【从零开始学Servlet笔记】ServletContext

每个web工程都只有一个ServletContext对象。 说白了也就是不管在哪个servlet里面，获取到的这个类的对象都是同一个。

如何得到对象

//1. 获取对象
	ServletContext context = getServletContext();

有什么作用

1. 获取全局配置参数

<context-param>
		<param-name>address
		</param-name>
		<param-value>深圳宝鸡
		</param-value>
	</context-param>

ServletContext sc = getServletContext();
		String s = sc.getInitParameter("address");
		System.out.println("ServletContext01 的 address:"+s);

2. 获取web工程中的资源

  1. 获取资源在tomcat里面的绝对路径

     先得到路径，然后自己new InpuStream

 	context.getRealPath("") //这里得到的是项目在tomcat里面的根目录。

 	D:	omcatapache-tomcat-7.0.52apache-tomcat-7.0.52wtpwebappsDemo03
 
  	String path = context.getRealPath("file/config.properties");

 	D:	omcatapache-tomcat-7.0.52apache-tomcat-7.0.52wtpwebappsDemo03fileconfig.properties

第一步：新建文件夹

name = zhangsan

第二步：编写servlet类

ServletContext sc = getServletContext();
		String path = sc.getRealPath("File/config.properties");
		System.out.println(path);
		
		Properties properties = new Properties();
		InputStream is = new FileInputStream(path);
		properties.load(is);
		
		String name = properties.getProperty("name");
		
		System.out.println("name= "+ name);

第三步：编写web.xml
第四步：输出

  1. getResourceAsStream 获取资源流对象

直接给相对的路径，然后获取流对象。

try {
			ServletContext sc = getServletContext();
						
			Properties properties = new Properties();
			//获取web工程下的资源，转化成流对象。前面 隐藏当前工程的根目录。
			InputStream is = sc.getResourceAsStream("File/config.properties");
			properties.load(is);
			String name = properties.getProperty("name");			
			System.out.println("name22= "+ name);
			is.close();
		}catch(Exception e){
			e.printStackTrace();
			
		}
  1. 通过classloader去获取web工程下的资源
Properties properties = new Properties();
			//类的加载器的路径和文件的相对路径不一样
			InputStream is = this.getClass().getClassLoader().getResourceAsStream("../../File/config.properties")
			properties.load(is);
			String name = properties.getProperty("name");			
			System.out.println("name3= "+ name);
			is.close();
		}catch(Exception e){
			e.printStackTrace();

3. 存取数据，servlet间共享数据(域对象)

ServletContext获取登录成功总数

步骤分析

  1. 定义一个登陆的html页面， 定义一个form表单
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>Insert title here</title>
</head>
<body>
<h2>请输入一下内容，完成登录</h2>

<form action="LoginServlet" method="get">
	账号：<input type="text" name="username"/><br>
	密码：<input type="text" name="password"/><br>
	<input type="submit" value="登录"/><br>
</form>

</body>
</html>
  1. 定义一个Servlet，名为LoginServlet，并接收客户端请求
String username = request.getParameter("username");
		String password = request.getParameter("password");
		
		System.out.println("username:"+username+","+"password:"+password);
  1. 针对成功或者失败，进行判断，然后跳转到不一样的网页
	if("admin".equals(username)&&"123".equals(password)) {
			//成功就跳转login_success.html
			
			//设置状态码？重新定位 状态码
			response.setStatus(302);
			//定位跳转到哪一个界面
			response.setHeader("Location", "login_success.html");
		}
		else {
			String s = "login failed!";
			pw.write(s);
		}
  1. ServletContext存取值分析

示例

工程文件

CountServlet类

package com.demo01;

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;

public class CountServlet extends HttpServlet {
	private static final long serialVersionUID = 1L;
   
	protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		
		//1.取值
		int count = (int) getServletContext().getAttribute("count");
		
		//2.输出到客户端
		response.getWriter().write("当前登录总次数："+count+"次");
		
		
	}

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

}

LoginServlet类

package com.demo01;

import java.io.IOException;
import java.io.PrintWriter;

import javax.script.ScriptContext;
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 LoginServlet extends HttpServlet {
	private static final long serialVersionUID = 1L;
	
	/**
	 * request :包含请求的信息
	   response :响应数据给浏览器，就靠这个对象

	 */
  
  
	protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		
		//1.获取数据
		String username = request.getParameter("username");
		String password = request.getParameter("password");
		
		System.out.println("username:"+username+","+"password:"+password);
		
		//2.数据校验
		
		//向客户端输送请求
		PrintWriter pw = response.getWriter();
				
		if("admin".equals(username)&&"123".equals(password)) {
			//1.成功登录次数类加
			
			
			//获取以前的值，再此基础上+1
			Object obj = getServletContext().getAttribute("count");
			
			//默认是0
			int totalCount = 0;
			if(obj != null) {
				totalCount = (int)obj;
			}
			
			System.out.println("已知登录成功的次数："+totalCount);
			
			//给这个count赋值
			getServletContext().setAttribute("count", totalCount+1);
			
			//2.成功就跳转login_success.html
			//设置状态码？重新定位 状态码
			response.setStatus(302);
			//定位跳转到哪一个界面
			response.setHeader("Location", "login_success.html");
		}
		else {
			String s = "login failed!";
			pw.write(s);
		}
	
	}


	protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		// TODO Auto-generated method stub
		doGet(request, response);
	}

}

web.xml

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns="http://xmlns.jcp.org/xml/ns/javaee"
	xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
	xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee http://xmlns.jcp.org/xml/ns/javaee/web-app_4_0.xsd"
	version="4.0">


	<servlet>
		<servlet-name>LoginServlet</servlet-name>
		<servlet-class>com.demo01.LoginServlet</servlet-class>
	</servlet>

	<servlet-mapping>
		<servlet-name>LoginServlet</servlet-name>
		<url-pattern>/LoginServlet</url-pattern>
	</servlet-mapping>
	
	<servlet>
		<servlet-name>CountServlet</servlet-name>
		<servlet-class>com.demo01.CountServlet</servlet-class>
	</servlet>

	<servlet-mapping>
		<servlet-name>CountServlet</servlet-name>
		<url-pattern>/CountServlet</url-pattern>
	</servlet-mapping>


</web-app>

login.html

<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>Insert title here</title>
</head>
<body>
<h2>请输入一下内容，完成登录</h2>

<form action="LoginServlet" method="get">
	账号：<input type="text" name="username"/><br>
	密码：<input type="text" name="password"/><br>
	<input type="submit" value="登录"/><br>
</form>

</body>
</html>

login_success.html

<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>Insert title here</title>
</head>
<body>
<h2>登录成功了</h2>
<a href="CountServlet">获取网站登录成功的次数</a>
</body>
</html>

ServletContext 何时创建， 何时销毁?

服务器启动的时候，会为托管的每一个web应用程序，创建一个ServletContext对象

从服务器移除托管，或者是关闭服务器。

  • ServletContext 的作用范围

只要在这个项目里面，都可以取。 只要同一个项目。 A项目存， 在B项目取，是取不到的？ ServletContext对象不同。

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原文地址：https://www.cnblogs.com/zllk/p/12821773.html