老师这样写可以嘛

package com.imooc.servlet;

import java.io.IOException;
import java.io.PrintWriter;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

public class ServletProj extends HttpServlet {

	@Override
	protected void service(HttpServletRequest resquest, HttpServletResponse response)
			throws ServletException, IOException {
		String category = resquest.getParameter("category");
		String html = "<p style='color:blue'>" + category + "</p>";
		PrintWriter out = response.getWriter();
		out.println(html);
	}
}
<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://xmlns.jcp.org/xml/ns/javaee" xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee http://xmlns.jcp.org/xml/ns/javaee/web-app_4_0.xsd" id="WebApp_ID" version="4.0">
  <display-name>ServletProj</display-name>
  <welcome-file-list>
    <welcome-file>index.html</welcome-file>
    <welcome-file>index.htm</welcome-file>
    <welcome-file>index.jsp</welcome-file>
    <welcome-file>default.html</welcome-file>
    <welcome-file>default.htm</welcome-file>
    <welcome-file>default.jsp</welcome-file>
  </welcome-file-list>
  <servlet>
  <servlet-name>second</servlet-name>
  <servlet-class>com.imooc.servlet.ServletProj</servlet-class>
  </servlet>
  <servlet-mapping>
  <servlet-name>second</servlet-name>
  <url-pattern>/ShopServlet</url-pattern>
  </servlet-mapping>
</web-app>