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【171期】面试官：小伙汁，Spring是怎么解决循环依赖的呢？
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前言
Spring大家族功能强大，模块复杂繁多。就Spring Framework模块而言，核心功能只有两个：IoC和AOP。
本篇主要从源码的角度讲解Spring容器中一些重要的接口、Spring如何解决循环依赖等
本篇使用的Spring版本为
5.2.2.RELEASE。
Spring的源码错综复杂，并且类名一般都比较长，并且调用层次较深。因此阅读起来有一定的难度，所以阅读的时候可以先从大体上理解整个流程，而不需要逐行的阅读。不然很容易陷入细节而无法自拔，导致事倍功半。
在深入Spring源码之前，需要先了解几个非常重要的接口，理解他们，是理解Spring容器启动的关键。
核心接口
BeanDefinition
BeanDefinition是Spring中非常重要的一个接口，定义于spring-beans模块中，其定义如下：
/**
* A BeanDefinition describes a bean instance, which has property values,
* constructor argument values, and further information supplied by
* concrete implementations.
*
* <p>This is just a minimal interface: The main intention is to allow a
* {@link BeanFactoryPostProcessor} to introspect and modify property values
* and other bean metadata.
*
*/
public interface BeanDefinition extends AttributeAccessor, BeanMetadataElement {
}
根据接口描述，我们可以知道
BeanDefinition描述了一个bean实例，它具有属性值，构造函数参数值以及具体实现提供的更多信息。
看完类的描述，我们似乎依然不知道这个接口是用来干嘛的。
就博主自己的理解，BeanDefinition主要做用是定义了一个Spring Bean的元信息（metadata）的抽象。使得不管是XML文件配置的Spring Bean、注解扫描的Spring Bean，还是Java Config类配置Spring Bean，都能一个统一的抽象来表示，这个抽象就是BeanDefinition。
接下来看一下这个接口里面的（部分）内容，可以帮助理解这个接口的作用
/** 返回当前bean实例是否是单例 */
boolean isSingleton();
/** 返回当前bean是否应该被懒加载 */
boolean isLazyInit();
/** 返回bean的类名称 */
@Nullable
String getBeanClassName();
从这个几个接口方法的描述就可以看出BeanDefinition可以描述Spring Bean的元信息。
在我们学习Java的时候其实已经接触过这样的类，那就是java.lang.Class类，Class就是用来描述JDK中各个类的元信息的抽象，我们可以从Class类获取类的名称、构造函数、字段等信息。
BeanFactory
BeanFactory是Spring中非常重要的一个接口，定义于spring-beans模块中，其定义如下：
/**
* The root interface for accessing a Spring bean container.
* This is the basic client view of a bean container;
* further interfaces such as {@link ListableBeanFactory} and
* {@link org.springframework.beans.factory.config.ConfigurableBeanFactory}
* are available for specific purposes.
*
* <p>Bean factory implementations should support the standard bean lifecycle interfaces
* as far as possible. The full set of initialization methods and their standard order is:
* <ol>
* <li>BeanNameAware's {@code setBeanName}
* <li>BeanClassLoaderAware's {@code setBeanClassLoader}
* <li>BeanFactoryAware's {@code setBeanFactory}
* <li>EnvironmentAware's {@code setEnvironment}
* <li>EmbeddedValueResolverAware's {@code setEmbeddedValueResolver}
* <li>ResourceLoaderAware's {@code setResourceLoader} (only applicable when running in an application context)
* <li>ApplicationEventPublisherAware's {@code setApplicationEventPublisher} (only applicable when running in an application context)
* <li>MessageSourceAware's {@code setMessageSource} (only applicable when running in an application context)
* <li>ApplicationContextAware's {@code setApplicationContext} (only applicable when running in an application context)
* <li>ServletContextAware's {@code setServletContext} (only applicable when running in a web application context)
* <li>{@code postProcessBeforeInitialization} methods of BeanPostProcessors
* <li>InitializingBean's {@code afterPropertiesSet}
* <li>a custom init-method definition
* <li>{@code postProcessAfterInitialization} methods of BeanPostProcessors
* </ol>
*
* <p>On shutdown of a bean factory, the following lifecycle methods apply:
* <ol>
* <li>{@code postProcessBeforeDestruction} methods of DestructionAwareBeanPostProcessors
* <li>DisposableBean's {@code destroy}
* <li>a custom destroy-method definition
* </ol>
*
*/
public interface BeanFactory {
}
从接口注释和接口定义就可以知道，BeanFactory是Spring容器的顶级接口。并且注释中也给出了全套初始化方法及其标准顺序。
由于是顶层接口，所以定义的方法比较少，最核心的方法当属getBean
/** 从Spring容器中获取bean实例 */
<T> T getBean(Class<T> requiredType) throws BeansException;
/** 从Spring容器中获取bean实例 */
Object getBean(String name) throws BeansException;
ApplicationContext
BeanFactory是Spring的顶级接口，从BeanFactory中已经能够获取Spring Bean实例了，但是Spring依然提供了一个用来扩展BeanFactory的接口，那就是ApplicationContext，该接口是BeanFactory的超集。定义在spring-context模块中。
/**
* Central interface to provide configuration for an application.
* This is read-only while the application is running, but may be
* reloaded if the implementation supports this.
*
* <p>An ApplicationContext provides:
* <ul>
* <li>Bean factory methods for accessing application components.
* Inherited from {@link org.springframework.beans.factory.ListableBeanFactory}.
* <li>The ability to load file resources in a generic fashion.
* Inherited from the {@link org.springframework.core.io.ResourceLoader} interface.
* <li>The ability to publish events to registered listeners.
* Inherited from the {@link ApplicationEventPublisher} interface.
* <li>The ability to resolve messages, supporting internationalization.
* Inherited from the {@link MessageSource} interface.
* <li>Inheritance from a parent context. Definitions in a descendant context
* will always take priority. This means, for example, that a single parent
* context can be used by an entire web application, while each servlet has
* its own child context that is independent of that of any other servlet.
* </ul>
*
* <p>In addition to standard {@link org.springframework.beans.factory.BeanFactory}
* lifecycle capabilities, ApplicationContext implementations detect and invoke
* {@link ApplicationContextAware} beans as well as {@link ResourceLoaderAware},
* {@link ApplicationEventPublisherAware} and {@link MessageSourceAware} beans.
*
*/
public interface ApplicationContext extends EnvironmentCapable, ListableBeanFactory, HierarchicalBeanFactory,
MessageSource, ApplicationEventPublisher, ResourcePatternResolver {
}
根据类的描述可以知道这是一个中央接口，为应用程序提供配置。这个接口主要用来扩展BeanFactory的功能。ApplicationContext接口提供了如下功能
用于访问应用程序组件的Bean工厂方法
以通用方式加载文件资源的能力
将事件发布给注册的侦听器的能力
处理消息的能力，支持国际化
BeanPostProcessor
BeanPostProcessor是Spring中非常重要的一个接口，定义于spring-beans模块中，其定义如下：
/**
* Factory hook that allows for custom modification of new bean instances —
* for example, checking for marker interfaces or wrapping beans with proxies.
*
* <p>Typically, post-processors that populate beans via marker interfaces
* or the like will implement {@link #postProcessBeforeInitialization},
* while post-processors that wrap beans with proxies will normally
* implement {@link #postProcessAfterInitialization}.
*
* <h3>Registration</h3>
* <p>An {@code ApplicationContext} can autodetect {@code BeanPostProcessor} beans
* in its bean definitions and apply those post-processors to any beans subsequently
* created. A plain {@code BeanFactory} allows for programmatic registration of
* post-processors, applying them to all beans created through the bean factory.
*
* <h3>Ordering</h3>
* <p>{@code BeanPostProcessor} beans that are autodetected in an
* {@code ApplicationContext} will be ordered according to
* {@link org.springframework.core.PriorityOrdered} and
* {@link org.springframework.core.Ordered} semantics. In contrast,
* {@code BeanPostProcessor} beans that are registered programmatically with a
* {@code BeanFactory} will be applied in the order of registration; any ordering
* semantics expressed through implementing the
* {@code PriorityOrdered} or {@code Ordered} interface will be ignored for
* programmatically registered post-processors. Furthermore, the
* {@link org.springframework.core.annotation.Order @Order} annotation is not
* taken into account for {@code BeanPostProcessor} beans.
*
*/
public interface BeanPostProcessor {
}
在Spring的源码中可以看到很多PostProcessor相关的接口和类，比如BeanPostProcessor、BeanFactoryPostProcessor。*PostProcessor是后置处理器，用来对已经被创建，但是尚未初始化完成的对象进行一些增强操作。
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启动Spring
想要启动一个Spring容器很简单，只需要几行代码。
public class SpringBeanLifecycle {
public static void main(String[] args) {
ClassPathXmlApplicationContext applicationContext = new ClassPathXmlApplicationContext("applicationContext.xml");
applicationContext.getBean(Student.class);
}
}
其中applicationContext.xml是Spring的配置文件
<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd">
<bean id="student" class="com.sicimike.bean.lifecycle.Student"></bean>
</beans>
而Student类仅仅是为了注入Spring容器，无实际内容。
其实仅仅一行代码，就已经启动了Spring，那就是new ClassPathXmlApplicationContext("applicationContext.xml")。所以我们需要深入连接这个构造方法执行了哪些内容
/**
* Create a new ClassPathXmlApplicationContext, loading the definitions
* from the given XML file and automatically refreshing the context.
* @param configLocation resource location
* @throws BeansException if context creation failed
*/
public ClassPathXmlApplicationContext(String configLocation) throws BeansException {
this(new String[] {configLocation}, true, null);
}
/**
* Create a new ClassPathXmlApplicationContext with the given parent,
* loading the definitions from the given XML files.
* @param configLocations array of resource locations
* @param refresh whether to automatically refresh the context,
* loading all bean definitions and creating all singletons.
* Alternatively, call refresh manually after further configuring the context.
* @param parent the parent context
* @throws BeansException if context creation failed
* @see #refresh()
*/
public ClassPathXmlApplicationContext(
String[] configLocations, boolean refresh, @Nullable ApplicationContext parent)
throws BeansException {
super(parent);
// configLocations是加载的配置文件的名称数字，因此该方法主要的作用就是设置配置文件
setConfigLocations(configLocations);
if (refresh) {
// 该方法就是启动Spring容器的核心方法
refresh();
}
}
refresh
refresh方法执行的逻辑就是Spring容器启动的完整过程，其定义如下
@Override
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.
// 完成包（类）的扫描
invokeBeanFactoryPostProcessors(beanFactory);
// Register bean processors that intercept bean creation.
// 注册后置处理器
registerBeanPostProcessors(beanFactory);
// Initialize message source for this context.
// 国际化
initMessageSource();
// Initialize event multicaster for this context.
初始化事件多播
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.
// 初始化所有的非懒加载的单例bean，核心方法
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();
}
}
}
该方法虽然调用了很多其他方法，但是真正创建Spring Bean的逻辑是finishBeanFactoryInitialization方法。
finishBeanFactoryInitialization
/**
* 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.
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.
// 初始化所有的非懒加载的单例bean，核心方法
beanFactory.preInstantiateSingletons();
}
preInstantiateSingletons
@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.
List<String> beanNames = new ArrayList<>(this.beanDefinitionNames);
// Trigger initialization of all non-lazy singleton beans...
// 触发所有非懒加载的单例bean
for (String beanName : beanNames) {
// RootBeanDefinition是BeanDefinition的子类
// 也就是spring bean的元信息的抽象，用来判断该bean是不是应该被初始化
RootBeanDefinition bd = getMergedLocalBeanDefinition(beanName);
if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) {
// 如果是非抽象、非懒加载的单例bean，就应该被初始化
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 {
// 该方法才是真正的实例化spring bean
getBean(beanName);
}
}
}
// Trigger post-initialization callback for all applicable beans...
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();
}
}
}
}
从调试信息可以看到，bd是RootBeanDefinition，也就是BeanDefinition，它定义了Bean的元信息。
getBean
@Override
public Object getBean(String name) throws BeansException {
return doGetBean(name, null, null, false);
}
doGetBean
该方法用于创建或者查询bean
/**
* Return an instance, which may be shared or independent, of the specified bean.
*/
@SuppressWarnings("unchecked")
protected <T> T doGetBean(final String name, @Nullable final Class<T> requiredType,
@Nullable final Object[] args, boolean typeCheckOnly) throws BeansException {
// 校验bean的名称
final String beanName = transformedBeanName(name);
Object bean;
// Eagerly check singleton cache for manually registered singletons.
// 从单例池中获取对象，该方法非常重要，后文详解
// 由于容器在这里第一次尝试创建或者获取bean，所以返回值为null
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)) {
// 判断该单例对象是否正在被创建
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.
if (mbd.isSingleton()) {
// student对象是单例，所以执行该逻辑，此处再次调用getSingleton方法
// 不过与上面不同的是，此处调用的是重载的方法
sharedInstance = getSingleton(beanName, () -> {
try {
// 真正创建对象的方法
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;
}
该方法中调用了三个非常重要的方法，分别是两个重载的getSingleton方法和createBean方法。
getSingleton
该方法定义如下
@Nullable
protected Object getSingleton(String beanName, boolean allowEarlyReference) {
// 从单例缓存池中获取单例对象
Object singletonObject = this.singletonObjects.get(beanName);
if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
// 获取的对象为null，且该单例对象正在被创建
// 但是它的调用时间是在getSingleton方法（也就是当前方法）之后
// 所以此处的isSingletonCurrentlyInCreation方法返回的是false
// 两个条件只满足了第一个，所以不会进入下面的逻辑
// 后面的逻辑会调用一个方法标识单例bean正在被创建，之后再调用isSingletonCurrentlyInCreation()方法会返回true
synchronized (this.singletonObjects) {
// 从缓存中获取
singletonObject = this.earlySingletonObjects.get(beanName);
if (singletonObject == null && allowEarlyReference) {
ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName);
if (singletonFactory != null) {
singletonObject = singletonFactory.getObject();
// 如果没有就加入缓存
this.earlySingletonObjects.put(beanName, singletonObject);
this.singletonFactories.remove(beanName);
}
}
}
}
// 直接返回null
return singletonObject;
}
从方法的实现可以看到，Spring取单例对象是从singletonObjects对象中取的，该对象定义如下
/** Cache of singleton objects: bean name to bean instance. */
private final Map<String, Object> singletonObjects = new ConcurrentHashMap<>(256);
实际上singletonObjects就是Spring Bean的单例对象缓存池，里面存放的就是所有已经被Spring创建的单例bean实例（经历了完整的Spring Bean初始化生命周期）。有的地方也叫做Spring的一级缓存，本质上就是一个Map。
除此之外，还在earlySingletonObjects中根据beanName查找，如果没有就加入。earlySingletonObjects对象定义如下
/** Cache of early singleton objects: bean name to bean instance. */
private final Map<String, Object> earlySingletonObjects = new HashMap<>(16);
可以看到，earlySingletonObjects也是一个缓存，存放的是尚未初始完成（已经被创建，但是尚未完成Spring初始化生命周期，也就是半成品）bean。有的地方也叫做二级缓存，本质也是个Map。
除此之外，还有一个缓存叫singletonFactories，其定义如下
/** Cache of singleton factories: bean name to ObjectFactory. */
private final Map<String, ObjectFactory<?>> singletonFactories = new HashMap<>(16);
该对象也是一个缓存，用于缓存尚未完成初始化对象的Bean工厂，也叫做三级缓存，本质也是个Map。至此，关于Spring解决循环依赖的三个缓存都已经出现了
/**
* 一级缓存，用于存放已经初始化完成的Spring Bean（经历了完整的Spring Bean初始化生命周期 ）
*/
private final Map<String, Object> singletonObjects = new ConcurrentHashMap<>(256);
/**
* 二级缓存，用于存放已经被创建，但是尚未初始化完成的Bean（尚未经历了完整的Spring Bean初始化生命周期 ）
* 这种对象提前暴露出来，就是为了解决循环引用，避免“鸡生蛋，蛋生鸡”的问题
*/
private final Map<String, Object> earlySingletonObjects = new HashMap<>(16);
/**
* 三级缓存，用于存放二级缓存中Bean的工厂
*/
private final Map<String, ObjectFactory<?>> singletonFactories = new HashMap<>(16);
这个方法在整个Spring Bean初始化的过程中被调用了很多次，应该算是最重要的方法之一了。想要读懂Spring解决循环依赖，务必反复阅读此方法。
第二个getSingleton方法定义如下
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 + "'");
}
// 该方法标记单例bean正在被创建
beforeSingletonCreation(beanName);
boolean newSingleton = false;
boolean recordSuppressedExceptions = (this.suppressedExceptions == null);
if (recordSuppressedExceptions) {
this.suppressedExceptions = new LinkedHashSet<>();
}
try {
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;
}
}
这个方法里调用了beforeSingletonCreation方法，作用是标记此单例bean正在被创建。是Spring解决循环依赖的关键操作之一
往期：001期~150期汇总，方便阅读，不断更新中.....
createBean
createBean方法是Spring真正创建Bean的方法，也是Spring Bean的生命周期的开始。方法定义如下
@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.
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Spring 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);
}
}
doCreateBean
该方法用于创建Spring Bean，经过层层套娃，终于来到了重点，该方法定义如下
protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final @Nullable Object[] args)
throws BeanCreationException {
// Instantiate the bean.
BeanWrapper instanceWrapper = null;
if (mbd.isSingleton()) {
// 调用构造方法创建对象
instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
}
if (instanceWrapper == null) {
instanceWrapper = createBeanInstance(beanName, mbd, args);
}
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.
// 此处是Spring解决循环引用的关键
// 第一个条件判断当前bean是否是单例，也就说明Spring只支持单例Bean的循环引用
// 第二个条件默认是true，也就说Spring默认是支持循环引用的，如果想要关闭循环引用，把这个值设置成false即可
// 第三个条件就是判断当前bean是否正在被创建，由于之前已经调用过beforeSingletonCreation方法，所以这个条件为true
boolean earlySingletonExposure = (mbd.isSingleton() && this.allowCircularReferences &&
isSingletonCurrentlyInCreation(beanName));
if (earlySingletonExposure) {
if (logger.isTraceEnabled()) {
logger.trace("Eagerly caching bean '" + beanName +
"' to allow for resolving potential circular references");
}
// 如果支持循环引用，就加入到一个集合，也就是【提前暴露出来】
addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
}
// Initialize the bean instance.
// 初始化bean实例
Object exposedObject = bean;
try {
// 填充属性，也就是自动注入
populateBean(beanName, mbd, instanceWrapper);
// 真正的初始Spring Bean
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;
}
在这个方法中，最重要的调用方法有三个createBeanInstance、populateBean和initializeBean。
createBeanInstance：调用构造方法创建对象populateBean：填充属性initializeBean：初始化给定的bean实例，应用工厂回调以及init方法和bean后置处理器
解决循环依赖的关键就是在第一步和第二步之间，判断当前bean是否需要支持循环引用，如果需要，就提前暴露出去，这时候暴露出去的bean是尚未完成初始化的bean，也就是所谓的半成品。
理解了这三个步骤，再结合getSingleton方法的逻辑，相信Spring解决循环依赖的思路已经非常明确了。
总结
Spring源码庞大且繁杂，想要在短时间内读懂不太可能，不如先带着问题去读某个部分。
来源：blog.csdn.net/Baisitao_/article/details/107349302
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