form的action处理表单跳转到servlet 返回404

Question

<form id="changekey" name="changekey" method="post" action="cks">

web.xml里面是这么写的

 <servlet>
    <servlet-name>changekeyservlet</servlet-name>
    <servlet-class>systemServlet.ChangeKeyServlet</servlet-class>
  </servlet>
  <servlet-mapping>
    <servlet-name>changekeyservlet</servlet-name>
    <url-pattern>/cks</url-pattern>
  </servlet-mapping>

为什么会返回The requested resource is not available.？