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Spring Gossip: BeanPostProcessor

在Bean的属性被Spring容器设定之后，您还有机会自订一些对Bean的修正，您可以实作org.springframework.beans.factory.config.BeanPostProcessor界面：

package org.springframework.beans.factory.config;
public interface BeanPostProcessor {
	public Object postProcessBeforeInitialization(Object bean, String name) throws BeansException;
	public Object postProcessAfterInitialization(Object bean, String name) throws BeansException;
}

postProcessBeforeInitialization()方法会在Bean初始化动作之前（例如InitializingBean的 afterPropertiesSet()方法或自定义的init方法）被呼叫，而postProcessAfterInitialization()方法会在Bean初始化之后立即被呼叫。

举个例子来说，您可以实作一个大写修正器，对于String型态的Bean属性，无论在定义档中是设定为大写或小写，在Bean属性设定之后，您可以在大写修正器中将所有的String改为大写，例如：

UpperCaseModifier.java

package onlyfun.caterpillar;

import java.lang.reflect.Field;

import org.springframework.beans.BeansException;
import org.springframework.beans.factory.config.BeanPostProcessor;

public class UpperCaseModifier implements BeanPostProcessor {

    public Object postProcessBeforeInitialization(Object bean, String name) throws BeansException {
        Field[] fields = bean.getClass().getDeclaredFields();
        
        for(int i = 0; i < fields.length; i++) {
            if(fields[i].getType().equals(String.class)) {
                fields[i].setAccessible(true);
                try {
                    String original = (String) fields[i].get(bean);
                    fields[i].set(bean, original.toUpperCase());
                } catch (IllegalArgumentException e) {
                    e.printStackTrace();
                } catch (IllegalAccessException e) {
                    e.printStackTrace();
                }
            }
        }
        
        return bean;
    }

    public Object postProcessAfterInitialization(Object bean, String name) throws BeansException {
        return bean;
    }

}

假设您定义了这么一个Bean类别：

HelloBean.java

package onlyfun.caterpillar; 

public class HelloBean { 
    private String helloWord; 
    
    public HelloBean() {
    }
    
    public void setHelloWord(String helloWord) { 
        this.helloWord = helloWord; 
    } 
    public String getHelloWord() { 
        return helloWord; 
    } 
}

ApplicationContext会自动侦测您是否在定义档中定义了实作BeanPostProcessor接口的类别，例如：

beans-config.xml

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE beans PUBLIC "-//SPRING/DTD BEAN/EN"
"http://www.springframework.org/dtd/spring-beans.dtd">
<beans>
	<bean id="upperCaseModifier"
	class="onlyfun.caterpillar.UpperCaseModifier"/>
	<bean id="helloBean" class="onlyfun.caterpillar.HelloBean">
		<property name="helloWord">
			<value>Hello!</value>
		</property>
	</bean>
</beans>

Spring容器会在每一个Bean被初始化之前之后分别呼叫upperCaseModifier的 postProcessBeforeInitialization()方法与postProcessAfterInitialization()方法，以对Bean进行指定的相关修正，可以实际来看看以下的测试程序：

SpringDemo.java

package onlyfun.caterpillar; 

import org.springframework.context.ApplicationContext;
import org.springframework.context.support.FileSystemXmlApplicationContext; 

public class SpringDemo { 
    public static void main(String[] args) { 
        ApplicationContext context = 
            new FileSystemXmlApplicationContext("beans-config.xml");
         
        HelloBean hello = 
            (HelloBean) context.getBean("helloBean");
        System.out.println(hello.getHelloWord());
    } 
}

执行结果如下：

HELLO!

虽然您在定义档中的helloBean之helloWord属性是设定小写字母，但upperCaseModifier将之改为大写字母了。