Java determine one dateformat from several dateformats

问题

We want to get a correct dateformat from LOG files generated by different systems with different language and log mechanism.

We thought this can be done be a SimpleDataFormat.parse and try-catch exception way. But the follow code shows a problem.

    String tryparseString = "18-01-22 00:03:34:071";
    try {
        SimpleDateFormat sdf1 = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
        System.out.println(sdf1.parse(tryparseString));
        System.out.println(sdf1.format(sdf1.parse(tryparseString)));
        System.out.println("Yeah! We found a CAN PARSE datefromat = " + sdf1.toPattern());
        SimpleDateFormat sdf2 = new SimpleDateFormat("yy-MM-dd HH:mm:ss:SSS");
        System.out.println(sdf2.parse(tryparseString));
        System.out.println(sdf2.format(sdf2.parse(tryparseString)));
        System.out.println("Yeah! We found a CAN PARSE datefromat = " + sdf2.toPattern());
    } catch (ParseException e) {
        e.printStackTrace();
    }

This got a result as:

Sat Jan 22 00:03:34 CST 18
0018-01-22 00:03:34
Yeah! We found a CAN PARSE datefromat = yyyy-MM-dd HH:mm:ss
Mon Jan 22 00:03:34 CST 2018
18-01-22 00:03:34:000
Yeah! We found a CAN PARSE datefromat = yy-MM-dd HH:mm:ss:SSS

So, "18-01-22 00:03:34:071" can both be formatted with yyyy-MM-dd HH:mm:ss and yy-MM-dd HH:mm:ss:SSS.

The former one is far away correct.

So, is there a way to determine a correct dateformat from serveral dateformats?

回答1:

java.time to the rescue.

    String[] formats = {
            "yyyy-MM-dd HH:mm:ss",
            "yy-MM-dd HH:mm:ss:SSS"
    };

    String tryparseString = "18-01-22 00:03:34:071";

    for (String format : formats) {
        try {
            LocalDateTime.parse(tryparseString, DateTimeFormatter.ofPattern(format));
            System.out.println("Yeah! We found a CAN PARSE date format = " + format);
        } catch (DateTimeParseException dtpe) {
            System.out.println(dtpe);
        }
    }

This prints:

java.time.format.DateTimeParseException: Text '18-01-22 00:03:34:071' could not be parsed at index 0
Yeah! We found a CAN PARSE datefromat = yy-MM-dd HH:mm:ss:SSS

In other words, the first format, yyyy-MM-dd HH:mm:ss, fails to parse your sample string, and the second succeeds as you wanted it to.

If you still experience a case where a DateTimeFormatter parses a string it shouldn’t, you may try DateTimeFormatter.ofPattern(format).withResolverStyle(ResolverStyle.STRICT). It may catch a few more non-matching strings. In this case you will need uuuu and uu instead of yyyy and yy in the format patterns, though.

回答2:

Check with below code snippet. You can have a list of your data format with a priority. After it finds one suitable it will break unless continuing.

for

 String tryparseString = "18-01-22 00:03:34:071";



     public void parseDate(String tryparseString) {

        SimpleDateFormat sdf1 = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");

        SimpleDateFormat sdf2 = new SimpleDateFormat("yy-MM-dd HH:mm:ss:SSS");

        ArrayList<SimpleDateFormat> simpleDateFormats = new ArrayList<>();
        simpleDateFormats.add(sdf1);//add as your priority
        simpleDateFormats.add(sdf2);

        for (int a = 0; a < simpleDateFormats.size(); a++) {

            SimpleDateFormat simpleDateFormat = simpleDateFormats.get(a);

            try {

                System.out.println(simpleDateFormat.parse(tryparseString));
                System.out.println(simpleDateFormat.format(simpleDateFormat.parse(tryparseString)));
                System.out.println("Yeah! We found a CAN PARSE datefromat = " + sdf1.toPattern());
                break;//When Meets it will breack the parsing
            } catch (ParseException e) {
                e.printStackTrace();
            }

        }


    }

来源：https://stackoverflow.com/questions/48678978/java-determine-one-dateformat-from-several-dateformats