<context-param>与<init-param>

<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:	omcat_bbswebappsBBSCS_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.