web.xml 的加载过程
当我们去启动一个 WEB 项目的时候, 容器(包括 JBoss, Tomcat 等)首先会去读项目的 web.xml 配置文件里面的信息,
当这一步骤没有出错并且完成之后, 项目才能正常的被启动起来。
1> 首先是, 容器会先读 <context-param></context-param> 节点, 并创建一个 ServletContext 实例, 以节点的 name 作为键, value 作为值,
存储到上下文环境中。
2> 接着, 容器会去读 <listener></listener> 节点, 根据配置的 class 类路径来创建监听。
3> 接着, 容器去读 <filter></filter> 节点, 根据指定的类路径来实例化过滤器。
以上都是在 WEB 项目还没有完全启动起来的时候就已经完成了的工作。如果系统中有用到 Servlet, 则 Servlet 是在第一次发起请求的时候被实例化的,
且一般不会被容器销毁, 它可以服务于多个用户的请求。所以, Servlet 的初始化都要比上面提到的那几个要迟。
总的来说, web.xml 的加载顺序是: context-param --> listener --> filter --> servlet
其中, 如果 web.xml 中出现了相同的节点, 则是按照在配置文件中出现的先后顺序来加载的。
下面引入一个小列子来说明:
<?xml version="1.0" encoding="UTF-8"?> <listener> <listener-class>net.yeah.fancydeepin.listener.AppStartListener</listener-class> </listener> <!-- 为了更好的说明, 特意将 context-param 放在 listener 后面 --> <context-param> <param-name>technology</param-name> <param-value>java,javascript,ajax,css,html</param-value> </context-param> <filter> <filter-name>ReDespatcherFilter</filter-name> <filter-class>net.yeah.fancydeepin.filter.ReDespatcherFilter</filter-class> <init-param> <param-name>it</param-name> <param-value>android, python, c</param-value> </init-param> </filter> <filter-mapping> <filter-name>ReDespatcherFilter</filter-name> <url-pattern>/*</url-pattern> </filter-mapping> <filter> <filter-name>ReDespatcherFilter2</filter-name> <filter-class>net.yeah.fancydeepin.filter.ReDespatcherFilter2</filter-class> <init-param> <param-name>mail</param-name> <param-value>fancydeepin@yeah.net</param-value> </init-param> </filter></web-app>
package net.yeah.fancydeepin.listener;import java.util.Arrays;import java.util.List;import javax.servlet.ServletContext;import javax.servlet.ServletContextEvent;import javax.servlet.ServletContextListener;public class AppStartListener implements ServletContextListener{ public void contextInitialized(ServletContextEvent contextEvent) { System.out.println("********************************************"); ServletContext context = contextEvent.getServletContext(); List<String> params = Arrays.asList(context.getInitParameter("technology").split(",")); for(String param : params){ System.out.print(param + "\t"); } System.out.println("\n********************************************"); } public void contextDestroyed(ServletContextEvent contextEvent) { }}package net.yeah.fancydeepin.filter;import java.io.IOException;import javax.servlet.Filter;import javax.servlet.FilterChain;import javax.servlet.FilterConfig;import javax.servlet.ServletException;import javax.servlet.ServletRequest;import javax.servlet.ServletResponse;public class ReDespatcherFilter implements Filter { public void init(FilterConfig filterConfig) throws ServletException { System.out.println("============================================"); System.out.println(filterConfig.getInitParameter("it")); System.out.println("============================================"); } public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException,ServletException{ chain.doFilter(request, response); } public void destroy() { }}package net.yeah.fancydeepin.filter;import java.io.IOException;import javax.servlet.Filter;import javax.servlet.FilterChain;import javax.servlet.FilterConfig;import javax.servlet.ServletException;import javax.servlet.ServletRequest;import javax.servlet.ServletResponse;public class ReDespatcherFilter2 implements Filter { public void init(FilterConfig filterConfig) throws ServletException { System.out.println("++++++++++++++++++++++++++++++++++++++++++++"); System.out.println(filterConfig.getInitParameter("mail")); System.out.println("++++++++++++++++++++++++++++++++++++++++++++"); } public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException,ServletException{ chain.doFilter(request, response); } public void destroy() { }}