在过滤器Filter里面定义变量 用@Value给其赋值，为什么一直都是NULL
必须是spring管理的bean才能注入
可以参考
[url]https://github.com/zhangkaitao/es/blob/master/web/src/main/webapp/WEB-INF/web.xml[/url]
[code="java"]
shiroFilter
org.springframework.web.filter.DelegatingFilterProxy
targetFilterLifecycle
true
[/code]
DelegatingFilterProxy自动到spring容器里找一个名字为shiroFilter的bean，完成任务
[url]https://github.com/zhangkaitao/es/blob/master/web/src/main/resources/spring-config-shiro.xml[/url]
[code="java"]
<!-- override these for application-specific URLs if you like:-->
<!-- The 'filters' property is not necessary since any declared javax.servlet.Filter bean -->
<!-- defined will be automatically acquired and available via its beanName in chain -->[/code]