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() { } }
后台启动输出结果: