在spring webmvc的controller读取context:property-placeholder定义的属性

SPRING 的context分两块， 一个是root webapp context, 一块是servelet app context, Controller属于后面这块， 在controller里要读取的@value属性值， 必须定定在servelet app context里。

参见：http://stackoverflow.com/questions/3652090/difference-between-applicationcontext-xml-and-spring-servlet-xml-in-spring

和
http://stackoverflow.com/questions/11890544/spring-value-annotation-in-controller-class-not-evaluating-to-value-inside-pro


Spring lets you define multiple contexts in a parent-child hierarchy.

The applicationContext.xml defines the beans for the "root webapp context", i.e. the context associated with the webapp.

The spring-servlet.xml (or whatever else you call it) defines the beans for one servlet's app context. There can be many of these in a webapp, one per Spring servlet (e.g. spring1-servlet.xml for servlet spring1, spring2-servlet.xml for servlet spring2).

Beans in spring-servlet.xml can reference beans in applicationContext.xml, but not vice versa.

All Spring MVC controllers must go in the spring-servlet.xml context.

In most simple cases, the applicationContext.xml context is unnecessary. It is generally used to contain beans that are shared between all servlets in a webapp. If you only have one servlet, then there's not really much point, unless you have a specific use for it.