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:
<!-- 加载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>
又如: --->自定义context-param,且自定义listener来获取这些信息 <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) {
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....
}
/*最终输出
-------------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]
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：<context-param> <param-name>context/param</param-name> <param-value>avalible during application</param-value></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：
<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中可以通过代码分别取用：
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("下面的两个参数param1是在servlet中存放的");
System.out.println(this.getInitParameter("param1")); System.out.println("下面的参数是存放在servletcontext中的");
System.out.println(this.getServletContext().getInitParameter("context/param")); }
}
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.
request.getSession().getServletContext();
Servlet：
this.getServletContext()this.getServletConfig().getServletContext()request.getSession().getServletContext()
JSP：
request.getSession().getServletContext()
Filter：
config.getServletContext()