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

Question

如何最好地将 java.util.Date 转换为 Java 8 java.time.YearMonth？

不幸的是，以下抛出 DateTimeException:

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

结果：

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)
...

我需要此功能，因为我想使用 JPA 将 YearMonth 值存储在数据库中。目前 JPA 不支持 YearMonth，所以我想出了以下 YearMonthConverter（省略导入）：

// 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);
  }
}

难道没有更好（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());

解释：

YearMonth.from(TemporalAccessor)-方法的 JavaDoc 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 超出 window。关于 how to convert Date to LocalDate 的这个优秀答案中解释了原因。

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，Date 可以转换为 Instant，但这对我们没有帮助，是吗？

然而，备选方案 2 被证明是成功的。将 Instant 转换为 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

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

How to convert java.util.Date to Java8 java.time.YearMonth

jpa

java-8

java.util.date