spring-扩展点-BeanFactoryPostProcessor

BeanFactoryPostProcessor：允许自定义对ApplicationContext的 bean definitions 进行修饰，扩展功能。

1、实现BeanFactoryPostProcessor 接口，会被Application contexts自动发现
2、BeanFactoryPostProcessor 仅仅对 bean definitions 发生关系，不能对bean instances 交互，对bean instances 的交互，由BeanPostProcessor的实现来处理
3、PropertyResourceConfigurer ：典型实现 属性替换的功能

4、CustomEditorConfigurer ：典型实现 添加自定义属性编辑器的功能
5、ConfigurationClassPostProcessor：启动@Configuration的扫描功能
举例：
有这样的也个业务场景：

 <bean id="user" class="com.gym.UserServiceImpl" >
       <property name="username" value="${username_}"/>
       <property name="password" value="${password_}"/>
</bean>

spring支持系统对username_进行占位符的配置为properties文件配置，试想如果我们有个配置中心，我们希望spring启动的时候，从远程配置中心取数据，而非本地文件，这里就需要我们自定义一个实现BeanFactoryPostProcessor的PropertyResourceConfigurer 实例。
看下面的例子：

xml配置：

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
	xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:context="http://www.springframework.org/schema/context"
	xsi:schemaLocation="http://www.springframework.org/schema/beans 
	http://www.springframework.org/schema/beans/spring-beans-3.0.xsd
	http://www.springframework.org/schema/context 
	http://www.springframework.org/schema/context/spring-context-3.0.xsd"
	default-autowire="byName">

     <bean id="user" class="com.gym.UserServiceImpl" >
       <property name="username" value="${username_}"/>
       <property name="password" value="${password_}"/>
     </bean>
     
     <bean id="myFactoryPostProcessor" class="com.gym.MyFilePlaceHolderBeanFactoryPostProcessor"/>
</beans>

模拟从远程取文件：

import org.springframework.beans.factory.InitializingBean;
import org.springframework.beans.factory.config.PropertyPlaceholderConfigurer;
import org.springframework.core.io.support.PropertiesLoaderUtils;

/**
 * @author xinchun.wang
 */
public class MyFilePlaceHolderBeanFactoryPostProcessor 
    extends PropertyPlaceholderConfigurer implements InitializingBean{
	
	public void afterPropertiesSet() throws Exception {
		List<Properties> list = new ArrayList<Properties>();
		Properties p = PropertiesLoaderUtils.loadAllProperties("config.properties");
		list.add(p);
		//这里是关键，这就设置了我们远程取得的List<Properties>列表
		setPropertiesArray(list.toArray(new Properties[list.size()]));
	}
	
}

javaBean配置：

public class UserServiceImpl implements IUserService{
	private static final Logger logger = LoggerFactory.getLogger(UserServiceImpl.class);

	public UserServiceImpl(){
		logger.info("UserServiceImpl 构造函数 ");
	}
	
	private String username;
	private String password;

	public String getUsername() {
		return username;
	}

	public String getPassword() {
		return password;
	}

	public void setUsername(String username) {
		logger.info("UserServiceImpl setUsername {}",username);
		this.username = username;
	}

	public void setPassword(String password) {
		logger.info("UserServiceImpl setPassword {}",password);
		this.password = password;
	}

	@Override
	public String toString() {
		return "UserServiceImpl [username=" + username + ", password="
				+ password + "]";
	}
	
}

测试：

public class TestApplicationContext {
	public static void main(String[] args) {
		ClassPathXmlApplicationContext applicationContext = new ClassPathXmlApplicationContext(
				"classpath:spring/applicationContext.xml");
		IUserService userService = applicationContext.getBean(IUserService.class);
		String password = userService.getPassword();
		applicationContext.destroy();
		System.out.println(password);
	}

}

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调用入口：