关于web.xml不同版本之间的区别

一、Servlet 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>Servlet 2.3 Web Application</display-name>
</web-app>

二、 Servlet 2.4

<web-app 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"
	      version="2.4">
<web-app>
  <display-name>Servlet 2.4 Web Application</display-name>
</web-app>

三、 Servlet 2.5 

<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>
  <display-name>Servlet 2.5 Web Application</display-name>
</web-app>

四、 Servlet 3.0

<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>
  <display-name>Servlet 3.0 Web Application</display-name>
</web-app>

参考于：http://www.mkyong.com/web-development/the-web-xml-deployment-descriptor-examples/

http://en.wikipedia.org/wiki/Java_Servlet