目录
applyBeanPostProcessorsBeforeInstantiation
postProcessBeforeInstantiation
applyBeanPostProcessorsAfterInitialization
postProcessAfterInitialization与wrapIfNecessary
无代理Bean以及无自定义TargetSource Bean的代理的生成
前言
上篇博客主要介绍了getBean方法，Bean的初始化会有三个主要的方法
- createBean：初始化Bean，此时还没有进行属性填充
- populateBean：进行属性填充
- initializeBean：调用生命周期回调
这篇博客主要介绍createBean方法
本文难免有错误，欢迎大家指出错误
createBean
该方法在AbstractAutowireCapableBeanFactory中
/**
* Central method of this class: creates a bean instance,
* populates the bean instance, applies post-processors, etc.
* @see #doCreateBean
*/
@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.
英文讲的有点抽象，这里举一个例子，并不是所有的BeanDefinition都有BeanClass属性值的，例如使用@Bean注解
初始化的Bean，这种情况下，就需要通过beanName获得Bean的类名
Class<?> resolvedClass = resolveBeanClass(mbd, beanName);
if (resolvedClass != null && !mbd.hasBeanClass() && mbd.getBeanClassName() != null) {
mbdToUse = new RootBeanDefinition(mbd);
mbdToUse.setBeanClass(resolvedClass);
}
// Prepare method overrides.
try {
对于 methodOverrides的校验，即 lookup-method和 replaced-method属性指定的方法，
如果指定的方法不存在，会抛出异常，这两个属性的效果都是通过动态代理实现的
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.
这里初始化有自定义TargetSource的代理类
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 {
这里会干两件事：
1、初始化Bean
2、如果有切面应用于Bean，则会为其生成代理
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);
}
}
关于TargetSource，请查看：https://my.oschina.net/lixin91/blog/688188，spring定义的TargetSource在大多数情况下已经够我们使了，截至5.1.0版本，TargetSource的种类远不止《spring揭秘》中所写的五种，这点会在springAOP原理学习时进行补充
有自定义TargetSource代理类的生成
resolveBeforeInstantiation
该方法位于AbstractAutowireCapableBeanFactory类中
@Nullable
protected Object resolveBeforeInstantiation(String beanName, RootBeanDefinition mbd) {
Object bean = null;
if (!Boolean.FALSE.equals(mbd.beforeInstantiationResolved)) {
// Make sure bean class is actually resolved at this point.
不是合成类，存在InstantiationAwareBeanPostProcessors实例，这个类用于处理动态代理
if (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {
Class<?> targetType = determineTargetType(beanName, mbd);
if (targetType != null) {
接下里展开此方法
bean = applyBeanPostProcessorsBeforeInstantiation(targetType, beanName);
if (bean != null) {
展开此方法
bean = applyBeanPostProcessorsAfterInitialization(bean, beanName);
}
}
}
mbd.beforeInstantiationResolved = (bean != null);
}
return bean;
}applyBeanPostProcessorsBeforeInstantiation
该方法位于AbstractAutowireCapableBeanFactory类中
/**
* Apply InstantiationAwareBeanPostProcessors to the specified bean definition
* (by class and name), invoking their {@code postProcessBeforeInstantiation} methods.
* <p>Any returned object will be used as the bean instead of actually instantiating
* the target bean. A {@code null} return value from the post-processor will
* result in the target bean being instantiated.
* @param beanClass the class of the bean to be instantiated
* @param beanName the name of the bean
* @return the bean object to use instead of a default instance of the target bean, or {@code null}
* @see InstantiationAwareBeanPostProcessor#postProcessBeforeInstantiation
*/
@Nullable
protected Object applyBeanPostProcessorsBeforeInstantiation(Class<?> beanClass, String beanName) {
for (BeanPostProcessor bp : getBeanPostProcessors()) {
if (bp instanceof InstantiationAwareBeanPostProcessor) {
InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp;
其中某个InstantiationAwareBeanPostProcessor的此方法用于实例化代理类
Object result = ibp.postProcessBeforeInstantiation(beanClass, beanName);
if (result != null) {
return result;
}
}
}
return null;
}postProcessBeforeInstantiation
该方法位于AbstractAutoProxyCreator中
@Override
public Object postProcessBeforeInstantiation(Class<?> beanClass, String beanName) {
Object cacheKey = getCacheKey(beanClass, beanName);
若targetSourcedBeans中含有beanName，说明beanName的代理对象已经生成了
if (!StringUtils.hasLength(beanName) || !this.targetSourcedBeans.contains(beanName)) {
adviseBeans中存储是否需要为某个Bean生成代理，如果里面存在cacheKey，
说明之前已经初始化过Bean的代理类了，此时不需要再次初始化
if (this.advisedBeans.containsKey(cacheKey)) {
return null;
}
不代理的条件一：不是 Advice、Pointcut、Advisor、AopInfrastructureBean类的子类
不代理的条件二：shouldSkip返回 true，默认返回 false
不代理的条件三：AspectJAwareAdvisorAutoProxyCreator：应该跳过所有 AspectJPointcutAdvisor指定的增强 bean
if (isInfrastructureClass(beanClass) || shouldSkip(beanClass, beanName)) {
this.advisedBeans.put(cacheKey, Boolean.FALSE);
return null;
}
}
// Create proxy here if we have a custom TargetSource.
// Suppresses unnecessary default instantiation of the target bean:
// The TargetSource will handle target instances in a custom fashion.
尝试根据beanClass、beanName获得自定义的TargetSource
TargetSource targetSource = getCustomTargetSource(beanClass, beanName);
if (targetSource != null) {
if (StringUtils.hasLength(beanName)) {
在这里放入targetSourcedBeans缓存中，这样在函数开头就可以返回
this.targetSourcedBeans.add(beanName);
}
getAdvicesAndAdvisorsForBean方法的作用：
Return whether the given bean is to be proxied, what additional advices (e.g. AOP Alliance interceptors) and advisors to apply.
Object[] specificInterceptors = getAdvicesAndAdvisorsForBean(beanClass, beanName, targetSource);
创建代理，这里暂不展开此方法，在springaop篇在解析
Object proxy = createProxy(beanClass, beanName, specificInterceptors, targetSource);
this.proxyTypes.put(cacheKey, proxy.getClass());
return proxy;
}
return null;
}applyBeanPostProcessorsAfterInitialization
该方法位于AbstractAutowireCapableBeanFactory类中
@Override
public Object applyBeanPostProcessorsAfterInitialization(Object existingBean, String beanName)
throws BeansException {
Object result = existingBean;
for (BeanPostProcessor processor : getBeanPostProcessors()) {
如果之前两步有返回代理Bean，那么将会跑到这个方法里，完成生命周期，但是有个BeanPostProcessor会处理无自定义TargetSource，但又想生成代理的情况，接下来展开介绍此postProcessAfterInitialization方法，
Object current = processor.postProcessAfterInitialization(result, beanName);
if (current == null) {
return result;
}
result = current;
}
return result;
}postProcessAfterInitialization与wrapIfNecessary
这两个方法位于均AbstractAutowireCapableBeanFactory类中
@Override
public Object applyBeanPostProcessorsAfterInitialization(Object existingBean, String beanName)
throws BeansException {
Object result = existingBean;
for (BeanPostProcessor processor : getBeanPostProcessors()) {
如果之前两部有返回代理Bean，那么将会跑到这个方法里接下来展开介绍postProcessAfterInitialization方法，
这个方法用于处理无自定义TargetSource，但又想生成代理的情况
Object current = processor.postProcessAfterInitialization(result, beanName);
if (current == null) {
return result;
}
result = current;
}
return result;
}
/**
* Wrap the given bean if necessary, i.e. if it is eligible for being proxied.
* @param bean the raw bean instance
* @param beanName the name of the bean
* @param cacheKey the cache key for metadata access
* @return a proxy wrapping the bean, or the raw bean instance as-is
*/
protected Object wrapIfNecessary(Object bean, String beanName, Object cacheKey) {
对于有自定义TargetSource的Bean来说，上文已经生成代理了，所以不在生成代理，直接返回
if (StringUtils.hasLength(beanName) && this.targetSourcedBeans.contains(beanName)) {
return bean;
}
在advisedBean中有缓存，并且显示不应该生成代理，直接返回
if (Boolean.FALSE.equals(this.advisedBeans.get(cacheKey))) {
return bean;
}
不代理的条件一：不是 Advice、Pointcut、Advisor、AopInfrastructureBean类的子类
不代理的条件二：shouldSkip返回 true，默认返回 false
不代理的条件三：AspectJAwareAdvisorAutoProxyCreator：应该跳过所有 AspectJPointcutAdvisor指定的增强 bean
if (isInfrastructureClass(bean.getClass()) || shouldSkip(bean.getClass(), beanName)) {
this.advisedBeans.put(cacheKey, Boolean.FALSE);
return bean;
}
创建代理
// Create proxy if we have advice.
Object[] specificInterceptors = getAdvicesAndAdvisorsForBean(bean.getClass(), beanName, null);
if (specificInterceptors != DO_NOT_PROXY) {
this.advisedBeans.put(cacheKey, Boolean.TRUE);
Object proxy = createProxy(
对于一般的代理类，其也会使用TargetSource，而且是SingletonTargetSource，这说明spring
的动态代理与我们平常的认知还是有点区别的（多了个TargetSource）
bean.getClass(), beanName, specificInterceptors, new SingletonTargetSource(bean));
this.proxyTypes.put(cacheKey, proxy.getClass());
return proxy;
}
放入缓存，表明不需要生成代理（由于已经生成，并且是单例）
this.advisedBeans.put(cacheKey, Boolean.FALSE);
return bean;
}
无代理Bean以及无自定义TargetSource Bean的代理的生成
doCreateBean
/**
* Actually create the specified bean. Pre-creation processing has already happened
* at this point, e.g. checking {@code postProcessBeforeInstantiation} callbacks.
* <p>Differentiates between default bean instantiation, use of a
* factory method, and autowiring a constructor.
* @param beanName the name of the bean
* @param mbd the merged bean definition for the bean
* @param args explicit arguments to use for constructor or factory method invocation
* @return a new instance of the bean
* @throws BeanCreationException if the bean could not be created
* @see #instantiateBean
* @see #instantiateUsingFactoryMethod
* @see #autowireConstructor
*/
protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final @Nullable Object[] args)
throws BeanCreationException {
// Instantiate the bean.
BeanWrapper负责Bean属性的填充
BeanWrapper instanceWrapper = null;
if (mbd.isSingleton()) {
instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
}
if (instanceWrapper == null) {
createBeanInstance方法会创建Bean实例，调用构造函数初始化Bean
instanceWrapper = createBeanInstance(beanName, mbd, args);
}
获得初始化好的Bean，此时并未进行构造函数以外的依赖注入
final Object bean = instanceWrapper.getWrappedInstance();
获得Bean的class类型，并将其保存到RootBeanDefinition中，后续就不需要通过解析获得class类型
Class<?> beanType = instanceWrapper.getWrappedClass();
if (beanType != NullBean.class) {
mbd.resolvedTargetType = beanType;
}
// Allow post-processors to modify the merged bean definition.
这个属性锁住了RootBeanDefinition的两个属性，之前博客有讲，这里不赘述
上锁的目的是确保MergedBeanDefinitionPostProcessor只处理一次该Bean的RootBeanDefinition对象
synchronized (mbd.postProcessingLock) {
if (!mbd.postProcessed) {
try {
让MergedBeanDefinitionPostProcessor处理RootBeanDefinition
applyMergedBeanDefinitionPostProcessors(mbd, beanType, beanName);
}
catch (Throwable ex) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"Post-processing of merged bean definition failed", ex);
}
mbd.postProcessed = true;
}
}
代码跑到这里，Bean已经初步实例化了，接下来进行构造函数以外的依赖注入
// Eagerly cache singletons to be able to resolve circular references
// even when triggered by lifecycle interfaces like BeanFactoryAware.
这里可以看到setter循环依赖的处理机制，允许setter循环依赖的条件是：
1、Bean是单例
2、允许循环依赖
3、当前的Bean正处于创建阶段（未进行依赖注入，故没有完全创建完毕）
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");
}
这一步是解决setter循环依赖的关键，在DefaultSingletonBeanRegistry的解析中，已经解析过spring是如何通过三级缓存解决循环依赖的
这里不在赘述
addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
}
// Initialize the bean instance.
Object exposedObject = bean;
try {
进行构造函数以外的依赖注入，下一篇博客会展开讲解
populateBean(beanName, mbd, instanceWrapper);
进行生命周期回调，包括：
1、BeanNameAware、BeanClassLoaderAware、BeanFactoryAware
2、生命周期：BeanPostProcessor.postProcessBeforeInitialization
3、生命周期：InitializingBean.afterPropertiesSet
4、生命周期：调用初始化方法：init-method
5、生命周期：BeanPostProcessor的postProcessAfterInitialization
这里可能会生成代理
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);
}
}
if (earlySingletonExposure) {
注意到getSingleton函数的allowEarlyReference属性值为false，意味着只能从singletonObjects缓存或是
earlySingletonObjects缓存中获得实例，此时singletonObjects缓存中是不存在实例的，
只能从earlySingletonObjects缓存中获得
Object earlySingletonReference = getSingleton(beanName, false);
如果earlySingletonReference不为空，说明出现setter循环依赖
if (earlySingletonReference != null) {
由上文可以看出这两者指向相同的内存，这里为什么要判断exposedObject与bean相等呢？
其实我们忽略了生成代理的情况，initializeBean方法中可能会生成代理，如果生成了代理，
这句if是不成立的，如果没有生成代理，earlySingletonReference才是最终的结果
if (exposedObject == bean) {
exposedObject = earlySingletonReference;
}
对于代理，需要确保所有的依赖都已经生成
else if (!this.allowRawInjectionDespiteWrapping && hasDependentBean(beanName)) {
String[] dependentBeans = getDependentBeans(beanName);
actualDependentBeans用于存储未创建的依赖Bean
Set<String> actualDependentBeans = new LinkedHashSet<>(dependentBeans.length);
for (String dependentBean : dependentBeans) {
已经创建完毕的Bean会存储到alreadyCreated中，若alreadyCreated不包含 dependentBean会返回 false
if (!removeSingletonIfCreatedForTypeCheckOnly(dependentBean)) {
actualDependentBeans.add(dependentBean);
}
}
如果actualDependentBeans不为空，则抛出异常，因为beanName已经创建完毕，但是beanName的依赖却没有创建完毕
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 {
如果bean实现了销毁回调，则会在这里注册，存储到某个缓存中，在spring销毁Bean时，
会根据该缓存，调用销毁回调
registerDisposableBeanIfNecessary(beanName, bean, mbd);
}
catch (BeanDefinitionValidationException ex) {
throw new BeanCreationException(
mbd.getResourceDescription(), beanName, "Invalid destruction signature", ex);
}
return exposedObject;
}createBeanInstance：创建Bean实例
/**
* Create a new instance for the specified bean, using an appropriate instantiation strategy:
* factory method, constructor autowiring, or simple instantiation.
* @param beanName the name of the bean
* @param mbd the bean definition for the bean
* @param args explicit arguments to use for constructor or factory method invocation
* @return a BeanWrapper for the new instance
* @see #obtainFromSupplier
* @see #instantiateUsingFactoryMethod
* @see #autowireConstructor
* @see #instantiateBean
*/
protected BeanWrapper createBeanInstance(String beanName, RootBeanDefinition mbd, @Nullable Object[] args) {
// Make sure bean class is actually resolved at this point.
Class<?> beanClass = resolveBeanClass(mbd, beanName);
isNonPublicAccessAllowed默认返回 true，即非 public的类同样可以实例化
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());
}
如果我们配置了AbstractBeanDefinition的InstanceSupplier属性，则会根据InstanceSupplier的值
初始化Bean，而不是构造函数与工厂方法，这是java8推出时出现的，以后有空看下咋回事
Supplier<?> instanceSupplier = mbd.getInstanceSupplier();
if (instanceSupplier != null) {
return obtainFromSupplier(instanceSupplier, beanName);
}
如果工厂方法不为空，则通过工厂方法实例化，使用@Bean初始化的Bean，在这里初始化
if (mbd.getFactoryMethodName() != null) {
return instantiateUsingFactoryMethod(beanName, mbd, args);
}
夹在横线中间的代码，在我们初始化自己的Bean时，是不会执行的
----------------------------------------------------------------------------------------------------------------------------
// Shortcut when re-creating the same bean...
boolean resolved = false;
boolean autowireNecessary = false;
第一次初始化时，传入的构造函数参数为空，会跑到下面的if语句中
if (args == null) {
在第一次初始化时，并不存在已经解析好的构造函数或是工厂方法，所以下面这些语句并不会执行
synchronized (mbd.constructorArgumentLock) {
判断是否存在已经解析好的构造函数或是工厂方法
if (mbd.resolvedConstructorOrFactoryMethod != null) {
resolved = true;
如果构造函数参数已解析，即知道要注入什么给构造函数参数，
则autowireNecessary为true，构造函数所需要的参数由constructorArgumentsResolved
提供，若constructorArgumentsResolved为空，说明我们没有配置构造函数参数，此时spring
会使用默认构造函数
autowireNecessary = mbd.constructorArgumentsResolved;
}
}
}
if (resolved) {
if (autowireNecessary) {
有参构造函数所需要的所有信息都存储在RootBeanDefinition中，
从而避免再次解析
return autowireConstructor(beanName, mbd, null, null);
}
else {
使用默认构造函数初始化
return instantiateBean(beanName, mbd);
}
}
---------------------------------------------------------------------------------------------------------------------------------
Bean首次初始化时，会在以下代码执行初始化
// Candidate constructors for autowiring?
获得构造函数列表，判断使用哪个构造器：
如果实现了 SmartInstantiationAwareBeanPostProcessor，
可以通过方法 determineCandidateConstructors指定，否则返回 null
Constructor<?>[] ctors = determineConstructorsFromBeanPostProcessors(beanClass, beanName);
if (ctors != null || mbd.getResolvedAutowireMode() == AUTOWIRE_CONSTRUCTOR ||
mbd.hasConstructorArgumentValues() || !ObjectUtils.isEmpty(args)) {
第一次初始化时，具有有参构造函数的Bean会在这里初始化，这个函数内部会通过createArgumentArray
方法解析获得构造函数参数，并且会有构造函数循环依赖的检查，这句代码执行完毕后，RootBeanDefinition的关于构造函数的四个属性会发生变化
1、constructorArgumentsResolved变为true
2、resolvedConstructorOrFactoryMethod不为空
3、constructorArgumentValues不为空
4、preparedConstructorArguments不为空
以后初始化时，就可以直接利用上述信息，跑到横向内的代码执行初始化
return autowireConstructor(beanName, mbd, ctors, args);
}
// Preferred constructors for default construction?
从5.1.0开始有这一段代码，但是这个函数一般返回null，if语句内的代码基本没用，所以还是会执行默认构造函数初始化
ctors = mbd.getPreferredConstructors();
if (ctors != null) {
return autowireConstructor(beanName, mbd, ctors, null);
}
使用默认构造函数初始化
// No special handling: simply use no-arg constructor.
return instantiateBean(beanName, mbd);
}收获
透过本次博客的学习，可以看出BeanDefinition并不是一开始就存储了Bean的所有信息，在Bean第一次进行初始化时，才会进行对应的解析，并且将解析的结果存储在BeanDefinition中
Bean初始化的轨迹
spring初始化Bean的结构十分复杂，这里画一张图，以捋清楚Bean初始化的顺序，对于搞清楚setter循环依赖的解决有一定帮助
