Token / TokenSession interceptor leading to null pointer to HttpSession

I am working on struts2 web application, I am handling CSRF vulnerability using an interceptor token.

what i am doing is success as well as error, i will redirect the user to the same page but with an error message or a success message.

<action name="saveApplicationForm" class="action.ApplicationFormAction" 
      method="saveApplicationForm">
        <interceptor-ref name="token" />
        <result name="invalid.token" type="tiles">applicationForm.tiles</result>    
        <result name="input" type="tiles">applicationForm.tiles</result>
</action>

      

        
        
        
      

    

without the token / tokenSession, the interceptor works fine , but when I use the interceptor, I get a NullPointerException.

StackTrace

java.lang.NullPointerException: null
at action.ApplicationFormAction.saveApplicationForm(ApplicationFormAction.java:218) ~[ApplicationFormAction.class:na]
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[na:1.7.0_67]
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57) ~[na:1.7.0_67]
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[na:1.7.0_67]
at java.lang.reflect.Method.invoke(Method.java:606) ~[na:1.7.0_67]
at com.opensymphony.xwork2.DefaultActionInvocation.invokeAction(DefaultActionInvocation.java:450) [xwork-core-2.3.16.3.jar:2.3.16.3]
at com.opensymphony.xwork2.DefaultActionInvocation.invokeActionOnly(DefaultActionInvocation.java:289) [xwork-core-2.3.16.3.jar:2.3.16.3]
at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:252) [xwork-core-2.3.16.3.jar:2.3.16.3]
at org.apache.struts2.interceptor.TokenInterceptor.handleValidToken(TokenInterceptor.java:193) [struts2-core-2.3.16.3.jar:2.3.16.3]
at org.apache.struts2.interceptor.TokenInterceptor.handleToken(TokenInterceptor.java:154) [struts2-core-2.3.16.3.jar:2.3.16.3]
at org.apache.struts2.interceptor.TokenInterceptor.doIntercept(TokenInterceptor.java:142) [struts2-core-2.3.16.3.jar:2.3.16.3]
at com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:98) [xwork-core-2.3.16.3.jar:2.3.16.3]
at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:246) [xwork-core-2.3.16.3.jar:2.3.16.3]
at org.apache.struts2.impl.StrutsActionProxy.execute(StrutsActionProxy.java:54) [struts2-core-2.3.16.3.jar:2.3.16.3]
at org.apache.struts2.dispatcher.Dispatcher.serviceAction(Dispatcher.java:562) [struts2-core-2.3.16.3.jar:2.3.16.3]
at org.apache.struts2.dispatcher.ng.ExecuteOperations.executeAction(ExecuteOperations.java:77) [struts2-core-2.3.16.3.jar:2.3.16.3]
at org.apache.struts2.dispatcher.ng.filter.StrutsPrepareAndExecuteFilter.doFilter(StrutsPrepareAndExecuteFilter.java:99) [struts2-core-2.3.16.3.jar:2.3.16.3]
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:241) [catalina.jar:7.0.63]
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208) [catalina.jar:7.0.63]
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:220) [catalina.jar:7.0.63]
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:122) [catalina.jar:7.0.63]
at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:505) [catalina.jar:7.0.63]
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:170) [catalina.jar:7.0.63]
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:103) [catalina.jar:7.0.63]
at org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:957) [catalina.jar:7.0.63]
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:116) [catalina.jar:7.0.63]
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:423) [catalina.jar:7.0.63]
at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1079) [tomcat-coyote.jar:7.0.63]
at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:620) [tomcat-coyote.jar:7.0.63]
at org.apache.tomcat.util.net.JIoEndpoint$SocketProcessor.run(JIoEndpoint.java:318) [tomcat-coyote.jar:7.0.63]
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145) [na:1.7.0_67]
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615) [na:1.7.0_67]
at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61) [tomcat-coyote.jar:7.0.63]
at java.lang.Thread.run(Thread.java:745) [na:1.7.0_67]

      

        
        
        
      

    

Action class

public class ApplicationFormAction extends ActionSupport implements ModelDriven<ApplicationFormBean>, SessionAware
{
   private Map<String, Object> session; 
    // getter and setter

   public String saveApplicationForm()
   {
      // getting nullpointer here
     ApplicationFormBean sessApplicationFormBean = (ApplicationFormBean) this.session.get(SESSION_KEY_APPLICANT);
   }

}

      

        
        
        
      

    

• what's wrong with the code?

• not redirecting to the same page, what will cause the problem (the presented token will be different from the newly generated token)?

• if so, how should I handle this situation without redirecting to another page?