BeanPostProcessor

在Bean的屬性被Spring容器設定之後，您還有機會自訂一些對Bean的修正，您可以實作org.springframework.beans.factory.config.BeanPostProcessor介面：

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()方法會在Bean初始化動作之前（例如InitializingBean的 afterPropertiesSet()方法或自定義的init方法）被呼叫，而postProcessAfterInitialization()方法會在Bean初始化之後立即被呼叫。

舉個例子來說，您可以實作一個大寫修正器，對於String型態的Bean屬性，無論在定義檔中是設定為大寫或小寫，在Bean屬性設定之後，您可以在大寫修正器中將所有的String改為大寫，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;
    }

}

假設您定義了這麼一個Bean類別：

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會自動偵測您是否在定義檔中定義了實作BeanPostProcessor介面的類別，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容器會在每一個Bean被初始化之前之後分別呼叫upperCaseModifier的 postProcessBeforeInitialization()方法與postProcessAfterInitialization()方法，以對Bean進行指定的相關修正，可以實際來看看以下的測試程式：

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());
    } 
}

執行結果如下：

HELLO!

雖然您在定義檔中的helloBean之helloWord屬性是設定小寫字母，但upperCaseModifier將之改為大寫字母了。

From Gossip@caterpillar

Spring Gossip: BeanPostProcessor