简单生成验证码，自定义位数随机大小写字母数字生成

import java.util.Random;
public class a {
    public static void main(String[] args) {

        code(10);

    }

    public static void code(int n){
       Random r=new Random();
       for(int i=0;i<n;i++){
           //几位数
           int a=r.nextInt(3);
           switch(a){
               case 0://数字
                   int temp1=r.nextInt(10);
                   System.out.print(temp1);
                   break;
               case 1://小写字母
                   char temp2 =(char)(r.nextInt(26)+97);
                   System.out.print(temp2);
                   break;
               case 2://大写字母
                   char temp3 =(char)(r.nextInt(26)+65);
                   System.out.print(temp3);
                   break;
               default://报错
                   System.out.print("出错了");
                   break;
           }
       }
    }
}

修改格式（ai立大功。。）实现接受验证码长度参数的接口

package com.example.demo;

import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RequestParam;
import org.springframework.web.bind.annotation.RestController;
import java.util.Random;

@RestController
public class CaptchaController {
    
    @GetMapping("/captcha")
    public String generateCaptcha(
        @RequestParam(defaultValue = "10") int length
    ) {
        return generate(length);
    }

    public static String generate(int n) {
        Random r = new Random();
        StringBuilder sb = new StringBuilder();
        for (int i = 0; i < n; i++) {
            int type = r.nextInt(3);
            switch (type) {
                case 0: // 数字
                    sb.append(r.nextInt(10));
                    break;
                case 1: // 小写字母
                    sb.append((char) (r.nextInt(26) + 97));
                    break;
                case 2: // 大写字母
                    sb.append((char) (r.nextInt(26) + 65));
                    break;
            }
        }
        return "验证码: " + sb.toString();
    }
}