ServletContext当全局变量的使用

ServletContext对象

1，作用：JavaWeb应用的一个全局变量，一个应用只有一个ServletContext对象，在应用启动时，容器就会创建该对象

2，获得ServletContext对象的引用

3，应用ServletContext的特点，可以将该对象看做是整个web应用的总管家

4，应用

　　实现多个Servlet之间的数据共享

　　配置web应用的全局参数

 Demo1向对象中存入参数

package com.itheima;

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 ServletContextDemo1 extends HttpServlet {

	@Override
	protected void doGet(HttpServletRequest req, HttpServletResponse resp)
			throws ServletException, IOException {
		//获取ServletConfig对象
		ServletConfig config=getServletConfig();
		//根据ServletConfig对象创建ServletContext对象
		ServletContext context=config.getServletContext();
		//向Servletcontext对象中添加属性和属性值
		context.setAttribute("abc", "你好");
	}

	@Override
	protected void doPost(HttpServletRequest req, HttpServletResponse resp)
			throws ServletException, IOException {
		doGet(req,resp);
	}
	
}

 Demo2取数据

package com.itheima;

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 ServletContextDemo2 extends HttpServlet {

	@Override
	protected void doGet(HttpServletRequest req, HttpServletResponse resp)
			throws ServletException, IOException {
		//创建ServletContext对象
		ServletContext context=getServletContext();
		//得到ServletContext对象中的属性名，用Object接收
		Object o=context.getAttribute("abc");
		//打印
		System.out.println(o);
	}

	@Override
	protected void doPost(HttpServletRequest req, HttpServletResponse resp)
			throws ServletException, IOException {
		
	}
	
}

 如果先取数据（执行Demo2），则取出的是null，如果先存数据（Demo1），则取出存入的内容。

定义全局变量

web.xml中的代码
 <!--  定义全局参数-->
 <context-param>
 	<param-name>url</param-name>
 	<param-value>jdbc:mysql:///test</param-value>
 </context-param>
 <context-param>
 	<param-name>username</param-name>
 	<param-value>root</param-value>
 </context-param>
 <context-param>
 	<param-name>password</param-name>
 	<param-value>123</param-value>
 </context-param>

//java中的获取
		Enumeration name=context.getInitParameterNames();
		while(name.hasMoreElements()){
			System.out.println("Demo2:"+context.getInitParameter((String)name.nextElement()));
		}