd, ctors, null);
}
// No special handling: simply use no-arg constructor.
// 調用無參構造函數
return instantiateBean(beanName, mbd);
}
1.2.3.4.5.6.7.8.9.10.11.12.13.14.15.16.17.18.19.20.21.22.23.24.25.26.27.28.29.30.31.32.33.34.35.36.37.38.39.40.41.42.43.44.45.46.47.48.49.50.51.52.53.54.55.56.57.58.59.60.61.62.63.64.65.看一下instantiateBean方法是怎麼做的。
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 {
// 實例化
beanInstance = getInstantiationStrategy().instantiate(mbd, beanName, this);
}
// 包裝一下，返回
BeanWrapper bw = new BeanWrapperImpl(beanInstance);
initBeanWrapper(bw);
return bw;
}
catch (Throwable ex) {
throw new BeanCreationException(
mbd.getResourceDescription(), beanName, "Instantiation of bean failed", ex);
}
}
1.2.3.4.5.6.7.8.9.10.11.12.13.14.15.16.17.18.19.20.21.22.我們可以看到，關鍵的地方在於：beanInstance = getInstantiationStrategy().instantiate(mbd, beanName, parent);
裡面是具體是實例化過程，我們進去看看。
@Override
public Object instantiate(RootBeanDefinition bd, @Nullable String beanName, BeanFactory owner) {
// Don't override the class with CGLIB if no overrides.
// 如果不存在方法覆寫，那就使用 java 反射進行實例化，否則使用 CGLIB,
// 方法覆寫 請參見附錄"方法注入"中對 lookup-method 和 replaced-method 的介紹
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 {
constructorToUse = clazz.getDeclaredConstructor();
}
bd.resolvedConstructorOrFactoryMethod = constructorToUse;
}
catch (Throwable ex) {
throw new BeanInstantiationException(clazz, "No default constructor found", ex);
}
}
}
// 利用構造方法進行實例化
return BeanUtils.instantiateClass(constructorToUse);
}
else {
// Must generate CGLIB subclass.
// 存在方法覆寫，利用 CGLIB 來完成實例化，需要依賴於 CGLIB 生成子類，這裡就不展開了。
// tips: 因為如果不使用 CGLIB 的話，存在 override 的情況 JDK 並沒有提供相應的實例化支持
return instantiateWithMethodInjection(bd, beanName, owner);
}
}
1.2.3.4.5.6.7.8.9.10.11.12.13.14.15.16.17.18.19.20.21.22.23.24.25.26.27.28.29.30.31.32.33.34.35.36.37.38.39.到這裡，我們就算實例化完成了。我們開始說怎麼進行屬性注入。
2、populateBean 方法
protected void populateBean(String beanName, RootBeanDefinition mbd, @Nullable BeanWrapper bw) {
if (bw == null) {
if (mbd.hasPropertyValues()) {
throw new BeanCreationException(
mbd.getResourceDescription(), beanName, "Cannot apply property values to null instance");
}
else {
// Skip property population phase for null instance.
return;
}
}
// Give any InstantiationAwareBeanPostProcessors the opportunity to modify the
// state of the bean before properties are set. This can be used, for example,
// to support styles of field injection.
if (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {
for (BeanPostProcessor bp : getBeanPostProcessors()) {
if (bp instanceof InstantiationAwareBeanPostProcessor) {
InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp;
if (!ibp.postProcessAfterInstantiation(bw.getWrappedInstance(), beanName)) {
return;
}
}
}
}
// bean 實例的所有屬性都在這裡了
PropertyValues pvs = (mbd.hasPropertyValues() ? mbd.getPropertyValues() : null);
int resolvedAutowireMode = mbd.getResolvedAutowireMode();
// 通過名字找到所有屬性值，如果是 bean 依賴，先初始化依賴的 bean。記錄依賴關係
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;
}
boolean hasInstAwareBpps = hasInstantiationAwareBeanPostProcessors();
boolean needsDepCheck = (mbd.getDependencyCheck() != AbstractBeanDefinition.DEPENDENCY_CHECK_NONE);
PropertyDescriptor[] filteredPds = null;
if (hasInstAwareBpps) {
if (pvs == null) {
pvs = mbd.getPropertyValues();
}
for (BeanPostProcessor bp : getBeanPostProcessors()) {
if (bp instanceof InstantiationAwareBeanPostProcessor) {
InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp;
PropertyValues pvsToUse = ibp.postProcessProperties(pvs, bw.getWrappedInstance(), beanName);
if (pvsToUse == null) {
if (filteredPds == null) {
filteredPds = filterPropertyDescriptorsForDependencyCheck(bw, mbd.allowCaching);
}
// 這裡有個非常有用的 BeanPostProcessor 進到這裡: AutowiredAnnotationBeanPostProcessor
// 對採用 @Autowired、@Value 註解的依賴進行設值，這裡的內容也是非常豐富的
pvsToUse = ibp.postProcessPropertyValues(pvs, filteredPds, bw.getWrappedInstance(), beanName);
if (pvsToUse == null) {
return;
}
}
pvs = pvsToUse;
}
}
}
if (needsDepCheck) {
if (filteredPds == null) {
filteredPds = filterPropertyDescriptorsForDependencyCheck(bw, mbd.allowCaching);
}
checkDependencies(beanName, mbd, filteredPds, pvs);
}
if (pvs != null) {
// 設置 bean 實例的屬性值
applyPropertyValues(beanName, mbd, bw, pvs);
}
}
1.2.3.4.5.6.7.8.9.10.11.12.13.14.15.16.17.18.19.20.21.22.23.24.25.26.27.28.29.30.31.32.33.34.35.36.37.38.39.40.41.42.43.44.45.46.47.48.49.50.51.52.53.54.55.56.57.58.59.60.61.62.63.64.65.66.67.68.69.70.71.72.73.74.75.76.77.78.79.80.81.82.83.屬性注入完成後，這一步其實就是處理各種回調了，這塊代碼比較簡單。
3、 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 {
// 如果 bean 實現了 BeanNameAware、BeanClassLoaderAware 或 BeanFactoryAware 接口，回調
invokeAwareMethods(beanName, bean);
}
Object wrappedBean = bean;
if (mbd == null || !mbd.isSynthetic()) {
// BeanPostProcessor 的 postProcessBeforeInitialization 回調
wrappedBean = applyBeanPostProcessorsBeforeInitialization(wrappedBean, beanName);
}
try {
// 處理 bean 中定義的 init-method，
// 或者如果 bean 實現了 InitializingBean 接口，調用 afterPropertiesSet() 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()) {
// BeanPostProcessor 的 postProcessAfterInitialization 回調
//BeanPostProcessor 的兩個回調都發生在這邊，只不過中間處理了 init-method
wrappedBean = applyBeanPostProcessorsAfterInitialization(wrappedBean, beanName);
}
return wrappedBean;
}
1.2.3.4.5.6.7.8.9.10.11.12.13.14.15.16.17.18.19.20.21.22.23.24.25.26.27.28.29.30.31.32.33.34.35.36.自此，Spring實例化單例非懶加載bean的過程也就完成了，這也是Spirng最最重要的方法了。在我們的日常使用Spring中，定義好各個類，然後在上面加上，@Controller,@Service,Autowired等註解，這些註解是怎麼起作用的呢？
想必大部分同學都是知其然，不知其所以然，想必通過本文，讀者心中能有一個清楚的認識。