前后端数据交互-----表单数据获取不到，出错的原因，在编写接口时，没有考虑数据如何返回，解决问题的思路，找到自己出错的地方，围绕着出错的地方进行考虑（很重要），找对解决问题的视频，理清返回数据的思路

1. <dependency>
2. <groupId>com.alibaba</groupId>
3. <artifactId>fastjson</artifactId>
4. <version>1.2.62</version>
5. </dependency>

package worldtolingyidianke.file.controller;
 
 
 
import com.alibaba.fastjson.JSON;
import worldtolingyidianke.file.bean.Brand;
import worldtolingyidianke.file.service.BrandService;
 
import javax.servlet.*;
import javax.servlet.http.*;
import javax.servlet.annotation.*;
import java.io.IOException;
import java.util.List;
 
@WebServlet("/selectAllServlet")
public class SelectAllServlet extends HttpServlet {
    private BrandService service = new BrandService();
 
    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        //1. 调用BrandService完成查询
        List<Brand> brands = service.selectAll();
 
        //2. 存入request域中
//        request.setAttribute("brands",brands);
        //2.将集合转换为JSON数据，序列化
        String jsonString = JSON.toJSONString(brands);
 
        //3. 响应数据
        response.setContentType("text/json;charset=utf-8");
        response.getWriter().write(jsonString);
    }
 
    @Override
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        this.doGet(request, response);
    }
}