HTTP Status 503报错

web链接时候，为什么HTTP Status 503呢？
配置文件
<?xml version="1.0" encoding="UTF-8"?>
<web-app 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">
<servlet>
<servlet-name>QQ</servlet-name>
<servlet-class>com.niit.myservlet.HelloServlet</servlet-class>
</servlet>
<servlet-mapping>
<servlet-name>QQ</servlet-name>
<url-pattern>/AA</url-pattern>

</servlet-mapping>
  <welcome-file-list>
    <welcome-file>index.jsp</welcome-file>
  </welcome-file-list>
</web-app>
为什么没办法链接呢？