ServletContext与三大作用域对象

1.ServletContext(Servlet上下文对象),是Web应用全局对象。一个Web应用只会创建一个ServletContext对象。ServletContext随着Web的启动而创建，随着Web的关闭而销毁

2.代码

ServletContextInit.java

package com.imooc.ServletContextFile;

import java.io.IOException;

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;

/**
 * 用于初始化网址Servlet基本信息
 */
@WebServlet("/ServletContextInit")
public class ServletContextInit extends HttpServlet {
	private static final long serialVersionUID = 1L;
       
    /**
     * @see HttpServlet#HttpServlet()
     */
    public ServletContextInit() {
        super();
        // TODO Auto-generated constructor stub
    }

	/**
	 * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
	 */
	protected void doGet(HttpServletRequest request, HttpServletResponse response) 
			throws ServletException, IOException {
		
		//获取全局对象
		ServletContext context = request.getServletContext();
		
		//向应用程序设置全局属性
		context.setAttribute("copyright", " 2003-2018 TMALL.COM 版权所有");
		context.setAttribute("title", "天猫");
		response.getWriter().println("<h1>init success</h1>");
	}

}

ServletContextDefaulServlet.java

package com.imooc.ServletContextFile;

import java.io.IOException;

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;

/**
 * Servlet implementation class ServletContextDefaulServlet
 */
@WebServlet("/ServletContextDefaulServlet")
public class ServletContextDefaulServlet extends HttpServlet {
	private static final long serialVersionUID = 1L;
       
    /**
     * @see HttpServlet#HttpServlet()
     */
    public ServletContextDefaulServlet() {
        super();
    }

	/**
	 * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
	 */
	protected void doGet(HttpServletRequest request, HttpServletResponse response) 
			throws ServletException, IOException {
		
		//获取应用程序全局对象
		ServletContext context = request.getServletContext();
		String copyright = (String) context.getAttribute("copyright");
		String title = (String) context.getAttribute("title");
		
		//防止乱码
		response.setContentType("text/html;charset=utf-8");
		
		response.getWriter().println("<h1>"+copyright+"</h1>");
		response.getWriter().println("<h1>"+title+"</h1>");
	}

}

比起Session，即使关闭当前浏览器窗口，打开新的窗口重新输入地址，也能获取到信息，这就是ServletContext全局变量，只有当Web关闭后才失效。

 3.三大作用域(从小到大)

HttpServletRequest——请求对象：请求和响应都是属于HttpServletRequest对象，当浏览器发送请求给服务器，服务器把信息响应给浏览器，这时请求对象就会被销毁。所以说请求对象是瞬时对象，用完就扔

HttpSession——用户会话对象：用于保存与浏览器窗口对应的数据。Session是第一次用户发来请求被创建的，默认情况下，30分钟没有被访问就会被自动销毁

ServleContext——Web应用全局变量：Web程序启动时就被创建了，当Web程序重启或销毁就销毁。