web.xml文件头声明各个版本参考

1. Servlet 3.1
   Java EE 7 XML schema, namespace is

<web-app xmlns="http://xmlns.jcp.org/xml/ns/javaee"
         
         xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee
         http://xmlns.jcp.org/xml/ns/javaee/web-app_3_1.xsd"
         version="3.1">
</web-app>

2. Servlet 3.0
   Java EE 6 XML schema, namespace is

<web-app 
          
          
          
          version="3.0">
</web-app>

3. Servlet 2.5
   Java EE 5 XML schema, namespace is

<web-app 
          
          
          
          version="2.5">
</web-app>

4. Servlet 2.4
   J2EE 1.4 XML schema, namespace is

<web-app 
          
          
          
          version="2.4">

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

5. Servlet 2.3
   J2EE 1.3 DTDs schema. This web.xml file is too old, highly recommend you to upgrade it.

<!DOCTYPE web-app PUBLIC
 "-//Sun Microsystems, Inc.//DTD Web Application 2.3//EN"
  >

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