前言
BeanPostProcessor也稱為Bean后置處理器，它是Spring中定義的接口，在Spring容器的創建過程中（具體為Bean初始化前后）會回調BeanPostProcessor中定義的兩個方法。BeanPostProcessor的源碼如下
public interface BeanPostProcessor {
Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException;
Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException;
}
其中postProcessBeforeInitialization方法會在每一個bean對象的初始化方法調用之前回調；postProcessAfterInitialization方法會在每個bean對象的初始化方法調用之后被回調。具體執行時期可以參考Spring中Bean的生命周期源碼探究。
自定義BeanPostProcessor
查看BeanPostProcessor源碼，可以看到它兩個方法的參數都相同，其中第一個參數Object bean表示當前正在初始化的bean對象。此外兩個方法都返回Object類型的實例，返回值既可以是將入參Object bean原封不動的返回出去，也可以對當前bean進行包裝再返回。來看看下面的自定義BeanPostProcessor
/**
* 后置處理器：初始化前后進行處理工作
* 需要將后置處理器加入到容器中
*/
@Component
public class MyBeanPostProcessor implements BeanPostProcessor {
@Override
public Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException {
System.out.println("postProcessBeforeInitialization..."+beanName+"..."+bean);
return bean;
}
@Override
public Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException {
System.out.println("postProcessAfterInitialization..."+beanName+"..."+bean);
return bean;
}
}
Spring容器中加入MyBeanPostProcessor之后，針對容器中每個創建的Bean對象（Spring自身創建的Bean和應用程序創建的Bean），都會回調postProcessBeforeInitialization和postProcessAfterInitialization方法。
執行原理
BeanPostProcessor的執行是定義在容器的刷新過程中，容器刷新對象具體的方法為：AbstractApplicationContext.refresh()。
在refresh方法執行的調用棧中會去調用AbstractAutowireCapableBeanFactory.doCreateBean()方法，該方法節選源碼如下
protected Object initializeBean(final String beanName, final Object bean, RootBeanDefinition mbd) {
if (mbd == null || !mbd.isSynthetic()) {
wrappedBean = applyBeanPostProcessorsBeforeInitialization(wrappedBean, beanName);
}
invokeInitMethods(beanName, wrappedBean, mbd);
if (mbd == null || !mbd.isSynthetic()) {
wrappedBean = applyBeanPostProcessorsAfterInitialization(wrappedBean, beanName);
}
return wrappedBean;
}
看到在調用初始化方法前后會分別調用applyBeanPostProcessorsBeforeInitialization()和applyBeanPostProcessorsAfterInitialization()。applyBeanPostProcessorsBeforeInitialization()方法的源碼如下
public Object applyBeanPostProcessorsBeforeInitialization(Object existingBean, String beanName)
throws BeansException {
Object result = existingBean;
//獲取所有的BeanPostProcessor進行遍歷
for (BeanPostProcessor beanProcessor : getBeanPostProcessors()) {
result = beanProcessor.postProcessBeforeInitialization(result, beanName);
if (result == null) {
return result;
}
}
return result;
}
可以看到其邏輯為遍歷得到容器中所有的BeanPostProcessor，然后一次執行postProcessBeforeInitialization，一但返回null，就跳出for循環不執行后面的BeanPostProcessor.postProcessorsBeforeInitialization()。也就是說如果返回的是null那么我們通過getBean方法將得不到目標Bean。
applyBeanPostProcessorsBeforeInitialization()方法的邏輯和上面一致，就是將循環執行的beanProcessor.postProcessBeforeInitialization()替換成beanProcessor.postProcessAfterInitialization()
Spring底層的很多功能特性都是借助BeanPostProcessor的子類來實現。
常見BeanPostProcessor分析
下圖是debug過程中，ApplicationContext對象中的包含的BeanPostProcessor。具體包含哪些BeanPostProcessor和具體應用程序相關，除了下標3中的MyBeanPostProcessor為自定義的BeanPostProcessor，其余均為Spring自帶的BeanPostProcessor。
下面來分析一下
ApplicationContextAwareProcessor和AutowiredAnnotationProcessor的執行原理。
ApplicationContextAwareProcessor
ApplicationContextAwareProcessor后置處理器的作用是，當應用程序定義的Bean實現ApplicationContextAware接口時注入ApplicationContext對象。
@Component
public class Car implements ApplicationContextAware {
private ApplicationContext applicationContext;
public Car(){
System.out.println("car instance...");
}
@Override
public void setApplicationContext(ApplicationContext applicationContext) throws BeansException {
System.out.println("ApplicationContextAware...setApplicationContext()");
this.applicationContext = applicationContext;
}
}
那Car是如何通過實現ApplicationContextAware接口就能獲得ApplicationContext對象呢？答案是通過ApplicationContextAwareProcessor后置處理器來實現，我們來看看ApplicationContextAwareProcessor的源碼
class ApplicationContextAwareProcessor implements BeanPostProcessor {
private final ConfigurableApplicationContext applicationContext;
private final StringValueResolver embeddedValueResolver;
/**
* Create a new ApplicationContextAwareProcessor for the given context.
*/
public ApplicationContextAwareProcessor(ConfigurableApplicationContext applicationContext) {
this.applicationContext = applicationContext;
this.embeddedValueResolver = new EmbeddedValueResolver(applicationContext.getBeanFactory());
}
@Override
public Object postProcessBeforeInitialization(final Object bean, String beanName) throws BeansException {
AccessControlContext acc = null;
// 這里bean是Car,它實現了ApplicationContextAware接口
if (System.getSecurityManager() != null &&
(bean instanceof EnvironmentAware || bean instanceof EmbeddedValueResolverAware ||
bean instanceof ResourceLoaderAware || bean instanceof ApplicationEventPublisherAware ||
bean instanceof MessageSourceAware || bean instanceof ApplicationContextAware)) {
invokeAwareInterfaces(bean);
}
return bean;
}
private void invokeAwareInterfaces(Object bean) {
if (bean instanceof Aware) {
if (bean instanceof EnvironmentAware) {
((EnvironmentAware) bean).setEnvironment(this.applicationContext.getEnvironment());
}
if (bean instanceof EmbeddedValueResolverAware) {
((EmbeddedValueResolverAware) bean).setEmbeddedValueResolver(this.embeddedValueResolver);
}
if (bean instanceof ResourceLoaderAware) {
((ResourceLoaderAware) bean).setResourceLoader(this.applicationContext);
}
if (bean instanceof ApplicationEventPublisherAware) {
((ApplicationEventPublisherAware) bean).setApplicationEventPublisher(this.applicationContext);
}
if (bean instanceof MessageSourceAware) {
((MessageSourceAware) bean).setMessageSource(this.applicationContext);
}
if (bean instanceof ApplicationContextAware) {
// 會執行這里回調car重寫的setApplicationContext方法，然后將this.applicationContext注入給Car
((ApplicationContextAware) bean).setApplicationContext(this.applicationContext);
}
}
}
@Override
public Object postProcessAfterInitialization(Object bean, String beanName) {
return bean;
}
}
InitDestroyAnnotationBeanPostProcessor
InitDestroyAnnotationBeanPostProcessor后置處理器是用來處理自定義的初始化方法和銷毀方法。Spring中提供了3種自定義初始化和銷毀方法：
- 通過@Bean指定init-method和destroy-method屬性
- Bean實現InitializingBean（定義初始化邏輯），DisposableBean（定義銷毀邏輯）;
- @PostConstruct：在bean創建完成并且屬性賦值完成；來執行初始化方法@PreDestroy：在容器銷毀bean之前通知我們進行清理工作
InitDestroyAnnotationBeanPostProcessor的作用就是讓第3種方式生效。先看看如何使用@PostConstruct和@PreDestroy注解。
@Component
public class Car {
public Car(){
System.out.println("car instance...");
}
/**
* 自定義的初始化方法
*/
@PostConstruct
public void init(){
System.out.println("car ... init...");
}
/**
* 自定義的銷毀方法
*/
@PreDestroy
public void detory(){
System.out.println("car ... detory...");
}
}
InitDestroyAnnotationBeanPostProcessor會在Bean創建的時候通過反射的方式查找包含@PostConstruct和@PreDestroy注解的方法，然后再通過反射執行方法。我們來看看InitDestroyAnnotationBeanPostProcessor.postProcessBeforeInitialization()的源碼
public Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException {
// 獲取bean的metadata
LifecycleMetadata metadata = findLifecycleMetadata(bean.getClass());
try {
// 執行@PostConstruct指定的init方法
metadata.invokeInitMethods(bean, beanName);
}
catch (InvocationTargetException ex) {
throw new BeanCreationException(beanName, "Invocation of init method failed", ex.getTargetException());
}
catch (Throwable ex) {
throw new BeanCreationException(beanName, "Failed to invoke init method", ex);
}
return bean;
}
metadata中已經解析出initMethods和destroyMethods，其具體內容如下。
metadata.invokeInitMethods(bean, beanName);就是根據反射執行init方法。
InstantiationAwareBeanPostProcessor
InstantiationAwareBeanPostProcessor是BeanPostProcessor的子接口。
public interface InstantiationAwareBeanPostProcessor extends BeanPostProcessor {
//實例化Bean之前調用
Object postProcessBeforeInstantiation(Class<?> beanClass, String beanName) throws BeansException;
//實例化Bean之后調用
boolean postProcessAfterInstantiation(Object bean, String beanName) throws BeansException;
//設置Bean對象中的某個屬性時調用
PropertyValues postProcessPropertyValues(
PropertyValues pvs, PropertyDescriptor[] pds, Object bean, String beanName) throws BeansException;
}
這兩個接口的功能基本類似，不過要注意InstantiationAwareBeanPostProcessor和BeanPostProcessor的方法名區別。InstantiationAwareBeanPostProcessor中是Instantiation， 而BeanPostProcessor是Initialization。
參考閱讀
https://blog.csdn.net/chjttony/article/details/6301591
Spring開閉原則的表現-BeanPostProcessor擴展點-2
