Spring import配置文件使用占位符

<import resource="classpath:META-INF/spring/spring-${db.connection.pool}.xml" />

db.connection.pool=druid

Caused by: java.lang.IllegalArgumentException: Could not resolve placeholder 'db.connection.pool' in value "classpath:META-INF/spring/spring-${db.connection.pool}.xml"

public class AppContextInitializer
        implements ApplicationContextInitializer<ConfigurableApplicationContext> {

    private Logger logger = Logger.getLogger(AppContextInitializer.class);

    @Override
    public void initialize(ConfigurableApplicationContext applicationContext) {
        ResourcePropertySource propertySource = null;
        try {
            propertySource = new ResourcePropertySource("classpath:config/db-config.properties");
        } catch (IOException e) {
            logger.error("加载配置文件[config/db-config.properties]失败");
        }
        applicationContext.getEnvironment().getPropertySources().addFirst(propertySource);
    }

}

context-param>  
    <param-name>contextInitializerClasses</param-name>  
    <param-value>com.example.AppContextInitializer</param-value>  
</context-param>