getServletContext allways return null with conf via spring (native mode)

Hi,
i am using shiro rev. 945078, with configuration which uses native sessions

filter is configured via springframework - DelegatingFilterProxy

(here is the snippet from web.xml)

<filter>
<filter-name>shiroFilter</filter-name>
<filter-class>org.springframework.web.filter.DelegatingFilterProxy</filter-class>
</filter>

(and here from spring conf. xml)

<bean id="shiroFilter" class="org.apache.shiro.spring.web.ShiroFilterFactoryBean">
<property name="securityManager" ref="securityManager"/>
</bean>
<bean id="securityManager" class="org.apache.shiro.web.DefaultWebSecurityManager">
<property name="realm" ref="trustRealm" />
<property name="subjectFactory" ref="trustSubjectFactory" />
<property name="sessionMode" value="native" />
<property name="sessionManager" ref="trustSessionManager" />
<property name="cacheManager">
<bean class="org.apache.shiro.cache.MemoryConstrainedCacheManager" />
</property>
</bean>

the problem i have is that the method HttpServletRequest.getSession().getServletContext() always return null,

(I was wondering why and i found that the ServletContext is set to AbstractFilter only in init(filterConfig) method.
But this method is never called in configuration via spring-DelegatingFilterProxy. I know that specifying the "targetFilterLifecycle" filter init-param as "true" will enforce invocation of the Filter.init. But this way my definitions will be overrided.)