web.xml文件头声明各个版本参考

Servlet 3.1
Java EE 7 XML schema, namespace is http://xmlns.jcp.org/xml/ns/javaee/

<web-app xmlns="http://xmlns.jcp.org/xml/ns/javaee"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         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>

Servlet 3.0
Java EE 6 XML schema, namespace is http://java.sun.com/xml/ns/javaee

<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>

Servlet 2.5
Java EE 5 XML schema, namespace is http://java.sun.com/xml/ns/javaee

<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>

Servlet 2.4
J2EE 1.4 XML schema, namespace is http://java.sun.com/xml/ns/j2ee

<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">

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

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"
 "http://java.sun.com/dtd/web-app_2_3.dtd" >

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