使用DateTimeFormatter解析任意長度的日期字符串

Question

我試圖解析（幾乎）任意長度的日期字符串。我使用SimpleDateFormat的方法是這樣的

private Date parseWithSimpleDateFormat(String dateString) throws ParseException { 
    String pattern = "yyyyMMddHHmmss".substring(0, dateString.length()); 
    SimpleDateFormat format = new SimpleDateFormat(pattern); 
    return format.parse(dateString); 
} 

...我想用新的Date API做「更好」。我想出的是以下

private static final DateTimeFormatter FLEXIBLE_FORMATTER = new DateTimeFormatterBuilder() 
    .appendPattern("yyyy[MM[dd[HH[mm[ss]]]]]") 
    .parseDefaulting(ChronoField.MONTH_OF_YEAR, 1) 
    .parseDefaulting(ChronoField.DAY_OF_MONTH, 1) 
    .parseDefaulting(ChronoField.HOUR_OF_DAY, 0) 
    .parseDefaulting(ChronoField.MINUTE_OF_HOUR, 0) 
    .parseDefaulting(ChronoField.SECOND_OF_MINUTE, 0) 
    .toFormatter(); 

private Date parseWithDateTimeFormatter(String dateString) { 
    LocalDateTime localDateTime = LocalDateTime.parse(dateString, FLEXIBLE_FORMATTER); 
    ZonedDateTime zonedDateTime = localDateTime.atZone(ZoneId.systemDefault()); 
    Instant instant = zonedDateTime.toInstant(); 
    return Date.from(instant); 
} 

有以下結果

parseWithDateTimeFormatter("2016"); // works as intended 
parseWithDateTimeFormatter("201605"); // Text '201605' could not be parsed at index 0 
parseWithDateTimeFormatter("20160504"); // Text '20160504' could not be parsed at index 0 
parseWithDateTimeFormatter("2016050416"); // Text '2016050416' could not be parsed at index 0 
parseWithDateTimeFormatter("201605041636"); // Text '201605041636' could not be parsed at index 0 

我在做什麼錯在這裏，或我將如何進一步解決？

Answer 1

你可以使用這個修改的格式，以避免解析超過4個位數的年份：

private static final DateTimeFormatter FLEXIBLE_FORMATTER = 
    new DateTimeFormatterBuilder() 
    .appendValue(ChronoField.YEAR, 4) 
    .appendPattern("[MM[dd[HH[mm[ss]]]]]") 
    .parseDefaulting(ChronoField.MONTH_OF_YEAR, 1) 
    .parseDefaulting(ChronoField.DAY_OF_MONTH, 1) 
    .parseDefaulting(ChronoField.HOUR_OF_DAY, 0) 
    .parseDefaulting(ChronoField.MINUTE_OF_HOUR, 0) 
    .parseDefaulting(ChronoField.SECOND_OF_MINUTE, 0) 
    .toFormatter(); 

相對於其他領域，如一個月（MM）等，年字段符號Y沒有限制爲四位數，如y字母數所示。