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();}画重点:
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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