Spring循环依赖主要流程
上篇文章中介绍了Spring初始化的整体流程，本篇文章主要是对初始化过程中finishBeanFactoryInitialization这个方法进行剖析，该方法主要是初始化所有非懒加载的单例Bean，创建Bean对象，完成属性填充，调用init-method进行初始化，然后调用后置处理器处理Bean，解决循环依赖就发生在这个过程中。大体流程如下：
这里我把一级、二级、三级缓存分别定义为：singletonObjects、earlySingletonObjects、singletonFactories
- 从缓存池（一级、二级、三级）中获取当前BeanA实例对象，此时为空
- 实例化当前的BeanA，此时的BeanA还没有填充属性
- 将创建好的BeanA加入到singletonFactory(三级缓存)中
- 获取BeanA的属性依赖值BeanB，为BeanA填充属性值
- 真正处理引用，执行getBean操作获取BeanA所依赖的BeanB
- 对BeanB执行doGetBean操作
- 调用步骤1方法，从缓存池（一级、二级、三级）中获取当前BeanB实例对象，此时为空
- 实例化当前的BeanB，此时的BeanB还没有填充属性
- 将创建好的BeanB加入到singletonFactory(三级缓存)中
- 获取BeanB的属性依赖值BeanA，为BeanB填充属性值
- 真正处理引用，执行getBean操作获取BeanB所依赖的BeanA
- 对BeanA执行doGetBean操作
- 调用步骤1，从缓存池（一级、二级、三级）中获取当前BeanA实例对象，此时可以获取到BeanA，所以将BeanA从三级缓存放入到二级缓存中，并完成对BeanB的属性填充
- 将BeanB二级缓存和三级缓存删除，并放到一级缓存中，然后将BeanB注入到BeanA的属性中，完成对BeanA的创建
Spring循环依赖核心源码
了解了循环依赖的大体流程，下面看一下循环依赖的源码过程，为了更准确的看循环依赖的核心源码，我会去掉一些不相干的源码，首先看入口方法：
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.
// 实例化所有立即加载的单例bean
beanFactory.preInstantiateSingletons();
}
主要看preInstantiateSingletons这个方法，对非懒加载的bean进行实例化：
public void preInstantiateSingletons() throws BeansException {
// 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.
// 所有bean的name
List<String> beanNames = new ArrayList<>(this.beanDefinitionNames);
// Trigger initialization of all non-lazy singleton beans...
// 触发所有非延迟加载单例bean的初始化，主要步骤为getBean
for (String beanName : beanNames) {
// 合并父BeanDefinition对象
// map.get(beanName)
RootBeanDefinition bd = getMergedLocalBeanDefinition(beanName);
// 判断是否是延迟加载，如果是延迟加载则不进行初始化
if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) {
// 如果是factoryBean则加&
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 {
// 实例化当前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();
}
}
}
}
继续往下看，进入到实例化getBean方法中，该方法对Bean进行实例化，真正创建Bean的是doGetBean方法：
protected <T> T doGetBean(final String name, @Nullable final Class<T> requiredType,
@Nullable final Object[] args, boolean typeCheckOnly) throws BeansException {
// 解析beanName，如果以&开头则去掉&开头，如果是别名则获取真正的名字
final String beanName = transformedBeanName(name);
Object bean;
// Eagerly check singleton cache for manually registered singletons.
// 单纯理解尝试从缓存（一级、二级、三级）中获取bean
Object sharedInstance = getSingleton(beanName);
// 如果已经存在则返回
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 + "'");
}
}
// 针对FactoryBean的处理
bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
} else {
// Fail if we're already creating this bean instance:
// We're assumably within a circular reference.
// 如果是prototype类型且开启允许循环依赖，则抛出异常
// prototype类型的循环依赖，Spring框架是没法处理的
if (isPrototypeCurrentlyInCreation(beanName)) {
throw new BeanCurrentlyInCreationException(beanName);
}
// Check if bean definition exists in this factory.
// 检查父工厂中是否存在该对象
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 {
// 合并父子的bean属性
final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
checkMergedBeanDefinition(mbd, beanName, args);
// Guarantee initialization of beans that the current bean depends on.
// 处理依赖配置
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.
// 创建单例bean
if (mbd.isSingleton()) {
sharedInstance = getSingleton(beanName, () -> {
try {
// 创建bean
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;
}
上面的方法中会先调用getSingleton方法从缓存获取Bean，如果为空则调用getSingleton方法创建Bean,注意这里的两个getSingleton方法非同一个方法,用的是重载,参数不同,实现也不同,然后我们先看到第一个getSingleton方法,该方法中如果不为空则从三级缓存放入到二级缓存,而此时是为空的:
protected Object getSingleton(String beanName, boolean allowEarlyReference) {
Object singletonObject = this.singletonObjects.get(beanName);
if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
synchronized (this.singletonObjects) {
singletonObject = this.earlySingletonObjects.get(beanName);
if (singletonObject == null && allowEarlyReference) {
ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName);
if (singletonFactory != null) {
// 从三级缓存放入二级缓存
singletonObject = singletonFactory.getObject();
this.earlySingletonObjects.put(beanName, singletonObject);
this.singletonFactories.remove(beanName);
}
}
}
}
return singletonObject;
}
继续看第二个getSingleton方法,该方法是在创建完Bean之后执行的，创建Bean的方法作为该方法的入参:
public Object getSingleton(String beanName, ObjectFactory<?> singletonFactory) {
synchronized (this.singletonObjects) {
Object singletonObject = this.singletonObjects.get(beanName);
if (singletonObject == null) {
// 是否正在销毁，是的话抛出异常
if (this.singletonsCurrentlyInDestruction) {
...
}
// 验证完要真正开始创建对象，先标识该bean正在被创建，因为spring bean创建过程复杂，步骤很多，需要标识
beforeSingletonCreation(beanName);
boolean newSingleton = false;
boolean recordSuppressedExceptions = (this.suppressedExceptions == null);
if (recordSuppressedExceptions) {
this.suppressedExceptions = new LinkedHashSet<>();
}
try {
// 传进来的调用，lambda表达式的调用
singletonObject = singletonFactory.getObject();
newSingleton = true;
}
catch (IllegalStateException ex) {
...
}
catch (BeanCreationException ex) {
...
}
finally {
if (recordSuppressedExceptions) {
this.suppressedExceptions = null;
}
afterSingletonCreation(beanName);
}
if (newSingleton) {
// 从二级缓存放到一级缓存
addSingleton(beanName, singletonObject);
}
}
return singletonObject;
}
}
下面看创建Bean的实际方法doCreateBean：
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) {
// 创建bean实例，仅仅调用构造方法，但是尚未设置属性
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) {
...
}
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) {
addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
}
// Initialize the bean instance.
// 初始化bean实例
Object exposedObject = bean;
try {
// bean属性填充
populateBean(beanName, mbd, instanceWrapper);
// 调用初始化方法，应用BeanPostProcessor后置处理器
exposedObject = initializeBean(beanName, exposedObject, mbd);
}
catch (Throwable ex) {
if (ex instanceof BeanCreationException && beanName.equals(((BeanCreationException) ex).getBeanName())) {
throw (BeanCreationException) ex;
}
else {
...
}
}
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()) {
...
}
}
}
}
// Register bean as disposable.
try {
registerDisposableBeanIfNecessary(beanName, bean, mbd);
}
catch (BeanDefinitionValidationException ex) {
...
}
return exposedObject;
}
这个方法中主要有两个方法，一个是createBeanInstance方法创建Bean，另一个是populateBean方法，对Bean属性进行填充，主要逻辑在applyPropertyValues方法中：
protected void applyPropertyValues(String beanName, BeanDefinition mbd, BeanWrapper bw, PropertyValues pvs) {
...
// Create a deep copy, resolving any references for values.
List<PropertyValue> deepCopy = new ArrayList<>(original.size());
boolean resolveNecessary = false;
for (PropertyValue pv : original) {
if (pv.isConverted()) {
deepCopy.add(pv);
} else {
// 依赖的属性名称
String propertyName = pv.getName();
// 依赖的属性值
Object originalValue = pv.getValue();
// 处理引用获取引用值
Object resolvedValue = valueResolver.resolveValueIfNecessary(pv, originalValue);
Object convertedValue = resolvedValue;
boolean convertible = bw.isWritableProperty(propertyName) &&
!PropertyAccessorUtils.isNestedOrIndexedProperty(propertyName);
if (convertible) {
convertedValue = convertForProperty(resolvedValue, propertyName, bw, converter);
}
// Possibly store converted value in merged bean definition,
// in order to avoid re-conversion for every created bean instance.
if (resolvedValue == originalValue) {
if (convertible) {
pv.setConvertedValue(convertedValue);
}
deepCopy.add(pv);
}
else if (convertible && originalValue instanceof TypedStringValue &&
!((TypedStringValue) originalValue).isDynamic() &&
!(convertedValue instanceof Collection || ObjectUtils.isArray(convertedValue))) {
pv.setConvertedValue(convertedValue);
deepCopy.add(pv);
}
else {
resolveNecessary = true;
deepCopy.add(new PropertyValue(pv, convertedValue));
}
}
}
...
}
处理引用的方法为resolveValueIfNecessary，进入该方法中主要处理逻辑在resolveReference方法：
private Object resolveReference(Object argName, RuntimeBeanReference ref) {
try {
Object bean;
String refName = ref.getBeanName();
refName = String.valueOf(doEvaluate(refName));
if (ref.isToParent()) {
if (this.beanFactory.getParentBeanFactory() == null) {
throw new BeanCreationException(
this.beanDefinition.getResourceDescription(), this.beanName,
"Can't resolve reference to bean '" + refName +
"' in parent factory: no parent factory available");
}
bean = this.beanFactory.getParentBeanFactory().getBean(refName);
}
else {
// A -> B
// 执行getBean操作获取所依赖的Bean B
bean = this.beanFactory.getBean(refName);
this.beanFactory.registerDependentBean(refName, this.beanName);
}
if (bean instanceof NullBean) {
bean = null;
}
return bean;
}
catch (BeansException ex) {
throw new BeanCreationException(
this.beanDefinition.getResourceDescription(), this.beanName,
"Cannot resolve reference to bean '" + ref.getBeanName() + "' while setting " + argName, ex);
}
}
从上面可以看出，又回到了getBean方法，也就是大体流程中的步骤7，后面的源码相同，就不贴出来了，最终获取到对象并完成属性填充，然后返回填充后的对象，并执行初始化方法和后置处理器方法。
Spring循环依赖时序图
下面为整个源码的时序图，建议同学下载源码到本地调试一遍会更好。
小结：上面为spring循环依赖的学习总结，包括了总体流程，源码细节，总结时序图。
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