java - Arraylist 不添加数据，调用时将我重定向到另一个页面

Question

创建一个超市软件，当我进入添加产品页面时，如果尚未添加类别和品牌，则页面重定向到类别添加并 品牌添加页面。

表格中已有数据，但页面重定向到添加类别页面。

类别的 UserDAO

    PreparedStatement ps =null;
    String query = "SELECT CategoryId, CategoryName from tbl_category";

    List <CategoryModel> categories = new ArrayList<CategoryModel>();
    CategoryModel categoryModel = new CategoryModel();

    CategoryModel u = null;
    try{
        ps = con.prepareStatement(query);
        ps.setString(1, categoryModel.getCategoryId());
        ps.setString(2, categoryModel.getCategoryName());

        ResultSet rs = ps.executeQuery();

        while(rs.next()){
            u.setCategoryId(rs.getString("CategoryId"));
            u.setCategoryName(rs.getString("CategoryName"));
            categories.add(u);
        }

    }catch(Exception e){
        e.printStackTrace();
        return null;
    }

    return categories;
}

品牌的 UserDAO

    PreparedStatement ps =null;
    String query = "SELECT BrandId, BrandName from tbl_brand";

    List <BrandModel> brands = new ArrayList<BrandModel>();
    BrandModel brandModel = new BrandModel();

    BrandModel u = null;    
    try{
        ps = con.prepareStatement(query);
        ps.setString(1, brandModel.getBrandId());
        ps.setString(2, brandModel.getBrandName());

        ResultSet rs = ps.executeQuery();

        while(rs.next()){

            u.setBrandId(rs.getString("BrandId"));
            u.setBrandName(rs.getString("BrandName"));
            brands.add(u);

        }
    }catch(Exception e){
        e.printStackTrace();
        return null;
    }

    return brands;
}

用于检索数据和重定向的 Servlet

     protected void doGet(HttpServletRequest request, HttpServletResponse        response) throws ServletException, IOException {
    session = request.getSession();
    List<BrandModel> brands=null;
    List<CategoryModel> categories=null;

    UserDAOImplementation brand = new UserDAOImplementation();

    brands = brand.brands();

    UserDAOImplementation category = new UserDAOImplementation();

    List<CategoryModel> catagories = category.catagories();


    if(category==null){
        response.sendRedirect("views/products/Category.jsp");
    }else if(brands==null){
        response.sendRedirect("views/products/Brands.jsp");
    }else if(categories.isEmpty()){
        response.sendRedirect("views/products/Category.jsp");
    }else if(brands.isEmpty()){
        response.sendRedirect("views/products/Brands.jsp");
    }else{
    session.setAttribute("brands", brand);
    session.setAttribute("categories", category);
    response.sendRedirect("views/products/Product.jsp");

    }


}

Answer 1

PreparedStatement ps =null; String query = "SELECT CategoryId, CategoryName from tbl_category";

List <CategoryModel> categories = new ArrayList<CategoryModel>(); CategoryModel categoryModel = new CategoryModel();

CategoryModel u = null; try{
    ps = con.prepareStatement(query);
    ps.setString(1, categoryModel.getCategoryId());
    ps.setString(2, categoryModel.getCategoryName());

    ResultSet rs = ps.executeQuery();

和

PreparedStatement ps =null; String query = "SELECT BrandId, BrandName from tbl_brand";

    List <BrandModel> brands = new ArrayList<BrandModel>();
    BrandModel brandModel = new BrandModel();

    BrandModel u = null;    
    try{
        ps = con.prepareStatement(query);
        ps.setString(1, brandModel.getBrandId());
        ps.setString(2, brandModel.getBrandName());

在这两种情况下，您都为查询设置输入参数，而没有输入参数的任何占位符 (?)。因此，我假设尝试调用 setString 会引发异常，您可以捕获该异常并将输出返回为 null，然后导致重定向。