servlet学习笔记_ServletContext

1 Web应用共享数据区ServletContext接口

ServletContext接口表示一个web应用的上下文。可以想象成一个web一个能用的共享数据区域,该区域保存整个web共享数据。

1.1 Servlet容器何时创建ServeltContext接口实现类

当一个web应用启动的时候,就会创建该ServletContext接口的实现类。我们可以在根据需求,读写ServletContext这个web共享区的数据,那么如何获得ServletContext这个实例呢?下面提供了两种方法

1.2 如何获得ServletContext接口实例

方法1:
可以通过ServletConfig接口的getServletContext()方法获得
方法2:
通过GenericServlet抽象类的getServletContext()方法获得,不过这个方法其实也是调用了ServletConfig的getServletContext()方法
在GenericServlet.java中

   public ServletConfig getServletConfig() {
	    return config;
    }

    public ServletContext getServletContext() {
	    return getServletConfig().getServletContext();
    }

1.3 一个计数器实例

package servletcontext;

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

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

public class CounterServlet extends HttpServlet
{
	public void doGet(HttpServletRequest request, HttpServletResponse response){
		try
		{
			response.setCharacterEncoding("gb2312");
			//获取ServletContext接口的实现类
			ServletContext sc = super.getServletContext();
			Integer count = (Integer) sc.getAttribute("count");
			if(null == count){
				count = new Integer(1);
			}else{
				count = new Integer(count.intValue()+1);
			}
			sc.setAttribute("count", count);
			PrintWriter out = response.getWriter();
			out.print("<body>");
			out.print("你登录的次数是:"+count.intValue());
			out.print("</body>");
			out.close();
			
		} catch (IOException e)
		{
			e.printStackTrace();
		}
	}
	
	public void doPost(HttpServletRequest request, HttpServletResponse response){
		doGet(request,response);
	}
}

 

<?xml version="1.0" encoding="UTF-8"?>
<web-app version="2.4" 
	xmlns="http://java.sun.com/xml/ns/j2ee" 
	xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
	xsi:schemaLocation="http://java.sun.com/xml/ns/j2ee 
	http://java.sun.com/xml/ns/j2ee/web-app_2_4.xsd">
  
  <servlet>
  	<servlet-name>CountServlet</servlet-name>
  	<servlet-class>servletcontext.CounterServlet</servlet-class>
  </servlet>
  
  <servlet-mapping>
  	<servlet-name>CountServlet</servlet-name>
  	<url-pattern>/count</url-pattern>
  </servlet-mapping>
  
</web-app>
 

 

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