不同版本（2.0,2.3,2.4,2.5,3.0）web.xml

<!-- 2.3 -->
<!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>
  <display-name>Archetype Created Web Application</display-name>
</web-app>

<!-- 2.4 -->
<?xml version="1.0" encoding="UTF-8"?>  
<web-app 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> 

<!-- 2.5 -->
<?xml version="1.0" encoding="UTF-8"?>  
<web-app version="2.5" 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">  
   
</web-app> 

<!--3.0 -->
<?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> 

<?xml version="1.0" encoding="UTF-8"?>  
<!-- web-fragment -->
<web-fragment 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" version="3.0">  
   
</web-fragment>