web.xml不同版本的头

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 >   
    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>