服务器三大作用域:Request，ServletContext，Session

context: 作用域

作用域是在服务器运行阶段时在内存中开辟的空间, 用于存储当前的信息(数据)!

Request 请求作用域

• 生命周期 短暂

创建: 当客户端发出请求时服务器会为当前的请求开辟一块存储空间!

销毁: 当服务器返回响应时, 立即从服务器端销毁请求作用域!

• 常用方法

package com.javakc.web1.context;

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 java.io.IOException;

@WebServlet(
        name = "UserServlet",
        urlPatterns = "/user.do",
        loadOnStartup = 0
)
public class UserServlet extends HttpServlet {

    @Override
    protected void service(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        //从作用域中获取属性值
        Object username1 = request.getAttribute("username");
        System.out.println("第一次从请求作用于获取属性:" + username1);

        //向作用域中写入属性值
        request.setAttribute("username", "admin");

        //从作用域中获取属性值
        Object username2 = request.getAttribute("username");
        System.out.println("第二次从请求作用于获取属性:" + username2);

        //从作用域中移除属性
        request.removeAttribute("username");

        //从作用域中获取属性值
        Object username3 = request.getAttribute("username");
        System.out.println("第三次从请求作用于获取属性:" + username3);
    }
}
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运行结果:
第一次从请求作用于获取属性:null
第二次从请求作用于获取属性:admin
第三次从请求作用于获取属性:null
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• 使用场景
  涉及到数据量较为庞大
  临时性数据

ServletContext 应用作用域

• 生命周期

创建: 在服务器启动阶段时, 创建一个全新的应用作用域空间, 用于该应用中数据共享!

销毁: 在服务器关闭阶段时, 销毁应用作用域!

• 常用方法

package com.javakc.web1.context;

import javax.servlet.ServletContext;
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 java.io.IOException;

@WebServlet(
        name = "UserServlet",
        urlPatterns = "/user.do",
        loadOnStartup = 0
)
public class UserServlet2 extends HttpServlet {

    @Override
    protected void service(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        //获取应用作用域
        ServletContext context = request.getServletContext();

        //从应用作用域获取属性
        Object username1 = context.getAttribute("username");
        System.out.println("第一次从请求作用于获取属性:" + username1);

        //向应用作用域中写入属性
        context.setAttribute("username", "admin");

        //从应用作用域获取属性
        Object username2 = context.getAttribute("username");
        System.out.println("第二次从请求作用于获取属性:" + username2);

        //从应用作用域中移除属性
        context.removeAttribute("username");

        //从作用域中获取属性值
        Object username3 = context.getAttribute("username");
        System.out.println("第三次从请求作用于获取属性:" + username3);
    }
}
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运行结果:
第一次从请求作用于获取属性:null
第二次从请求作用于获取属性:admin
第三次从请求作用于获取属性:null
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• 应用场景
  适合共享的数据
  不适合临时性数据，隐私性数据

Session 会话作用域

• 生命周期
  ​创建: 在用户需要存储查询隐私性的内容时候, 动态创建会话作用域!

package com.javakc.web1.context;

import javax.servlet.ServletContext;
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(
        name = "UserServlet",
        urlPatterns = "/user.do",
        loadOnStartup = 0
)
//会话作用域创建
public class UserServlet3 extends HttpServlet {
    @Override
    protected void service(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        //1.假如客户端没有携带凭证(cookie), 服务器会创建全新的会话作用域
        //2.假如客户端携带了凭证(cookie), 服务器会获取之前创建过会话作用域
        HttpSession session1 = request.getSession();
        HttpSession session2 = request.getSession(true);

        //1.假如客户端没有携带凭证(cookie), 服务器会返回null
        //2.假如客户端携带了凭证(cookie), 服务器会获取之前创建过会话作用域
        HttpSession session3 = request.getSession(false);

        //判断会话作用域是否是新创建的
        HttpSession session4 = request.getSession();
        System.out.println(session4.isNew());
        System.out.println(session4.getId());
    }
}
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销毁

• 全局会话销毁方式 保证服务器不会出现内存溢出

web.xml

  <session-config>
        
        <session-timeout>30session-timeout>
    session-config>
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• 单独销毁 立即销毁

package com.javakc.web1.context;

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(
        name = "LogoutServlet",
        urlPatterns = "/logout.do",
        loadOnStartup = 0
)
//会话作用域销毁
public class UserServlet4 extends HttpServlet {
    @Override
    protected void service(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        HttpSession session = request.getSession(false);

        //单独销毁,立即销毁
        if (null != session) {
            //立即销毁
            session.invalidate();
        }
    }
}
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• 单独销毁 计时销毁

package com.javakc.web1.context;

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(
        name = "LogoutServlet",
        urlPatterns = "/logout.do",
        loadOnStartup = 0
)
//会话作用域销毁
public class UserServlet4 extends HttpServlet {
    @Override
    protected void service(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        HttpSession session = request.getSession(false);
       
        //单独销毁,计时销毁
        if (null != session) {
            //倒计时十分钟后销毁
            session.setMaxInactiveInterval(60 * 10);
        }
    }
}
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