问题
Our client found an interesting bug today. Consider the following method:
final DateTimeFormatter englishFormatter = DateTimeFormatter.ofLocalizedDate(FormatStyle.SHORT).withLocale(Locale.ENGLISH);
System.out.println(LocalDate.parse("04/01/17", englishFormatter));
System.out.println(LocalDate.parse("4/1/17", englishFormatter));
final DateTimeFormatter germanFormatter = DateTimeFormatter.ofLocalizedDate(FormatStyle.SHORT).withLocale(Locale.GERMAN);
System.out.println(LocalDate.parse("01.04.17", germanFormatter));
System.out.println(LocalDate.parse("1.4.17", germanFormatter));
(If you don't know, these are indeed correct dates in English and German. I'd even say they're the same date [April 1 2017]. Take a moment to consider what you'd think this application should return.)
What it does return is the following:
Exception in thread "main" java.time.format.DateTimeParseException: Text '1.4.17' could not be parsed at index 0
at java.time.format.DateTimeFormatter.parseResolved0(DateTimeFormatter.java:1949)
at java.time.format.DateTimeFormatter.parse(DateTimeFormatter.java:1851)
at java.time.LocalDate.parse(LocalDate.java:400)
at Main.main(Main.java:20)
The English date format works with and without leading zeroes. The German format works only with leading zeroes.
I can't seem to find a property to change this behavior to work correctly.
How can I make the DateTimeFormatter understand German dates without leading zeroes?
Note: Our application supports multiple locales, so using a specific DateTimeFormatter (like DateTimeFormatter.ofPattern("d.M.yy")) is completely out of the question, especially since we want to parse the default date format.
回答1:
I tried the opposite: formatting the date with your localized formatters.
LocalDate testDate = LocalDate.of(2017, Month.APRIL, 1);
final DateTimeFormatter englishFormatter
= DateTimeFormatter.ofLocalizedDate(FormatStyle.SHORT)
.withLocale(Locale.ENGLISH);
System.out.println("English: " + testDate.format(englishFormatter));
final DateTimeFormatter germanFormatter
= DateTimeFormatter.ofLocalizedDate(FormatStyle.SHORT)
.withLocale(Locale.GERMAN);
System.out.println("German: " + testDate.format(germanFormatter));
I got
English: 4/1/17 German: 01.04.17
So it’s quite clear that Java thinks that standard German date formatting uses leading zeroes. If you are certain that this is wrong, you may consider filing a bug with Oracle.
To circumvent the behaviour you don’t like, with multiple locales I am afraid you will need some sort of hack. The best hack I could think of was the following. It’s not beautiful. It works.
private static final Map<Locale, DateTimeFormatter> STEFFI_S_LOCALIZED_FORMATTERS
= createSteffiSFormatters();
private static Map<Locale, DateTimeFormatter> createSteffiSFormatters() {
Map<Locale, DateTimeFormatter> formatters = new HashMap<>(2);
formatters.put(Locale.GERMAN, DateTimeFormatter.ofPattern("d.M.uu"));
return formatters;
}
public static DateTimeFormatter getLocalizedFormatter(Locale formattingLocale) {
DateTimeFormatter localizedFormatter
= STEFFI_S_LOCALIZED_FORMATTERS.get(formattingLocale);
if (localizedFormatter == null) {
localizedFormatter = DateTimeFormatter.ofLocalizedDate(FormatStyle.SHORT)
.withLocale(formattingLocale);
}
return localizedFormatter;
}
Now you can do:
final DateTimeFormatter englishFormatter = getLocalizedFormatter(Locale.ENGLISH);
System.out.println(LocalDate.parse("04/01/17", englishFormatter));
System.out.println(LocalDate.parse("4/1/17", englishFormatter));
final DateTimeFormatter germanFormatter = getLocalizedFormatter(Locale.GERMAN);
System.out.println(LocalDate.parse("01.04.17", germanFormatter));
System.out.println(LocalDate.parse("1.4.17", germanFormatter));
This prints:
2017-04-01
2017-04-01
2017-04-01
2017-04-01
回答2:
One solution is to trap the DateTimeParseException and then try again with a modified/reduced date pattern.
import java.text.Format;
import java.time.LocalDate;
import java.time.chrono.IsoChronology;
import java.time.format.DateTimeFormatter;
import java.time.format.DateTimeFormatterBuilder;
import java.time.format.DateTimeParseException;
import java.time.format.FormatStyle;
import java.util.ListResourceBundle;
import java.util.Locale;
public class Demo {
public static void main(String[] args) {
Locale[] locales = {Locale.ENGLISH, Locale.GERMAN};
for (Locale locale : locales)
{
DateTimeFormatter formatter = DateTimeFormatter.ofLocalizedDate(FormatStyle.SHORT).withLocale(locale);
String [] englishDates = {"01/04/17","1/4/17"};
String [] germanDates = {"01.04.17","1.4.17"};
String datePattern = DateTimeFormatterBuilder.getLocalizedDateTimePattern(FormatStyle.SHORT,null, IsoChronology.INSTANCE, locale);
System.out.println("Locale " + locale.getDisplayName() + ": Default Date Pattern (short): " + datePattern);
String[] dates = null;
if (locale == Locale.ENGLISH)
dates = englishDates;
else
dates = germanDates;
for (String date : dates) {
try {
System.out.printf(" " + date + " -> ");
System.out.println(LocalDate.parse(date, formatter));
}
catch (DateTimeParseException e)
{
System.out.println("Error!");
// Try alternate pattern
datePattern = datePattern.replace("dd", "d").replace("MM", "M");
System.out.println(" Modified Date Pattern (short): " + datePattern);
// Allow single digits in day and month
DateTimeFormatter modifiedFormatter = DateTimeFormatter.ofPattern(datePattern);
System.out.println(" " + date + " -> " + LocalDate.parse(date, modifiedFormatter)); // OK
}
}
}
}
}
This gives:
Locale English: Default Date Pattern (short): M/d/yy
01/04/17 -> 2017-01-04
1/4/17 -> 2017-01-04
Locale German: Default Date Pattern (short): dd.MM.yy
01.04.17 -> 2017-04-01
1.4.17 -> Error!
Modified Date Pattern (short): d.M.yy
1.4.17 -> 2017-04-01
来源：https://stackoverflow.com/questions/47773148/datetimeformatter-cannot-parse-dates-without-leading-zeroes-in-german