Cannot change version of project facet Dynamic Web Module to 3.0

步骤：

1.打开web.xml,查看下version是否是3.0

<web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://java.sun.com/xml/ns/javaee" 
xmlns:web="http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd" 
xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd" 
id="WebApp_ID" version="3.0">

2.打开项目下.setting目录下的org.eclipse.wst.common.project.facet.core.xml ,"jst.web"对应的version改为“3.0”

<?xml version="1.0" encoding="UTF-8"?>
<faceted-project>
  <fixed facet="wst.jsdt.web"/>
  <installed facet="wst.jsdt.web" version="1.0"/>
  <installed facet="jst.web" version="3.0"/>
  <installed facet="java" version="1.7"/>
</faceted-project>

3.执行项目右键Maven的Update Project.

dynamic web module 版本之间的区别：

Servlet 3.0	December 2009	JavaEE 6, JavaSE 6	Pluggability, Ease of development, Async Servlet, Security, File Uploading
Servlet 2.5	September 2005	JavaEE 5, JavaSE 5	Requires JavaSE 5, supports annotations
Servlet 2.4	November 2003	J2EE 1.4, J2SE 1.3	web.xml