Activates replacement of ${...} placeholders by registering a PropertySourcesPlaceholderConfigurer within the application context. Properties will be resolved against the specified properties file or Properties object -- so called "local properties", if any, and against the Spring Environment‘s current set of PropertySources. Note that as of Spring 3.1 the system-properties-mode attribute has been removed in favor of the more flexible PropertySources mechanism. However, Spring 3.1-based applications may continue to use the 3.0 (and older) versions of the spring-context schema in order to preserve system-properties-mode behavior. In this case, the traditional PropertyPlaceholderConfigurer component will be registered instead of the new PropertySourcesPlaceholderConfigurer. See ConfigurableEnvironment javadoc for more information on using.
原文：http://www.cnblogs.com/zno2/p/4691904.html