String sql="select c.id, c.user_id,c.motorcycle_id,c.motorcycle_comment,u.username from comment c left join user u on c.user_id=u.id where c.motorcycle_id=?";

添加评论：

String sql = "insert into comment(user_id,motorcycle_id,motorcycle_comment) values(?,?,?)"

删除评论：

String sql = "delete from comment where id=?";

service层直接调用，不做处理。

clist = cDao.getMotorcycleComment(motorcycleId);

3.servlet层编写核心代码

将查询结果放到request域里。

List<Comment> clist=commentService.getMotorcycleComment(id);
//        for (Comment c:clist
//             ) {
//            System.out.println(c);
//        }
        request.getSession().setAttribute("MotorcycleId", id);
        request.setAttribute("clist", clist);

调用删除后重定向到详情页。

 commentService.deleteComment(commentId);
        resp.sendRedirect("/motorcycle_detail?id="+motorcycleId);

添加也是，添加完后重定向到商品详情页。

CommentService commentService=new CommentService();
        int userId= Integer.parseInt(req.getParameter("userId"));
        int motorcycleId= Integer.parseInt(req.getParameter("motorcycleId"));
        String motorcycleComment=req.getParameter("comment");

        commentService.addComment(userId,motorcycleId,motorcycleComment);
//        req.getRequestDispatcher("/motorcycle_detail?id="+motorcycleId).forward(req, resp);
        resp.sendRedirect("/motorcycle_detail?id="+motorcycleId);