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# Java Date operation

## 前情提要

Java Date 物件底層都是透過TimeZone 來去做時區判斷，但是這會依賴系統時間，即使透過TimeZone setDefaultTimeZone 來去做完時區轉換的方式，但是這無法一勞永逸且不具有彈性的設計，所以才會有這篇文章的誕生。\
我這邊也不賣關子，我自己測試下來的結論，Date物件不好操作，日後有關時區問題，我是會棄用他的。

### 人狠，話不多，上扣！！

<pre class="language-java"><code class="lang-java">/**
 * convert date String with time zone to LocalDateTime Object
 *
 * @param dateStr
 * @param formatterPattern
 * @param dateStrTimeZone
 * @param toTimeZon
 * @return
 * @throws ParseException
 */
private static LocalDateTime convertDateStrWithTimeZone(String dateStr, String formatterPattern, TimeZone dateStrTimeZone, ZoneId toTimeZon) throws ParseException {
    /**
     * 這邊會發生兩件事情
     * 1.SimpleDateFormat 認定字串 為 GMT+0
     * 2.Date Object 將轉換的GMT+0 加上系統預設時區
     * Ex. "2023-03-23 12:00:00" -> 是GMT+0 時區的時間
     * 當下系統 default 時區是 GMT+8
     * 轉換結果就會變成 2023-03-23 20:00:00[GMT+8]
     */

    // Create a SimpleDateFormat object for parsing the date string
    SimpleDateFormat sdf = new SimpleDateFormat(formatterPattern);

    // Set the time zone of the SimpleDateFormat object to GMT+0
    sdf.setTimeZone(dateStrTimeZone);

    // Parse the date string into an Instant object and transfer to LocalDataTime
    return sdf.parse(dateStr).toInstant().atZone(toTimeZon).toLocalDateTime();
}

/**
 * @param dateTime
 * @param fromZone
 * @param toZone
 * @return
 */
private static LocalDateTime convertTimeZoneUseZoneId(LocalDateTime dateTime, ZoneId fromZone, ZoneId toZone) {
    ZonedDateTime originZoneDateTime = <a data-footnote-ref href="#user-content-fn-1">ZonedDateTime</a>.of(dateTime, fromZone);
    ZonedDateTime transZoned = originZoneDateTime.withZoneSameInstant(toZone);
    return transZoned.toLocalDateTime();
}

/**
 * @param dateTime
 * @param fromOffset
 * @param toOffset
 * @return
 */
public static LocalDateTime convertTimeZoneUseZoneOffset(LocalDateTime dateTime, ZoneOffset fromOffset, ZoneOffset toOffset) {
    long timeDifference = toOffset.getTotalSeconds() - fromOffset.getTotalSeconds();
    return dateTime.plusSeconds(timeDifference);
}
</code></pre>

其實 `convertTimeZoneUseZoneId`跟 `convertTimeZoneUseZoneOffset` 背後是繼承關係，所以基本上是一樣的東西，有可能差別在於效能上，但這部分我就沒有花太多時間著墨了。

### 測試！

```java
import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.time.LocalDateTime;
import java.time.ZoneId;
import java.time.ZoneOffset;
import java.time.ZonedDateTime;
import java.time.format.DateTimeFormatter;
import java.util.TimeZone;


public class DateTimeConvertWithTimeZoneUtil {
    final static DateTimeFormatter df = DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss.SSS");
    static SimpleDateFormat sf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");

    final static String GMT0_STR = "GMT+0";
    final static String GMT8_STR = "GMT+8";
    final static ZoneId gmt8Zone = ZoneId.of(GMT8_STR);
    final static ZoneId gmt0Zone = ZoneId.of(GMT0_STR);
    final static TimeZone timeZoneGMT0 = TimeZone.getTimeZone(GMT0_STR);
    final static TimeZone timeZoneGMT8 = TimeZone.getTimeZone(GMT8_STR);
    final static TimeZone timeZoneGMT_M8 = TimeZone.getTimeZone("GMT-8");

    public static void main(String[] args) throws ParseException {

//        String dateStr = "1970-01-01 00:00:00.000";
        String dateStr = "2023-01-05 8:00:00.000";
        String dateStrPattern = "yyyy-MM-dd HH:mm:ss";

        System.out.println("dateStr >> " + dateStr);
        System.out.println("GMT+8 dateStr to GMT+0 LocalDateTime >>> " + df.format(convertDateStrWithTimeZone(dateStr, dateStrPattern, timeZoneGMT8
                , gmt0Zone)));
        System.out.println("GMT+8 dateStr to GMT+8 LocalDateTime >>> " + df.format(convertDateStrWithTimeZone(dateStr, dateStrPattern,
        timeZoneGMT8, gmt8Zone)));
        System.out.println("GMT+0 dateStr to GMT+8 LocalDateTime >>> " + df.format(convertDateStrWithTimeZone(dateStr, dateStrPattern,
        timeZoneGMT0, gmt8Zone)));
        System.out.println("GMT+0 dateStr to GMT+0 LocalDateTime >>> " + df.format(convertDateStrWithTimeZone(dateStr, dateStrPattern,
        timeZoneGMT0, gmt0Zone)));

        System.out.println("=================================================");
        LocalDateTime GMT8LDT = convertDateStrWithTimeZone(dateStr, dateStrPattern, timeZoneGMT8, gmt8Zone);
        LocalDateTime GMT0LDT = convertDateStrWithTimeZone(dateStr, dateStrPattern, timeZoneGMT0, gmt0Zone);

        long start2 = System.currentTimeMillis();
        for (int i = 18; i >= -18; i--) {
            System.out.println("GMT+8 dateStr to GMT" + (i > 0 ? "+" + i : i) + " LocalDateTime >>> " +
                    df.format(convertTimeZoneUseZoneId(GMT8LDT, ZoneOffset.ofHours(8), ZoneOffset.ofHours(i))));
        }

        for (int i = 18; i >= -18; i--) {
            System.out.println("GMT+0 dateStr to GMT" + (i > 0 ? "+" + i : i) + " LocalDateTime >>> " +
                    df.format(convertTimeZoneUseZoneId(GMT0LDT, ZoneOffset.ofHours(0), ZoneOffset.ofHours(i))));
        }
        System.out.println("two end: >>>" + (System.currentTimeMillis() - start2));

        System.out.println("======== change ZoneId =======");

        long start1 = System.currentTimeMillis();
        for (int i = 18; i >= -18; i--) {
            System.out.println("GMT+8 dateStr to GMT" + (i > 0 ? "+" + i : i) + " LocalDateTime >>> " + df.format(convertTimeZoneUseZoneOffset(GMT8LDT,
                    ZoneOffset.ofHours(8), ZoneOffset.ofHours(i))));
        }

        System.out.println(" reverse ~~ ");

        for (int i = 18; i >= -18; i--) {
            System.out.println("GMT+0 dateStr to GMT" + (i > 0 ? "+" + i : i) + " LocalDateTime >>> " + df.format(convertTimeZoneUseZoneOffset(GMT0LDT,
                    ZoneOffset.ofHours(0), ZoneOffset.ofHours(i))));
        }

        System.out.println("one end: >>>" + (System.currentTimeMillis() - start1));

    }
}
```

## 結論

不斷的測試及驗證，最後發現Date 物件對於時區操作，並不是這麼靈活且適合，但我本身就不是那麼愛用Date，Java 8 過後新增的LocalDateTime 等等新增的時間類別，相較於Date 比較好用，但是呢～～～\
永遠都有但是工作上既有產品還是使用Date 所以你還是無法避免這狀況，只能將狀況變成最適合自己的操作方式，盡可能提高自己工作的效率。🔥

[^1]: