学session,看这一往篇就够了

学session,看这一往篇就够了

Session的原理：

原理:

• 服务器为浏览器分配一种对象叫session
• 每个session有id，可以存key-value
• 为了每个浏览器访问自己的数据不受他人影响，必须让Cookie存jessionld

Session存数据

• 获取Session
  HttpSession getSession()获取session对象

• 存数据
  setAttribute(key,value)

• 读数据
  getAttribute(key)

存session代码：

package com.lbl.servlet;

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 java.io.IOException;

@WebServlet("/sessionSetServlet")
public class SessionSetServlet extends HttpServlet {
    
    
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    
    

    }

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    
    
        //访问服务器，服务器为每个浏览器分配一个session，有自己的id
        //1获取session对象
        HttpSession session = request.getSession();
        session.setAttribute("name","jack");
        session.setAttribute("age","18");
        //2获取session的编号jsession工d
        String jsessionId = session.getId();
        response.getWriter( ).println(jsessionId);
    }
}

效果图：

##取session代码：

package com.lbl.servlet;

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 java.io.IOException;

@WebServlet("/sessionGetServlet")
public class SessionGetServlet extends HttpServlet {
    
    
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    
    

    }

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    
    
        //1获取session对象
        HttpSession session = request.getSession();
        //2获取key对应的vaLue
        String name = (String) session.getAttribute("name");
        String age = (String) session.getAttribute("age");
        System.out.println(name);
        System.out.println(age);

    }
}

效果图:

使用Cookie保存sessionID：

• 每次关闭浏览器，再访问项目，服务器会分配一个新的sessionld如果关闭浏览器之后，再启动，想访问到之前的session的数据有一个要求，使用cookie，将sessionld保存到浏览器，浏览器在访问项目时会自动将sessionid菜到请求头中。

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• getSession()先根据jsessionld查找是否之前有session存在，如果有就重用该session,否则就创建一个新的。

• Cookie cookie = new Cookie("JSESSIONID",jsessionld);参数名必须是JSESSIONID

存的代码如下：

package com.lbl.servlet;

import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.*;
import java.io.IOException;

@WebServlet("/sessionStoreServlet")
public class SessionStoreServlet extends HttpServlet {
    
    
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    
    

    }

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    
    
        //打开浏览器，向服务器发一个jsessionID，让服务器根据jsessionID打开之前的session
        //取里面的数据，得使用技术保存jsessionID在浏览器 Cookie

        //1 获取session对象
        HttpSession session = request.getSession();
        //2 获取session的编号jsessionId
        String jsessionId = session.getId();

        //3 保存key-value
        session.setAttribute("name","jack");
        session.setAttribute("age","20");

        response.getWriter().println(jsessionId);

        Cookie cookie = new Cookie("JSESSIONID",jsessionId);

        //存活时间
        cookie.setMaxAge(10*60);

        //项目地址
        cookie.setPath(request.getContextPath());

        response.addCookie(cookie);

    }
}

取的代码：

package com.lbl.servlet;

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 java.io.IOException;

@WebServlet("/sessionGetServlet")
public class SessionGetServlet extends HttpServlet {
    
    
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    
    

    }

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    
    
        //1获取session对象
        HttpSession session = request.getSession();
        //2获取key对应的vaLue
        String name = (String) session.getAttribute("name");
        String age = (String) session.getAttribute("age");
        System.out.println(name);
        System.out.println(age);

    }
}

运行效果：

关闭浏览器后，还可以再取到！

清除Session

使用session的session.invalidate();方法可以清除session

完成代码如下：

package com.lbl.servlet;

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 java.io.IOException;

@WebServlet("/sessionClearServlet")
public class SessionClearServlet extends HttpServlet {
    
    
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    
    

    }

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    
    
        //获取session对象
        HttpSession session = request.getSession();
        //1:清除session
        session.invalidate();
    }
}

Session的钝化与活化

• tomcat中关闭服务会自动把session持久化也叫钝化

• 本质是将session数据存储在work目录下的一个session.ser

• 使用idea发布工程的朋友查看log控制台找work目录：

图解：