Illegal state exception create session after response

This is probably something duh but please bare with me I am new...

I am trying to create a webservice that returns a UUID as a response my 
code looks as follows...

@RequestMapping(value="/getAuthUser.htm", method=RequestMethod.GET)
        @ResponseBody
        public String authenticateUserGet( ModelMap modelMap, 
 HttpServletRequest httpReq,
                                                                        
@RequestParam("custPermId") String custPermId,
                                                                        
@RequestParam("sessionKey") String sessKey)
         throws Exception {
                String sessionKey = sessKey;
                String customerPermId = custPermId;
                // If the user is coming from H.com with no sessionKey and 
custPermId forward them to Login page
                if (sessionKey == null || customerPermId == null){
                        return null;
                }
                userSession = populateClientDetails(httpReq, sessionKey);
                userSession.setCustomer( new Customer() );
                userSession.getCustomer().setCustPermId( customerPermId );
                userSession.setUuid(uuid());
                modelMap.addAttribute(AppConstants.USER_SESSION, 
userSession);
                String intelligentAuthenticationStatus = null; 
                return userSession.getUuid();
        }

However, after it returns the UUID I see the following in my error 
window,and when I look for the session added to the model map later I 
cannot find it.

java.lang.IllegalStateException: Cannot create a session after the 
response has been committed
        at org.apache.catalina.connector.Request.doGetSession(
Request.java:2381)
        at org.apache.catalina.connector.Request.getSession(
Request.java:2098)
        at org.apache.catalina.connector.RequestFacade.getSession(
RequestFacade.java:833)
        at 
org.springframework.web.context.request.ServletRequestAttributes.getSession(
ServletRequestAttributes.java:79)
        at 
org.springframework.web.context.request.ServletRequestAttributes.setAttribute(
ServletRequestAttributes.java:129)
        at 
org.springframework.web.bind.support.DefaultSessionAttributeStore.storeAttribute(
DefaultSessionAttributeStore.java:54)
        at 
org.springframework.web.bind.annotation.support.HandlerMethodInvoker.updateModelAttributes(
HandlerMethodInvoker.java:206)
        at 
org.springframework.web.servlet.mvc.annotation.AnnotationMethodHandlerAdapter.invokeHandlerMethod(
AnnotationMethodHandlerAdapter.java:424)
        at 
org.springframework.web.servlet.mvc.annotation.AnnotationMethodHandlerAdapter.handle(
AnnotationMethodHandlerAdapter.java:409)
        at org.springframework.web.servlet.DispatcherServlet.doDispatch(
DispatcherServlet.java:771)
        at org.springframework.web.servlet.DispatcherServlet.doService(
DispatcherServlet.java:716)
        at 
org.springframework.web.servlet.FrameworkServlet.processRequest(
FrameworkServlet.java:644)
        at org.springframework.web.servlet.FrameworkServlet.doGet(
FrameworkServlet.java:549)
        at javax.servlet.http.HttpServlet.service(HttpServlet.java:617)
        at javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
        at 
org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(
ApplicationFilterChain.java:290)
        at org.apache.catalina.core.ApplicationFilterChain.doFilter(
ApplicationFilterChain.java:206)
        at org.apache.catalina.core.StandardWrapperValve.invoke(
StandardWrapperValve.java:233)
        at org.apache.catalina.core.StandardContextValve.invoke(
StandardContextValve.java:191)
        at org.apache.catalina.core.StandardHostValve.invoke(
StandardHostValve.java:127)
        at org.apache.catalina.valves.ErrorReportValve.invoke(
ErrorReportValve.java:102)
        at org.apache.catalina.core.StandardEngineValve.invoke(
StandardEngineValve.java:109)
        at org.apache.catalina.connector.CoyoteAdapter.service(
CoyoteAdapter.java:298)
        at org.apache.coyote.http11.Http11Processor.process(
Http11Processor.java:852)
        at 
org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.process(
Http11Protocol.java:588)
        at org.apache.tomcat.util.net.JIoEndpoint$Worker.run(
JIoEndpoint.java:489)
        at java.lang.Thread.run(Unknown Source)

Thanks in advance for any help

Jackie


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