访问Spring中placeholder里的属性值（也可以是任何合法的SpringEL表达式）的方法。

PropertiesAccessor.java

package cn.boxiao.bxn.common.boot;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.beans.factory.support.AbstractBeanFactory;
import org.springframework.stereotype.Component;

import java.util.Map;
import java.util.concurrent.ConcurrentHashMap;

/**
 * 获取属性文件的值，利用了spring的表达式功能
 */
@Component
public class PropertiesAccessor {

    private Logger logger = LoggerFactory.getLogger(getClass());

    private final AbstractBeanFactory beanFactory;

    private final Map<String,String> cache = new ConcurrentHashMap<>();

    @Autowired
    protected PropertiesAccessor(@SuppressWarnings("SpringJavaAutowiringInspection") AbstractBeanFactory beanFactory) {
        this.beanFactory = beanFactory; 
    }

    /**
     * 得到一个key对应的值
     * 如果key在properties文件中配置，那么返回null
     * @param key
     * @return
     */
    public  String getProperty(String key) {
        if(cache.containsKey(key)){ 
            return cache.get(key); 
        } 

        String foundProp = null; 
        try { 
            foundProp = beanFactory.resolveEmbeddedValue("${" + key.trim() + "}");        
            cache.put(key,foundProp);        
        } catch (IllegalArgumentException ex) { 
           logger.error("Can't get property[{}]'s value,has config it in properties file?", key);
        } 

        return foundProp; 
    } 
}

PropertiesAccessorUtil.java

package cn.boxiao.bxn.common.util;

import cn.boxiao.bxn.common.boot.PropertiesAccessor;
import org.springframework.web.context.WebApplicationContext;
import org.springframework.web.context.request.RequestContextHolder;
import org.springframework.web.context.request.ServletRequestAttributes;
import org.springframework.web.servlet.support.RequestContextUtils;

import javax.servlet.http.HttpServletRequest;

/**
 * 对{@link cn.boxiao.bxn.common.boot.PropertiesAccessor}的方法做了静态方法的包装
 * 常用于不使用Spring注入的对象使用（比如VO）
 * 但要特别注意，此方法不能脱离Spring的环境而使用，且必须配置{@link org.springframework.web.context.request.RequestContextListener}
 * Created by yanhua on 14-5-8.
 */
public class PropertiesAccessorUtil {

    private static PropertiesAccessor propertiesAccessor;

    /**
     * 对{@link cn.boxiao.bxn.common.boot.PropertiesAccessor#getProperty(String)}的封装
     * @param key
     * @return
     */
    public static String getProperty(String key){
        if(propertiesAccessor == null){
            HttpServletRequest request = ((ServletRequestAttributes)RequestContextHolder.getRequestAttributes()).getRequest();
            WebApplicationContext applicationContext = RequestContextUtils.getWebApplicationContext(request);
            propertiesAccessor = applicationContext.getBean(PropertiesAccessor.class);
        }
        return propertiesAccessor.getProperty(key);
    }

}