BeanPostProcessor 是spring容器的容器的一个扩展点，可以进行自定义的实例化、初始化、依赖装配、依赖检查等流程，即可以覆盖默认的实例化，也可以增强初始化、依赖注入、依赖检查等流程。
Spring提供了很多BeanPostProcesser的扩展接口及其实现，用于完成除实例化之外的其他功能。
public interface BeanPostProcessor {
@Nullable
default Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException {
return bean;
}
@Nullable
default Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException {
return bean;
}
bean的创建和初始化
第1次调用后置处理器：
InstantiationAwareBeanPostProcessor#postProcessBeforeInstantiation
第1次调用后置处理器在目标对象实例化之前调用，可以返回任意类型的值，如果不为空，此时可以代替原本应该生成的目标对象（一般是代理对象），并且会调用postProcessAfterInitialization 方法，否则走正常实例化流程
源码跟踪：
调用 AbstractAutowireCapableBeanFactory的createBean方法创建bean实例
protected Object createBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args)
throws BeanCreationException {
。。。。。。。
try {
// Give BeanPostProcessors a chance to return a proxy instead of the target bean instance.
// 实例化前的后置处理器调用 InstantiationAwareBeanPostProcessor
// 第1次调用后置处理器
Object bean = resolveBeforeInstantiation(beanName, mbdToUse);
if (bean != null) {
// 直接返回替代的对象实例，不再实例化目标对象
return bean;
}
}
}
AbstractAutowireCapableBeanFactory#resolveBeforeInstantiation 代码如下
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.
//确保此时bean类已经被解析
if (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {
Class<?> targetType = determineTargetType(beanName, mbd);
if (targetType != null) {
// 在目标对象实例化之前调用，可以返回任意类型的值，如果不为空，
// 此时可以代替原本应该生成的目标对象实例（一般是代理对象）
bean = applyBeanPostProcessorsBeforeInstantiation(targetType, beanName);
if (bean != null) {
// 如果bean不为空，调用 postProcessAfterInitialization 方法，否则走正常实例化流程
bean = applyBeanPostProcessorsAfterInitialization(bean, beanName);
}
}
}
mbd.beforeInstantiationResolved = (bean != null);
}
return bean;
}
@Nullable
protected Object applyBeanPostProcessorsBeforeInstantiation(Class<?> beanClass, String beanName) {
for (BeanPostProcessor bp : getBeanPostProcessors()) {
if (bp instanceof InstantiationAwareBeanPostProcessor) {
InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp;
Object result = ibp.postProcessBeforeInstantiation(beanClass, beanName);
if (result != null) {
return result;
}
}
}
return null;
}
InstantiationAwareBeanPostProcessor 是BeanPostProcesser的子类，如果bean继承了该类，则会走 applyBeanPostProcessorsBeforeInstantiation方法，不会走正常的实例化bean的流程，在此返回object对象
通过如上代码可以进行实例化的预处理（自定义实例化bean，如创建相应的代理对象）和后处理（如 进行自定义实例化的bean的依赖装配）。
第2次调用后置处理器：
第2次调用后置处理器，决定以哪种方式创建bean对象，(先通过工厂创建；如果不能，使用构造方法；最后使用无参构造 )假如类中有多个构造方法，生效的优先级（@Autowired > autowireMode =3 > 无参构造）
SmartInstantiationAwareBeanPostProcessor#determineCandidateConstructors
确定要为给定bean使用的候选构造器，检查所有已注册的构造器，实现类 AutowiredAnnotationBeanPostProcessor，扫描@Autowired修饰的构造器,判断创建对象所用的构造 器（deafult，primary）
源码跟踪 ：
AbstractAutowireCapableBeanFactory的doCreateBean方法代码如下：
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) {
/**
* 第2次调用后置处理器
* 创建bean实例，并将实例放在包装类BeanWrapper中返回
* 1.通过工厂方法创建bean实例 method.invoke()
* 2.通过构造方法自动注入创建bean实例 clazz.newInstance()
* 3.通过无参构造器创建bean实例 clazz.newInstance()
*/
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 {
//允许后置处理器修改合并的bean定义 MergedBeanDefinitionPostProcessor#postProcessMergedBeanDefinition
// 第3次调用后置处理器
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.
// 急切地缓存单例，以便能够解析循环引用,即使是由生命周期接口(如BeanFactoryAware)触发的。
// 如果当前是单例，且允许循环依赖而且当前bean处于创建状态
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singletonFactories SmartInstantiationAwareBeanPostProcessor#getEarlyBeanReference
//第4次调用后置处理器 提前暴露对象的引用到 singletonFactory 单例工厂中，解决循环引用的问题
addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
}
// Initialize the bean instance.
// 初始化bean实例
Object exposedObject = bean;
try {
// 填充bean 设置属性 InstantiationAwareBeanPostProcessors
// 第5次，第6次调用后置处理器 注入依赖
populateBean(beanName, mbd, instanceWrapper);
// 初始化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);
}
}
// 提前暴露单例
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 {
// 将bean注册为可以销毁 DestructionAwareBeanPostProcessor bean的销毁后置处理器
// 第九次调用后置处理器
registerDisposableBeanIfNecessary(beanName, bean, mbd);
}
catch (BeanDefinitionValidationException ex) {
throw new BeanCreationException(
mbd.getResourceDescription(), beanName, "Invalid destruction signature", ex);
}
return exposedObject;
}
AbstractAutowireCapableBeanFactory.createBeanInstance()
protected BeanWrapper createBeanInstance(String beanName, RootBeanDefinition mbd, @Nullable Object[] args) {
..........
// Candidate constructors for autowiring?
// 自动装配的候选构造器 SmartInstantiationAwareBeanPostProcessor#determineCandidateConstructors
// 第2次调用后置处理器
Constructor<?>[] ctors = determineConstructorsFromBeanPostProcessors(beanClass, beanName);
// AutowireMode设置为3，采用构造器贪婪模式 最多参数构造器注入 Autowire)constructor = 3
if (ctors != null || mbd.getResolvedAutowireMode() == AUTOWIRE_CONSTRUCTOR ||
mbd.hasConstructorArgumentValues() || !ObjectUtils.isEmpty(args)) {
return autowireConstructor(beanName, mbd, ctors, args);
}
// Preferred constructors for default construction?
// 默认构造的首选构造器
ctors = mbd.getPreferredConstructors();
if (ctors != null) {
return autowireConstructor(beanName, mbd, ctors, null);
}
// No special handling: simply use no-arg constructor.
// 无参构造器
return instantiateBean(beanName, mbd);
}
第3次调用后置处理器：见上面代码：
MergedBeanDefinitionPostProcessor#postProcessMergedBeanDefinition
允许后置处理器修改合并的bean定义 ，实现类 AutowiredAnnotationBeanPostProcessor，用于扫描 @Autowired和@Value修饰的属性和方法，封装到InjectionMetadata。
AutowiredAnnotationBeanPostProcessor 将扫描 Spring 容器中所有 Bean，当发现 Bean 中拥有@Autowired 注解时就找到和其匹配（默认按类型匹配）的 Bean，并注入到对应的地方中去。会调用到AutowiredAnnotationBeanPostProcessor 的buildAutowiringMetadata()方法，buildAutowiringMetadata方法解析等待自动注入类的所有属性。它通过分析所有字段和方法并初始化org.springframework.beans.factory.annotation.InjectionMetadata类的实例来实现。
详细代码如下：
详细代码如下：
private InjectionMetadata buildAutowiringMetadata(final Class<?> clazz) {
List<InjectionMetadata.InjectedElement> elements = new ArrayList<>();
Class<?> targetClass = clazz;
do {
final List<InjectionMetadata.InjectedElement> currElements = new ArrayList<>();
ReflectionUtils.doWithLocalFields(targetClass, field -> {
// 找到@Autowired 注解修饰的属性，
AnnotationAttributes ann = findAutowiredAnnotation(field);
if (ann != null) {
if (Modifier.isStatic(field.getModifiers())) {
if (logger.isInfoEnabled()) {
logger.info("Autowired annotation is not supported on static fields: " + field);
}
return;
}
boolean required = determineRequiredStatus(ann);
currElements.add(new AutowiredFieldElement(field, required));
}
});
ReflectionUtils.doWithLocalMethods(targetClass, method -> {
Method bridgedMethod = BridgeMethodResolver.findBridgedMethod(method);
if (!BridgeMethodResolver.isVisibilityBridgeMethodPair(method, bridgedMethod)) {
return;
}// 找到@Autowired 修饰的方法
AnnotationAttributes ann = findAutowiredAnnotation(bridgedMethod);
if (ann != null && method.equals(ClassUtils.getMostSpecificMethod(method, clazz))) {
if (Modifier.isStatic(method.getModifiers())) {
if (logger.isInfoEnabled()) {
logger.info("Autowired annotation is not supported on static methods: " + method);
}
return;
}
if (method.getParameterCount() == 0) {
if (logger.isInfoEnabled()) {
logger.info("Autowired annotation should only be used on methods with parameters: " +
method);
}
}
boolean required = determineRequiredStatus(ann);
PropertyDescriptor pd = BeanUtils.findPropertyForMethod(bridgedMethod, clazz);
currElements.add(new AutowiredMethodElement(method, required, pd));
}
});
elements.addAll(0, currElements);
targetClass = targetClass.getSuperclass();
}
while (targetClass != null && targetClass != Object.class);
// 生成一个 InjectionMetadata对象，进行属性赋值（通过反射）
return new InjectionMetadata(clazz, elements);
}
第4次调用后置处理器：解决循环引用的问题，提前暴露对象的应用到 SingletonFactory 单例工厂中，此时对象的属性未赋值；详细见spring循环依赖；
第5次调用后置处理器：
InstantiationAwareBeanPostProcessor#postProcessAfterInstantiation
在目标对象实例化之后调用，此时对象被实例化，但是对象的属性还未被设置。如果该方法返回false，则会忽略之后的属性设置；返回true，则进行正常的属性设置。
源码跟踪：
// 初始化bean实例
Object exposedObject = bean;
try {
// 填充bean 设置属性 InstantiationAwareBeanPostProcessors
// 第5次，第6次调用后置处理器 注入依赖
populateBean(beanName, mbd, instanceWrapper);
// 初始化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);
}
}
protected void populateBean(String beanName, RootBeanDefinition mbd, @Nullable BeanWrapper bw) { if (bw == null) { if (mbd.hasPropertyValues()) { throw new BeanCreationException( mbd.getResourceDescription(), beanName, "Cannot apply property values to null instance"); } else { // Skip property population phase for null instance. return; } } // Give any InstantiationAwareBeanPostProcessors the opportunity to modify the // state of the bean before properties are set. This can be used, for example, // to support styles of field injection. boolean continueWithPropertyPopulation = true; // 在属性设置之前修改bean的状态 InstantiationAwareBeanPostProcessor#postProcessAfterInstantiation // 第5次调用后置处理器 if (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) { for (BeanPostProcessor bp : getBeanPostProcessors()) { if (bp instanceof InstantiationAwareBeanPostProcessor) { InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp; //在目标对象实例化之后调用，此时对象被实例化，但是对象的属性还未设置。如果该方法返回 //fasle,则会忽略之后的属性设置。返回true，按正常流程设置属性值 if (!ibp.postProcessAfterInstantiation(bw.getWrappedInstance(), beanName)) { continueWithPropertyPopulation = false; break; } } } } if (!continueWithPropertyPopulation) { // InstantiationAwareBeanPostProcessor#postProcessAfterInstantiation方法返回false, // 直接返回，将忽略实例的属性设置 return; } PropertyValues pvs = (mbd.hasPropertyValues() ? mbd.getPropertyValues() : null); // 判断autowireMode否是 by_name或者by_type if (mbd.getResolvedAutowireMode() == AUTOWIRE_BY_NAME || mbd.getResolvedAutowireMode() == AUTOWIRE_BY_TYPE) { MutablePropertyValues newPvs = new MutablePropertyValues(pvs); // Add property values based on autowire by name if applicable. if (mbd.getResolvedAutowireMode() == AUTOWIRE_BY_NAME) { autowireByName(beanName, mbd, bw, newPvs); } // Add property values based on autowire by type if applicable. if (mbd.getResolvedAutowireMode() == AUTOWIRE_BY_TYPE) { autowireByType(beanName, mbd, bw, newPvs); } pvs = newPvs; } boolean hasInstAwareBpps = hasInstantiationAwareBeanPostProcessors(); boolean needsDepCheck = (mbd.getDependencyCheck() != AbstractBeanDefinition.DEPENDENCY_CHECK_NONE); PropertyDescriptor[] filteredPds = null; if (hasInstAwareBpps) { if (pvs == null) { pvs = mbd.getPropertyValues(); } //InstantiationAwareBeanPostProcessor.postProcessProperties // 第6次调用后置处理器 // 可以在该方法内对属性值进行修改（此时属性值还未设置，但可以修改原本会设置的进去的属性值）。 // 如果postProcessAfterInstantiation方法返回false，该方法不会调用 for (BeanPostProcessor bp : getBeanPostProcessors()) { if (bp instanceof InstantiationAwareBeanPostProcessor) { //处理 @Autowired 属性注入逻辑 // AutowiredAnnotationBeanPostProcessor.postProcessProperties //InjectionMetadata#inject //AutowiredAnnotationBeanPostProcessor.AutowiredFieldElement#inject //DefaultListableBeanFactory#resolveDependency //DefaultListableBeanFactory#doResolveDependency //DependencyDescriptor#resolveCandidate //此方法直接返回 beanFactory.getBean(beanName); InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp; PropertyValues pvsToUse = ibp.postProcessProperties(pvs, bw.getWrappedInstance(), beanName); if (pvsToUse == null) { if (filteredPds == null) { filteredPds = filterPropertyDescriptorsForDependencyCheck(bw, mbd.allowCaching); } pvsToUse = ibp.postProcessPropertyValues(pvs, filteredPds, bw.getWrappedInstance(), beanName); if (pvsToUse == null) { return; } } pvs = pvsToUse; } } } if (needsDepCheck) { if (filteredPds == null) { filteredPds = filterPropertyDescriptorsForDependencyCheck(bw, mbd.allowCaching); } checkDependencies(beanName, mbd, filteredPds, pvs); } if (pvs != null) { // 普通属性填充 propertyValues applyPropertyValues(beanName, mbd, bw, pvs); } }
第6次调用后置处理器，用来处理Bean中 属性注入，包括 @Autowired 注入；此时如果存在循环引用的问题，也会一并处理掉（从单例工厂中获取属性bean，生成earlySingletonObject，添加到earlySingletonObjects中，解决循环依赖问题）；（代码见上）
第7次调用后置处理器： 主要用于处理aware接口，bean.setXXXAware()
protected Object initializeBean(final String beanName, final Object bean, @Nullable RootBeanDefinition mbd) {
if (System.getSecurityManager() != null) {
AccessController.doPrivileged((PrivilegedAction<Object>) () -> {
invokeAwareMethods(beanName, bean);
return null;
}, getAccessControlContext());
}
else {
invokeAwareMethods(beanName, bean);
}
Object wrappedBean = bean;
if (mbd == null || !mbd.isSynthetic()) {
// BeanPostProcessor.postProcessBeforeInitialization
//第7次调用后置处理器
// 如果配置了@PostConstruct 会调用
// InitDestroyAnnotationBeanPostProcessor#postProcessBeforeInitialization
wrappedBean = applyBeanPostProcessorsBeforeInitialization(wrappedBean, beanName);
}
try {
// 执行bean生命周期回调的init方法
invokeInitMethods(beanName, wrappedBean, mbd);
}
catch (Throwable ex) {
throw new BeanCreationException(
(mbd != null ? mbd.getResourceDescription() : null),
beanName, "Invocation of init method failed", ex);
}
if (mbd == null || !mbd.isSynthetic()) {
// BeanPostProcessor.postProcessAfterInitialization
//第8次调用后置处理器
// aop实现：AbstractAutoProxyCreator#postProcessAfterInitialization
wrappedBean = applyBeanPostProcessorsAfterInitialization(wrappedBean, beanName);
}
return wrappedBean;
}
BeanPostProcessor#postProcessBeforeInitialization
在bean new出来并完成属性填充之后，回调init方法之前调用；
其他扩展：处理aware方法调用，比如实现了ApplicationContextAware或者EnvironmentAware接口，
ApplicationContextAwareProcessor#postProcessBeforeInitialization
如果配置了@PostConstruct 会调用
InitDestroyAnnotationBeanPostProcessor#postProcessBeforeInitialization
说明 ：构造方法 > @Autowired > @PostContruct 方法 执行顺序 （此时还未执行bean的 init 初始化方法），@PostContruct 修饰的方法，主要用于bean实例化 属性注入之后的调用
源码追踪：
AbstractAutowireCapableBeanFactory#initializeBean(String, Object, RootBeanDefinition) > wrappedBean = applyBeanPostProcessorsBeforeInitialization(wrappedBean, beanName)
第8次调用后置处理器：在执行bean的 init方法之后调用；扩展：spring的aop基于此实现；源码见 上
第9次调用后置处理器：标记单例bean为可销毁的，
类 AbstractBeanFactory protected void registerDisposableBeanIfNecessary(String beanName, Object bean, RootBeanDefinition mbd) { AccessControlContext acc = (System.getSecurityManager() != null ? getAccessControlContext() : null); //requiresDestruction中调用DestructionAwareBeanPostProcessor#requiresDestruction //第九次调用后置处理器 确定给定的bean实例是否需要销毁后处理器 单例bean才会加标记 为可销毁，
// （这里是registerDisposableBean方法加入到 DefaultSingletonBeanFactory 中的 disposableBeans 中，是个map，k-beanName,v-bean） if (!mbd.isPrototype() && requiresDestruction(bean, mbd)) { if (mbd.isSingleton()) { // Register a DisposableBean implementation that performs all destruction // work for the given bean: DestructionAwareBeanPostProcessors, // DisposableBean interface, custom destroy method. registerDisposableBean(beanName, new DisposableBeanAdapter(bean, beanName, mbd, getBeanPostProcessors(), acc)); } else { // A bean with a custom scope... Scope scope = this.scopes.get(mbd.getScope()); if (scope == null) { throw new IllegalStateException("No Scope registered for scope name '" + mbd.getScope() + "'"); } scope.registerDestructionCallback(beanName, new DisposableBeanAdapter(bean, beanName, mbd, getBeanPostProcessors(), acc)); } } }