web.xml v2.3
<?xml version="1.0" encoding="ISO-8859-1"?> ?<!DOCTYPE web-app PUBLIC "-//Sun Microsystems, Inc.//DTD Web Application 2.3//EN" "http://java.sun.com/dtd/web-app_2_3.dtd"> ????<web-app> ????</web-app>
web.xml v2.4
<?xml version="1.0" encoding="UTF-8"?> ?<web-app id="WebApp_9" version="2.4" xmlns="http://java.sun.com/xml/ns/j2ee" ??????xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" ?????xsi:schemaLocation="http://java.sun.com/xml/ns/j2ee http://java.sun.com/xml/ns/j2ee/web-app_2_4.xsd"> ????</web-app>
web.xml v2.5
<?xml version="1.0" encoding="UTF-8"?> ????<web-app xmlns="http://java.sun.com/xml/ns/javaee" ????xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" ????xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd" ????version="2.5"> ????</web-app>
web.xml v3.0
<?xml version="1.0" encoding="UTF-8"?> ????<web-app ?????????version="3.0" ?????????xmlns="http://java.sun.com/xml/ns/javaee" ?????????xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" ?????????xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd"> ????</web-app>
web.xml不同版本的头
原文地址：http://www.cnblogs.com/cowshed/p/7990130.html