ServletContext对象

自我理解：

　　ServletContext类似字节码文件对象，在web创建的时候就自动生成了，并且是唯一的，跟随着项目和服务器共存亡了。通过这个对象，我们可以向里面存数据（键值对）,也可以通过别的Servlet来获取这个数据；也可以根据相对（服务器）路径继来获取绝对路径。根据这个信息我们可以在以后创建文件的过程中，将静态资源的文件尽量创建在web-content文件夹下，而项目文件、配置文件创建在src下。不要直接创建在web文件夹下（不会在服务器上生成）。

一、ServletContext对象：

　　ServletContext代表是一个web应用的环境（上下文）对象，ServletContext对象内部封装是该web应用的信息，ServletContext对象一个web应用只有一个。

二、ServletContext对象的生命周期：

创建：该web应用被加载且服务器开启时创建；

销毁：web应用被卸载（移除该项目应用）或者服务器关闭。

三、获得ServletContext对象：

1、Servlet的init方法中获得ServletConfig 

ServletContext servletContext = config.getServletContext ()

2、

ServletContext  servletContext = this.getservletContext ()

四、ServletContext的作用：

1、获得web应用全局的初始化参数；

2、获得web应用中任何资源的绝对路径：

（通过服务器的相对路径，得到一个在服务器上的绝对路径）

String path = context.getRealPath(相对于该web应用的相对地址);

package com.oracle.web;

import java.io.IOException;

import javax.servlet.ServletConfig;
import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

public class Servlet extends HttpServlet {
	
	public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		//获取ServletContext对象
		ServletContext context=getServletConfig().getServletContext();
		/*//1.获取ServletConfig对象
		ServletConfig config=getServletConfig();
		//2.获取ServletContext对象
		ServletContext context=config.getServletContext();*/
		//获取相对路径的绝对路径
		String patha=context.getRealPath("a.txt");
		System.out.println(patha);
		String pathb=context.getRealPath("WEB-INF/b.txt");
		System.out.println(pathb);
		String pathc=context.getRealPath("WEB-INF/classes/c.txt");
		System.out.println(pathc);
		response.getWriter().write("hello dandan...");
	}

	public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		doGet(request, response);
	}
}

　　

3、ServletContext是一个域对象（存储数据的区域）：

ServletContext域对象的作用范围：整个web

（所有的web资源都可以随意向 ServletContext 域中存取数据，数据可以分享）。

域对象的通用方法：

package com.oracle.web;

import java.io.IOException;

import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

public class Servlet03 extends HttpServlet {

	public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		//hqServletContextdx
		ServletContext context=getServletContext();
		//取值
		String value=(String)context.getAttribute("name");
		response.getWriter().write(value);
	}

	public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		doGet(request, response);
	}
}package com.oracle.web;

import java.io.IOException;

import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

public class Servlet02 extends HttpServlet {

	public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		//hqServletContextdx
		ServletContext context=getServletContext();
		//存值
		context.setAttribute("name","zhangsan");
	}

	public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		doGet(request, response);
	}
}

　登录统计：

package com.oracle.web;

import java.io.IOException;

import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

import com.oracle.service.UserService;

public class LoginServlet extends HttpServlet {
	private UserService userService=new UserService();
	@Override
	public void init() throws ServletException {
		int num=0;
		//获取ServletContextdx
		ServletContext context=getServletContext();
		context.setAttribute("num", num);
	}
	public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		//获取用户名和密码
		String name=request.getParameter("username");
		System.out.println(name);
		String pwd=request.getParameter("pwd");
		//调用service层方法
		int count=userService.login(name, pwd);
		if (count>0) {
			//获取ServletContextdx
			ServletContext context=getServletContext();
			int num=(int)context.getAttribute("num");
			num++;
			context.setAttribute("num", num);
			response.getWriter().write("you are the "+num+"success");
		}else {
			response.getWriter().write("fail");
		}
	}

	public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		doGet(request, response);
	}
}