<context-param>的作用:
web.xml的配置中<context-param>配置作用
1. 啟動(dòng)一個(gè)WEB項(xiàng)目的時(shí)候,容器(如:Tomcat)會(huì)去讀它的配置文件web.xml.讀兩個(gè)節(jié)點(diǎn): <listener></listener> 和 <context-param></context-param>

5.在監(jiān)聽中會(huì)有contextInitialized(ServletContextEvent args)初始化方法,在這個(gè)方法中獲得ServletContext = ServletContextEvent.getServletContext();
context-param的值 = ServletContext.getInitParameter("context-param的鍵");

6.得到這個(gè)context-param的值之后,你就可以做一些操作了.注意,這個(gè)時(shí)候你的WEB項(xiàng)目還沒有完全啟動(dòng)完成.這個(gè)動(dòng)作會(huì)比所有的Servlet都要早.
換句話說,這個(gè)時(shí)候,你對(duì)<context-param>中的鍵值做的操作,將在你的WEB項(xiàng)目完全啟動(dòng)之前被執(zhí)行.

7.舉例.你可能想在項(xiàng)目啟動(dòng)之前就打開數(shù)據(jù)庫.
那么這里就可以在<context-param>中設(shè)置數(shù)據(jù)庫的連接方式,在監(jiān)聽類中初始化數(shù)據(jù)庫的連接.

8.這個(gè)監(jiān)聽是自己寫的一個(gè)類,除了初始化方法,它還有銷毀方法.用于關(guān)閉應(yīng)用前釋放資源.比如說數(shù)據(jù)庫連接的關(guān)閉.

如:
<!-- 加載spring的配置文件 -->
<context-param>
    <param-name>contextConfigLocation</param-name>
    <param-value>/WEB-INF/applicationContext.xml,/WEB-INF/action-servlet.xml,/WEB-

INF/jason-servlet.xml</param-value>
</context-param>
<listener>
    <listener-class>org.springframework.web.context.ContextLoaderListener</listener-class>
</listener>

<context-param>
    <param-name>urlrewrite</param-name>
    <param-value>false</param-value>
</context-param>
<context-param>
    <param-name>cluster</param-name>
    <param-value>false</param-value>
</context-param>
<context-param>
    <param-name>servletmapping</param-name>
    <param-value>*.bbscs</param-value>
</context-param>
<context-param>
    <param-name>poststoragemode</param-name>
    <param-value>1</param-value>
</context-param>
<listener>
    <listener-class>com.laoer.bbscs.web.servlet.SysListener</listener-class>
</listener>

public class SysListener extends HttpServlet implements ServletContextListener {

private static final Log logger = LogFactory.getLog(SysListener.class);

public void contextDestroyed(ServletContextEvent sce) {

   //用于在容器關(guān)閉時(shí),操作
}

public void contextInitialized(ServletContextEvent sce) {
   String rootpath = sce.getServletContext().getRealPath("/");
   System.out.println("-------------rootPath:"+rootpath);

   if (rootpath != null) {
    rootpath = rootpath.replaceAll("\\\\", "/");
   } else {
    rootpath = "/";
   }
   if (!rootpath.endsWith("/")) {
    rootpath = rootpath + "/";
   }
   Constant.ROOTPATH = rootpath;
   logger.info("Application Run Path:" + rootpath);
   String urlrewrtie = sce.getServletContext().getInitParameter("urlrewrite");
   boolean burlrewrtie = false;
   if (urlrewrtie != null) {
    burlrewrtie = Boolean.parseBoolean(urlrewrtie);
   }
   Constant.USE_URL_REWRITE = burlrewrtie;
   logger.info("Use Urlrewrite:" + burlrewrtie);
   其它略之....

   }

}
   /*最終輸出
   -------------rootPath:D:\tomcat_bbs\webapps\BBSCS_8_0_3\
   2009-06-09 21:51:46,526 [com.laoer.bbscs.web.servlet.SysListener]-[INFO]

Application Run Path:D:/tomcat_bbs/webapps/BBSCS_8_0_3/
   2009-06-09 21:51:46,526 [com.laoer.bbscs.web.servlet.SysListener]-[INFO]

Use Urlrewrite:true
   2009-06-09 21:51:46,526 [com.laoer.bbscs.web.servlet.SysListener]-[INFO]

Use Cluster:false
   2009-06-09 21:51:46,526 [com.laoer.bbscs.web.servlet.SysListener]-[INFO]

SERVLET MAPPING:*.bbscs
   2009-06-09 21:51:46,573 [com.laoer.bbscs.web.servlet.SysListener]-[INFO]

Post Storage Mode:1
   */

context-param和init-param區(qū)別
web.xml里面可以定義兩種參數(shù)：
(1)application范圍內(nèi)的參數(shù)，存放在servletcontext中，在web.xml中配置如下：
<context-param>
           <param-name>context/param</param-name>
           <param-value>avalible during application</param-value>
</context-param>

(2)servlet范圍內(nèi)的參數(shù)，只能在servlet的init()方法中取得，在web.xml中配置如下：
<servlet>
    <servlet-name>MainServlet</servlet-name>
    <servlet-class>com.wes.controller.MainServlet</servlet-class>
    <init-param>
       <param-name>param1</param-name>
       <param-value>avalible in servlet init()</param-value>
    </init-param>
    <load-on-startup>0</load-on-startup>
</servlet>

在servlet中可以通過代碼分別取用：
package com.wes.controller;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;

public class MainServlet extends HttpServlet ...{

    public MainServlet() ...{
        super();
     }
    public void init() throws ServletException ...{
         System.out.println("下面的兩個(gè)參數(shù)param1是在servlet中存放的");
         System.out.println(this.getInitParameter("param1"));
         System.out.println("下面的參數(shù)是存放在servletcontext中的");
        System.out.println(getServletContext().getInitParameter("context/param"));
      }
}

第一種參數(shù)在servlet里面可以通過getServletContext().getInitParameter("context/param")得到
第二種參數(shù)只能在servlet的init()方法中通過this.getInitParameter("param1")取得.