web.xml根标签web-app规范

WEB-INF/web.xml文件其实就是一个描述web应用如何被部署的配置文件。web.xml根元素web-app各版本写法如下：

1. Servlet 3.1 部署描述符

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

web-app_3.1.xsd和version="3.1"其实对应的就是Servlet版本。

This is the XML Schema for the Servlet 3.1 deployment descriptor.
The deployment descriptor must be named "WEB-INF/web.xml" in the
web application's war file.  All Servlet deployment descriptors
must indicate the web application schema by using the Java EE
namespace:

http://xmlns.jcp.org/xml/ns/javaee 

and by indicating the version of the schema by 
using the version element as shown below: 

 
...


The instance documents may indicate the published version of
the schema using the xsi:schemaLocation attribute for Java EE
namespace with the following location:

http://xmlns.jcp.org/xml/ns/javaee/web-app_3_1.xsd

2. Servlet 3.0 部署描述符

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

3. Servlet 2.5 部署描述符

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

4. Servlet 2.4 部署描述符

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

  Servlet 2.4 Web Application

5. Servlet 2.3 部署描述符

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

  Servlet 2.3 Web Application

原文链接： http://www.mkyong.com/web-development/the-web-xml-deployment-descriptor-examples/