使用servlet需要两部分：配置文件（web.xml）和servlet代码。
将配置设置到web.xml中：

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns="http://java.sun.com/xml/ns/javaee"
xmlns:web="http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd"
xsi:schemaLocation="http://java.sun.com/xml/ns/javaee
http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd" id="WebApp_ID" version="3.0">
  <display-name>Servlet Example</display-name>
  <welcome-file-list>
    <!-- you will want to take a look at jsp as well --> 
    <welcome-file>login.jsp</welcome-file>
  </welcome-file-list>

  <servlet>
    <servlet-name>Login Servlet</servlet-name>
    <!-- dont forget to put the canonical name, including the package name -->
    <servlet-class>com.example.LoginServlet</servlet-class>
  </servlet>
</web-app>

然后编写servlet代码：

package com.example;
public class LoginServlet extends HttpServlet {

// GET
protected void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
    response.sendRedirect("index.jsp");
}

// POST
protected void doPost(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
    String username = request.getParameter("username");
    String password = request.getParameter("password");

    // method credentialsOk validate the user/password
    if(credentialsOk(username, password)){
        response.sendRedirect("home.jsp");
    }else{
        response.sendRedirect("login.jsp");
    }
 }

}
请看这里的servlet介绍：https://www.baeldung.com/intro-to-servlets