不同servlet版本的web.xml的头部信息

<?xml version="1.0" encoding="UTF-8"?>
<web-app 
    version="2.5"
    xmlns="http://java.sun.com/xml/ns/javaee" 
    xmlns:xml="http://www.w3.org/XML/1998/namespace"
    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">


</web-app>

<?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>

<?xml version="1.0" encoding="UTF-8"?>
<web-fragment 
    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-fragment_3_0.xsd" >  
   
</web-fragment>