How to get session attribute in struts 2 tag

                           How to get session attribute in struts 2  <s:if test> tag

<s:if test='%{#session.USERID != null}' >

        // do something

</s:if>

<s:else if test='%{#session.USERID =="admin"}' >

   // if user is admin in USERID attribute of session, do something
  </s:else if>

<s:else>

   //do other things

</s:else>