如何使用mpmovieplayervewcontroller在本地播放电影?

-(IBAction)videoButtonClicked:(id)sender{
    NSString *url = [[NSBundle mainBundle] 
                     pathForResource:@"lebron" 
                     ofType:@"mp4"];

    MPMoviePlayerViewController* moviePlayerViewController = [[MPMoviePlayerViewController alloc] initWithContentURL:[NSURL URLWithString:url]];

    moviePlayerViewController.view.backgroundColor = [UIColor blackColor];
    [self presentMoviePlayerViewControllerAnimated:moviePlayerViewController];

    [moviePlayerViewController release];    
}

显示电影播放器,但电影不播放。为什么?