JSP九大内置对象详解全析（四）：application对象

电玩女神 2022-06-01 02:17 196阅读 0赞

## 1、application对象概述 ##

application对象作用范围是整个项目，该对象提供了项目环境属性的访问方法。比如在web.xml文件中，提供的初始化参数(< context-parama>标签），连接数据库的URL、用户名、密码。对应javax.servlet.ServletConfig.class对象。application对象常用的方法如下：

<table> 
 <thead> 
  <tr> 
   <th>方法</th> 
   <th>返回值</th> 
   <th>说明</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td>getAttribute（String name）</td> 
   <td>Object</td> 
   <td>通过关键字返回保存在application对象中的值</td> 
  </tr> 
  <tr> 
   <td>getAttributeNames（）</td> 
   <td>Enumeration</td> 
   <td>获得所有application对象使用的属性名</td> 
  </tr> 
  <tr> 
   <td>setAttribute（String key，Object obj）</td> 
   <td>void</td> 
   <td>指定一个名称，将一个对象保存在application</td> 
  </tr> 
  <tr> 
   <td>getMajorVersion（）</td> 
   <td>int</td> 
   <td>获得服务器支持的Servlet版本号</td> 
  </tr> 
  <tr> 
   <td>getServletInfo（）</td> 
   <td>String</td> 
   <td>获得JSP引擎相关信息</td> 
  </tr> 
  <tr> 
   <td>removeAttribute（String name）</td> 
   <td>void</td> 
   <td>删除application对象中指定名称的属性</td> 
  </tr> 
  <tr> 
   <td>getRealPath（）</td> 
   <td>String</td> 
   <td>返回虚拟路径的真实路径</td> 
  </tr> 
  <tr> 
   <td>getInitParameter（String name）</td> 
   <td>String</td> 
   <td>获得指定name的参数值</td> 
  </tr> 
 </tbody> 
</table>

## 2、实例示范： ##

web.xml文件中配置初始化参数：

<?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">
        <context-param>             
            <param-name>url</param-name>
            <param-value>jdbc:mysql://localhost:3306/db_database15</param-value>
        </context-param>
        <context-param>         
            <param-name>name</param-name>
            <param-value>root</param-value>
        </context-param>
        <context-param>         
            <param-name>password</param-name>
            <param-value>111</param-value>
        </context-param>
      <welcome-file-list>
        <welcome-file>index.jsp</welcome-file>
      </welcome-file-list>
    </web-app>

在index.jsp页面访问初始化参数：

<%@ page language="java" import="java.util.*" pageEncoding="gbk" %>
    <% String path = request.getContextPath(); String basePath = request.getScheme()+"://"+request.getServerName()+":"+request.getServerPort()+path+"/"; %>
    
    <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
    <html>
      <head>
        <base href="<%=basePath%>">
    
        <title>My JSP 'index.jsp' starting page</title>
        <meta http-equiv="pragma" content="no-cache">
        <meta http-equiv="cache-control" content="no-cache">
        <meta http-equiv="expires" content="0">    
        <meta http-equiv="keywords" content="keyword1,keyword2,keyword3">
        <meta http-equiv="description" content="This is my page">
        
      </head>
    
      <body>
    <% String url = application.getInitParameter("url"); //获取初始化参数，与web.xml文件中内容对应 String name = application.getInitParameter("name"); String password = application.getInitParameter("password"); out.println("URL: "+url+"<br>"); out.println("name: "+name+"<br>"); out.println("password: "+password+"<br>"); %>
    
      </body>
    </html>