<context-param>的作用:
web.xml的配置中<context-param>配置作用
1. 启动一个WEB项目的时候,容器(如:Tomcat)会去读它的配置文件web.xml.读两个节点: <listener></listener> 和 <context-param></context-param>
2.紧接着,容器创建一个ServletContext(上下文),这个WEB项目所有部分都将共享这个上下文.
3.容器将<context-param></context-param>转化为键值对,并交给ServletContext.
4.容器创建<listener></listener>中的类实例,即创建监听.
5.在监听中会有contextInitialized(ServletContextEvent args)初始化方法,在这个方法中获得ServletContext = ServletContextEvent.getServletContext();
context-param的值 = ServletContext.getInitParameter("context-param的键");
6.得到这个context-param的值之后,你就可以做一些操作了.注意,这个时候你的WEB项目还没有完全启动完成.这个动作会比所有的Servlet都要早.
换句话说,这个时候,你对<context-param>中的键值做的操作,将在你的WEB项目完全启动之前被执行.
7.举例.你可能想在项目启动之前就打开数据库.
那么这里就可以在<context-param>中设置数据库的连接方式,在监听类中初始化数据库的连接.
8.这个监听是自己写的一个类,除了初始化方法,它还有销毁方法.用于关闭应用前释放资源.比如说数据库连接的关闭.
如:
1 <!-- 加载spring的配置文件 --> 2 <context-param> 3 <param-name>contextConfigLocation</param-name> 4 <param-value>/WEB-INF/applicationContext.xml,/WEB-INF/action-servlet.xml,/WEB- 5 INF/jason-servlet.xml</param-value> 6 </context-param> 7 <listener> 8 <listener-class>org.springframework.web.context.ContextLoaderListener</listener-class> 9 </listener>
又如: --->自定义context-param,且自定义listener来获取这些信息
1 <context-param> 2 <param-name>urlrewrite</param-name> 3 <param-value>false</param-value> 4 </context-param> 5 <context-param> 6 <param-name>cluster</param-name> 7 <param-value>false</param-value> 8 </context-param> 9 <context-param> 10 <param-name>servletmapping</param-name> 11 <param-value>*.bbscs</param-value> 12 </context-param> 13 <context-param> 14 <param-name>poststoragemode</param-name> 15 <param-value>1</param-value> 16 </context-param> 17 <listener> 18 <listener-class>com.laoer.bbscs.web.servlet.SysListener</listener-class> 19 </listener>
1 public class SysListener extends HttpServlet implements ServletContextListener 2 { 3 private static final Log logger = LogFactory.getLog(SysListener.class); 4 5 public void contextDestroyed(ServletContextEvent sce) 6 { 7 // 用于在容器关闭时,操作 8 } 9 // 用于在容器开启时,操作 10 public void contextInitialized(ServletContextEvent sce) 11 { 12 String rootpath = sce.getServletContext().getRealPath("/"); 13 System.out.println("-------------rootPath:"+rootpath); 14 if (rootpath != null) 15 { 16 rootpath = rootpath.replaceAll("\\\\", "/"); 17 } else 18 { 19 rootpath = "/"; 20 } 21 if (!rootpath.endsWith("/")) 22 { 23 rootpath = rootpath + "/"; 24 } 25 Constant.ROOTPATH = rootpath; 26 logger.info("Application Run Path:" + rootpath); 27 String urlrewrtie = sce.getServletContext().getInitParameter("urlrewrite"); 28 boolean burlrewrtie = false; 29 if (urlrewrtie != null) 30 { 31 burlrewrtie = Boolean.parseBoolean(urlrewrtie); 32 } 33 Constant.USE_URL_REWRITE = burlrewrtie; 34 logger.info("Use Urlrewrite:" + burlrewrtie); 35 其它略之.... 36 } 37 } 38 /* 39 * 最终输出 -------------rootPath:D:\tomcat_bbs\webapps\BBSCS_8_0_3\ 2009-06-09 40 * 21:51:46,526 [com.laoer.bbscs.web.servlet.SysListener]-[INFO] Application Run 41 * Path:D:/tomcat_bbs/webapps/BBSCS_8_0_3/ 2009-06-09 21:51:46,526 42 * [com.laoer.bbscs.web.servlet.SysListener]-[INFO] Use Urlrewrite:true 43 * 2009-06-09 21:51:46,526 [com.laoer.bbscs.web.servlet.SysListener]-[INFO] Use 44 * Cluster:false 2009-06-09 21:51:46,526 45 * [com.laoer.bbscs.web.servlet.SysListener]-[INFO] SERVLET MAPPING:*.bbscs 46 * 2009-06-09 21:51:46,573 [com.laoer.bbscs.web.servlet.SysListener]-[INFO] Post 47 * Storage Mode:1 48 */
context-param和init-param区别
web.xml里面可以定义两种参数:
(1)application范围内的参数,存放在servletcontext中,在web.xml中配置如下:
1 <context-param> 2 <param-name>context/param</param-name> 3 <param-value>avalible during application</param-value> 4 </context-param>
(2)servlet范围内的参数,只能在servlet的init()方法中取得,在web.xml中配置如下:
1 <servlet> 2 <servlet-name>MainServlet</servlet-name> 3 <servlet-class>com.wes.controller.MainServlet</servlet-class> 4 <init-param> 5 <param-name>param1</param-name> 6 <param-value>avalible in servlet init()</param-value> 7 </init-param> 8 <load-on-startup>0</load-on-startup> 9 </servlet>
在servlet中可以通过代码分别取用:
1 package com.wes.controller; 2 import javax.servlet.ServletException; 3 import javax.servlet.http.HttpServlet; 4 public class MainServlet extends HttpServlet ... 5 { 6 public MainServlet() ... 7 { 8 super(); 9 } 10 public void init() throws ServletException ... 11 { 12 System.out.println("下面的两个参数param1是在servlet中存放的"); 13 System.out.println(this.getInitParameter("param1")); 14 System.out.println("下面的参数是存放在servletcontext中的"); 15 System.out.println(getServletContext().getInitParameter("context/param")); 16 } 17 }
第一种参数在servlet里面可以通过getServletContext().getInitParameter("context/param")得到
第二种参数只能在servlet的init()方法中通过this.getInitParameter("param1")取得.