Yii :: app（） - > user-> id没有返回空结果

I've a problem, it might be a small one but I couldn't find the solution for this. Yii:app()->user->getId() or Yii:app()->user->id is not returning value. it just returning empty result. But I've set the id in my UserIdentity class.I've a problem, it might be a small one but I c