我这里没有显示form表单页面，直接在浏览器显示null，是哪里出问题了
package com.study;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
import java.io.PrintWriter;
public class GetRequestServlet extends HttpServlet {
@Override
protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
doPost(req, resp);
}
@Override
protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
String username = req.getParameter("username");
PrintWriter out = resp.getWriter();
out.println("<!DOCTYPE HTML PUBLIC \"-//W3C//DTD HTML 4.01 Transitional//EN\">");
out.println("<HTML>");
out.println("<HEAD><TITLE>A Servlet</TITLE></HEAD>");
out.println("<BODY>");
out.println("UserName:" + username + "<br/>");
out.println("</BODY>");
out.println("</HTML>");
out.flush();
out.close();
}
}<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Title</title>
</head>
<body>
<form action="getInfo.do" method="post">
用户名：<input type="text" name="username"/><br/>
<input type="submit" value="ok"/>
</form>
</body>
</html>
<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns="http://xmlns.jcp.org/xml/ns/javaee"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee http://xmlns.jcp.org/xml/ns/javaee/web-app_4_0.xsd"
version="4.0">
<servlet>
<servlet-name>demoServlet</servlet-name>
<servlet-class>com.study.demoServlet</servlet-class>
</servlet>
<servlet-mapping>
<servlet-name>demoServlet</servlet-name>
<url-pattern>/demo.do</url-pattern>
</servlet-mapping>
<servlet>
<servlet-name>GetRequestServlet</servlet-name>
<servlet-class>com.study.GetRequestServlet</servlet-class>
</servlet>
<servlet-mapping>
<servlet-name>GetRequestServlet</servlet-name>
<url-pattern>/getInfo.do</url-pattern>
</servlet-mapping>
</web-app>
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