案例：网页提交用户名和密码信息，后端校验密码长度需在6-12位之间

1. 后端部分

   @WebServlet("/valid")
   public class SimpleServlet extends HttpServlet{public void service(HttpServletRequest req, HttpServletResponse resp) throws IOException{// 获取请求参数String username = req.getParameter("username");String password = req.getParameter("password");// 校验密码长度需在6-12位之间String result = "NO";if (password.length() >= 6 && password.length() <= 12) {result = "YES";}// 将结果写入到响应体中resp.setHeader("Content-Type", "text/html");PrintWriter writer = resp.getWriter();writer.write(result);}
   }
   

2. 前端部分

   <form action="valid" method="post"><div><span>用户名：</span><input type="text" name="username"></div><div><span>密码：</span><input type="password" name="password"></div><button type="submit">提交</button>
   </form>