jpa - 如何将 java.util.Date 转换为 Java8 java.time.YearMonth

Question

我怎样才能最好地转换 java.util.Date到 Java 8 java.time.YearMonth ?

不幸的是，以下抛出 DateTimeException :

YearMonth yearMonth = YearMonth.from(date.toInstant());

结果是:

java.time.DateTimeException: Unable to obtain YearMonth from TemporalAccessor: 2015-01-08T14:28:39.183Z of type java.time.Instant
at java.time.YearMonth.from(YearMonth.java:264)
...

我需要这个功能，因为我想存储 YearMonth使用 JPA 的数据库中的值。目前 JPA 不支持 YearMonth 's，所以我想出了以下 YearMonthConverter(省略了导入):

// TODO (future): delete when next version of JPA (i.e. Java 9?) supports YearMonth. See https://java.net/jira/browse/JPA_SPEC-63
@Converter(autoApply = true)
public class YearMonthConverter implements AttributeConverter<YearMonth, Date> {

  @Override
  public Date convertToDatabaseColumn(YearMonth attribute) {
    // uses default zone since in the end only dates are needed
    return attribute == null ? null : Date.from(attribute.atDay(1).atStartOfDay(ZoneId.systemDefault()).toInstant());
  }

  @Override
  public YearMonth convertToEntityAttribute(Date dbData) {
    // TODO: check if Date -> YearMonth can't be done in a better way
    if (dbData == null) return null;
    Calendar calendar = Calendar.getInstance();
    calendar.setTime(dbData);
    return YearMonth.of(calendar.get(Calendar.YEAR), calendar.get(Calendar.MONTH) + 1);
  }
}

没有更好(更干净、更短)的解决方案(双向)吗？

Answer 1

简答:

// From Date to YearMonth
YearMonth yearMonth =
        YearMonth.from(date.toInstant()
                           .atZone(ZoneId.systemDefault())
                           .toLocalDate());

// From YearMonth to Date
// The same as the OP:s answer
final Date convertedFromYearMonth = 
        Date.from(yearMonth.atDay(1).atStartOfDay(ZoneId.systemDefault()).toInstant());

说明:

YearMonth.from(TemporalAccessor)的JavaDoc -方法说:

The conversion extracts the YEAR and MONTH_OF_YEAR fields. The extraction is only permitted if the temporal object has an ISO chronology, or can be converted to a LocalDate.

因此，您需要能够:

提取 YEAR和 MONTH_OF_YEAR字段，或

你应该使用可以转换为 LocalDate 的东西.

让我们试试吧!

final Date date = new Date();
final Instant instant = date.toInstant();
instant.get(ChronoField.YEAR); // causes an error

这是不可能的，抛出异常:

java.time.temporal.UnsupportedTemporalTypeException: Unsupported field: Year at java.time.Instant.get(Instant.java:571) ...

这意味着备选方案 1 已被排除在外。在这个关于 how to convert Date to LocalDate 的优秀回答中解释了原因。 .

Despite its name, java.util.Date represents an instant on the time-line, not a "date". The actual data stored within the object is a long count of milliseconds since 1970-01-01T00:00Z (midnight at the start of 1970 GMT/UTC).

The equivalent class to java.util.Date in JSR-310 is Instant, thus there is a convenient method toInstant() to provide the conversion.

所以，一个 Date可以转换为 Instant但这对我们没有帮助，是吗？

然而，方案 2 证明是成功的。转换 Instant到 LocalDate然后使用 YearMonth.from(TemporalAccessor) -方法。

    Date date = new Date();

    LocalDate localDate = date.toInstant()
                              .atZone(ZoneId.systemDefault())
                              .toLocalDate();

    YearMonth yearMonth = YearMonth.from(localDate);
    System.out.println("YearMonth: " + yearMonth);

输出是(因为代码是在 2015 年 1 月执行的；):

YearMonth: 2015-01