前言
Spring如何解决循环依赖的？这个也是一个Spring的高频面试题。下面我们就从源码的角度去剖析下这个问题。
Spring创建Bean 中的三级缓存 解决循环依赖
Bean 创建的简单介绍
Spring 创建的主要流程在 AbstractApplicationContext.refresh
Bash
public void refresh() throws BeansException, IllegalStateException {
synchronized (this.startupShutdownMonitor) {
// 一些准备工作，读取一些配置变量，设置启动标识等
prepareRefresh();
// 创建BeanFactory 并将BeanDefinition 放入对应的Map 中
// 方法最终会调用 AbstractRefreshableApplicationContext.refreshBeanFactory
ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();
// 设置BeanFactory 的类加载器 ， 设置几个特殊的BeanPostProcessor
// BeanPostProcessor 用于加入到Bean 的生命周期，初始化前postProcessBeforeInitialization
//初始化后执行 postProcessAfterInitialization 可以用于创建Bean 改变Bean 属性
//这里也会有ApplicationListenerDetector （BeanPostProcessor）用于注册监听器
prepareBeanFactory(beanFactory);
try {
// 留给子类的扩展点，可以在这里添加BeanPostProcessor和BeanFactoryPostProcessor
// BeanFactoryPostProcessor 可以对beanFactory进行相关操作
postProcessBeanFactory(beanFactory);
// 调用 BeanFactoryPostProcessor 实现类的 postProcessBeanFactory(factory)方法
invokeBeanFactoryPostProcessors(beanFactory);
// 注册 BeanPostProcessor 的实现类，Bean 还没有初始化
registerBeanPostProcessors(beanFactory);
// 国际化相关操作
initMessageSource();
// 初始化 ApplicationContext 的事件广播器，对ApplicationListener 的处理，这个会在后面的registerListeners（）进行添加
initApplicationEventMulticaster();
// 模板方法 留给子类扩展
onRefresh();
//注册ApplicationListener 事件监听器
registerListeners();
//初始化所有的 singleton beans （lazy-init 的除外）
finishBeanFactoryInitialization(beanFactory);
// 最后，广播事件ApplicationContext 初始化完成，并做一些清理操作等
finishRefresh();
}
catch (BeansException ex) {
if (logger.isWarnEnabled()) {
logger.warn("Exception encountered during context initialization - " +
"cancelling refresh attempt: " + ex);
}
// 销魂Bean
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();
}
}
}一些方法说明
invokeBeanFactoryPostProcessors：会创建和调用BeanFactoryPostProcessor 的相关方法
registerBeanPostProcessors：会注册创建所有的BeanPostProcessor ，这里我们普通用到的Bean 是还没有创建生成的。
invokeBeanFactoryPostProcessors和registerBeanPostProcessors中 普通的bean都是没有创建的，所以不要在这里使用getBean 和自动注入，因为这样你可能会打乱Spring的流程造成Bean的创建异常
finishBeanFactoryInitialization:开始创建普通的Bean 不包括懒加载的bean
AbstractApplicationContext.finishBeanFactoryInitialization 中主要的步骤在DefaultListableBeanFactory.preInstantiateSingletons
Bash
public void preInstantiateSingletons() throws BeansException {
if (logger.isTraceEnabled()) {
logger.trace("Pre-instantiating singletons in " + this);
}
//获取所有的定义的Bean的名字
List<String> beanNames = new ArrayList<>(this.beanDefinitionNames);
// Trigger initialization of all non-lazy singleton beans...
for (String beanName : beanNames) {
// 这个对BeanDefinition 进行合并 ，合并parent（设置的，不是类中的继承关系，xml 中用parent 定义的）<bean id="" class="" parent="" />
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);//通过getBean 触发bean的初始化
}
}
}
else {
getBean(beanName);
}
}
}
// 对所有继承了SmartInitializingSingleton bean 的进行回调处理
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();
}
}
}
}这个方法中比较重要的方法是getBean，最后是一直调用到AbstractBeanFactory.doGetBean
@SuppressWarnings("unchecked")
protected <T> T doGetBean(
String name, @Nullable Class<T> requiredType, @Nullable Object[] args, boolean typeCheckOnly)
throws BeansException {
// 将别名转成通用的beanName ，这步主要的目的是确定通用的beanName
String beanName = transformedBeanName(name);
//这个是返回值
Object bean;
// 查询创建的缓存中是否已经存在这个bean，这里有三级缓存，用于解决循环引用的问题
//里面有三个缓存，后面把代码贴出来
//Map<String, Object> singletonObjects 最终的bean 完全创建初始化完那种
//Map<String, ObjectFactory<?>> singletonFactories 存放创建bean的工厂，bean未被创建
//Map<String, Object> earlySingletonObjects 早期暴露单未被初始化完的bean，比如依赖注入未完成之类的，一般从singletonFactories 来
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 + "'");
}
}
// 如果是 FactoryBean 的话，返回它创建的那个实例对象，name 如果是&开头返回FactoryBean 实例
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)) {
// 创建过了此 beanName 的 prototype 类型的 bean，那么抛异常，一般是循环引用造成
throw new BeanCurrentlyInCreationException(beanName);
}
// Check if bean definition exists in this factory.
BeanFactory parentBeanFactory = getParentBeanFactory();
//当前容器中不存在相关的bean 的定义 检查parentBeanFactory（父亲容器）
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);
}
// 到这里准备开始创建Bean 了，之前都没有创建
try {
RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
checkMergedBeanDefinition(mbd, beanName, args);
// 注意这里DependsOn 是指<bean depends-on =""/>,这个一般用不到，不是分析重点
String[] dependsOn = mbd.getDependsOn();
if (dependsOn != null) {
for (String dep : dependsOn) {
// 检查是不是有循环依赖
if (isDependent(beanName, dep)) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"Circular depends-on relationship between '" + beanName + "' and '" + dep + "'");
}
//注册依赖
registerDependentBean(dep, beanName);
try {
// 先初始化被依赖项
getBean(dep);
}
catch (NoSuchBeanDefinitionException ex) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"'" + beanName + "' depends on missing bean '" + dep + "'", ex);
}
}
}
// Create bean instance.
if (mbd.isSingleton()) {
//单例创建，getSingleton 总体逻辑就是执行createBean ，创建完之后会调用DefaultSingletonBeanRegistry.addSingleton(beanName, singletonObject);用于同步一下三级缓存的状态，主体逻辑就是放进singletonObjects （一级缓存）中，清楚其他缓存中这个beanName 的相关信息
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()) {
//Prototype 的创建
// 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 {
// 不是 Prototype 和 singleton 的创建
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;
}
}
//最后检查一下bean 的类型是否正确
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;
}三级缓存的主要代码
/** Cache of singleton objects: bean name to bean instance. */
private final Map<String, Object> singletonObjects = new ConcurrentHashMap<>(256);
/** Cache of singleton factories: bean name to ObjectFactory. */
private final Map<String, ObjectFactory<?>> singletonFactories = new HashMap<>(16);
/** Cache of early singleton objects: bean name to bean instance. */
private final Map<String, Object> earlySingletonObjects = new ConcurrentHashMap<>(16);
protected Object getSingleton(String beanName, boolean allowEarlyReference) {
// Quick check for existing instance without full singleton lock
Object singletonObject = this.singletonObjects.get(beanName);
if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
singletonObject = this.earlySingletonObjects.get(beanName);
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;
}不管哪种方式创建都会调用AbstractAutowireCapableBeanFactory.createBean，所以看看createBean
@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.
// 确保 BeanDefinition 中的 Class 被加载
Class<?> resolvedClass = resolveBeanClass(mbd, beanName);
if (resolvedClass != null && !mbd.hasBeanClass() && mbd.getBeanClassName() != null) {
mbdToUse = new RootBeanDefinition(mbd);
mbdToUse.setBeanClass(resolvedClass);
}
// Prepare method overrides.
//spring 中的一个功能，通过配置replaced-method 和实现org.springframework.beans.factory.support.MethodReplacer 可以替换目标类的方法，这个不是我们的重点，略过
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.
// 执行实现了InstantiationAwareBeanPostProcessor 的类的相关方法postProcessBeforeInstantiation，这个是返回代理类的地方，也是Spring中返回aop 代理的地方
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 {
// 创建 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);
}
}/** Cache of unfinished FactoryBean instances: FactoryBean name to BeanWrapper. */
private final ConcurrentMap<String, BeanWrapper> factoryBeanInstanceCache = new ConcurrentHashMap<>();
protected Object doCreateBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args)
throws BeanCreationException {
// Instantiate the bean.
BeanWrapper instanceWrapper = null;
if (mbd.isSingleton()) {
//查看ConcurrentMap<String, BeanWrapper> factoryBeanInstanceCache 是否存在
instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
}
if (instanceWrapper == null) {
//这里实例化 Bean，里面分情况看是否需不需要自动注入的构造方法创建对象，一般都是无参构造创建，所以略过
instanceWrapper = createBeanInstance(beanName, mbd, args);
}
// 这个就是我们定义的bean 的实例
Object bean = instanceWrapper.getWrappedInstance();
Class<?> beanType = instanceWrapper.getWrappedClass();
if (beanType != NullBean.class) {
mbd.resolvedTargetType = beanType;
}
// Allow post-processors to modify the merged bean definition.
//MergedBeanDefinitionPostProcessor 的回调 允许改变beandefinition，这里有用于实现@Autowired和@Value 功能的AnnotationInjectedBeanPostProcessor， 扫描类中关于@Autowired和@Value的定义，并将其封装成对应的数据类InjectionMetadata保存进beandefinition 中，用于实现自动注入
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.
//是否可以提前暴露引用=是否单例，容器是否允许循环引用出现（默认可以），是否当前创建的bean
// 这里提前暴露未完全初始化完的bean实例 是为了解决循环引用的问题
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");
}
// 添加到三级缓存中，用于解决循环引用，getEarlyBeanReference 会回调SmartInstantiationAwareBeanPostProcessor.getEarlyBeanReference 这个方法用于拿到早期暴露的需要被aop 代理的代理类的对象，这样就不会导致循环引用的是原始对象
// () -> getEarlyBeanReference(beanName, mbd, bean) 这个是ObjectFactory 的实现类，没有马上运行getEarlyBeanReference ，后面才运行的，结合之前的doGetBean 中的Object singletonInstance = getSingleton(beanName); 解决循环引用
addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
}
// Initialize the bean instance.
// 这个就是我们之前创建的bean实例
Object exposedObject = bean;
try {
// 负责属性装配(包括自动注入)，前面的实例只是实例化了，并没有设值
//会回调InstantiationAwareBeanPostProcessor.postProcessAfterInstantiation这个如果返回false，就可能导致后面的自动注入出现问题
//然后在回调InstantiationAwareBeanPostProcessor.postProcessPropertyValues
populateBean(beanName, mbd, instanceWrapper);
// 完成对init-method ，还有 InitializingBean 接口,还有 BeanPostProcessor 接口的回处理
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);
}
}
// 处理循环引用
if (earlySingletonExposure) {
// 检查 需要aop的类的代理类是否出现，false 不触发早期getEarlyBeanReference的执行
// earlySingletonReference 就是早期暴露的对象
Object earlySingletonReference = getSingleton(beanName, false);
if (earlySingletonReference != null) {
//earlySingletonReference 不空 getEarlyBeanReference 执行力 并 已经创建相应对象
if (exposedObject == bean) {
exposedObject = earlySingletonReference;
}
else if (!this.allowRawInjectionDespiteWrapping && hasDependentBean(beanName)) {
//处理之前定义的<bean depends-on =""/>问题，这个不经常用到，直接跳过
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;
}
还记得 前面AbstractBeanFactory.doGetBean 中的方法吗，这里就是放进一级缓存的地方，可以回去看看if (mbd.isSingleton()) {
//单例创建，getSingleton 总体逻辑就是执行createBean ，创建完之后会调用DefaultSingletonBeanRegistry.addSingleton(beanName, singletonObject);用于同步一下三级缓存的状态，主体逻辑就是放进singletonObjects （一级缓存）中，清楚其他缓存中这个beanName 的相关信息
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);
}原文链接：https://blog.csdn.net/weixin_36927395/article/details/114062678
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