忍耐和堅持雖是痛苦的事情，但卻能漸漸地為你帶來好處。——奧維德

屬性占位符

　　Spring一直支持將屬性定義到外部的屬性的文件中，並使用占位符值將其插入到Spring bean中。

　　占位符的形式為使用"${}"包裝的屬性名稱，為了使用屬性占位符，我們必須配置一個PropertyPlaceholderConfigurer或PropertySourcesPlaceholderConfigurer實例，從Spring 3.0開始，推薦使用PropertySourcesPlaceholderConfigurer，因為它能夠基於Spring Environment及其屬性源來解析占位符。

　　1.在基於Java配置中使用屬性占位符註入屬性

package chapter3.prctice6;

import org.springframework.beans.factory.annotation.Value;
import org.springframework.context.annotation.Bean;
import org.springframework.context.support.PropertySourcesPlaceholderConfigurer;
import org.springframework.stereotype.Component;

@Component
public class AppleMobile implements Mobile {
    
    
 
private String color;
    
    private String type;
        
    public AppleMobile(@Value("${mobile.color}") String color, @Value("${mobile.type}") String type) {
        this.color = color;
        this.type = type;
    }
    
    @Bean
    public PropertySourcesPlaceholderConfigurer placeholderConfigurer() {
        
 
return new PropertySourcesPlaceholderConfigurer();
    }
    
    public void play() {
        System.out.println(color+"-"+type);
    }

}

　　2.在基於XML配置中使用占位符註入屬性

<?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:aop="http://www.springframework.org/schema/aop"
    xmlns:c="http://www.springframework.org/schema/c"
    xmlns:util="http://www.springframework.org/schema/util"
    xmlns:p="http://www.springframework.org/schema/p"
    xmlns:jdbc="http://www.springframework.org/schema/jdbc"
    xmlns:jee="http://www.springframework.org/schema/jee"
    xmlns:context="http://www.springframework.org/schema/context"
    xsi:schemaLocation="http://www.springframework.org/schema/jdbc 
        http://www.springframework.org/schema/jdbc/spring-jdbc-4.3.xsd
        http://www.springframework.org/schema/jee http://www.springframework.org/schema/jee/spring-jee-4.3.xsd
        http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd
        http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context-4.3.xsd
        http://www.springframework.org/schema/aop http://www.springframework.org/schema/aop/spring-aop.xsd
        http://www.springframework.org/schema/util http://www.springframework.org/schema/util/spring-util-4.3.xsd">
    
        <bean id="appleMobile" class="chapter3.prctice6.AppleMobile" 
        c:color="${moble.color}"
        c:type="${mobile.type}">
        </bean>
        <context:property-placeholder/>
</beans>

　　解析外部屬性能夠將值的處理推遲到運行時，它的關註點在於根據名稱解析來自於Spring Environment和屬性源的屬性。

Spring_屬性占位符