Spring项目报错：Required parameter 'username' is not present 求助

提交表单时，表单设置action="userlogin"、method="post"，但无法跳转到预期的index页面。通过Postman测试时，出现报错：

trace信息显示org.springframework.web.bind.MissingServletRequestParameterException: Required request parameter 'username' for method parameter type String is not present，message为"Required parameter 'username' is not present"

相关代码如下：

package com.home_project.oop_project.controllers;

import java.sql.*;
import org.springframework.stereotype.Controller;
import org.springframework.ui.Model;
import org.springframework.web.bind.annotation.*;

@Controller
public class UserController {

int adminlogcheck = 0;
String usernameforclass = "";

//Some code
@GetMapping("userlogin")
public String userlog(Model model) {
    
    return "userLogin";
}
@RequestMapping(value = "userlogin", method = RequestMethod.POST)
public String userlogin( @RequestParam("username") String username, @RequestParam("password") String pass,Model model) {
    try
    {
        System.out.println(username);
        Class.forName("com.mysql.jdbc.Driver");
        Connection con = DriverManager.getConnection("jdbc:mysql://localhost:3306/springproject","root","");
        Statement stmt = con.createStatement();
        ResultSet rst = stmt.executeQuery("select * from users where username = '"+username+"' and password = '"+ pass+"' ;");
        if(rst.next()) {
            usernameforclass = rst.getString(2);
            return "redirect:/index";
            }
        else {
            model.addAttribute("message", "Invalid Username or Password");
            return "userLogin";
        }
        
    }
    catch(Exception e)
    {
        System.out.println("Exception:"+e);
    }
    return "userLogin";
    }
}

1. 检查前端表单字段的name属性 #

报错核心是Spring无法匹配到username参数，需确认前端表单中用户名输入框的name属性严格为username，密码输入框为password，示例：

<form action="userlogin" method="post">
  <input type="text" name="username" placeholder="用户名">
  <input type="password" name="password" placeholder="密码">
  <button type="submit">登录</button>
</form>

若name属性拼写错误（如userName、user），Spring无法绑定到@RequestParam注解的参数。

2. 修复SQL注入风险与语法问题 #

当前代码直接拼接SQL语句，存在严重注入风险，同时原代码中DriverManager.getConnection的密码参数有语法错误（原代码为""""，需改为""）。建议改用PreparedStatement：

// 替换原Statement相关代码
String sql = "select * from users where username = ? and password = ?";
PreparedStatement pstmt = con.prepareStatement(sql);
pstmt.setString(1, username);
pstmt.setString(2, pass);
ResultSet rst = pstmt.executeQuery();

3. 验证Postman请求参数格式 #

用Postman测试时，需选择form-data或x-www-form-urlencoded格式传递参数，而非raw的JSON格式（当前代码用@RequestParam接收表单参数，不支持JSON格式直接绑定）。

4. 优化资源关闭与异常处理 #

当前代码未关闭数据库连接、Statement、ResultSet，会导致资源泄漏，建议在finally块中统一关闭：

finally {
    try {
        if (rst != null) rst.close();
        if (stmt != null) stmt.close();
        if (con != null) con.close();
    } catch (SQLException e) {
        e.printStackTrace();
    }
}

同时，异常处理可添加前端提示信息：

catch(Exception e) {
    e.printStackTrace();
    model.addAttribute("message", "服务器异常，请稍后重试");
}

内容的提问来源于stack exchange，提问作者Lê Trung Hiếu