Yii::app()->user->id is not returning the empty result

Yii::app()->user->id is not returning the empty result

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.
in my UserIdentity class I've this,
public function getId()
{
return $this->id;
}
I'm setting id in the autheticate() function. if i displayed there itself
it's displaying the id, but not in getId() function. Please help me in
this.
Thanks in advance.
Dhanendran Rajagopal.