I have done some R&D to fix this issue,finally i found a solution.We cant call/redirect the portal page from struts2Portlet Action class.We can have to give a result type in strut action class,then we have to configure the action in the result.
<action name="view" class="com.ibm.rock.ViewAction" method="prepareview">
<result name="view">*/view/viewportalpage.action*</result>
</action>
<action name="viewportalpage" class="com.ibm.rock.ViewAction" method="prepareview">
<result name="preview">/_Rock/jsp/html/Preview.jsp</result>
</action>
Synatax:
<result name="success">*/Namespace/view.action*</result>
<action name="view" class="com.ibm.rock.ViewAction" method="prepareview">
<result name="preview">/_Rock/jsp/html/Preview.jsp</result>
</action>