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转自：Telami，

链接：telami.cn/2020/smart_localdate/

前两天线上出了个小问题，有个统计页面报错了。简单一看，原来是前端传了个无效日期，2020-06-31。

代码抛异常在这一行。

LocalDate.parse(param.getEndDate())

错误信息如下：

java.time.format.DateTimeParseException: Text '2020-06-31' could not be parsed: Invalid date 'JUNE 31'

先不管为啥前端传了个0631，为啥我这转换日期会报错呢？已经加了校验了啊。

public static final DateTimeFormatter dateTimeFormat = DateTimeFormatter.ofPattern("yyyy-MM-dd");
public static boolean isDateTimeFormat2(String date) {String regex = "[0-9]{4}-[0-9]{2}-[0-9]{2}";Pattern pattern = Pattern.compile(regex);Matcher m = pattern.matcher(date);boolean dateFlag = m.matches();if (!dateFlag) {return false;} else {try {LocalDate.parse(date, dateTimeFormat);return true;} catch (DateTimeParseException var6) {return false;}}
}

上面就是校验代码，用了好久了，debug了一下，发现确实校验通过了。

嗯？等等，我明明传的【2020-06-31】，怎么变成【2020-06-30】了，咋回事？

看看源码吧。

/*** Obtains an instance of {@code LocalDate} from a text string such as {@code 2007-12-03}.* <p>* The string must represent a valid date and is parsed using* {@link java.time.format.DateTimeFormatter#ISO_LOCAL_DATE}.** @param text  the text to parse such as "2007-12-03", not null* @return the parsed local date, not null* @throws DateTimeParseException if the text cannot be parsed*/
public static LocalDate parse(CharSequence text) {return parse(text, DateTimeFormatter.ISO_LOCAL_DATE);
}
/*** Obtains an instance of {@code LocalDate} from a text string using a specific formatter.* <p>* The text is parsed using the formatter, returning a date.** @param text  the text to parse, not null* @param formatter  the formatter to use, not null* @return the parsed local date, not null* @throws DateTimeParseException if the text cannot be parsed*/
public static LocalDate parse(CharSequence text, DateTimeFormatter formatter) {Objects.requireNonNull(formatter, "formatter");return formatter.parse(text, LocalDate::from);
}

卖关子好累，不卖了。

LocalDate.parse 方法有两个，区别就是指没指定 DateTimeFormatter。

很明显上面的没指定，下面那个指定了。

/*** Creates a formatter using the specified pattern.* <p>* This method will create a formatter based on a simple* <a href="#patterns">pattern of letters and symbols</a>* as described in the class documentation.* For example, {@code d MMM uuuu} will format 2011-12-03 as '3 Dec 2011'.* <p>* The formatter will use the {@link Locale#getDefault(Locale.Category) default FORMAT locale}.* This can be changed using {@link DateTimeFormatter#withLocale(Locale)} on the returned formatter* Alternatively use the {@link #ofPattern(String, Locale)} variant of this method.* <p>* The returned formatter has no override chronology or zone.* It uses {@link ResolverStyle#SMART SMART} resolver style.** @param pattern  the pattern to use, not null* @return the formatter based on the pattern, not null* @throws IllegalArgumentException if the pattern is invalid* @see DateTimeFormatterBuilder#appendPattern(String)*/
public static DateTimeFormatter ofPattern(String pattern) {return new DateTimeFormatterBuilder().appendPattern(pattern).toFormatter();
}

划重点：

It uses {@link ResolverStyle#SMART SMART} resolver style.

SMART（聪明的，智能的），话说我经历好几个叫SMART的项目了……DateTimeFormatter.ofPattern，使用了智能解析模式。

public enum ResolverStyle {/*** Style to resolve dates and times strictly.* <p>* Using strict resolution will ensure that all parsed values are within* the outer range of valid values for the field. Individual fields may* be further processed for strictness.* <p>* For example, resolving year-month and day-of-month in the ISO calendar* system using strict mode will ensure that the day-of-month is valid* for the year-month, rejecting invalid values.*/STRICT,/*** Style to resolve dates and times in a smart, or intelligent, manner.* <p>* Using smart resolution will perform the sensible default for each* field, which may be the same as strict, the same as lenient, or a third* behavior. Individual fields will interpret this differently.* <p>* For example, resolving year-month and day-of-month in the ISO calendar* system using smart mode will ensure that the day-of-month is from* 1 to 31, converting any value beyond the last valid day-of-month to be* the last valid day-of-month.*/SMART,/*** Style to resolve dates and times leniently.* <p>* Using lenient resolution will resolve the values in an appropriate* lenient manner. Individual fields will interpret this differently.* <p>* For example, lenient mode allows the month in the ISO calendar system* to be outside the range 1 to 12.* For example, month 15 is treated as being 3 months after month 12.*/LENIENT;
}

怎么个智能法呢？

1 to 31, converting any value beyond the last valid day-of-month to be the last valid day-of-month.

超出这个月的最后有效日，会被转化为这个月的最后有效日。就是说 31 就变成 30 了，但是 32 不会，因为不在 1~31 之间。

现在我们知道了，为啥会开篇所提的 2020-06-31 会通过了校验，因为它是 SMART 模式。

public static final DateTimeFormatter ISO_LOCAL_DATE;
static {ISO_LOCAL_DATE = new DateTimeFormatterBuilder().appendValue(YEAR, 4, 10, SignStyle.EXCEEDS_PAD).appendLiteral('-').appendValue(MONTH_OF_YEAR, 2).appendLiteral('-').appendValue(DAY_OF_MONTH, 2).toFormatter(ResolverStyle.STRICT, IsoChronology.INSTANCE);
}

而没指定 DateTimeFormatter 的，则使用了默认的 ISO_LOCAL_DATE。可以看出，它使用了 ResolverStyle.STRICT，严格模式。

到这里，就是全部真相了，看来JDK Smart与否，还得看使用者啊。

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