本章目的
认识SpringBPP,掌握BeanPostProcessor。
系统版本
- jdk1.8
- springboot 2.1.6.RELEASE
- 开发工具（IntelliJ IDEA 2018.1.5 x64）
- apache maven（3.6.0，本章采用maven形式管理jar包，具体配置环境变量以及使用请自行查找资料）
开始。。。
在spring世界中，当我们要对bean的实例化，初始化过程中做点什么的时候，作为开发人员，显然不需要继承ApplicationContext或者BeanFactory，这个时候，bpp提供对外的钩子，就被派上用场了。下面我们介绍几个比较常见的bpp接口。
1. BeanFactoryPostProcessor
@FunctionalInterface
public interface BeanFactoryPostProcessor {
/**
* Modify the application context's internal bean factory after its standard
* initialization. All bean definitions will have been loaded, but no beans
* will have been instantiated yet. This allows for overriding or adding
* properties even to eager-initializing beans.
* @param beanFactory the bean factory used by the application context
* @throws org.springframework.beans.BeansException in case of errors
*/
void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) throws BeansException;
}
实现该接口，可以在spring的bean创建之前，修改bean的定义属性。也就是说，Spring允许BeanFactoryPostProcessor在容器实例化任何其它bean之前读取配置元数据，并可以根据需要进行修改，例如可以把bean的scope从singleton改为prototype，也可以把property的值给修改掉。可以同时配置多个BeanFactoryPostProcessor，并通过设置'order'属性来控制各个BeanFactoryPostProcessor的执行次序。
注意：BeanFactoryPostProcessor是在spring容器加载了bean的定义文件之后，在bean实例化之前执行的。接口方法的入参是ConfigurrableListableBeanFactory，使用该参数，可以获取到相关bean的定义信息
spring中，有内置的一些BeanFactoryPostProcessor实现类，常用的有：
-
org.springframework.beans.factory.config.PropertyPlaceholderConfigurer:${}属性覆盖 org.springframework.beans.factory.config.PropertyOverrideConfigurer-
org.springframework.beans.factory.config.CustomEditorConfigurer：用来注册自定义的属性编辑器
2. BeanPostProcessor :bean的后置处理器
public interface BeanPostProcessor {
/**
* Apply this BeanPostProcessor to the given new bean instance <i>before</i> any bean
* initialization callbacks (like InitializingBean's {@code afterPropertiesSet}
* or a custom init-method). The bean will already be populated with property values.
* The returned bean instance may be a wrapper around the original.
* <p>The default implementation returns the given {@code bean} as-is.
* @param bean the new bean instance
* @param beanName the name of the bean
* @return the bean instance to use, either the original or a wrapped one;
* if {@code null}, no subsequent BeanPostProcessors will be invoked
* @throws org.springframework.beans.BeansException in case of errors
* @see org.springframework.beans.factory.InitializingBean#afterPropertiesSet
*/
@Nullable
default Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException {
return bean;
}
/**
* Apply this BeanPostProcessor to the given new bean instance <i>after</i> any bean
* initialization callbacks (like InitializingBean's {@code afterPropertiesSet}
* or a custom init-method). The bean will already be populated with property values.
* The returned bean instance may be a wrapper around the original.
* <p>In case of a FactoryBean, this callback will be invoked for both the FactoryBean
* instance and the objects created by the FactoryBean (as of Spring 2.0). The
* post-processor can decide whether to apply to either the FactoryBean or created
* objects or both through corresponding {@code bean instanceof FactoryBean} checks.
* <p>This callback will also be invoked after a short-circuiting triggered by a
* {@link InstantiationAwareBeanPostProcessor#postProcessBeforeInstantiation} method,
* in contrast to all other BeanPostProcessor callbacks.
* <p>The default implementation returns the given {@code bean} as-is.
* @param bean the new bean instance
* @param beanName the name of the bean
* @return the bean instance to use, either the original or a wrapped one;
* if {@code null}, no subsequent BeanPostProcessors will be invoked
* @throws org.springframework.beans.BeansException in case of errors
* @see org.springframework.beans.factory.InitializingBean#afterPropertiesSet
* @see org.springframework.beans.factory.FactoryBean
*/
@Nullable
default Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException {
return bean;
}
BeanPostProcessor，可以在spring容器实例化bean之后，在执行bean的初始化方法前后，添加一些自己的处理逻辑。这里说的初始化方法，指的是下面两种：
- bean实现了InitializingBean接口，对应的方法为afterPropertiesSet
- 在bean定义的时候，通过init-method设置的方法
注意：BeanPostProcessor是在spring容器加载了bean的定义文件并且实例化bean之后执行的。BeanPostProcessor的执行顺序是在BeanFactoryPostProcessor之后。 postProcessBeforeInitialization 是在bean实例化之前，属性设置之后但是初始化之前
-pstProcessAfterInitialization是在所有初始化方法之后
这个接口很重要，所以 接下来，我们重点介绍下这个接口。先看下 他的继承关系。
继承实现关系
从图中可以看出,
BeanPostProcessor的实现类有很多，我们从左向右挑几个比较常用的:
1）. DestructionAwareBeanPostProcessor :销毁Bean后置处理器
public interface DestructionAwareBeanPostProcessor extends BeanPostProcessor {
/**
* Apply this BeanPostProcessor to the given bean instance before its
* destruction, e.g. invoking custom destruction callbacks.
...
*/
void postProcessBeforeDestruction(Object bean, String beanName) throws BeansException;
/**
* Determine whether the given bean instance requires destruction by this
* post-processor.
* <p>The default implementation returns {@code true}.
...
*/
default boolean requiresDestruction(Object bean) {
return true;
}
}
注释已经很清楚了，就是销毁bean之前，你还想做什么。
2）. MergedBeanDefinitionPostProcessor :合并Bean定义后置处理器
public interface MergedBeanDefinitionPostProcessor extends BeanPostProcessor {
/**
* Post-process the given merged bean definition for the specified bean.
...
*/
void postProcessMergedBeanDefinition(RootBeanDefinition beanDefinition, Class<?> beanType, String beanName);
/**
* A notification that the bean definition for the specified name has been reset,
* and that this post-processor should clear any metadata for the affected bean.
...
*/
default void resetBeanDefinition(String beanName) {
}
}
合并Bean定义后置处理器，感觉 概念很模糊。大家可以看下，它的一个实现类AutuwiredAnnotationBeanPostProcessor
//在每个bean实例化后，初始化前执行，获取并记录该bean属性注入的元数据（成员），在随后的属性注入时使用
@Override
public void postProcessMergedBeanDefinition(RootBeanDefinition beanDefinition, Class<?> beanType, String beanName) {
InjectionMetadata metadata = findAutowiringMetadata(beanName, beanType, null);
metadata.checkConfigMembers(beanDefinition);
}
这个方法，检查并注册需要注入的成员，然后调用InstantiationAwareBeanPostProcessor#postProcessProperties方法，完成所有成员的属性注入（依赖注入）
@Override
public PropertyValues postProcessProperties(PropertyValues pvs, Object bean, String beanName) {
InjectionMetadata metadata = findAutowiringMetadata(beanName, bean.getClass(), pvs);
try {
metadata.inject(bean, beanName, pvs);
}
catch (BeanCreationException ex) {
throw ex;
}
catch (Throwable ex) {
throw new BeanCreationException(beanName, "Injection of autowired dependencies failed", ex);
}
return pvs;
}
3）. InstantiationAwareBeanPostProcessor :实例化bean后置处理器
public interface InstantiationAwareBeanPostProcessor extends BeanPostProcessor {
/**
* Apply this BeanPostProcessor <i>before the target bean gets instantiated</i>.
* The returned bean object may be a proxy to use instead of the target bean,
* effectively suppressing default instantiation of the target bean.
* <p>If a non-null object is returned by this method, the bean creation process
* will be short-circuited. The only further processing applied is the
...
调用机制为bean实例化(Instantiation)之前
默认null,即什么不做，代表走默认实例化。
如果返回了bean实例, 则会替代原来正常通过target bean生成的bean的流程. 典型的例如aop返回proxy对象.
*/
@Nullable
default Object postProcessBeforeInstantiation(Class<?> beanClass, String beanName) throws BeansException {
return null;
}
/**
* Perform operations after the bean has been instantiated, via a constructor or factory method,
* but before Spring property population (from explicit properties or autowiring) occurs.
...
//调用时机为bean实例化(Instantiation)之后和任何初始化(Initialization)之前。
//实例化完了之后，如果这里返回true，代表这个bean的属性会被正常set进去，如果是fa//lse，那么就会跳过，spring给的建议是一般都要返回true。
*/
default boolean postProcessAfterInstantiation(Object bean, String beanName) throws BeansException {
return true;
}
/**
* Post-process the given property values before the factory applies them
* to the given bean, without any need for property descriptors.
* <p>Implementations should return {@code null} (the default) if they provide a custom
* {@link #postProcessPropertyValues} implementation, and {@code pvs} otherwise.
* In a future version of this interface (with {@link #postProcessPropertyValues} removed),
* the default implementation will return the given {@code pvs} as-is directly.
...
//调用时机为postProcessAfterInstantiation执行之后并返回true, //返回的PropertyValues将作用于给定bean属性赋值.
//spring 5.1之后出现以替换@Deprecated标注的postProcessPropertyValues
*/
@Nullable
default PropertyValues postProcessProperties(PropertyValues pvs, Object bean, String beanName)
throws BeansException {
return null;
}
/**
* Post-process the given property values before the factory applies them
* to the given bean. Allows for checking whether all dependencies have been
* satisfied, for example based on a "Required" annotation on bean property setters.
* <p>Also allows for replacing the property values to apply, typically through
* creating a new MutablePropertyValues instance based on the original PropertyValues,
* adding or removing specific values.
* <p>The default implementation returns the given {@code pvs} as-is.
...
*/
@Deprecated
@Nullable
default PropertyValues postProcessPropertyValues(
PropertyValues pvs, PropertyDescriptor[] pds, Object bean, String beanName) throws BeansException {
return pvs;
}
4）. SmartInstantiationAwareBeanPostProcessor :智能实例化bean后置处理器(InstantiationAwareBeanPostProcessor的子类)
public interface SmartInstantiationAwareBeanPostProcessor extends InstantiationAwareBeanPostProcessor {
/**
* Predict the type of the bean to be eventually returned from this
* processor's {@link #postProcessBeforeInstantiation} callback.
* <p>The default implementation returns {@code null}.
...
//预测Bean的类型，返回第一个预测成功的Class类型，如果不能预测返回null
*/
@Nullable
default Class<?> predictBeanType(Class<?> beanClass, String beanName) throws BeansException {
return null;
}
/**
* Determine the candidate constructors to use for the given bean.
* <p>The default implementation returns {@code null}.
...
// 选择合适的构造器，比如目标对象有多个构造器，在这里可以进行一些定制化，选择合适的构造器
// beanClass参数表示目标实例的类型，beanName是目标实例在Spring容器中的name
// 返回值是个构造器数组，如果返回null，会执行下一个PostProcessor的determineCandidateConstructors方法；否则选取该PostProcessor选择的构造器
*/
@Nullable
default Constructor<?>[] determineCandidateConstructors(Class<?> beanClass, String beanName)
throws BeansException {
return null;
}
/**
* Obtain a reference for early access to the specified bean,
* typically for the purpose of resolving a circular reference.
* <p>This callback gives post-processors a chance to expose a wrapper
* early - that is, before the target bean instance is fully initialized.
* The exposed object should be equivalent to the what
* {@link #postProcessBeforeInitialization} / {@link #postProcessAfterInitialization}
* would expose otherwise. Note that the object returned by this method will
* be used as bean reference unless the post-processor returns a different
* wrapper from said post-process callbacks. In other words: Those post-process
* callbacks may either eventually expose the same reference or alternatively
* return the raw bean instance from those subsequent callbacks (if the wrapper
* for the affected bean has been built for a call to this method already,
* it will be exposes as final bean reference by default).
* <p>The default implementation returns the given {@code bean} as-is.
...
//获得提前暴露的bean引用。主要用于解决循环引用的问题
//只有单例对象才会调用此方法
*/
default Object getEarlyBeanReference(Object bean, String beanName) throws BeansException {
return bean;
}
}
可以看出继承InstantiationAwareBeanPostProcessor接口，多了三个方法。这个接口，有一个比较厉害的实现类AbstractAutoProxyCreator,它是AOP实现的核心。在下一个系列中，我们再细说这个类。
3. AbstractAutowireCapableBeanFactory ：BeanPostProcessor的入口，spring世界中的生产者
类中方法调用顺序
/**
* Abstract bean factory superclass that implements default bean creation,
* with the full capabilities specified by the {@link RootBeanDefinition} class.
* Implements the {@link org.springframework.beans.factory.config.AutowireCapableBeanFactory}
* interface in addition to AbstractBeanFactory's {@link #createBean} method.
*
* <p>Provides bean creation (with constructor resolution), property population,
* wiring (including autowiring), and initialization. Handles runtime bean
* references, resolves managed collections, calls initialization methods, etc.
* Supports autowiring constructors, properties by name, and properties by type.
*
* <p>The main template method to be implemented by subclasses is
* {@link #resolveDependency(DependencyDescriptor, String, Set, TypeConverter)},
* used for autowiring by type. In case of a factory which is capable of searching
* its bean definitions, matching beans will typically be implemented through such
* a search. For other factory styles, simplified matching algorithms can be implemented.
*
* <p>Note that this class does <i>not</i> assume or implement bean definition
* registry capabilities. See {@link DefaultListableBeanFactory} for an implementation
* of the {@link org.springframework.beans.factory.ListableBeanFactory} and
* {@link BeanDefinitionRegistry} interfaces, which represent the API and SPI
* view of such a factory, respectively.
...
*/
public abstract class AbstractAutowireCapableBeanFactory extends AbstractBeanFactory
implements AutowireCapableBeanFactory {
/**
* 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.
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.
//bean的实例化后前置处理器，如果有，这里会返回代理的实例。AOP的实现基础就是基于这里来实现的
Object bean = resolveBeforeInstantiation(beanName, mbdToUse);
//如果不为空 直接返回 修饰后的实例，并中断主线
if (bean != null) {
return bean;
}
}
catch (Throwable ex) {
throw new BeanCreationException(mbdToUse.getResourceDescription(), beanName,
"BeanPostProcessor before instantiation of bean failed", ex);
}
try {
//正常创建实例
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);
}
}
...
/**
* Populate the bean instance in the given BeanWrapper with the property values
* from the bean definition.
* @param beanName the name of the bean
* @param mbd the bean definition for the bean
* @param bw the BeanWrapper with bean instance
*/
@SuppressWarnings("deprecation") // for postProcessPropertyValues
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;
//BeanDefinition为应用程序bean，而非基础框架bean信息
//注册过InstantiationAwareBeanPostProcessor类型接口
if (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {
for (BeanPostProcessor bp : getBeanPostProcessors()) {
if (bp instanceof InstantiationAwareBeanPostProcessor) {
InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp;
if (!ibp.postProcessAfterInstantiation(bw.getWrappedInstance(), beanName)) {
continueWithPropertyPopulation = false;
break;
}
}
}
}
if (!continueWithPropertyPopulation) {
return;
}
PropertyValues pvs = (mbd.hasPropertyValues() ? mbd.getPropertyValues() : null);
//ioc依赖注入
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;
}
//容器是否注册了InstantiationAwareBeanPostProcessor
boolean hasInstAwareBpps = hasInstantiationAwareBeanPostProcessors();
// 是否进行依赖检查
boolean needsDepCheck = (mbd.getDependencyCheck() != AbstractBeanDefinition.DEPENDENCY_CHECK_NONE);
PropertyDescriptor[] filteredPds = null;
if (hasInstAwareBpps) {
if (pvs == null) {
pvs = mbd.getPropertyValues();
}
for (BeanPostProcessor bp : getBeanPostProcessors()) {
if (bp instanceof InstantiationAwareBeanPostProcessor) {
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);
}
// 检查是否满足相关依赖关系，对应的depends-on属性，需要确保所有依赖的Bean先完成初始化
checkDependencies(beanName, mbd, filteredPds, pvs);
}
if (pvs != null) {
//将pvs上所有的属性填充到BeanWrapper对应的Bean实例中
applyPropertyValues(beanName, mbd, bw, pvs);
}
}
}
总结
- BPP是Spring container的一个扩展，在容器初始化bean的过程过，对每个bean都会执行一次，是容器本身的回调行为。aplication和beanfacory，处理BPP的时候，有些区别。感兴趣的同学可以看下相关文章。
-
BeanPostProcessor调用机制为 spring容器实例化bean之后，在执行bean的初始化方法前后。 -
MergedBeanDefinitionPostProcessor、InstantiationAwareBeanPostProcessor都继承自BeanPostProcessor但是，调用机制不太一样。InstantiationAwareBeanPostProcessor为bean实例化前后，MergedBeanDefinitionPostProcessor为bean实例化后，初始化前执行。 -
BeanFactoryPostProcessor是在spring容器加载了bean的定义文件之后，在bean实例化之前执行的。
这篇文章写起来，很枯燥，很吃力，参考了一些大神的博客，spring官网的文档，加上自己对源码的理解，断断续续大概用了一天的时间，如果有问题，欢迎大家在下方指摘。谢谢！
本来打算这篇文章把上一篇文章提到的问题一次性讲了，但是 背后的东西容太多了，下一篇我们实战下，搞点轻松加愉快的！
参考内容：
