Java实现优雅日期处理的方案详解

// 旧代码片段（线程不安全的经典写法）
public class OrderService {

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

    public void saveOrder(Order order) {
        // 线程A和线程B同时进入该方法
        String createTime = sdf.format(order.getCreateTime()); 
        // 可能出现"2023-02-30 12:00:00"这种根本不存在的日期
        orderDao.insert(createTime);**
    }

}

// 错误示范：简单加减8小时
public Date convertToBeijingTime(Date utcDate) {
    Calendar cal = Calendar.getInstance();
    cal.setTime(utcDate);
    cal.add(Calendar.HOUR, 8); // 没考虑夏令时切换问题
    return cal.getTime();
}

// ThreadLocal封装方案（适用于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);
    }
}

// 新时代写法（线程安全+表达式增强）
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);
    }
}

// 正确示范：基于时区计算营业时长
public Duration calculateBusinessHours(ZonedDateTime start, ZonedDateTime end) {
    // 显式指定时区避免歧义
    ZonedDateTime shanghaiStart = start.withZoneSameInstant(ZoneId.of("Asia/Shanghai"));
    ZonedDateTime newYorkEnd = end.withZoneSameInstant(ZoneId.of("America/New_York"));
    
    // 自动处理夏令时切换
    return Duration.between(shanghaiStart, newYorkEnd);
}

// 预编译模式（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);
    }
}

// 全局时区上下文传递
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拦截器中设置时区
@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的不可变设计
LocalDate date = LocalDate.now();
date.plusDays(1); // 返回新实例，原对象不变
System.out.println(date); // 输出当前日期，不受影响

// Stream API处理时间序列
List<Transaction> transactions = 
    list.stream()
        .filter(t -> t.getTimestamp().isAfter(yesterday)) // 声明式过滤
        .sorted(Comparator.comparing(Transaction::getTimestamp)) // 自然排序
        .collect(Collectors.toList()); // 延迟执行