JDK8 开始使用LcoalDateTime Insant DateTimeFormatter

SimpleDateFormat是非线程安全的

private static final ThreadLocal<DateFormat> DATE_FORMAT_THREAD_LOCAL = ThreadLocal.withInitial(() -> new SimpleDateFormat("yyyy-MM-dd"));

    // Called from Format after creating a FieldDelegateprivate StringBuffer format(Date date, StringBuffer toAppendTo,FieldDelegate delegate) {// Convert input date to time field listcalendar.setTime(date);boolean useDateFormatSymbols = useDateFormatSymbols();for (int i = 0; i < compiledPattern.length; ) {int tag = compiledPattern[i] >>> 8;int count = compiledPattern[i++] & 0xff;if (count == 255) {count = compiledPattern[i++] << 16;count |= compiledPattern[i++];}switch (tag) {case TAG_QUOTE_ASCII_CHAR:toAppendTo.append((char)count);break;case TAG_QUOTE_CHARS:toAppendTo.append(compiledPattern, i, count);i += count;break;default:subFormat(tag, count, delegate, toAppendTo, useDateFormatSymbols);break;}}return toAppendTo;}

DateTimeFormatter dateTimeFormatter = DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss");
String format = dateTimeFormatter.format(LocalDateTime.now());

Formats a date-time object using this formatter.
This formats the date-time to a String using the rules of the formatter.temporal  the temporal object to format, not null
the formatted string, not null
DateTimeException if an error occurs during formattingpublic String format(TemporalAccessor temporal) {StringBuilder buf = new StringBuilder(32);formatTo(temporal, buf);return buf.toString();
}

Formats a date-time object to an Appendable using this formatter.This outputs the formatted date-time to the specified destination.
Appendable is a general purpose interface that is implemented by all
key character output classes including StringBuffer, StringBuilder,
PrintStream and Writer.Although {@code Appendable} methods throw an {@code IOException}, this method does not.
Instead, any {@code IOException} is wrapped in a runtime exception.temporal  the temporal object to format, not null
appendable  the appendable to format to, not null
DateTimeException if an error occurs during formattingpublic void formatTo(TemporalAccessor temporal, Appendable appendable) {Objects.requireNonNull(temporal, "temporal");Objects.requireNonNull(appendable, "appendable");try {DateTimePrintContext context = new DateTimePrintContext(temporal, this);if (appendable instanceof StringBuilder) {printerParser.format(context, (StringBuilder) appendable);} else {// buffer output to avoid writing to appendable in case of errorStringBuilder buf = new StringBuilder(32);printerParser.format(context, buf);appendable.append(buf);}} catch (IOException ex) {throw new DateTimeException(ex.getMessage(), ex);}
}

　　在DateTimeFormatter#formatTo函数中重点注意DateTimePrintContext context = new DateTimePrintContext(temporal, this);这句代码表明传入的日期被封装到一个新的DateTimePrintContext对象中，所以你可以理解DateTimeFormatter#format函数就是一个简单的函数调用，并没有使用到共享数据；所以DateTimeFormatter#format是线程安全的。

MySQL 8 如何存储LocalDateTime

LocalDateTime 可以精确到纳秒（纳秒，十亿分之一秒），可是如果在数据库中使用datetime类型，那么会出先什么情况？

@Contract(value = "_,_,_,_,_,_,_->new", pure = true)  
@NotNull  
public static LocalDateTime of(     @Range(from = -999999999, to = 999999999)  int year,@Range(from = 1, to = 12)  int month,@Range(from = 1, to = 31)  int dayOfMonth,@Range(from = 0, to = 23)  int hour,@Range(from = 0, to = 59)  int minute,@Range(from = 0, to = 59)  int second,@Range(from = 0, to = 999999999)  int nanoOfSecond )
Obtains an instance of LocalDateTime from year, month, day, hour, minute, second and nanosecond.
This returns a LocalDateTime with the specified year, month, day-of-month, hour, minute, second and nanosecond. The day must be valid for the year and month, otherwise an exception will be thrown.
Params:
year – the year to represent, from MIN_YEAR to MAX_YEAR month – the month-of-year to represent, from 1 (January) to 12 (December) dayOfMonth – the day-of-month to represent, from 1 to 31 hour – the hour-of-day to represent, from 0 to 23 minute – the minute-of-hour to represent, from 0 to 59 second – the second-of-minute to represent, from 0 to 59 nanoOfSecond – the nano-of-second to represent, from 0 to 999,999,999
Returns:
the local date-time, not null

问题：存入数据库时只能精确到秒，所以会出现四舍五入的情况；例如2022-12-17T21:34:33.664475400，存入数据库是会被保存为2022-12-17T21:34:34。

解决方案：MySQL为了不丢失精度保存LocalDateTime，则需要使用bigint数据类型