JSR-310 - parsing seconds fraction with variable l

问题:

Is there a way how to create JSR-310 formatter that is able to parse both following date/times with variable length of seconds fraction?

2015-05-07 13:20:22.276052

or

2015-05-07 13:20:22.276

Example code:

DateTimeFormatter formatter
= new java.time.format.DateTimeFormatterBuilder()
        .append( java.time.format.DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss") )
        .appendOptional( java.time.format.DateTimeFormatter.ofPattern(".SSSSSS") )
        .toFormatter();
formatter.parse("2015-05-07 13:20:22.276052", LocalDateTime::from);

回答1:

This solves the problem:

DateTimeFormatter formatter = new DateTimeFormatterBuilder()
    .appendPattern("yyyy-MM-dd HH:mm:ss")
    .appendFraction(ChronoField.MICRO_OF_SECOND, 0, 6, true)
    .toFormatter();

System.out.println(LocalDateTime.parse("2015-05-07 13:20:22.276052", formatter));
System.out.println(LocalDateTime.parse("2015-05-07 13:20:22.276", formatter));
System.out.println(LocalDateTime.parse("2015-05-07 13:20:22", formatter));

// output
2015-05-07T13:20:22.276052
2015-05-07T13:20:22.276
2015-05-07T13:20:22

The answer by JiriS is incorrect, as it uses appendValue whereas the correct way is to use DateTimeFormatterBuilder.appendFraction (which also handles the decimal point). The difference can be seen in the second system out, where appendValue incorrectly parses "2015-05-07T13:20:22.000276".

When parsing, LocalDateTime.parse(str, formatter) is a neater approach than using the formatter directly in most cases.

When using the builder, take advantage of appendPattern() and optionalStart() to keep things neat.

回答2:

And this one works

DateTimeFormatter formatter
= new java.time.format.DateTimeFormatterBuilder()
        .append( java.time.format.DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss") )
        .appendOptional(
                new java.time.format.DateTimeFormatterBuilder()
                    .appendLiteral('.')
                    .appendValue( ChronoField.MICRO_OF_SECOND, 1, 6, SignStyle.NOT_NEGATIVE).toFormatter())
        .toFormatter();