【Java.Web】Session —— 示例 —— Servlet —— 购物车示例，与JSP区别
JSP文件在默认情况下都支持Session，而HttpServlet类在默认情况下不支持Session，这是JSP与Servlet的一个小小的区别。
当Servlet容器调用HttpServlet类的服务方法时，会传递一个HttpSevletRequest类型的参数，HttpServlet可通过HttpServletRequest对象来获得HttpSession对象。
在HttpServletResponse接口中提供了两个与Session有关的方法：
- getSession() —— 使得当前HttpServlet支持Session；假如Session已经存在，就返回相应的HttpSession对象，否则就创建一个新Session，并返回新建的HttpSession对象。该方法等价于调用HttpServletRequest.getSession(true)方法；
- getSession(boolean create) —— 如果参数为true，等价于调用HttpServletRequest的getSession()方法；如果参数为false，那么假如Session已经存在，就返回相应的HttpSession对象，否则就返回null；
Servlet 购物车示例
实现一个简单的购物网站应用；
- ShoopingServlet负责把客户选购的商品放入购物车，并且把购物车存放在会话范围内。
- ShoppintCart用来保存购买的商品；
- shopping.html用来让用户选购商品；
package com.gof.test.bean;
import java.util.*;
import java.io.*;
public class ShoppingCart implements Serializable {
Map<String, Integer> items = new HashMap<String, Integer>();
int numberOfItems = 0;
public synchronized void add(String itemNames){
if (items.containsKey(itemNames)){
Integer itemCount = (Integer)items.get(itemNames);
items.put(itemNames, new Integer(itemCount + 1));
}else{
items.put(itemNames, new Integer(1));
}
numberOfItems++;
}
public synchronized int getNumberOfItems(){
return numberOfItems;
}
public synchronized Map<String, Integer> getItems(){
return items;
}
}
package com.gof.test.servlet;
import java.io.PrintWriter;
import java.util.*;
import java.io.IOException;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;
import com.gof.test.bean.ShoppingCart;
public class ShoppingServlet extends HttpServlet {
protected void doGet(HttpServletRequest req, HttpServletResponse resp)
throws ServletException, IOException{
String[] itemNames={"糖果", "收音机", "练习簿"};
// get the session object
HttpSession session = req.getSession(true);
// get the ShoppingCart object from the session
ShoppingCart cart = (ShoppingCart)session.getAttribute("cart");
// if these is cart object, then create it and save it into session
if (cart == null){
cart = new ShoppingCart();
session.setAttribute("cart", cart);
}
resp.setContentType("text/html;charset=utf-8");
PrintWriter out = resp.getWriter();
String[] itemsSelected;
String itemIndex;
String itemName;
itemsSelected = req.getParameterValues("item");
if (itemsSelected != null){
for (int i = 0; i < itemsSelected.length; i++){
itemIndex = itemsSelected[i];
itemName = itemNames[Integer.parseInt(itemIndex)];
// add into the ShoppingCart
cart.add(itemName);
}
}
// print the info in ShoppingCart
out.println("<html>");
out.println("<head>");
out.println("<title>The content in Shopping Cart</title>");
out.println("</head>");
out.println("<body>");
out.println("Session ID: " + session.getId() + "<br/>");
out.println("<center><h1>You owns: " + cart.getNumberOfItems() + "items: </h1></center>");
Map<String,Integer> items = cart.getItems();
Iterator<Map.Entry<String, Integer>> it = items.entrySet().iterator();
while (it.hasNext()){
Map.Entry entry = it.next();
out.println(entry.getKey() + ": " + entry.getValue() + "<br/>");
}
out.println("<br><a href='shopping.html'> Go on shopping!</a></br>");
out.println("</body>");
out.println("</html>");
}
protected void doPost(HttpServletRequest req, HttpServletResponse resp)
throws ServletException, IOException{
doGet(req, resp);
}
}
在web.xml中注册该Servlet：
<!-- Shopping Cart -->
<servlet>
<servlet-name>shopping</servlet-name>
<servlet-class>com.gof.test.servlet.ShoppingServlet</servlet-class>
</servlet>
<servlet-mapping>
<servlet-name>shopping</servlet-name>
<url-pattern>/shopping</url-pattern>
</servlet-mapping>
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>Shopping</title>
</head>
<body>
<h1>Shopping Mall</h1>
<form action="shopping" method="post">
Select items:
<p>
<input type="checkbox" name="item" value="0"/>
第一种：糖果
</p>
<p>
<input type="checkbox" name="item" value="1"/>
第一种：收音机
</p>
<p>
<input type="checkbox" name="item" value="2"/>
第一种：练习簿
</p>
<hr/>
<input type="submit" name="submit" value="加入购物车"/>
</form>
</body>
</html>
通过浏览器访问如下URL：
http://localhost:8080/base-webapp/shopping.html
选择若干商品，点击“加入购物车”，ShoppingServlet负责响应客户请求，显示购物车内容：
点击“Go on shopping”，返回shopping.html页面，再选择若干商品，
加入购物车：
从ShoppingServlet显示的购物车内容可以看出，购物车能够积累存放客户在多次HTTP请求中选购的商品。这是因为在同一个浏览器进程中，客户的多次选购商品的HTTP请求都在同一个Session中。