Automatic date format recognition in java-CodePudding

I want to parse some dates in Java, but the format is not defined and could be a lot of them (any ISO-8601 format which is already a lot, Unix timestamp in any unit, and more) Here are some samples :

1970-01-01T00:00:00.00Z
1234567890
1234567890000
1234567890000000
2021-09-20T17:27:00.000Z 02:00

The perfect parsing might be impossible because of ambiguous cases but, a solution to parse most of the common dates with some logical might be achievable (for example timestamps are considered in seconds / milli / micro / nano in order to give a date close to the 2000 era, dates like '08/07/2021' could have a default for month and day distinction). I didn't find any easy way to do it in Java while in python it is kind of possible (not working on all my samples but at least some of them) using infer_datetime_format of panda function to_datetime (https://pandas.pydata.org/docs/reference/api/pandas.to_datetime.html).

Are there some easy approach in Java?

CodePudding user response：

Well, first of all, I agree with rzwitserloot here that date parsing in free format is extremely difficult and full of ambiguities. So you are skating on thin ice and will eventually run into trouble if you just assume that a user input will be correctly parsed the way you think it will.

Nevertheless, we could make it work if I assume either of the following:

You simply don't care if it will be parsed incorrectly; or
You are doing this for fun or for learning purposes; or
You have a banner, saying:

If the parsing goes wrong, it's your fault. Don't blame us.

Anyway, the DateTimeFormatterBuilder is able to build a DateTimeFormatter which could be able to parse a lot of different patterns. Since a formatter supports optional parsing, it could be instructed to try to parse a certain value, or skip that part if no valid value could be found.

For instance, this builder is able to parse a fairly wide range of ISO-like dates, with many optional parts:

DateTimeFormatterBuilder builder = new DateTimeFormatterBuilder()
    .appendPattern("uuuu-M-d")
    .optionalStart()
        .optionalStart().appendLiteral(' ').optionalEnd()
        .optionalStart().appendLiteral('T').optionalEnd()
        .appendValue(ChronoField.HOUR_OF_DAY)
        .optionalStart()
            .appendLiteral(':')
            .appendValue(ChronoField.MINUTE_OF_HOUR)
            .optionalStart()
                .appendLiteral(':')
                .appendValue(ChronoField.SECOND_OF_MINUTE)
                .optionalStart()
                    .appendFraction(ChronoField.NANO_OF_SECOND, 1, 9, true)
                .optionalEnd()
            .optionalEnd()
        .optionalEnd()
        .appendPattern("[XXXXX][XXXX][XXX][XX][X]")
    .optionalEnd();
DateTimeFormatter formatter = builder.toFormatter(Locale.ROOT);

All of the strings below can be successfully parsed by this formatter.

Stream.of(
    "2021-09-28",
    "2021-07-04T14",
    "2021-07-04T14:06",
    "2001-09-11 00:00:15",
    "1970-01-01T00:00:15.446-08:00",
    "2021-07-04T14:06:15.2017323Z",
    "2021-09-20T17:27:00.000 02:00"
).forEach(testcase -> System.out.println(formatter.parse(testcase)));

Als you can see, with optionalStart() and optionalEnd(), you could define optional portions of the format.

There are many more patterns you probably want to parse. You could add those patterns to the abovementioned builder. Alternatively, the appendOptional(DateTimeFormatter) method could be used to include multiple builders.

CodePudding user response：

The perfect parsing might be impossible because of ambiguous cases but, a solution to parse most of the common dates with some logical might be achievable

Sure, and such wide-ranging guesswork should most definitely not be part of a standard java.* API. I think you're also wildly underestimating the ambiguity. 1234567890? It's just flat out incorrect to say that this can reasonably be parsed.

You are running into many, many problems here:

Java in general prefers throwing an error instead of guessing. This is inherent in the language (java has few optional syntax constructs; semicolons aren't optional, () for method invocations are not optional, java intentionally does not have 'truthy/false', i.e. if (foo) is only valid if foo is an expression of the boolean type, unlike e.g. python where you can stick anything in there and there's a big list of what counts as falsy, with the rest being considering truthy. When in rome, be like the romans: If this tenet annoys you, well, either learn to love it, begrudgingly accept it, or program in another language. This idea is endemic in the entire ecosystem. For what it is worth, given that debugging tends to take far longer than typing the optional constructs, java is objectively correct or at least making rational decisions for being like this.
Either you can't bring in the notion that 'hey, this number is larger than 12, therefore it cannot possibly be the month', or, you have to accept that whether a certain date format parsers properly depends on whether the day-of-month value is above or below 12. I would strongly advocate that you avoid a library that fails this rule like the plague. What possible point is there, in the end? "My app will parse your date correctly, but only for about 3/5ths of all dates?" So, given that you can't/should not take that into account, 1234567890, is that seconds-since-1970? milliseconds-since-1970? Is that the 12th of the 34th month of the year 5678, the 90th hour, and assumed zeroes for minutes, seconds, and millis? If a library guesses, that library is wrong, because you should not guess unless you're 95% sure.
The obvious and perennial "do not guess" example is, of course, 101112. Is that November 10th, 2012 (european style)? Is that October 11th, 2012 (American style), or is that November 12th, 2010 (ISO style)? These are all reasonable guesses and therefore guessing is just wrong here. Do. Not. Guess. Unless you're really sure. Given that this is a somewhat common way to enter dates, thus: Guessing at all costs is objectively silly (see above). Guessing only when it's pretty clear and erroring out otherwise is mostly useless, given that ambiguity is so easy to introduce.
The concept of guessing may be defensible but only with a lot more information. For example, if you give me the input '101112100000', there's no way it's correct to guess here. But if you also tell me that a human entered this input, and that human is clearly clued into, say, german locale, then I can see the need to be able to turn that into '10th of november 2012, 10 o'clock in the morning': Interpreting as seconds or millis since some epoch is precluded by the human factor, and the day-month-year order by locale.

You asked:

Are there some easy approach in Java?

This entire question is incorrect. The in Java part needs to be stripped from this question, and then the answer is a simple: No. There is no simple way to parse strings into date/times without a lot more information than just the input string. If another library says they can do that, they are lying, or at least, operating under a list of cultural and source assumptions as long as my leg, and you should not be using that library.

CodePudding user response：

I don't know any standard library with this functionality, but you can always use DateTimeFormatter class and guess the format looping over a list of predefined formats, or using the ones provides by this class.

This is a typichal approximation of what you want to archive.

Here you can see and old implementation https://balusc.omnifaces.org/2007/09/dateutil.html