不同版本（2.3/2.4/2.5/3.0/3.1）web.xml头信息

Web App 3.1

 <web-app xmlns="http://xmlns.jcp.org/xml/ns/javaee"
          xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
          xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee
          http://xmlns.jcp.org/xml/ns/javaee/web-app_3_1.xsd"
          version="3.1">

   ...
 </web-app>   

Web App 3.0

 <?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_3_0.xsd"
 　　　　　　version="3.0">

   ...
 </web-app>

Web App 2.5

 <?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 App 2.4

 <?xml version="1.0" encoding="UTF-8"?>
 <web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xmlns="http://java.sun.com/xml/ns/j2ee"
         xmlns:web="http://java.sun.com/xml/ns/j2ee/web-app_2_4.xsd"
         xsi:schemaLocation="http://java.sun.com/xml/ns/j2ee
         http://java.sun.com/xml/ns/j2ee/web-app_2_4.xsd"
         version="2.4">

   ...
 </web-app>

Web App  2.3

 <?xml version="1.0" encoding="UTF-8"?>
 <!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>