Why did Ali recommend using LocalDateTime instead of Date?

SimpleDateFormat (SimpleDateFormat) is a format that is static and SimpleDateFormat is not allowed

By reading this article you will learn:

• LocalDate, LocalTime, LocalDateTime
• Java8 uses the new time API, including creating, formatting, parsing, calculating, and modifying

Why LocalDate, LocalTime, LocalDateTime

Date If no format is used, the printed Date is not readable

Tue Sep 10 09:34:04 CST 2019
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Time is formatted using SimpleDateFormat, but SimpleDateFormat is thread-safe. The format method of SimpleDateFormat finally calls the code:

private StringBuffer format(Date date, StringBuffer toAppendTo,
                          FieldDelegate delegate) {
    // Convert input date to time field list
    calendar.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;
}
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Calendar is a shared variable, and this shared variable has no thread-safe control.

When the format method is called by multiple threads simultaneously using the same SimpleDateFormat object, such as SimpleDateFormat modified static.

Multiple threads may call the calendar.setTime method at the same time. It is possible that as soon as one thread sets the time value, another thread changes the time value, resulting in the return of the formatting time may be wrong.

Using SimpleDateFormat in a multithreaded environment requires special attention. In addition to the fact that SimpleDateFormat is thread-unsafe, the parse method is thread-unsafe as well.

The parse method actually calls alb.establish(calendar).getTime() to parse, which is mostly done in the alb.establish(calendar) method

• Resets the attribute value of the date object CAL
• Set the CAL using the properties in calB
• Returns the set CAL object

But these three steps are not atomic operations

How to keep threads safe from sharing a SimpleDateFormat object between threads:

• Create the SimpleDateFormat object once per thread =>Creating and destroying objects is expensive
• Lock where format and parse methods are used =>Poor thread blocking performance
• Use ThreadLocal to ensure that the SimpleDateFormat object is created at most once per thread =>Better way

Date specifies the Date, month, week, and Date after n days.

The Date class has methods like getYear and getMonth. It’s Easy to get the year, month and day, but those methods are deprecated

Come On uses java8’s new date and time API together

LocalDate

It only gets the year, month and day

Create a LocalDate

// Get the current year, month and date
LocalDate localDate = LocalDate.now();
// Construct the specified year, month and day
LocalDate localDate1 = LocalDate.of(2019.9.10);
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Gets the year, month, day, and day of the week

int year = localDate.getYear();
int year1 = localDate.get(ChronoField.YEAR);
Month month = localDate.getMonth();
int month1 = localDate.get(ChronoField.MONTH_OF_YEAR);
int day = localDate.getDayOfMonth();
int day1 = localDate.get(ChronoField.DAY_OF_MONTH);
DayOfWeek dayOfWeek = localDate.getDayOfWeek();
int dayOfWeek1 = localDate.get(ChronoField.DAY_OF_WEEK);
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LocalTime

It only takes a few minutes, a few seconds

Create a LocalTime

LocalTime localTime = LocalTime.of(13.51.10);
LocalTime localTime1 = LocalTime.now();
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Get time minute second

// Get the hour
int hour = localTime.getHour();
int hour1 = localTime.get(ChronoField.HOUR_OF_DAY);
/ / to get points
int minute = localTime.getMinute();
int minute1 = localTime.get(ChronoField.MINUTE_OF_HOUR);
/ / for seconds
int second = localTime.getSecond();
int second1 = localTime.get(ChronoField.SECOND_OF_MINUTE);
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LocalDateTime

Get year, month, day, hour, minute, second, equal to LocalDate+LocalTime

Create LocalDateTime

LocalDateTime localDateTime = LocalDateTime.now();
LocalDateTime localDateTime1 = LocalDateTime.of(2019, Month.SEPTEMBER, 10.14.46.56);
LocalDateTime localDateTime2 = LocalDateTime.of(localDate, localTime);
LocalDateTime localDateTime3 = localDate.atTime(localTime);
LocalDateTime localDateTime4 = localTime.atDate(localDate);
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To obtain a LocalDate

LocalDate localDate2 = localDateTime.toLocalDate();
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To obtain a LocalTime

LocalTime localTime2 = localDateTime.toLocalTime();
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Instant

Get number of seconds

Creating Instant Objects

Instant instant = Instant.now();
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Get number of seconds

long currentSecond = instant.getEpochSecond();
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Get milliseconds

long currentMilli = instant.toEpochMilli();
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I think it’s more convenient to use System.currentTimemillis () if you’re just trying to get seconds or milliseconds

Modify LocalDate, LocalTime, LocalDateTime, and Instant

LocalDate, LocalTime, LocalDateTime, and Instant are immutable objects. Modifying these objects will return a copy

Add or subtract years, months, and days. LocalDateTime is used as an example.

LocalDateTime localDateTime = LocalDateTime.of(2019, Month.SEPTEMBER, 10.14.46.56);
// Add a year
localDateTime = localDateTime.plusYears(1);
localDateTime = localDateTime.plus(1, ChronoUnit.YEARS);
// Reduce by one month
localDateTime = localDateTime.minusMonths(1);
localDateTime = localDateTime.minus(1, ChronoUnit.MONTHS);  
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Modify some values with with

// Change the year to 2019
localDateTime = localDateTime.withYear(2020);
// Change to 2022
localDateTime = localDateTime.with(ChronoField.YEAR, 2022);
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You can also modify the month and day

Time to calculate

For example, when you want to know what day the last day of the month is and what day the next weekend is, the time and date API provides quick answers

For example, firstDayOfYear() returns the first day of the current date, and there are many other methods I won’t illustrate here

LocalDate localDate = LocalDate.now();
LocalDate localDate1 = localDate.with(firstDayOfYear());
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Formatting time

LocalDate localDate = LocalDate.of(2019.9.10);
String s1 = localDate.format(DateTimeFormatter.BASIC_ISO_DATE);
String s2 = localDate.format(DateTimeFormatter.ISO_LOCAL_DATE);
// Custom formatting
DateTimeFormatter dateTimeFormatter =   DateTimeFormatter.ofPattern("dd/MM/yyyy");
String s3 = localDate.format(dateTimeFormatter);
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The DateTimeFormatter provides a variety of formatting methods by default. If the default formatting method does not meet your requirements, you can use the DateTimeFormatter’s ofPattern method to create a custom formatting method

Parsing time

LocalDate localDate1 = LocalDate.parse("20190910", DateTimeFormatter.BASIC_ISO_DATE);
LocalDate localDate2 = LocalDate.parse("2019-09-10", DateTimeFormatter.ISO_LOCAL_DATE);
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Compared to SimpleDateFormat, DateTimeFormatter is thread-safe

summary

LocalDateTime: LocalDateTime: LocalDateTime: LocalDateTime: LocalDateTime: LocalDateTime: LocalDateTime: LocalDateTime: LocalDateTime: LocalDateTime

LocalDateTime is applied to SpringBoot

Return the LocalDateTime field as a timestamp to the front-end add date conversion class

public class LocalDateTimeConverter extends JsonSerializer<LocalDateTime> {

    @Override
    public void serialize(LocalDateTime value, JsonGenerator gen, SerializerProvider serializers) throws IOException {
    gen.writeNumber(value.toInstant(ZoneOffset.of("+ 8")).toEpochMilli()); }}Copy the code

And add @ JsonSerialize on LocalDateTime field using = LocalDateTimeConverter. Class notes, are as follows:

@JsonSerialize(using = LocalDateTimeConverter.class)
protected LocalDateTime gmtModified;
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The LocalDateTime field is returned to the front end with a formatted date specified.

@jsonFormat (shape= jsonformat.shape. STRING, pattern=” YYYY-MM-DD HH: MM :ss”);

@JsonFormat(shape=JsonFormat.Shape.STRING, pattern="yyyy-MM-dd HH:mm:ss")
protected LocalDateTime gmtModified;
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Format @datetimeFormat (pattern = “YYYY-MM-DD HH: MM :ss”);

@DateTimeFormat(pattern = "yyyy-MM-dd HH:mm:ss")
protected LocalDateTime gmtModified;
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From: juejin. Cn/post / 6844903939402383368