不同版本(2.3、2.4、2.5) web.xml 的web-app头信息

Servlet 2.3 
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE web-app
    PUBLIC "-//Sun Microsystems, Inc.//DTD Web Application 2.3//EN"
    "http://java./dtd/web-app_2_3.dtd">
<web-app>
  ...
</web-app>


Servlet 2.4 
<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns:xsi="http://www./2001/XMLSchema-instance"
        xmlns="http://java./xml/ns/j2ee"
        xmlns:web="http://java./xml/ns/j2ee/web-app_2_4.xsd"
        xsi:schemaLocation="http://java./xml/ns/j2ee http://java./xml/ns/j2ee/web-app_2_4.xsd "
        version="2.4">
  ...
</web-app>

Servlet 2.5 
<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns="http://java./xml/ns/javaee"
        xmlns:xsi="http://www./2001/XMLSchema-instance"
        xsi:schemaLocation="http://java./xml/ns/javaee http://java./xml/ns/javaee/web-app_2_5.xsd "
        version="2.5">
  ...
</web-app>