BeanPostProcessor的作用是在调用初始化方法的前后添加一些逻辑，这里初始化方法是指在配置文件中配置init-method，或者实现了InitializingBean接口的afterPropertiesSet方法,注意不包括@PostConstruct这种初始化方式。可以看源码AbstractAutowireCapableBeanFactory类中的invokeInitMethods方法。
1.用法
新建Student类
package com.demo.spring.entity; import javax.annotation.PostConstruct; import org.springframework.beans.factory.InitializingBean;
import org.springframework.stereotype.Component; /**
* @author chenyk
* @date 2018年5月8日
*/
@Component("student")
public class Student{
private String name;
private int age; private School school; public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public int getAge() {
return age;
}
public void setAge(int age) {
this.age = age;
}
public School getSchool() {
return school;
}
public void setSchool(School school) {
this.school = school;
} public void init(){
System.out.println("执行 init方法");
} }
创建实现了BeanPostProcessor接口的后置处理器StudentBeanPostProcessor
package com.demo.spring.entity; import org.springframework.beans.BeansException;
import org.springframework.beans.factory.config.BeanPostProcessor; /**
* Bean后置处理器
* @author chenyk
* @date 2018年6月29日
*/
//以下这两个方法不能返回null，否则在调用初始化方法时会报空指针异常；因为后置处理器从springIOC容器中取出bean实例后需要再把bean实例放回去
public class StudentBeanPostProcessor implements BeanPostProcessor{ /**
* 在bean实例调用初始化方法之前调用
*/
public Object postProcessBeforeInitialization(Object bean, String beanName)
throws BeansException {
if(beanName.equals("student")){
System.out.println("开始调用init方法");
}
return bean;
} /**
* 在bean实例调用初始化方法之后调用
*/
public Object postProcessAfterInitialization(Object bean, String beanName)
throws BeansException {
if(beanName.equals("student")){
System.out.println("init方法调用结束");
}
return bean;
}
}
在配置文件aop.xml中添加以下信息
<bean id="student" class="com.demo.spring.entity.Student" init-method="init" >
<property name="name" value="张三"></property>
</bean>
<!-- spring会自动检测实现了BeanPostProcessor接口的类，并将其注册为后置处理器 -->
<bean id="studentBeanPostProcessor" class="com.demo.spring.entity.StudentBeanPostProcessor" />
测试方法
@Test
public void testAop5(){
ClassPathXmlApplicationContext applicationContext = new ClassPathXmlApplicationContext("aop.xml");
}
看下打印信息：
开始调用init方法
2018-07-02 13:58:47,519 DEBUG [AbstractAutowireCapableBeanFactory.java:1731] : Invoking init method 'init' on bean with name 'student' //这一行信息，就是即将调用init-method方法的日志打印
执行 init方法
init方法调用结束
需要注意的是，如果一个类实现了BeanPostProcessor接口，那么这个类就作用于所有的bean实例，我们可以在BeanPostProcessor接口的两个方法中添加自己的控制和逻辑；
2.源码分析
2.1 找到实现BeanPostProcessor接口的类，并将其注册为后置处理器，看源码
@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);
//发现注册bean后置处理器，在这个方法中进行
// Register bean processors that intercept bean creation.
registerBeanPostProcessors(beanFactory); // Initialize message source for this context.
initMessageSource(); // Initialize event multicaster for this context.
initApplicationEventMulticaster(); // Initialize other special beans in specific context subclasses.
onRefresh(); // Check for listener beans and register them.
registerListeners(); // Instantiate all remaining (non-lazy-init) singletons.
finishBeanFactoryInitialization(beanFactory); // Last step: publish corresponding event.
finishRefresh();
} catch (BeansException ex) {
if (logger.isWarnEnabled()) {
logger.warn("Exception encountered during context initialization - " +
"cancelling refresh attempt: " + ex);
} // Destroy already created singletons to avoid dangling resources.
destroyBeans(); // Reset 'active' flag.
cancelRefresh(ex); // Propagate exception to caller.
throw ex;
} finally {
// Reset common introspection caches in Spring's core, since we
// might not ever need metadata for singleton beans anymore...
resetCommonCaches();
}
}
}
2.2 后置处理器生效的地方，进入AbstractAutowireCapableBeanFactory类中的initializeBean方法
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;
//后置处理器的postProcessBeforeInitialization方法
if (mbd == null || !mbd.isSynthetic()) {
wrappedBean = applyBeanPostProcessorsBeforeInitialization(wrappedBean, beanName);
}
//调用初始化方法有:init-method,实现了InitializingBean接口的方法
try {
invokeInitMethods(beanName, wrappedBean, mbd);
}
catch (Throwable ex) {
throw new BeanCreationException(
(mbd != null ? mbd.getResourceDescription() : null),
beanName, "Invocation of init method failed", ex);
}
//后置处理器的postProcessAfterInitialization方法
if (mbd == null || !mbd.isSynthetic()) {
wrappedBean = applyBeanPostProcessorsAfterInitialization(wrappedBean, beanName);
}
return wrappedBean;
}3.最后，我们看下@PostConstruct执行的时间点。
进入AbstractAutowireCapableBeanFactory类的initializeBean方法
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()) {
//这个是后置处理器的方法，其实@PostConstruct是在CommonAnnotationBeanPostProcessor后置处理器的父类InitDestroyAnnotationBeanPostProcessor中实现的，
//也就是说@PostConstruct是由后置处理器来处理实现的，进入该方法
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;
}进入applyBeanPostProcessorsBeforeInitialization方法
public Object applyBeanPostProcessorsBeforeInitialization(Object existingBean, String beanName)
throws BeansException { Object result = existingBean;
for (BeanPostProcessor beanProcessor : getBeanPostProcessors()) {
//在循环中会发现一个类 CommonAnnotationBeanPostProcessor，然后进入其父类InitDestroyAnnotationBeanPostProcessor的postProcessBeforeInitialization该方法 进入该方法
result = beanProcessor.postProcessBeforeInitialization(result, beanName);
if (result == null) {
return result;
}
}
return result;
}
进入postProcessBeforeinitialization方法
public Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException {
//该类封装着一些通过注解的方式标识的初始化方法和销毁方法，以及相关属性
LifecycleMetadata metadata = findLifecycleMetadata(bean.getClass());
try {
//@PostConstrutct注解就是在此处生效调用
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;
}