Analysis of the common methods and scopes of the three servlet containers

在JavaWeb中,ServletThe three scope objects arerequest,session,application,Its main purpose is to share data.

三大作用域的使用,其本质是根据作用域的范围,生命周期决定其使用的方式.：

How scoped objects share data：

        1.设置作用域中的共享数据.

        作用域对象名.setAttribute(String name,Object value);

        2.获取作用域中的共享数据.

       Object value=作用域对象名.getAttribute(String name);

       3.获取作用域中的共享数据.

        作用域对象.removeAttribute(String name);

注意：In which scope to set shared data,就只能从该作用域中取出数据.

三大作用域：

request:每一次请求都是一个新的request对象,如果在Web组件之间需要共享同一个请求中的数据,只能使用请求转发.
session：每一次会话都是一个新的session对象,如果需要在一次会话中的多个请求之间需要共享数据,只能使用session.
application：应用对象,Tomcat 启动到关闭,表示一个应用,在一个应用中有且只有一个applic-ation对象,作用于整个Web应用,可以实现多次会话之间的数据共享.
 

代码演示：

我们新建两个类：ScopeServlet  ResultServlet

package com.ape.view;

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

/**
 * Servlet implementation class ScopeServlet
 */
@WebServlet("/scope.do")
public class ScopeServlet extends HttpServlet {
	@Override
    protected void service(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        
        //1.request
        Integer numInRequest=(Integer)req.getAttribute("NUM_IN_REQUEST") ;
        if (numInRequest == null) {
            req.setAttribute("NUM_IN_REQUEST",1);
        }
        else {
            req.setAttribute("NUM_IN_REQUEST",numInRequest+1);
        }
        //2.Session
        Integer numInSession=(Integer)req.getSession().getAttribute("NUM_IN_SESSION") ;
        if (numInSession == null) {
            req.getSession().setAttribute("NUM_IN_SESSION",1);
        }
        else {
            req.getSession().setAttribute("NUM_IN_SESSION",numInSession+1);
        }
        //3.application

        Integer numInApplication=(Integer) req.getServletContext().getAttribute("NUM_IN_APPLICATION");
        if (numInApplication == null) {
            req.getServletContext().setAttribute("NUM_IN_APPLICATION",1);
        }
        else {
            req.getServletContext().setAttribute("NUM_IN_APPLICATION",numInApplication+1);
        }

        req.getRequestDispatcher("/result").forward(req,resp);

    }

}

package com.ape.view;

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;

/**
 * Servlet implementation class ResultServlet
 */
@WebServlet("/result.do")
public class ResultServlet extends HttpServlet {

    @Override
    protected void service(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        resp.setContentType("text/html;charset=utf-8");
        PrintWriter out=resp.getWriter();

        Integer numInRequest=(Integer) req.getAttribute("NUM_IN_REQUEST");
        Integer numInSession=(Integer) req.getSession().getAttribute("NUM_IN_SESSION");
        Integer numInApplication=(Integer) req.getServletContext().getAttribute("NUM_IN_APPLICATION");

        out.println("Request="+numInRequest);
        out.println("Session="+numInSession);
        out.println("Application="+numInApplication);
    }

}

然后我们运行一下

第一次访问 The three scopes are1

刷新了5次 会看到 request一次 session与Application都是6次

Next we change the browser 会发现 session 是1了 而 Appplicationadded again

 refreshed several times,发现session从0开始重新计数

 Back to our firstGoogle浏览器 Google里面的sessionAnd add1了

 因此 我们会发现 

Request是一次请求

Session是一个会话

Applicationis multiple sessions(Tomcat开启到关闭)

常用方法：

1. Request：

request.setAttribute(); //向requestSetting data in the field
request.getAttribute(); //向requestExtract data from the field
request.removeAttribute(); //向requestremove data from domain
request.getParameter(). //从requestGet the passed data in the field

2.Session：

session.setAttribute(); //向sessionIn the domain store Shared data
session.getAttribute(); //从sessionExtract shared data from the domain
session.removeAttribute();//从sessionRemove Shared data in the field

3. Application（ServletContext）：

1. 设置作用域中的共享数据
   作用域对象.setAttribute(String name,Object value);
2. 获取作用域中的共享数据
   Object value = 作用域对象.getAttribute(String name);
3. removes the specified shared data from the scope
   作用域对象.removeAttribute(String name);

完事.

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