Question

我的团队正在寻求从Joda时间切换到java.time，但我们在使用相同模式的格式化中看到了不同的行为。当我们使用星期一周w符号时会出现问题：

final String dateString = "2016-01-04 00:00:00";
final String inputPattern = "yyyy-MM-dd HH:mm:ss";

// parse the input string using Joda
final org.joda.time.format.DateTimeFormatter jodaInputFormatter = org.joda.time.format.DateTimeFormat.forPattern(inputPattern);
final org.joda.time.DateTime jodaDateTime = jodaInputFormatter.parseDateTime(dateString);

// parse the input string using two different java.time classes
final java.time.format.DateTimeFormatter javaTimeInputFormatter = java.time.format.DateTimeFormatter.ofPattern(inputPattern).withZone(java.time.ZoneOffset.UTC);
final java.time.LocalDateTime localDateTime = java.time.LocalDateTime.parse(dateString, javaTimeInputFormatter);
final java.time.ZonedDateTime zonedDateTime = java.time.ZonedDateTime.parse(dateString, javaTimeInputFormatter);

final String outputPattern = "'week' w - dd/MM/yyyy HH:mm:ss";
final org.joda.time.format.DateTimeFormatter jodaOutputFormatter = org.joda.time.format.DateTimeFormat.forPattern(outputPattern);
final java.time.format.DateTimeFormatter javaTimeOutputFormatter = java.time.format.DateTimeFormatter.ofPattern(outputPattern);

// output: week 1 - 04/01/2016 00:00:00
System.out.println("With joda: " + jodaOutputFormatter.print(jodaDateTime));
// output: week 2 - 04/01/2016 00:00:00
System.out.println("With LocalDateTime: " + javaTimeOutputFormatter.format(localDateTime));
// output: week 2 - 04/01/2016 00:00:00
System.out.println("With ZonedDateTime: " + javaTimeOutputFormatter.format(zonedDateTime));

出于某种原因，w符号的输出在两个实现中是逐个的。

造成这种不一致的原因是什么？ Joda时间和w之间是否java.time符号不一致？

Answer 1

Well, it is a little bit speculative, but since you told me that your system timezone is EST (-05:00) I assume that you are sitting in US (New York?). And US does not apply ISO-8601-week rules. Weeks start on Sunday, and the first week of the year does not need to contain at least 4 days (even one day is enough to be counted as first week of year).

So let's look at your example date of 4th of January. It is a Monday. The first US-week is from 2016-01-01 until 2016-01-02 (2 days - enough for US). And the second US-week starts on Sunday the 3rd of January, so the fourth of January is in the second week, too.

And now the critical point: java.time (JSR-310) uses a localized week of week-based-year for the pattern symbol w, see also its backport which should have the same code. Code excerpt:

} else if (cur == 'w') {
    if (count > 2) {
        throw new IllegalArgumentException("Too many pattern letters: " + cur);
    }
    appendInternal(new WeekFieldsPrinterParser('w', count));

...

static final class WeekFieldsPrinterParser implements DateTimePrinterParser {
    private final char letter;
    private final int count;

    public WeekFieldsPrinterParser(char letter, int count) {
        this.letter = letter;
        this.count = count;
    }

    @Override
    public boolean print(DateTimePrintContext context, StringBuilder buf) {
        WeekFields weekFields = WeekFields.of(context.getLocale());
        DateTimePrinterParser pp = evaluate(weekFields);
        return pp.print(context, buf);
    }

The use of WeekFields.of(context.getLocale()) for the pattern symbol "w" is evident.

In contrast, Joda-Time only uses ISO-8601-week-definition which let weeks start on Monday and count that week as first week of year which contains at least four days in current calendar year. So the Monday 4th of January is the start of the first week-of-year because the three days before are not enough for ISO-8601 to be counted as week. Those preceding days are instead considered as last week of previous year.

Consequently, Joda-Time displays week 1 for 4th of January while java.time uses the US-week 2.

Solution of your problem is to specify the locale such that the formatter will use ISO-weeks so you get the same result as in Joda-Time. For example, you could choose Locale.UK which also uses English but other week rules. Don't rely on your default locale. This can fool you.

Answer 2

编辑：正如理查德指出的那样，我错了 - 爪哇SE actually does say基于一周的第一周是第一周的第一周，至少包含四周天，就像Joda-Time。

来自Java SE的文档IsoFields.WEEK_OF_WEEK_BASED_YEAR：

以周为基础的年份的值为1到52，如果以周为基础的年份为53周，则为53。

没有提及排除任何周数，所以假设所有周都被计算在内是有意义的。

来自Joda-Time Fields overview：

基于一周的一周，周数从1到52-53。一周的第一天定义为星期一，并给出值1.一年的第一周定义为一年中至少有四天的第一周。

1月1日和2日是2016年的第一个部分周，由于这不到4天，Joda-Time根本不算一周。 1月4日是第一周，包含四天或更长时间。

从joda时间到java.time的`w`符号格式不一致

2 个答案: