java.lang.NoClassDefFoundError: IUrlGeneratorService

I am using the following Code :-

IUrlGeneratorService urlGenService = (IUrlGeneratorService) PortalRuntime. getRuntimeResources().getService(IUrlGeneratorService.KEY);

IPortalUrlGenerator urlGenerator =(IPortalUrlGenerator) urlGenService.getSpecializedUrlGenerator(PortalUrlGenerator.KEY);

I have added the com.sap.portal.appintegrator.sap.bwc_api.jar in the classpath .

But still I am getting the error :-

java.lang.NoClassDefFoundError: com/sapportals/portal/prt/service/urlgenerator/IUrlGeneratorService

Kindly let me how to resolve the error.

Thanks.