@Autowired注释在Spring中如何工作？

真正的问题是,为什么允许我通过以下两种方式编写此代码：

@Controller
public class PostController {
@Autowired 
private PostService postService;

@Autowired 
private CommentService commentService;

....
}

和

@Controller
public class PostController {

private PostService postService;
private CommentService commentService;

@Autowired 
public PostController(PostService postService,CommentService commentService){
    this.postService = postService;
    this.commentService = commentService;
}

....
}

这些摘要是否相同？推荐哪一个？