ajax+servlet报404路径错误

Question

    **这是验证函数：**
        function validate(){
        createXMLHttpRequest();
        var date=document.getElementById("birthDate");
        var url="<%=request.getContextPath()%>/ValidationServlet?birthDate="+escape(date.value);
        xmlHttp.open("GET",url,true);
        xmlHttp.onreadystatechange=callback();
        xmlHttp.send(null);
    }

        **这是xml配置文件：**
        <servlet>
<servlet-name>ValidationServlet</servlet-name>
<servlet-class>ch04.ValidationServlet</servlet-class>

ValidationServlet
/ValidationServlet

请问哪里出问题了呢，一直报错。。。。