Spring BeanPostProcessor接口使用详解

Spring中提供了很多PostProcessor供开发者进行拓展，例如：BeanPostProcessor、BeanFactoryPostProcessor、BeanValidationPostProcessor等一系列后处理器。他们的使用方式大多类似，了解其中一个并掌握他的使用方式，其他的可以触类旁通。

这里以BeanPostProcessor为例展示其使用方式。

BeanPostProcessor接口提供了两个供开发者自定义的方法：postProcessBeforeInitialization、postProcessAfterInitialization。

postProcessBeforeInitialization：该方法主要针对spring在bean初始化时调用初始化方法前进行自定义处理。

postProcessAfterInitialization：该方法主要针对spring在bean初始化时调用初始化方法后进行自定义处理。

示例代码：

/**
* 测试bean
*/
public class Cat {
 private String name;
 private int age;
 public void say() {
   System.out.println("name:" + name);
   System.out.println("age:" + age);
 }

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;
 }
}

/**
* 自定义后处理器
*/
public class CatBeanPostProcessor implements BeanPostProcessor {

@Nullable
 @Override
 public Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException {
   if (bean instanceof Cat) {
     //输出原始属性
     Cat cat = (Cat) bean;
     cat.say();
     return bean;
   }
   return bean;
 }

@Nullable
 @Override
 public Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException {
   if (bean instanceof Cat) {
     //修改属性值，并返回
     Cat cat = (Cat) bean;
     cat.setName("hello maomi");
     cat.setAge(3);
     return cat;
   }
   return bean;
 }
}

/**
* 运行
*/
public class Run {
 public static void main(String[] args) {
   ApplicationContext applicationContext = new ClassPathXmlApplicationContext("spring-bean.xml");
   Cat cat = (Cat) applicationContext.getBean("cat");
   cat.say();
 }
}

xml配置信息

 <!--配置bean并初始化-->
 <bean id="cat" class="com.source.postprocessor.Cat" >
   <property name="name" value="HelloKitty" />
   <property name="age" value="1" />
 </bean>

<bean id="catBeanPostProcessor" class="com.source.postprocessor.CatBeanPostProcessor" />

输出结果：

name:HelloKitty
age:1
name:hello maomi
age:3

可以看到通过后处理器处理过后的bean信息已经改变。最后，看看源码中如何调用自定义实现的。

在初始化bean方法中：AbstractAutowireCapableBeanFactory.java

 /**
 * 初始化bean
 */
protected Object initializeBean(final String beanName, final Object bean, @Nullable RootBeanDefinition mbd) {
  //省略部分无关代码
  Object wrappedBean = bean;
  //初始化前
  if (mbd == null || !mbd.isSynthetic()) {
    wrappedBean = applyBeanPostProcessorsBeforeInitialization(wrappedBean, beanName);
  }

try {
    //调用初始化方法初始化bean
    invokeInitMethods(beanName, wrappedBean, mbd);
  }
  catch (Throwable ex) {
    throw new BeanCreationException(
        (mbd != null ? mbd.getResourceDescription() : null),
        beanName, "Invocation of init method failed", ex);
  }
  //初始化后
  if (mbd == null || !mbd.isSynthetic()) {
    wrappedBean = applyBeanPostProcessorsAfterInitialization(wrappedBean, beanName);
  }
  return wrappedBean;
}
//postProcessBeforeInitialization方法调用
@Override
public Object applyBeanPostProcessorsBeforeInitialization(Object existingBean, String beanName)
    throws BeansException {

Object result = existingBean;
  for (BeanPostProcessor beanProcessor : getBeanPostProcessors()) {
    //调用自定义postProcessBeforeInitialization方法
    Object current = beanProcessor.postProcessBeforeInitialization(result, beanName);
    if (current == null) {
      return result;
    }
    result = current;
  }
  return result;
}
//postProcessAfterInitialization方法调用
@Override
public Object applyBeanPostProcessorsAfterInitialization(Object existingBean, String beanName)
    throws BeansException {

Object result = existingBean;
  for (BeanPostProcessor beanProcessor : getBeanPostProcessors()) {
    //自定义postProcessAfterInitialization方法调用
    Object current = beanProcessor.postProcessAfterInitialization(result, beanName);
    if (current == null) {
      return result;
    }
    result = current;
  }
  return result;
}

来源：https://www.jianshu.com/p/e1c3c6e90e8a