public class AttemptTrackingService {
    // 假設這個服務可以追蹤并返回給定sessionId對應的用戶嘗試次數
    public int getUserAttempts(String sessionId) {
        // 這里應該連接到你的數據存儲，以獲取實際的嘗試次數
        // 此處只為演示目的，返回一個固定值或模擬值
        return 3; // 假設用戶已經嘗試了3次
    }
}

package com.example.captchaservice;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Service;

import java.awt.*;
import java.awt.image.BufferedImage;
import java.util.Random;

@Service
public class DynamicCaptchaService {

    @Autowired
    private AttemptTrackingService attemptTrackingService; // 這個是假想的服務，用來追蹤用戶嘗試次數

    private static final int WIDTH = 160;
    private static final int HEIGHT = 70;

    public BufferedImage generateCaptcha(String sessionId) {
        int attempts = attemptTrackingService.getUserAttempts(sessionId);
        int difficulty = calculateDifficulty(attempts);

        return createCaptchaImage(difficulty);
    }

    private AttemptTrackingService attemptTrackingService = new AttemptTrackingService();

    private int calculateDifficulty(int attempts) {
        int baseDifficulty = 5; // 作為基礎難度，一開始驗證碼有5個字符
        int difficultyAdjustmentFactor = 1; // 每增加3次嘗試，難度（字符數量）就增加1
        int difficultyIncreaseCap = 3; // 最大難度增加上限，即額外字符上限

        // 計算難度增加值，使用嘗試次數除以3（向下取整）
        int difficultyIncrease = Math.min(attempts / 3, difficultyIncreaseCap);

        // 返回總難度值，即基礎字符數加上根據嘗試次數調整后增加的字符數
        return baseDifficulty + difficultyIncrease * difficultyAdjustmentFactor;
    }

    private BufferedImage createCaptchaImage(int numberOfChars) {
        BufferedImage bufferedImage = new BufferedImage(WIDTH, HEIGHT, BufferedImage.TYPE_INT_RGB);
        Graphics2D g2d = bufferedImage.createGraphics();

        g2d.setColor(Color.WHITE);
        g2d.fillRect(0, 0, WIDTH, HEIGHT);

        Random rand = new Random();
        StringBuilder captchaString = new StringBuilder();
        g2d.setFont(new Font("Arial", Font.BOLD, 48));
        
        // Simple captcha generating random digits
        for (int i = 0; i < numberOfChars; i++) {
            int character = (rand.nextInt(10) + 48); // ASCII range for digits
            g2d.setColor(new Color(rand.nextInt(256), rand.nextInt(256), rand.nextInt(256)));
            g2d.drawString(Character.toString((char) character), (i * 30) + 20, 50);
            captchaString.append((char) character);
        }

        g2d.dispose(); //完成圖形修改

        // 這里captchaString.toString()就是生成的驗證碼文本，可以存儲下來以便之后驗證
        // 注意：實際做法中應采取安全措施保護驗證碼文本，避免安全風險
        
        return bufferedImage;
    }
}

import org.springframework.web.bind.annotation.*;

@RestController
public class CaptchaVerificationController {

    @PostMapping("/verifyCaptcha")
    public boolean verifyCaptcha(@RequestParam("sessionId") String sessionId, @RequestParam("userInput") String userInput) {
        boolean isCorrect = captchaService.verifyCaptcha(sessionId, userInput);
        if(isCorrect) {
            // 如果驗證碼正確
            return true;
        } else {
            // 驗證碼錯誤，可以根據情況調整邏輯，如記錄嘗試次數等
            return false;
        }
    }
}