How to create Java time instant from pattern?

问题

Consider a code:

TemporalAccessor date = DateTimeFormatter.ofPattern("yyyy-MM-dd").parse("9999-12-31");
Instant.from(date);

The last line throws an exception:

Unable to obtain Instant from TemporalAccessor: {},ISO resolved to 9999-12-31 of type java.time.format.Parsed

How to create Instant from yyyy-MM-dd pattern?

回答1:

The string "9999-12-31" only contains information about a date. It does not contain any information about the time-of-day or offset. As such, there is insufficient information to create an Instant. (Other date and time libraries are more lenient, but java.time avoids defaulting these values)

Your first choice is to use a LocalDate instead of an `Instant:

LocalDate date = LocalDate.parse("9999-12-31");

Your second choice is to post process the date to convert it to an instant, which requires a time-zone, here chosen to be Paris:

LocalDate date = LocalDate.parse("9999-12-31");
Instant instant = date.atStartOfDay(ZoneId.of("Europe/Paris")).toInstant();

Your third choice is to add the time-zone to the formatter, and default the time-of-day:

static final DateTimeFormatter FMT = new DateTimeFormatterBuilder()
    .appendPattern("yyyy-MM-dd")
    .parseDefaulting(ChronoField.NANO_OF_DAY, 0)
    .toFormatter()
    .withZone(ZoneId.of("Europe/Paris"));
Instant instant = FMT.parse("9999-31-12", Instant::from);

(If this doesn't work, ensure you have the latest JDK 8 release as a bug was fixed in this area).

It is worth noting that none of these possibilities use TemporalAccessor directly, because that type is a low-level framework interface, not one for most application developers to use.

回答2:

The problem isn't the fact that you are using the year 9999. The Instant.MAX field evaluates to the timestamp 1000000000-12-31T23:59:59.999999999Z, so 9999 as a year is fine.

Dealing with TemporalAccessors instead of the more semantically rich types like LocalDateTime or ZonedDateTime is like using a Map to model an object and its properties instead of writing a class -- you have to assure that the value has the fields (like seconds, nanoseconds, etc) that are expected by something receiving it, rather than depending on formally declared operations in a higher level class to prevent dependencies from going unmet.

In your case it is likely that the temporal accessor contained the parsed date fields it was given, but didn't have a "seconds" field that the Instant needed. It is best to use the more semantically rich types like LocalDateTime in most instances.

Since you only have date fields, you should parse it as a date, then add the time fields before converting it to an Instant. Here is one way, using LocalDate to parse the date:

LocalDate localDate = LocalDate.parse("2016-04-17");
LocalDateTime localDateTime = localDate.atStartOfDay();
Instant instant = localDateTime.toInstant(ZoneOffset.UTC);

回答3:

public static void main(String[] args) throws ParseException {
        System.out.println(new SimpleDateFormat("yyyy-MM-dd").parse("2016-12-31").toInstant());
}

the above code gives the following output:

2016-12-31T00:00:00Z

i have answered this question using features('toInstant' method) of java 8. hope this answers your question...

回答4:

Either you are only interested in the date itself (31st of December 9999), in which case the appropriate type would be a LocalDate:

LocalDate date = LocalDate.parse("9999-12-31");

Or you do want an Instant, in which case you need to set a time and time zone, for example, 00:00 in Tokyo:

Instant instant = date.atStartOfDay(ZoneId.of("Asia/Tokyo")).toInstant();

来源：https://stackoverflow.com/questions/37672012/how-to-create-java-time-instant-from-pattern

标签

java

java-time