web.xml是DTD约束的

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE web-app SYSTEM "web-app_2_3.dtd">
<web-app>
    <servlet>
        <servlet-name>MyServlet1</servlet-name>
        <servlet-class>demo1.person</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>MyServlet1</servlet-name>
        <url-pattern>/hello</url-pattern>
    </servlet-mapping>
</web-app>