JDK：1.8
SpringBoot：2.1.5
把整個過程總結在前面：
搭建源碼閱讀環境
這裏你也可以不用搭建，直接使用Idea的源碼下載功能也可以，但這樣拿到的源碼是隻讀的，如果你想在閱讀過的地方加上註釋則很困難。另外我們搭建的是Springboot的源碼閱讀環境，並不是Spring-framework的，前者是基於maven構建的項目，後者是基於gradle，兩者搭建方法不一樣。
1.到github上下載源碼：https://github.com/spring-projects/spring-boot。選擇release：
找到2.1.5版本，當然你也可以選擇其他版本，這個隨意。拉到版本頁面的最底部，現在源碼：
2.下載下來後，解壓到某個位置，然後用idea打開主目錄下的pom文件，選擇open as project。首次打開項目可能會比較久，要解析許多依賴。
很多教程裏面還會教你如何安裝maven和grdle，2019版本的idea並不需要額外安裝這兩個東西，它已經自帶了。
3.項目打開之後，看到的目錄是這樣的：
設置項目的jdk版本爲java8，選擇maven選項卡，點擊那個m圖標：
輸入mvn clean install -DskipTests -Pfast
這步操作也需要挺長的時間，等maven執行完畢，這個項目已經配置了一個啓動配置，直接點擊debug就能運行起來：
4.寫一個新的maven項目作爲測試項目。選擇選擇最上層的目錄，右鍵，基於maven新添加一個module：spring-boot-study。如圖：
5.在pom文件裏面加上依賴：
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-web</artifactId>
<version>2.1.5.RELEASE</version>
</dependency>
2.1.5.RELAESE是通過我們下載的源碼生成的，如果你選擇其他的版本，maven會從遠程倉庫裏面下載，那麼你調試的代碼也就是不是你下載的代碼了。
你選擇的時候，idea提示器會提示你哪個版本本地就有，比如2.1.5這個版本和其他的版本標誌是不一樣的。看上圖。
6.在項目的App類裏面添加如下代碼：
@SpringBootApplication
public class App {
public static void main(String[] args)
{
SpringApplication.run(App.class,args);
}
}
這裏有個坑，原來我們創建的module，其目錄結構是繼承自主項目的，這會導致這個項目運行的時候報一個錯誤：
這個問題的解決辦法也比較簡單，只需要把原來的目錄結構刪了，重新創建一下App這個類，其目錄結構如下：
然後鼠標放到main方法內部，右鍵，Debug ‘App’，程序就能正常debug了，結果如圖：
到現在爲止，SpringBoot的源碼閱讀環境就搭建好了。現在可以在這個study模塊上寫一些測試代碼，跟蹤代碼，分析源碼了。
值得一提的是，SpringBoot這個項目大量依賴Spring Framework，因此，如果單純下載下來SpringBoot的源碼，很多代碼仍然無法跟蹤進去，這點後面我們在分析ioc源碼的時候你就會發現。
分析SpringBoot的IOC過程：
1.run方法返回值類型是ConfigurableApplicationContext，這裏的繼承關係是：
ConfigurableApplicationContext=》ApplicationContext=》ListableBeanFactory=》BeanFactory
上面列舉的全是接口，BeanFactory是最頂層的接口，spring裏面各式各樣的context，比如AnnotationConfigApplicationContext、XmlServletWebServerApplicationContext等都繼承自它。那麼這個ConfigurableApplicationContext具體是由哪個類實現的呢，持續跟蹤代碼會跟蹤到下面的方法：
protected ConfigurableApplicationContext createApplicationContext() {
Class<?> contextClass = this.applicationContextClass;
if (contextClass == null) {
try {
switch (this.webApplicationType) {
case SERVLET:
contextClass = Class.forName(DEFAULT_SERVLET_WEB_CONTEXT_CLASS);
break;
case REACTIVE:
contextClass = Class.forName(DEFAULT_REACTIVE_WEB_CONTEXT_CLASS);
break;
default:
contextClass = Class.forName(DEFAULT_CONTEXT_CLASS);
}
}
catch (ClassNotFoundException ex) {
throw new IllegalStateException(
"Unable create a default ApplicationContext, "
+ "please specify an ApplicationContextClass",
ex);
}
}
return (ConfigurableApplicationContext) BeanUtils.instantiateClass(contextClass);
}
這個方法實際上告訴了我們，在2.1.5版本中，總共有三種context的實現，具體使用哪一種，取決於webApplicationType，而webApplicationType則是通過掃描包裏面的依賴，根據依賴內容而得出的（是否包含web相關的依賴等）。具體怎麼得到的，由於這塊和ioc關係不大，這裏不追究了。
2持續跟蹤run方法，一直跟蹤到
refreshContext(context);
這個方法，在這個方法之前的工作都是spring相關的準備工作，和ioc關係不大，這個方法是用來實現ioc相關功能的核心方法，整個ioc的生命週期，都在這個方法裏。
3.持續跟蹤代碼，最後跟蹤到AbstractApplicationContext這個抽象類的refresh方法，這個類直接繼承自ConfigurableApplicationContext接口，該類屬於spring framework，不屬於springboot，因此如果你嘗試修改它，會發現它是隻讀的，是因爲我們並沒有把spring framework的代碼下載下來。方法的具體實現如下，這個ioc的生命週期發生的操作，都在這裏了。具體每步的功能是做什麼，官方都添加了英文註釋，但是這個註釋有些簡單，我又額外添加了一些中文註釋進來：
public void refresh() throws BeansException, IllegalStateException {
synchronized (this.startupShutdownMonitor) {
// 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);
// Invoke factory processors registered as beans in the context.
//非常重要的一步，進行包掃描，把bean定義掃描出來，
//什麼是bean定義，在spring裏面表現爲BeanDefination，
//它用來描述bean，比如是否是單例，是否是懶加載等等信息，
//這些信息都是從xml配置信息或者註解信息裏面讀出來。
invokeBeanFactoryPostProcessors(beanFactory);
// Register bean processors that intercept bean creation.
registerBeanPostProcessors(beanFactory);
// 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.
//非常重要的一步，完成bean的實例化
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();
}
}
}
4.第一步掃描包，它的代碼實現在下面這個方法裏面：
invokeBeanFactoryPostProcessors(beanFactory);
掃描包的目的是把所有的bean定義掃描出來，然後放到下面這個map裏面：
private final Map<String, BeanDefinition> beanDefinitionMap = new ConcurrentHashMap<>(256);
由此可見，bean定義的緩存類型是ConcurrentHashMap類型，值的類型是BeanDefinition，它是一個接口，具體的實現有很多類，spring會根據不同的bean定義方案實現不同的類。它總共有哪些實現，看下圖：
在執行這步代碼之前，2.1.5版本的springboot裏面它的值是6，具體內容如下：
其中就包含了我們的入口類，app。
在這個方法執行完之後，會把我們定義的bean也放進來以及系統定義的bean加載進來，看下執行後它的值：
我標註的bean是我自己定義的一個測試類，其他基本都是系統自己的bean。
5.這些bean定義緩存決定了ioc容器處理bean的方案，那麼是不是意味着它只能通過配置或者註解來決定呢，答案是否定的，其實在這個過程中我們是可以干預的，干預是使用beanfactory後置處理器（注意不是bean的後置處理器）來處理的，參考下面的代碼：
@Component
public class MyBeanFactoryPostProcessor implements BeanFactoryPostProcessor {
@Override
public void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) throws BeansException {
GenericBeanDefinition genericBeanDefinition=(GenericBeanDefinition)beanFactory.getBeanDefinition("singletonTest");
genericBeanDefinition.setLazyInit(true);
}
}
這樣就可以把我們定義的bean設置爲懶加載了。當然了，你也可以通過在bean定義的時候加上註解實現這個功能，這倆效果是一樣的：
@Component
@Lazy
public class SingletonTest {
public SingletonTest()
{
System.out.println("SingletonTest被實例化");
}
public void test()
{
}
}
5.包掃描結束後，後來又到了一步非常關鍵的方法：finishBeanFactoryInitialization(beanFactory)。這個方法的作用是完成非懶加載的類的單例類的實例化。
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();
// Instantiate all remaining (non-lazy-init) singletons.
//關鍵步驟，上面的基本都是準備工作。
beanFactory.preInstantiateSingletons();
}
6.重點關注下preInstantiateSingletons這個方法，其代碼如下：
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...
//實例化所有的單例的、非懶加載的、非抽象的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) {
final FactoryBean<?> factory = (FactoryBean<?>) bean;
boolean isEagerInit;
if (System.getSecurityManager() != null && factory instanceof SmartFactoryBean) {
isEagerInit = AccessController.doPrivileged((PrivilegedAction<Boolean>)
((SmartFactoryBean<?>) factory)::isEagerInit,
getAccessControlContext());
}
else {
isEagerInit = (factory instanceof SmartFactoryBean &&
((SmartFactoryBean<?>) factory).isEagerInit());
}
if (isEagerInit) {
getBean(beanName);
}
}
}
else {
getBean(beanName);
}
}
}
// Trigger post-initialization callback for all applicable beans...
//後置處理所有的後置處理方法
for (String beanName : beanNames) {
Object singletonInstance = getSingleton(beanName);
if (singletonInstance instanceof SmartInitializingSingleton) {
final SmartInitializingSingleton smartSingleton = (SmartInitializingSingleton) singletonInstance;
if (System.getSecurityManager() != null) {
AccessController.doPrivileged((PrivilegedAction<Object>) () -> {
smartSingleton.afterSingletonsInstantiated();
return null;
}, getAccessControlContext());
}
else {
smartSingleton.afterSingletonsInstantiated();
}
}
}
}
在第一次輪詢中，做了一個判斷，判斷是否是factorybean，什麼是factorybean，參考我另一篇博文https://blog.csdn.net/dap769815768/article/details/100862413。如果不是直接通過getBean方法就可以了。
7.getBean最終會走到doGetBean方法：
protected <T> T doGetBean(final String name, @Nullable final Class<T> requiredType,
@Nullable final Object[] args, boolean typeCheckOnly) throws BeansException {
final 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 {
final 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);
}
}
}
// Create bean instance.
//重要，真實地創建bean實例的方法。
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();
final Scope scope = this.scopes.get(scopeName);
if (scope == null) {
throw new IllegalStateException("No Scope registered for scope name '" + scopeName + "'");
}
try {
Object scopedInstance = scope.get(beanName, () -> {
beforePrototypeCreation(beanName);
try {
return createBean(beanName, mbd, args);
}
finally {
afterPrototypeCreation(beanName);
}
});
bean = getObjectForBeanInstance(scopedInstance, name, beanName, mbd);
}
catch (IllegalStateException ex) {
throw new BeanCreationException(beanName,
"Scope '" + scopeName + "' is not active for the current thread; consider " +
"defining a scoped proxy for this bean if you intend to refer to it from a singleton",
ex);
}
}
}
catch (BeansException ex) {
cleanupAfterBeanCreationFailure(beanName);
throw ex;
}
}
// Check if required type matches the type of the actual bean instance.
if (requiredType != null && !requiredType.isInstance(bean)) {
try {
T convertedBean = getTypeConverter().convertIfNecessary(bean, requiredType);
if (convertedBean == null) {
throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
}
return convertedBean;
}
catch (TypeMismatchException ex) {
if (logger.isTraceEnabled()) {
logger.trace("Failed to convert bean '" + name + "' to required type '" +
ClassUtils.getQualifiedName(requiredType) + "'", ex);
}
throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
}
}
return (T) bean;
}
這裏需要關注的是getSingleton這個方法，它是bean的實際實例化方法，包含了屬性注入的部分。
8.getSingleton方法：
public Object getSingleton(String beanName, ObjectFactory<?> singletonFactory) {
Assert.notNull(beanName, "Bean name must not be null");
synchronized (this.singletonObjects) {
Object singletonObject = this.singletonObjects.get(beanName);
if (singletonObject == null) {
if (this.singletonsCurrentlyInDestruction) {
throw new BeanCreationNotAllowedException(beanName,
"Singleton bean creation not allowed while singletons of this factory are in destruction " +
"(Do not request a bean from a BeanFactory in a destroy method implementation!)");
}
if (logger.isDebugEnabled()) {
logger.debug("Creating shared instance of singleton bean '" + beanName + "'");
}
//解決循環依賴問題，使用一個set保存正在實例化的bean。
beforeSingletonCreation(beanName);
boolean newSingleton = false;
boolean recordSuppressedExceptions = (this.suppressedExceptions == null);
if (recordSuppressedExceptions) {
this.suppressedExceptions = new LinkedHashSet<>();
}
try {
//調用傳進來的回調createBean，創建bean。
singletonObject = singletonFactory.getObject();
newSingleton = true;
}
catch (IllegalStateException ex) {
// Has the singleton object implicitly appeared in the meantime ->
// if yes, proceed with it since the exception indicates that state.
singletonObject = this.singletonObjects.get(beanName);
if (singletonObject == null) {
throw ex;
}
}
catch (BeanCreationException ex) {
if (recordSuppressedExceptions) {
for (Exception suppressedException : this.suppressedExceptions) {
ex.addRelatedCause(suppressedException);
}
}
throw ex;
}
finally {
if (recordSuppressedExceptions) {
this.suppressedExceptions = null;
}
afterSingletonCreation(beanName);
}
if (newSingleton) {
addSingleton(beanName, singletonObject);
}
}
return singletonObject;
}
}
首先是通過調用下面的方法，把正在實例化的bean緩存起來：
protected void beforeSingletonCreation(String beanName) {
if (!this.inCreationCheckExclusions.contains(beanName) && !this.singletonsCurrentlyInCreation.add(beanName)) {
throw new BeanCurrentlyInCreationException(beanName);
}
}
然後調用
singletonObject = singletonFactory.getObject();
執行回調createBean(beanName, mbd, args)，進入到真正的bean創建方法中。
9.createBean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 {
//完成創建bean，完整的bean，包括屬性注入。
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);
}
}
在doCreateBean方法，整個bean就被創建了，該方法源碼如下：
protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final @Nullable Object[] args)
throws BeanCreationException {
// Instantiate the bean.
BeanWrapper instanceWrapper = null;
if (mbd.isSingleton()) {
instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
}
if (instanceWrapper == null) {
//通過某個策略創建一個bean的包裝實例
instanceWrapper = createBeanInstance(beanName, mbd, args);
}
//通過類的包裝實例獲取到類的實例，但未完成屬性注入。
final Object bean = instanceWrapper.getWrappedInstance();
Class<?> beanType = instanceWrapper.getWrappedClass();
if (beanType != NullBean.class) {
mbd.resolvedTargetType = beanType;
}
// Allow post-processors to modify the merged bean definition.
synchronized (mbd.postProcessingLock) {
if (!mbd.postProcessed) {
try {
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 {
//完成屬性注入
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 " +
"'getBeanNamesOfType' 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;
}
8.循環依賴。spring中如果循環依賴的方式是構造器注入，那麼會拋出異常：The dependencies of some of the beans in the application context form a cycle。如果是屬性注入，要求兩邊不能全部是多例，否則也會拋出上面的異常。如果A是多例，B是單例，那麼不可以對A進行自動注入，否則仍然會拋出上面的異常。原因很簡單：
實例化A=》注入A的依賴B=》實例化B=》注入B的依賴A=》由於A是多例，所以不會獲取之前實例化一半的A，而是重新實例化一個A=》程序回到最初的一步，進入死循環。
而如果反過來，則沒問題：
實例化B=》注入B的依賴A=》實例化A=》注入A的依賴B=》B是單例，所以直接將之前實例化一半的B的引用傳給A=》自動注入完成。
這裏面其實就是誰先實例化的問題。多例的那個不能先實例化，必須後實例化，纔不會導致報循環依賴的異常。
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