Spring中Bean的实例化过程如下：

 

其中BeanPostProcessor的前置和后置处理的方法中都要返回该bean，不能是null，不然在getBean的时候获取不到。

public interface PasswordDecodable{	public String getEncodedPassword();		public void setDecodedPassword(String password);}

public class LoginModel implements PasswordDecodable{	private String password;		public LoginModel()	{			}		public LoginModel(String password)	{		this.password = password;	}		@Override	public String getEncodedPassword()	{		// TODO Auto-generated method stub		return this.password;	}	@Override	public void setDecodedPassword(String password)	{		// TODO Auto-generated method stub		this.password = password;	}}

import org.springframework.beans.BeansException;import org.springframework.beans.factory.config.BeanPostProcessor;import com.uqee.spring.beans.PasswordDecodable;import com.uqee.utils.Md5Token;public class PasswordDecodePostProcessor implements BeanPostProcessor{	@Override	public Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException	{		// TODO Auto-generated method stub		return bean;	}	@Override	public Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException	{		// TODO Auto-generated method stub		if(bean instanceof PasswordDecodable)		{			String encodedPassword = ((PasswordDecodable)bean).getEncodedPassword();			String decodePassword = decodePassword(encodedPassword);			((PasswordDecodable)bean).setDecodedPassword(decodePassword);		}		return bean;	}		private String decodePassword(String encodePassword)	{		return Md5Token.getInstance().getLongToken(encodePassword);	}}

    	    
     	        
      
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PasswordDecodePostProcessor的postProcessAfterInitialization和postProcessBeforeInitialization均不能返回null，不然下列代码：

ConfigurableListableBeanFactory factory = new XmlBeanFactory(new ClassPathResource("applicationContext3.xml"));		factory.addBeanPostProcessor(new PasswordDecodePostProcessor());		PasswordDecodable passwordDecodable = (PasswordDecodable)factory.getBean("loginModel");

中passwordDecodable会是null引用。