Java 獲取當前日期時間完整指南：從基礎到進階

核心類對比 (Java 8 前 vs 後)

Java 8+ 新 API 推薦用法 (主流標準)

1. 獲取當前日期時間

// 本地日期時間（無時區，適合業務邏輯）
LocalDateTime now = LocalDateTime.now();        // 2024-01-15T14:30:45.123
LocalDate today = LocalDate.now();              // 2024-01-15
LocalTime time = LocalTime.now();               // 14:30:45.123

// 帶時區（準確時間點）
ZonedDateTime zdt = ZonedDateTime.now();        // 2024-01-15T14:30:45.123+08:00[Asia/Shanghai]
Instant instant = Instant.now();                // 2024-01-15T06:30:45.123Z (UTC)

// 指定時區
LocalDateTime shanghai = LocalDateTime.now(ZoneId.of("Asia/Shanghai"));

2. 格式化輸出

DateTimeFormatter fmt = DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss");
String formatted = now.format(fmt);  // "2024-01-15 14:30:45"

// 內建格式器
String iso = now.format(DateTimeFormatter.ISO_LOCAL_DATE_TIME); // "2024-01-15T14:30:45.123"
String chinese = now.format(DateTimeFormatter.ofPattern("yyyy年MM月dd日 HH時mm分ss秒", Locale.CHINESE));

3. 解析字符串

String str = "2024-01-15 14:30:45";
LocalDateTime parsed = LocalDateTime.parse(str, DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss"));

// 容錯解析：多格式嘗試
public LocalDateTime parseFlexible(String str) {
    DateTimeFormatter[] formatters = {
        DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss"),
        DateTimeFormatter.ofPattern("yyyy/MM/dd HH:mm:ss"),
        DateTimeFormatter.ISO_LOCAL_DATE_TIME
    };
    for (DateTimeFormatter f : formatters) {
        try { return LocalDateTime.parse(str, f); } catch (DateTimeParseException ignored) {}
    }
    throw new IllegalArgumentException("無法解析: " + str);
}

4. 日期計算與調整

LocalDate date = LocalDate.now();

// 加減操作（不可變，返回新對象）
LocalDate tomorrow = date.plusDays(1);
LocalDate nextMonth = date.plusMonths(1);
LocalDate lastYear = date.minusYears(1);

// TemporalAdjusters 複雜調整
LocalDate firstDayOfMonth = date.with(TemporalAdjusters.firstDayOfMonth());
LocalDate lastDayOfMonth = date.with(TemporalAdjusters.lastDayOfMonth());
LocalDate nextMonday = date.with(TemporalAdjusters.next(DayOfWeek.MONDAY));
LocalDate firstDayOfYear = date.withDayOfYear(1);

// 兩日期間隔
long daysBetween = ChronoUnit.DAYS.between(startDate, endDate);
long monthsBetween = ChronoUnit.MONTHS.between(start, end);

5. 舊 API 互操作 (數據庫/JDBC)

// LocalDateTime → Timestamp
Timestamp ts = Timestamp.valueOf(localDateTime);

// Timestamp → LocalDateTime
LocalDateTime ldt = timestamp.toLocalDateTime();

// Instant → Date
Date date = Date.from(instant);

// Date → Instant
Instant ins = date.toInstant();

舊 API (java.util.Date/Calendar) 維護代碼指南

若項目仍使用舊 API，請遵循以下規範：

1. 線程安全：SimpleDateFormat 非線程安全

// 錯誤：共享實例導致並發錯誤
static final SimpleDateFormat SDF = new SimpleDateFormat("yyyy-MM-dd");

// 正確：ThreadLocal 隔離
static final ThreadLocal SDF_THREAD_LOCAL = 
    ThreadLocal.withInitial(() -> new SimpleDateFormat("yyyy-MM-dd"));

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

2. 常用操作模板

// 獲取當前時間毫秒數 (三種等價方式)
long ms1 = System.currentTimeMillis();
long ms2 = new Date().getTime();
long ms3 = Calendar.getInstance().getTimeInMillis();

// 格式化輸出
SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
String str = sdf.format(new Date());

// 解析
Date date = sdf.parse("2024-01-15 14:30:45");

// Calendar 字段操作
Calendar cal = Calendar.getInstance();
cal.setTime(new Date());
int year = cal.get(Calendar.YEAR);
int month = cal.get(Calendar.MONTH) + 1; // 0-based!
int day = cal.get(Calendar.DAY_OF_MONTH);
cal.add(Calendar.DAY_OF_MONTH, -7); // 一周前
Date weekAgo = cal.getTime();

實用工具類封裝 (生產級)

public final class DateUtils {
    private DateUtils() {}

    // 線程安全格式器緩存
    private static final Map> FORMATTER_CACHE = new ConcurrentHashMap<>();

    public static DateTimeFormatter getFormatter(String pattern) {
        return FORMATTER_CACHE.computeIfAbsent(pattern, 
            p -> ThreadLocal.withInitial(() -> DateTimeFormatter.ofPattern(p)))
            .get();
    }

    // 格式化
    public static String format(LocalDateTime ldt, String pattern) {
        return ldt.format(getFormatter(pattern));
    }

    public static String format(Date date, String pattern) {
        return format(date.toInstant()
            .atZone(ZoneId.systemDefault())
            .toLocalDateTime(), pattern);
    }

    // 解析
    public static LocalDateTime parse(String str, String pattern) {
        return LocalDateTime.parse(str, getFormatter(pattern));
    }

    // 相對時間描述
    public static String relativeTime(LocalDateTime target) {
        Duration dur = Duration.between(target, LocalDateTime.now());
        long seconds = dur.getSeconds();
        if (seconds < 60) return "剛剛";
        if (seconds < 3600) return seconds / 60 + "分鐘前";
        if (seconds < 86400) return seconds / 3600 + "小時前";
        if (seconds < 604800) return seconds / 86400 + "天前";
        return format(target, "yyyy-MM-dd");
    }
}

常見陷阱總結