// 舊代碼片段（線程不安全的經(jīng)典寫法）
public class OrderService {

  private SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");*

    public void saveOrder(Order order) {
        // 線程A和線程B同時進入該方法
        String createTime = sdf.format(order.getCreateTime()); 
        // 可能出現(xiàn)"2023-02-30 12:00:00"這種根本不存在的日期
        orderDao.insert(createTime);**
    }

}

// 錯誤示范：簡單加減8小時
public Date convertToBeijingTime(Date utcDate) {
    Calendar cal = Calendar.getInstance();
    cal.setTime(utcDate);
    cal.add(Calendar.HOUR, 8); // 沒考慮夏令時切換問題
    return cal.getTime();
}

// ThreadLocal封裝方案（適用于JDK7及以下）
public class SafeDateFormatter {
    private static final ThreadLocal<DateFormat> THREAD_LOCAL = ThreadLocal.withInitial(() -> 
        new SimpleDateFormat("yyyy-MM-dd HH:mm:ss")
    );

    public static String format(Date date) {
        return THREAD_LOCAL.get().format(date);
    }
}

// 新時代寫法（線程安全+表達式增強）
public class ModernDateUtils {
    public static String format(LocalDateTime dateTime) {
        return dateTime.format(DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss"));
    }

    public static LocalDateTime parse(String str) {
        return LocalDateTime.parse(str, DateTimeFormatter.ISO_LOCAL_DATE_TIME);
    }
}

// 正確示范：基于時區(qū)計算營業(yè)時長
public Duration calculateBusinessHours(ZonedDateTime start, ZonedDateTime end) {
    // 顯式指定時區(qū)避免歧義
    ZonedDateTime shanghaiStart = start.withZoneSameInstant(ZoneId.of("Asia/Shanghai"));
    ZonedDateTime newYorkEnd = end.withZoneSameInstant(ZoneId.of("America/New_York"));
    
    // 自動處理夏令時切換
    return Duration.between(shanghaiStart, newYorkEnd);
}

// 預編譯模式（性能提升300%）
public class CachedDateFormatter {
    private static final Map<String, DateTimeFormatter> CACHE = new ConcurrentHashMap<>();

    public static DateTimeFormatter getFormatter(String pattern) {
        return CACHE.computeIfAbsent(pattern, DateTimeFormatter::ofPattern);
    }
}

// 全局時區(qū)上下文傳遞
public class TimeZoneContext {
    private static final ThreadLocal<ZoneId> CONTEXT_HOLDER = new ThreadLocal<>();

    public static void setTimeZone(ZoneId zoneId) {
        CONTEXT_HOLDER.set(zoneId);
    }

    public static ZoneId getTimeZone() {
        return CONTEXT_HOLDER.get();
    }
}

// 在Spring Boot攔截器中設置時區(qū)
@Component
public class TimeZoneInterceptor implements HandlerInterceptor {
    @Override
    public boolean preHandle(HttpServletRequest request, HttpServletResponse response, Object handler) {
        String timeZoneId = request.getHeader("X-Time-Zone");
        TimeZoneContext.setTimeZone(ZoneId.of(timeZoneId));
        return true;
    }
}

// LocalDate的不可變設計
LocalDate date = LocalDate.now();
date.plusDays(1); // 返回新實例，原對象不變
System.out.println(date); // 輸出當前日期，不受影響

// Stream API處理時間序列
List<Transaction> transactions = 
    list.stream()
        .filter(t -> t.getTimestamp().isAfter(yesterday)) // 聲明式過濾
        .sorted(Comparator.comparing(Transaction::getTimestamp)) // 自然排序
        .collect(Collectors.toList()); // 延遲執(zhí)行