session的学习

session:

在web应用开发中，当用户首次访问应用时，此应用服务器会为此用户创建一个session，保存此用户的相关数据为此用户服务，当用户再次访问此应用时，可以直接从保存后的session中直接取出数据使用

session和cookie的区别：

cookie：是将用户的数据直接保存在浏览器客户端中。cookie只能存字符串，cookie不安全储存在浏览器客户端

session：直接将用户的数据保存在应用的服务器上，session对象是又服务器创建，开发人员可直接调用request的getSession方法获取session对象。

session可以存对象，session安全

session的实现原理：

服务器如何实现一个session为一个用户服务

用户一访问时，会创建一个session，创建session的同时会创建一个sessionId，随机生成，将此id保存在cookie当中。

用户二访问时，会创建一个session，创建session的同时会创建一个sessionId，随机生成，将此id保存在cookie当中。

用户一再次访问时，首先会查看cookie中是否有开始创建session时生成的id，有不再创建session。

package qingxia.tang;

import java.io.IOException;
import java.io.PrintWriter;

import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;

/**
 * Servlet implementation class cookieDemo1
 */
@WebServlet("/cookieDemo1")
public class sessionDemo1 extends HttpServlet {
    private static final long serialVersionUID = 1L;
       
    /**
     * @see HttpServlet#HttpServlet()
     */
    public sessionDemo1() {
        super();
        // TODO Auto-generated constructor stub
    }

    /**
     * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
     */
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        //创建session
        HttpSession session = request.getSession();
        PrintWriter pw=response.getWriter();
        String parameter = request.getParameter("name");
        parameter=new String(parameter.getBytes("iso-8859-1"),"UTF-8");
        //将name保存到session中
        session.setAttribute("name", parameter);
        //获取创建session时生成的id
        String id = session.getId();
        //pw.print(parameter);
        pw.print(id);
    }

    /**
     * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
     */
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        // TODO Auto-generated method stub
        doGet(request, response);
    }

}

package qingxia.tang;

import java.io.IOException;
import java.io.PrintWriter;

import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;

/**
 * Servlet implementation class sessionDemo2
 */
@WebServlet("/sessionDemo2")
public class sessionDemo2 extends HttpServlet {
    private static final long serialVersionUID = 1L;
       
    /**
     * @see HttpServlet#HttpServlet()
     */
    public sessionDemo2() {
        super();
        // TODO Auto-generated constructor stub
    }

    /**
     * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
     */
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        response.setContentType("text/html;charset=UTF-8");
        //获取session对象
        HttpSession session = request.getSession();
        //从session中获取name值
        String name = (String)session.getAttribute("name");
        PrintWriter pw=response.getWriter();
        //获取创建session时生成的id
        String id = session.getId();
        pw.write(name+":id:"+id);
    }

    /**
     * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
     */
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        // TODO Auto-generated method stub
        doGet(request, response);
    }

}

保存name到session中和从session中取name值，从session中取到的id是同一个id

httpsession实现购物车

package qingxia.tang;

import java.io.IOException;
import java.io.PrintWriter;
import java.util.Map;
import java.util.Map.Entry;
import java.util.Set;

import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

import qingxia.tang.entity.Book;
import qingxia.tang.util.DbUtils;

/**
 * Servlet implementation class ShowAllBook
 */
@WebServlet("/ShowAllBook")
public class ShowAllBook extends HttpServlet {
    private static final long serialVersionUID = 1L;
       
    /**
     * @see HttpServlet#HttpServlet()
     */
    public ShowAllBook() {
        super();
        // TODO Auto-generated constructor stub
    }

    /**
     * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
     */
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        response.setContentType("text/html;charset=UTF-8");
        PrintWriter out = response.getWriter();
        out.print("本网站有以下书籍:</br>");
        Map<String, Book> allBook = DbUtils.getAllBook();
        for (Entry<String, Book> entry : allBook.entrySet()) {
            out.print("<a href='"+request.getContextPath()+"/addCart?id="+entry.getKey()+"' target='_blank'>"+entry.getValue().getBookName()+"</a></br>");
        }
        out.print("<a href='"+request.getContextPath()+"/ShowCart' target='_blank'>购物车有以下书籍:</a>");
    }

    /**
     * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
     */
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        // TODO Auto-generated method stub
        doGet(request, response);
    }

}

package qingxia.tang;

import java.io.IOException;
import java.io.PrintWriter;
import java.util.ArrayList;
import java.util.List;

import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;

import qingxia.tang.entity.Book;
import qingxia.tang.util.DbUtils;

/**
 * Servlet implementation class addCart
 */
@WebServlet("/addCart")
public class addCart extends HttpServlet {
    private static final long serialVersionUID = 1L;
       
    /**
     * @see HttpServlet#HttpServlet()
     */
    public addCart() {
        super();
        // TODO Auto-generated constructor stub
    }

    /**
     * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
     */
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        response.setContentType("text/html;charset=UTF-8");
        PrintWriter out = response.getWriter();
        String id = request.getParameter("id");
        Book bookById = DbUtils.getBookById(id);
        HttpSession session = request.getSession();
        //从session中获取key值为cart的list
        List<Book> list=(List<Book>) session.getAttribute("cart");
        //如果session中无key值为cart的list，则创建list
        if(list==null){
            list=new ArrayList<Book>();
        }
        //将要添加的书添加到list
        list.add(bookById);
        //将书的list添加到session中
        session.setAttribute("cart", list);
        out.print("添加成功!");
        response.setHeader("refresh", "2;url='"+request.getContextPath()+"/ShowAllBook'");

        
    }

    /**
     * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
     */
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        // TODO Auto-generated method stub
        doGet(request, response);
    }

}

package qingxia.tang;

import java.io.IOException;
import java.io.PrintWriter;
import java.util.List;

import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;

import qingxia.tang.entity.Book;

/**
 * Servlet implementation class ShowCart
 */
@WebServlet("/ShowCart")
public class ShowCart extends HttpServlet {
    private static final long serialVersionUID = 1L;
       
    /**
     * @see HttpServlet#HttpServlet()
     */
    public ShowCart() {
        super();
        // TODO Auto-generated constructor stub
    }

    /**
     * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
     */
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        response.setContentType("text/html;charset=UTF-8");
        HttpSession session = request.getSession();
        PrintWriter out = response.getWriter();
        //从session中获取key为cart的list值
        List<Book> list = (List<Book>) session.getAttribute("cart");
        if(list==null){
            out.print("你还没有买东西呢!");
            response.setHeader("refresh", "2;url='"+request.getContextPath()+"/ShowAllBook'");
            return;
        }
        for (Book book : list) {
            out.print(book.getBookName()+"</br>");
        }
    }

    /**
     * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
     */
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        // TODO Auto-generated method stub
        doGet(request, response);
    }

}

为什么要学习HttpSession？

• 它是一个域对象，request、response、servletContext
• 同一个会话下，可以使一个应用的多个资源共享数据
• cookie是客户端技术，只能储存字符串，储存在客户端不安全；httpSession是服务端技术，它可以储存对象。储存在服务端安全。

httpSession常用方法

把数据保存在HttpSession对象中，该对象也是一个域对象。

void setAttribute(String name,Object value);

Object getAttribute(String name);

void removeAttribute(String name); //从会话中将某一session移除

HttpSession.getId():

setMaxInactiveInterval(int interval)  //设置session的存活时间   

应用如：session中储存的是用户对象，当用户长时间没操作应用，可以设置session的存活时间，超出存活时间此用户失效，用户需重新登录应用。

invalidate()  //使此会话无效

应用如：session中储存的是用户对象，当用户点击退出当前应用，可设置session失效，用户需重新登录应用。

session的状态

当停服务器应用时，服务器会将session值持久化到存盘里，同时会实现实体的序列化serializable(即要求实体要能够序列化)，这样序列化成功，服务重启时会反序列化数据，用户信息和购物车中的内容就不会消失

服务停掉时，保存session数据的文件位置

服务器重启时，反序列化数据，此文件消失

购物车中的书还在

session存活时间的设置方式有四种

• 默认session存活时间为30分钟

• Session.invalidate();强制销毁

• setMaxInactiveInterval(int ）单位秒

• 在web.xml中配置

 总结

当停服务器应用时，会将应用中的session数据持久化到tomcat下项目的work文件中，同时要求保存在session中的实体能够序列化，当服务重启时会反序列化读取存在内容中的文件，此过程保存在session中的数据不会丢失，如淘宝中的购物车。