# 改善Servlet的編寫
###### tags: `Java Web-常用知識點`
###### tags: `Java Web-mvc`

#### 原始架構
把每項功能都獨立作為一Servlet類來編寫，並且將資料傳輸到DAO，傳到DB後，再依序回傳

缺點：非常繁雜

#### 結構優化
整合成一個FruitServlet類，並且在裏頭編寫需要的方法

@WebServlet("/fruit.do")
public class FruitServlet extends ViewBaseServlet {
    private FruitDAO fruitDAO = new FruitDAOImpl();

    @Override
    protected void service(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        //设置编码
        request.setCharacterEncoding("UTF-8");

        String operate = request.getParameter("operate");
        if(StringUtil.isEmpty(operate)){
            operate = "index" ;
        }
        //根據operate獲得元素來判斷作法
        switch(operate){
            case "index":
                index(request,response);
                break;
            case "add":
                add(request,response);
                break;
            case "del":
                del(request,response);
                break;
            case "edit":
                edit(request,response);
                break;
            case "update":
                update(request,response);
                break;
            default:
                throw new RuntimeException("operate值非法!");
        }
    }

    private void update(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        //1.设置编码
        request.setCharacterEncoding("utf-8");

        //2.获取参数
        String fidStr = request.getParameter("fid");
        Integer fid = Integer.parseInt(fidStr);
        String fname = request.getParameter("fname");
        String priceStr = request.getParameter("price");
        int price = Integer.parseInt(priceStr);
        String fcountStr = request.getParameter("fcount");
        Integer fcount = Integer.parseInt(fcountStr);
        String remark = request.getParameter("remark");

        //3.执行更新
        fruitDAO.updateFruit(new Fruit(fid,fname, price ,fcount ,remark ));

        //4.资源跳转
        //super.processTemplate("index",request,response);
        //request.getRequestDispatcher("index.html").forward(request,response);
        //此处需要重定向，目的是重新给IndexServlet发请求，重新获取furitList，然后覆盖到session中，这样index.html页面上显示的session中的数据才是最新的
        response.sendRedirect("fruit.do");
    }

    private void edit(HttpServletRequest request , HttpServletResponse response)throws IOException, ServletException {
        String fidStr = request.getParameter("fid");
        if(StringUtil.isNotEmpty(fidStr)){
            int fid = Integer.parseInt(fidStr);
            Fruit fruit = fruitDAO.getFruitByFid(fid);
            request.setAttribute("fruit",fruit);
            super.processTemplate("edit",request,response);
        }
    }

    private void del(HttpServletRequest request , HttpServletResponse response)throws IOException, ServletException {
        String fidStr = request.getParameter("fid");
        if(StringUtil.isNotEmpty(fidStr)){
            int fid = Integer.parseInt(fidStr);
            fruitDAO.delFruit(fid);

            //super.processTemplate("index",request,response);
            response.sendRedirect("fruit.do");
        }
    }

    private void add(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        request.setCharacterEncoding("UTF-8");

        String fname = request.getParameter("fname");
        Integer price = Integer.parseInt(request.getParameter("price")) ;
        Integer fcount = Integer.parseInt(request.getParameter("fcount"));
        String remark = request.getParameter("remark");

        Fruit fruit = new Fruit(0,fname , price , fcount , remark ) ;

        fruitDAO.addFruit(fruit);

        response.sendRedirect("fruit.do");

    }

    private void index(HttpServletRequest request , HttpServletResponse response)throws IOException, ServletException {
        HttpSession session = request.getSession() ;

        // 设置当前页，默认值1
        Integer pageNo = 1 ;

        String oper = request.getParameter("oper");

        //如果oper!=null 说明 通过表单的查询按钮点击过来的
        //如果oper是空的，说明 不是通过表单的查询按钮点击过来的
        String keyword = null ;
        if(StringUtil.isNotEmpty(oper) && "search".equals(oper)){
            //说明是点击表单查询发送过来的请求
            //此时，pageNo应该还原为1 ， keyword应该从请求参数中获取
            pageNo = 1 ;
            keyword = request.getParameter("keyword");
            //如果keyword为null，需要设置为空字符串""，否则查询时会拼接成 %null% , 我们期望的是 %%
            if(StringUtil.isEmpty(keyword)){
                keyword = "" ;
            }
            //将keyword保存（覆盖）到session中
            session.setAttribute("keyword",keyword);
        }else{
            //说明此处不是点击表单查询发送过来的请求（比如点击下面的上一页下一页或者直接在地址栏输入网址）
            //此时keyword应该从session作用域获取
            String pageNoStr = request.getParameter("pageNo");
            if(StringUtil.isNotEmpty(pageNoStr)){
                pageNo = Integer.parseInt(pageNoStr);   //如果从请求中读取到pageNo，则类型转换。否则，pageNo默认就是1
            }
            //如果不是点击的查询按钮，那么查询是基于session中保存的现有keyword进行查询
            Object keywordObj = session.getAttribute("keyword");
            if(keywordObj!=null){
                keyword = (String)keywordObj ;
            }else{
                keyword = "" ;
            }
        }

        // 重新更新当前页的值
        session.setAttribute("pageNo",pageNo);

        FruitDAO fruitDAO = new FruitDAOImpl();
        List<Fruit> fruitList = fruitDAO.getFruitList(keyword , pageNo);
        session.setAttribute("fruitList",fruitList);

        //总记录条数
        int fruitCount = fruitDAO.getFruitCount(keyword);
        //总页数
        int pageCount = (fruitCount+5-1)/5 ;
        /*
        总记录条数       总页数
        1               1
        5               1
        6               2
        10              2
        11              3
        fruitCount      (fruitCount+5-1)/5
         */
        session.setAttribute("pageCount",pageCount);

        //此处的视图名称是 index
        //那么thymeleaf会将这个 逻辑视图名称 对应到 物理视图 名称上去
        //逻辑视图名称 ：   index
        //物理视图名称 ：   view-prefix + 逻辑视图名称 + view-suffix
        //所以真实的视图名称是：      /       index       .html
        super.processTemplate("index",request,response);
    }
}

### 再次優化
原因：如果不斷新增功能，FruitServlet的switch代碼會變得太長
解決方法：
1.先獲取這個類(FruitServlet)中的所有方法
2.透過if找到對應方法，並且使用反射來調用

protected void service(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        //設置編碼
        req.setCharacterEncoding("UTF-8");

        String operate = req.getParameter("operate");
        if(StringUtil.isEmpty(operate)){
            operate = "index";
        }

        //獲取當前類中所有的方法
        Method[] methods = this.getClass().getDeclaredMethods();
        for(Method m:methods){
            //獲取方法名稱
            String methodName = m.getName();
            if(operate.equals(methodName)){
                try {
                    //表示找到和operate同名的方法，那麼通過反射技術調用它
                    m.invoke(this,req,resp);
                    return;
                } catch (IllegalAccessException e) {
                    e.printStackTrace();
                } catch (InvocationTargetException e) {
                    e.printStackTrace();
                }
            }
        }
        throw new RuntimeException("operate值非法");
    }