下面是从2.3,2.4.2.5 3.0集中版本的web.xml头信息的细节，当发现系统启动不报错但是该生成的功能没有正常生成，特别是在这次配置润乾报表的时候发现用2.4版本的时候，在web.xml中配置的reportServlet负责生成润乾报表页面需要使用的js函数库失败，后来去掉表头只留下<web-app>就可以了。如果有需要可以换成2.3,2.4,2.5 或者3.0
其实这里的版本就是servlet的版本，我们用专业的眼光回顾一下servlet版本的发展：
servlet 1.0 june 1997 undefined ->servlet 2.0 JDK 1.1 part of java servlet development kit 2.0->Servlet 2.1 first official specification added requestDispatcher, ServletContext->Servlet 2.4 Nov 2003 J2EE 1.4 J2SE1.3 web.xml->Servlet 2.5 Sept 2005 JavaEE5 required javaSe5 支持注解了。->Servlet3.0 Dec 2009 JavaEE6 JavaSE6 pluggability, Ease of Development, AsyncServlet, more security and File uploading
web.xml v2.3
------------------------------------------------------------------
<?xml version="1.0" encoding="ISO-8859-1"?>
<!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>
</web-app>
web.xml v2.4
-----------------------------------------------------------------------------------------------------
<?xml version="1.0" encoding="UTF-8"?>
<web-app id="WebApp_9" version="2.4" 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">
</web-app>
web.xml v2.5
----------------------------------------------------------------------------------------------
<?xml version="1.0" encoding="UTF-8"?>
<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>
web.xml v3.0
------------------------------------------------------------------------------
<?xml version="1.0" encoding="UTF-8"?>
<web-app
version="3.0"
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">
</web-app>
web-fragment.xml
--------------------------------------------------------------------------------------
<web-fragment 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-fragment_3_0.xsd" version="3.0">
</web-fragment>