实例讲解JSP Model2体系结构(中)


理解“音乐无国界”
  “音乐无国界”的主界面是JSP页 Eshop.jsp(见代码清单1)。你会注意到,这个页面几乎只作为专门的用户界面,不承担任何处理任务――是一个最理想的JSP方案。另外,请注意另一个JSP页Cart.jsp(见代码清单2)被Eshop.jsp通过指令<jsp:include page="Cart.jsp" flush="true" />包含于其中。

  代码清单 1:EShop.jsp

  <%@ page session="true" %>

  <html>

  <head>

   <title>Music Without Borders</title>

  </head>

  <body bgcolor="#33CCFF">

   <font face="Times New Roman,Times" size="+3">

   Music Without Borders

   </font>

   <hr><p>

   <center>

   <form name="shoppingForm"

   action="/examples/servlet/ShoppingServlet"

   method="POST">

   <b>CD:</b>

   <select name=CD>

   <option>Yuan | The Guo Brothers | China | $14.95</option>

   <option>Drums of Passion | Babatunde Olatunji | Nigeria | $16.95</option>

   <option>Kaira | Tounami Diabate| Mali | $16.95</option>

   <option>The Lion is Loose | Eliades Ochoa | Cuba | $13.95</option>

   <option>Dance the Devil Away | Outback | Australia | $14.95</option>

   <option>Record of Changes | Samulnori | Korea | $12.95</option>

   <option>Djelika | Tounami Diabate | Mali | $14.95</option>

   <option>Rapture | Nusrat Fateh Ali Khan | Pakistan | $12.95</option>

   <option>Cesaria Evora | Cesaria Evora | Cape Verde | $16.95</option>

   <option>Ibuki | Kodo | Japan | $13.95</option>

   </select>

   <b>Quantity: </b><input type="text" name="qty" SIZE="3" value=1>

   <input type="hidden" name="action" value="ADD">

   <input type="submit" name="Submit" value="Add to Cart">

   </form>

   </center>

   <p>

   <jsp:include page="Cart.jsp" flush="true" />

  </body>

  </html>

  代码清单 2:Cart.jsp

  <%@ page session="true" import="java.util.*, shopping.CD" %>

  <%

   Vector buylist = (Vector) session.getValue("shopping.shoppingcart");

   if (buylist != null && (buylist.size() > 0)) {

  %>

  <center>

  <table border="0" cellpadding="0" width="100%" bgcolor="#FFFFFF">

   <tr>

   <td><b>ALBUM</b></td>

   <td><b>ARTIST</b></td>

   <td><b>COUNTRY</b></td>

   <td><b>PRICE</b></td>

   <td><b>QUANTITY</b></td>

   <td></td>

   </tr>

   <%

   for (int index=0; index < buylist.size();index++) {

   CD anOrder = (CD) buylist.elementAt(index);

   %>

   <tr>

   <td><b><%= anOrder.getAlbum() %></b></td>

   <td><b><%= anOrder.getArtist() %></b></td>

   <td><b><%= anOrder.getCountry() %></b></td>

   <td><b><%= anOrder.getPrice() %></b></td>

   <td><b><%= anOrder.getQuantity() %></b></td>

   <td>

   <form name="deleteForm"

    action="/examples/servlet/ShoppingServlet"

    method="POST">

   <input type="submit" value="Delete">

   <input type="hidden" name= "delindex" value='<%= index %>'>

   <input type="hidden" name="action" value="DELETE">

   </form>

     </td>

    </tr>

    <% } %>

   </table>

   <p>

   <form name="checkoutForm"

    action="/examples/servlet/ShoppingServlet"

    method="POST">

    <input type="hidden" name="action" value="CHECKOUT">

    <input type="submit" name="Checkout" value="Checkout">

   </form>

   </center>

  <% } %>

  这里,Cart.jsp操纵着基于会话的购物车的表达,在MVC体系中,购物车就充当Model的角色。

  观察Cart.jsp开头处的脚本片段:

  <%

   Vector buylist = (Vector) session.getValue("shopping.shoppingcart");

   if (buylist != null && (buylist.size() > 0)) {

  %>

  这段脚本主要是从会话中取出购物车。如果购物车是空的或尚未创建,则它什么都不显示;因此,当用户第一次访问这个应用程序时,呈现给他的视图如图3所示:


  图3:音乐无国界,主视图

  图中按钮文字:放入购物车

  如果购物车不为空,则选中的物品被依次从购物车中取出,如下面的脚本片段所示:

  <%

   for (int index=0; index < buylist.size(); index++) {

    CD anOrder = (CD) buylist.elementAt(index);

  %>

  描述物品的变量一旦被创建,就会被用JSP表达式直接嵌入静态HTML模板中去。图4显示了当用户向购物车中放入一些物品后的视图。


  图4:音乐无国界,购物车视图

  图中文字:Music Without Borders:音乐无国界;Quantity:数量;ALBUM:唱片;ARTIST:演唱者;COUNTRY:国家;PRICE:价格;Delete:删除;Checkout:结帐。

  这里需要注意的重要一点是,在Eshop.jsp和Cart.jsp中实现的对所有动作的处理都由一个servlet――ShoppingServlet.java控制,如代码清单3所示:

  代码清单3:ShoppingServlet.java

  import java.util.*;

  import java.io.*;

  import javax.servlet.*;

  import javax.servlet.http.*;

  import shopping.CD;

  public class ShoppingServlet extends HttpServlet {

   public void init(ServletConfig conf) throws ServletException {

    super.init(conf);

   }

   public void doPost (HttpServletRequest req, HttpServletResponse res)

     throws ServletException, IOException {

    HttpSession session = req.getSession(false);

    if (session == null) {

     res.sendRedirect("http://localhost:8080/error.html");

    }

    Vector buylist=

     (Vector)session.getValue("shopping.shoppingcart");

    String action = req.getParameter("action");

    if (!action.equals("CHECKOUT")) {

     if (action.equals("DELETE")) {

      String del = req.getParameter("delindex");

      int d = (new Integer(del)).intValue();

      buylist.removeElementAt(d);

     } else if (action.equals("ADD")) {

      //以前是否购买了同样的cd?

      boolean match=false;

      CD aCD = getCD(req);

      if (buylist==null) {

       //将第一张CD放入购物车

       buylist = new Vector(); //第一份定单

       buylist.addElement(aCD);

      } else { // 不是第一次购买

       for (int i=0; i< buylist.size(); i++) {

        CD cd = (CD) buylist.elementAt(i);

        if (cd.getAlbum().equals(aCD.getAlbum())) {

         cd.setQuantity(cd.getQuantity()+aCD.getQuantity());

         buylist.setElementAt(cd,i);

         match = true;

        } //if name matches结束

       } // for循环结束

       if (!match)

        buylist.addElement(aCD);

      }

     }

     session.putValue("shopping.shoppingcart", buylist);

     String url="/jsp/shopping/EShop.jsp";

     ServletContext sc = getServletContext();

     RequestDispatcher rd = sc.getRequestDispatcher(url);

     rd.forward(req, res);

    } else if (action.equals("CHECKOUT")) {

     float total =0;

     for (int i=0; i< buylist.size();i++) {

      CD anOrder = (CD) buylist.elementAt(i);

      float price= anOrder.getPrice();

      int qty = anOrder.getQuantity();

      total += (price * qty);

     }

     total += 0.005;

     String amount = new Float(total).toString();

     int n = amount.indexOf('.');

     amount = amount.substring(0,n+3);

     req.setAttribute("amount",amount);

     String url="/jsp/shopping/Checkout.jsp";

     ServletContext sc = getServletContext();

     RequestDispatcher rd = sc.getRequestDispatcher(url);

     rd.forward(req,res);

    }

   }

   private CD getCD(HttpServletRequest req) {

    //想象一下如果这些都在一个脚本片段中会有多么难看

    String myCd = req.getParameter("CD");

    String qty = req.getParameter("qty");

    StringTokenizer t = new StringTokenizer(myCd,"|");

    String album= t.nextToken();

    String artist = t.nextToken();

    String country = t.nextToken();

    String price = t.nextToken();

    price = price.replace('$',' ').trim();

    CD cd = new CD();

    cd.setAlbum(album);

    cd.setArtist(artist);

    cd.setCountry(country);

    cd.setPrice((new Float(price)).floatValue());

    cd.setQuantity((new Integer(qty)).intValue());

    return cd;

   }

  }



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