servlet版本与tomcat版本对应关系，各版本web.xml头信息写法

The mapping between the specifications and the respective Apache Tomcat versions is:

Servlet Spec	JSP Spec	EL Spec	WebSocket Spec	JASPIC Spec	Apache Tomcat version	Actual release revision	Supported Java Versions
4.0	TBD (2.4?)	TBD (3.1?)	TBD (1.2?)	1.1	9.0.x	9.0.0.M9 (alpha)	8 and later
3.1	2.3	3.0	1.1	1.1	8.5.x	8.5.4	7 and later
3.1	2.3	3.0	1.1	N/A	8.0.x (superseded)	8.0.35 (superseded)	7 and later
3.0	2.2	2.2	1.1	N/A	7.0.x	7.0.70	6 and later (7 and later for WebSocket)
2.5	2.1	2.1	N/A	N/A	6.0.x	6.0.45	5 and later
2.4	2.0	N/A	N/A	N/A	5.5.x (archived)	5.5.36 (archived)	1.4 and later
2.3	1.2	N/A	N/A	N/A	4.1.x (archived)	4.1.40 (archived)	1.3 and later
2.2	1.1	N/A	N/A	N/A	3.3.x (archived)	3.3.2 (archived)	1.1 and later

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 id="WebApp_9" 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>

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>