客戶端發(fā)起請求
     ↓
download(...) 方法被調(diào)用
     ↓
從 Redis 增加并發(fā)數(shù) DOWNLOAD_CONCURRENT_KEY
     ↓
根據(jù)并發(fā)數(shù)計算限速值（getDownloadSpeedLimit）
     ↓
調(diào)用 limitReadFile(...) 開始下載文件
     ↓
limitReadFile(...) 內(nèi)部調(diào)用 FileUtil.limitDownloadSpeed(...)
     ↓
讀取文件內(nèi)容 → 分段寫入輸出流 → 控制每秒發(fā)送的數(shù)據(jù)量
     ↓
下載完成或客戶端斷開連接
     ↓
finally 中減少 Redis 的并發(fā)數(shù)

// 增加并發(fā)計數(shù)器
Long currentConcurrent = redisTemplate.opsForValue().increment(DOWNLOAD_CONCURRENT_KEY);
// 更新過期時間
redisTemplate.expire(DOWNLOAD_CONCURRENT_KEY, EXPIRE_TIME_IN_SECONDS, TimeUnit.SECONDS);
// 計算限速值
int speedLimit = getDownloadSpeedLimit(currentConcurrent.intValue());

private int getDownloadSpeedLimit(int concurrentCount) {
    int totalBandwidth = 1024 * 1024 * 5; // 總帶寬 5MB/s
    int perUserSpeed = totalBandwidth / Math.max(1, concurrentCount);
    return Math.max(perUserSpeed, 1024 * 50); // 最低限速 50KB/s
}

public static void limitDownloadSpeed(InputStream in, OutputStream out, int bytesPerSecond) throws IOException {
    byte[] buffer = new byte[BUFFER_SIZE]; // 默認(rèn)是 1024 字節(jié)
    int bytesRead;
    long bytesSent = 0;
    long startTime = System.currentTimeMillis();
    while ((bytesRead = in.read(buffer)) != -1) {
        out.write(buffer, 0, bytesRead);
        bytesSent += bytesRead;
        if (bytesSent >= bytesPerSecond) {
            long elapsedTime = System.currentTimeMillis() - startTime;
            if (elapsedTime < 1000) {
                Thread.sleep(1000 - elapsedTime);
            }
            bytesSent = 0;
            startTime = System.currentTimeMillis();
        }
    }
}