web.xml在不同的Servlet版本下的区别

1. Servlet 3.1 deployment descriptor

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>

2. Servlet 3.0 deployment descriptor

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>

3. Servlet 2.5 deployment descriptor

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>

4. Servlet 2.4 deployment descriptor

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>

5. Servlet 2.3 deployment descriptor

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>

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