BeanPostProcessor接口作用：
如果我们想在Spring容器中完成bean实例化、配置以及其他初始化方法前后要添加一些自己逻辑处理。我们需要定义一个或多个BeanPostProcessor接口实现类，然后注册到Spring IoC容器中。
/**
* bean后置处理器
* @author zss
*
*/
public class PostProcessor implements BeanPostProcessor {
@Override
public Object postProcessBeforeInitialization(Object bean,
String beanName) throws BeansException {
if ("narCodeService".equals(beanName)) {//过滤掉bean实例ID为narCodeService
return bean;
}
System.out.println("后置处理器处理bean=【"+beanName+"】开始");
try {
Thread.sleep(1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
return bean;
}
@Override
public Object postProcessAfterInitialization(Object bean,
String beanName) throws BeansException {
if ("narCodeService".equals(beanName)) {
return bean;
}
System.out.println("后置处理器处理bean=【"+beanName+"】完毕!");
try {
Thread.sleep(1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
return bean;
}
}
注意:接口中两个方法不能返回null，如果返回null那么在后续初始化方法将报空指针异常或者通过getBean()方法获取不到bena实例对象
因为后置处理器从Spring IoC容器中取出bean实例对象没有再次放回IoC容器中
将Spring的后置处理器PostProcessor配置到Spring配置文件中
<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://www.springframework.org/schema/beans
http://www.springframework.org/schema/beans/spring-beans.xsd">
<!-- 定义一个bean -->
<bean id="narCodeService" class="com.test.service.impl.NarCodeServiceImpl">
</bean>
<bean id="beanLifecycle" class="com.test.spring.BeanLifecycle" init-method="init" destroy-method="close">
<property name="name" value="张三"></property>
<property name="sex" value="男"></property>
</bean>
<!-- Spring后置处理器 -->
<bean id="postProcessor" class="com.test.spring.PostProcessor"/>
</beans>
源码分析：
@Override
public void refresh() throws BeansException, IllegalStateException {
synchronized (this.startupShutdownMonitor) {
// Prepare this context for refreshing.
prepareRefresh();
// Tell the subclass to refresh the internal bean factory.
ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();
// Prepare the bean factory for use in this context.
prepareBeanFactory(beanFactory);
try {
// Allows post-processing of the bean factory in context subclasses.
postProcessBeanFactory(beanFactory);
// Invoke factory processors registered as beans in the context.
invokeBeanFactoryPostProcessors(beanFactory);
// 在这里把所有实现了BeanPostProcessor接口注册到容器的beanPostProcessors里面
registerBeanPostProcessors(beanFactory);
protected Object initializeBean(final String beanName, final Object bean, RootBeanDefinition mbd) { if (System.getSecurityManager() != null) {
AccessController.doPrivileged(new PrivilegedAction<Object>() {
@Override public Object run() {
invokeAwareMethods(beanName, bean); return null;
}
}, getAccessControlContext());
} else {
invokeAwareMethods(beanName, bean);
}
Object wrappedBean = bean; if (mbd == null || !mbd.isSynthetic()) {
wrappedBean = applyBeanPostProcessorsBeforeInitialization(wrappedBean, beanName);
} try {
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()) {
wrappedBean = applyBeanPostProcessorsAfterInitialization(wrappedBean, beanName);
} return wrappedBean;
}
applyBeanPostProcessorsBeforeInitialization和applyBeanPostProcessorsAfterInitialization的具体代码分别为:
public Object applyBeanPostProcessorsBeforeInitialization(Object existingBean, String beanName) throws BeansException {
Object result = existingBean; for (BeanPostProcessor beanProcessor : getBeanPostProcessors()) {
result = beanProcessor.postProcessBeforeInitialization(result, beanName); if (result == null) { return result;
}
} return result;
}
public Object applyBeanPostProcessorsAfterInitialization(Object existingBean, String beanName) throws BeansException {
Object result = existingBean; for (BeanPostProcessor beanProcessor : getBeanPostProcessors()) {
result = beanProcessor.postProcessAfterInitialization(result, beanName); if (result == null) { return result;
}
} return result;
}
根据以上代码，我们得知，在invokeInitMethods的执行前后，spring会分别调用所有的BeanPostProcessor，执行其中的方法，那么invokeInitMethods的具体内容我们仍需要看下，发现此方法主要作用有两个：1、判断bean是否继承了InitializingBean，如果继承接口，执行afterPropertiesSet()方法，2、获得是否设置了init-method属性，如果设置了，就执行设置的方法
protected void invokeInitMethods(String beanName, final Object bean, RootBeanDefinition mbd) throws Throwable { boolean isInitializingBean = (bean instanceof InitializingBean); if (isInitializingBean && (mbd == null || !mbd.isExternallyManagedInitMethod("afterPropertiesSet"))) { if (logger.isDebugEnabled()) {
logger.debug("Invoking afterPropertiesSet() on bean with name '" + beanName + "'");
} if (System.getSecurityManager() != null) { try {
AccessController.doPrivileged(new PrivilegedExceptionAction<Object>() {
@Override public Object run() throws Exception {
((InitializingBean) bean).afterPropertiesSet(); return null;
}
}, getAccessControlContext());
} catch (PrivilegedActionException pae) { throw pae.getException();
}
} else {
((InitializingBean) bean).afterPropertiesSet();
}
} if (mbd != null) {
String initMethodName = mbd.getInitMethodName(); if (initMethodName != null && !(isInitializingBean && "afterPropertiesSet".equals(initMethodName)) &&
!mbd.isExternallyManagedInitMethod(initMethodName)) {
invokeCustomInitMethod(beanName, bean, mbd);
}
}
}
根据以上描述，我们可以看到原有推断有一些问题，两个方法的执行主要是在bean完成初始化之后，准备执行默认方法时候对bean进行包装。
三、应用场景
几个典型的应用如：
1、解析bean的注解，将注解中的字段转化为属性
2、统一将属性在执行前，注入bean中，如数据库访问的sqlMap，如严重服务，这样不需要每个bean都配置属性
3、打印日志，记录时间等。
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