Why?

在项目开发过程中中经常遇到时间处理。一般是用了Date配合Calendar再配合SimpleDateFormat使用。但是

1. 阿里巴巴开发手册中却禁用SimpleDateFormat.因为线程不安全。
2. 而且Date中好多方法已经过时
3. 方法返回的值不直观,可读性差。
4. jdk8新出的比较先进, 更新了好多东西。

What?

LocalDate

使用

//获取当前年月日
LocalDate localDate = LocalDate.now();
//构造指定的年月日
LocalDate localDate1 = LocalDate.of(2020, 1, 10);
System.out.println(localDate);//2020-01-21
System.out.println(localDate1);//2020-01-10

System.out.println(localDate.get(ChronoField.YEAR));//2020
System.out.println(localDate.getDayOfMonth());//21
System.out.println(localDate.getDayOfYear());//21
System.out.println(localDate.getLong(ChronoField.DAY_OF_MONTH));//21
System.out.println(localDate.getMonthValue());//1
System.out.println(localDate.getYear());//2020
System.out.println(localDate.getChronology());//ISO
System.out.println(localDate.getDayOfWeek());//TUESDAY
System.out.println(localDate.getEra());//CE
System.out.println(localDate.getMonth());//JANUARY
System.out.println(localDate.lengthOfMonth());//31
System.out.println(localDate.lengthOfYear());//366
System.out.println(localDate.range(ChronoField.MONTH_OF_YEAR));//1 - 12
System.out.println(localDate.compareTo(localDate1));//11
System.out.println(localDate.format(DateTimeFormatter.ofPattern("YYYY--MM--dd")));//2020--01--21
System.out.println(localDate.isAfter(localDate1));//true
System.out.println(localDate.isBefore(localDate1));//false
System.out.println(localDate.isEqual(localDate1));//false

源码

public final class LocalDate implements Temporal, TemporalAdjuster, ChronoLocalDate, Serializable {
    //支持的最小日期：最小年的1月1日==>+999999999-12-31
    public static final LocalDate MIN = LocalDate.of(Year.MIN_VALUE, 1, 1);
    //支持的最大日期：最大年的12月31日==>-999999999-01-01
    public static final LocalDate MAX = LocalDate.of(Year.MAX_VALUE, 12, 31);
    //序列化版本UID
    private static final long serialVersionUID = 2942565459149668126L;
    //每400年一个大周期(平年闰年等等)等于146097天
    private static final int DAYS_PER_CYCLE = 146097;
    //从0年到1970年经过的天数==>5个400年大周期减去30年(其中1970-2000有7个闰年所以加上7天)
    static final long DAYS_0000_TO_1970 = (DAYS_PER_CYCLE * 5L) - (30L * 365L + 7L);
    //年月日用不同数据类型保存节省资源
    private final int year;
    private final short month;
    private final short day;
    //获取当地的当前时间,调用了Clock.systemDefaultZone()-->SystemClock[Asia/Shanghai]
    //然后return new SystemClock(ZoneId.systemDefault());
    //然后return TimeZone.getDefault().toZoneId();
    //此时区的字符串标识符。这是一个编程标识符，用于在内部从系统表中查找时区对象，并将它们映射到其本地化的显示名称。ID值在系统表中是唯一的，但不能用于动态创建的区域
    public static LocalDate now() {
        return now(Clock.systemDefaultZone());
    }
    //获取指定地区的当前时间,调用了Clock.system(zone)
    //然后return new SystemClock(zone);
    public static LocalDate now(ZoneId zone) {
        return now(Clock.system(zone));
    }
    //获取指定的Clock的当前时间,Clock是时钟抽象类,实现类有4个,其中SystemClock是用System.currentTimeMillis()来实现获取时间。
    public static LocalDate now(Clock clock) {
        Objects.requireNonNull(clock, "clock");
        // clock.instant()-->Instant.ofEpochMilli(millis())-->Instant.ofEpochMilli(System.currentTimeMillis())
        //其中Instant.ofEpochMilli()把精确到微妙强行精确到纳秒,存到Instant.nanoOfSecond里面
        final Instant now = clock.instant();  // called once
        ZoneOffset offset = clock.getZone().getRules().getOffset(now);  // 根据时区获取不同地区偏移量规则来获取秒偏移量
        long epochSec = now.getEpochSecond() + offset.getTotalSeconds();  // 加上去
        //一天多少秒 static final int SECONDS_PER_DAY = SECONDS_PER_HOUR * HOURS_PER_DAY;
        long epochDay = Math.floorDiv(epochSec, SECONDS_PER_DAY); // 计算出当前日
        return LocalDate.ofEpochDay(epochDay);  // 转化为年月日返回
    }
    //获取指定年月日的LocalDate对象
    public static LocalDate of(int year, Month month, int dayOfMonth) {
        YEAR.checkValidValue(year);  //return (value >= getMinimum() && value <= getMaximum());
        Objects.requireNonNull(month, "month");
        DAY_OF_MONTH.checkValidValue(dayOfMonth);
        //检查年、日合理，且月非空后返回创建后的LocalDate, create方法有检查月合理且二月29日等错误
        return create(year, month.getValue(), dayOfMonth);
    }
    //一样，检查年月日合理
    public static LocalDate of(int year, int month, int dayOfMonth) {
        YEAR.checkValidValue(year);
        MONTH_OF_YEAR.checkValidValue(month);
        DAY_OF_MONTH.checkValidValue(dayOfMonth);
        return create(year, month, dayOfMonth);
    }
    //获取指定年日的LocalDate对象,月份根据日来计算
    public static LocalDate ofYearDay(int year, int dayOfYear) {
        YEAR.checkValidValue(year);
        DAY_OF_YEAR.checkValidValue(dayOfYear);
        boolean leap = IsoChronology.INSTANCE.isLeapYear(year);
        if (dayOfYear == 366 && leap == false) {
            throw new DateTimeException("Invalid date 'DayOfYear 366' as '" + year + "' is not a leap year");
        }
        Month moy = Month.of((dayOfYear - 1) / 31 + 1);
        int monthEnd = moy.firstDayOfYear(leap) + moy.length(leap) - 1;
        if (dayOfYear > monthEnd) {
            moy = moy.plus(1);
        }
        int dom = dayOfYear - moy.firstDayOfYear(leap) + 1;
        return new LocalDate(year, moy.getValue(), dom);
    }
    //获取指定日的LocalDate对象,年月根据日来计算(日期从1970年开始)
    public static LocalDate ofEpochDay(long epochDay) {
        long zeroDay = epochDay + DAYS_0000_TO_1970;
        zeroDay -= 60;
        long adjust = 0;
        if (zeroDay < 0) {
            long adjustCycles = (zeroDay + 1) / DAYS_PER_CYCLE - 1;
            adjust = adjustCycles * 400;
            zeroDay += -adjustCycles * DAYS_PER_CYCLE;
        }
        long yearEst = (400 * zeroDay + 591) / DAYS_PER_CYCLE;
        long doyEst = zeroDay - (365 * yearEst + yearEst / 4 - yearEst / 100 + yearEst / 400);
        if (doyEst < 0) {
            yearEst--;
            doyEst = zeroDay - (365 * yearEst + yearEst / 4 - yearEst / 100 + yearEst / 400);
        }
        yearEst += adjust;
        int marchDoy0 = (int) doyEst;
        int marchMonth0 = (marchDoy0 * 5 + 2) / 153;
        int month = (marchMonth0 + 2) % 12 + 1;
        int dom = marchDoy0 - (marchMonth0 * 306 + 5) / 10 + 1;
        yearEst += marchMonth0 / 10;
        int year = YEAR.checkValidIntValue(yearEst);
        return new LocalDate(year, month, dom);
    }
    //获取指定时间的LocalDate对象
    public static LocalDate from(TemporalAccessor temporal) {
        Objects.requireNonNull(temporal, "temporal");
        LocalDate date = temporal.query(TemporalQueries.localDate());
        if (date == null) {
            throw new DateTimeException("Unable to obtain LocalDate from TemporalAccessor: " +
                    temporal + " of type " + temporal.getClass().getName());
        }
        return date;
    }
    //获取指定字符串的LocalDate对象,调用了下面那个方法,默认用ISO_LOCAL_DATE
    public static LocalDate parse(CharSequence text) {
        return parse(text, DateTimeFormatter.ISO_LOCAL_DATE);
    }
    //获取指定字符串的LocalDate对象,验证DateTimeFormatter非空,然后通过formatter.parse解析。
    public static LocalDate parse(CharSequence text, DateTimeFormatter formatter) {
        Objects.requireNonNull(formatter, "formatter");
        return formatter.parse(text, LocalDate::from);
    }
    //通过年月日创建的LocalDate对象
    private static LocalDate create(int year, int month, int dayOfMonth) {
        if (dayOfMonth > 28) {
            int dom = 31;
            switch (month) {
                case 2:
                    dom = (IsoChronology.INSTANCE.isLeapYear(year) ? 29 : 28);
                    break;
                case 4:
                case 6:
                case 9:
                case 11:
                    dom = 30;
                    break;
            }
            if (dayOfMonth > dom) {
                if (dayOfMonth == 29) {
                    throw new DateTimeException("Invalid date 'February 29' as '" + year + "' is not a leap year");
                } else {
                    throw new DateTimeException("Invalid date '" + Month.of(month).name() + " " + dayOfMonth + "'");
                }
            }
        }
        return new LocalDate(year, month, dayOfMonth);
    }
    ////通过年月日创建的LocalDate对象,保证日期正确
    private static LocalDate resolvePreviousValid(int year, int month, int day) {
        switch (month) {
            case 2:
                day = Math.min(day, IsoChronology.INSTANCE.isLeapYear(year) ? 29 : 28);
                break;
            case 4:
            case 6:
            case 9:
            case 11:
                day = Math.min(day, 30);
                break;
        }
        return new LocalDate(year, month, day);
    }
    //年月日构造LocalDate
    private LocalDate(int year, int month, int dayOfMonth) {
        this.year = year;
        this.month = (short) month;
        this.day = (short) dayOfMonth;
    }
    //检查是否支持指定的字段,比如ChronoField.DAY_OF_WEEK
    @Override  // override for Javadoc
    public boolean isSupported(TemporalField field) {
        return ChronoLocalDate.super.isSupported(field);
    }
    //检查是否支持指定的字段,比如ChronoUnit.DAYS
    @Override  // override for Javadoc
    public boolean isSupported(TemporalUnit unit) {
        return ChronoLocalDate.super.isSupported(unit);
    }
    //获取指定时间的范围localDate.range(ChronoField.DAY_OF_WEEK)输出为1 - 7
    @Override
    public ValueRange range(TemporalField field) {
        if (field instanceof ChronoField) {
            ChronoField f = (ChronoField) field;
            if (f.isDateBased()) {
                switch (f) {
                    case DAY_OF_MONTH: return ValueRange.of(1, lengthOfMonth());
                    case DAY_OF_YEAR: return ValueRange.of(1, lengthOfYear());
                    case ALIGNED_WEEK_OF_MONTH: return ValueRange.of(1, getMonth() == Month.FEBRUARY && isLeapYear() == false ? 4 : 5);
                    case YEAR_OF_ERA:
                        return (getYear() <= 0 ? ValueRange.of(1, Year.MAX_VALUE + 1) : ValueRange.of(1, Year.MAX_VALUE));
                }
                return field.range();
            }
            throw new UnsupportedTemporalTypeException("Unsupported field: " + field);
        }
        return field.rangeRefinedBy(this);
    }
    //获取LocalDate指定字段的信息,验证一下调get0
    @Override  // override for Javadoc and performance
    public int get(TemporalField field) {
        if (field instanceof ChronoField) {
            return get0(field);
        }
        return ChronoLocalDate.super.get(field);
    }
    //获取LocalDate指定字段的信息(返回Long)
    @Override
    public long getLong(TemporalField field) {
        if (field instanceof ChronoField) {
            if (field == EPOCH_DAY) {
                return toEpochDay();
            }
            if (field == PROLEPTIC_MONTH) {
                return getProlepticMonth();
            }
            return get0(field);
        }
        return field.getFrom(this);
    }
    //获取LocalDate指定字段的信息
    private int get0(TemporalField field) {
        switch ((ChronoField) field) {
            case DAY_OF_WEEK: return getDayOfWeek().getValue();
            case ALIGNED_DAY_OF_WEEK_IN_MONTH: return ((day - 1) % 7) + 1;
            case ALIGNED_DAY_OF_WEEK_IN_YEAR: return ((getDayOfYear() - 1) % 7) + 1;
            case DAY_OF_MONTH: return day;
            case DAY_OF_YEAR: return getDayOfYear();
            case EPOCH_DAY: throw new UnsupportedTemporalTypeException("Invalid field 'EpochDay' for get() method, use getLong() instead");
            case ALIGNED_WEEK_OF_MONTH: return ((day - 1) / 7) + 1;
            case ALIGNED_WEEK_OF_YEAR: return ((getDayOfYear() - 1) / 7) + 1;
            case MONTH_OF_YEAR: return month;
            case PROLEPTIC_MONTH: throw new UnsupportedTemporalTypeException("Invalid field 'ProlepticMonth' for get() method, use getLong() instead");
            case YEAR_OF_ERA: return (year >= 1 ? year : 1 - year);
            case YEAR: return year;
            case ERA: return (year >= 1 ? 1 : 0);
        }
        throw new UnsupportedTemporalTypeException("Unsupported field: " + field);
    }
    //不公开，获取以月为计时的时间
    private long getProlepticMonth() {
        return (year * 12L + month - 1);
    }
    //获取时间标准：ISO国际标准时间
    @Override
    public IsoChronology getChronology() {
        return IsoChronology.INSTANCE;
    }
    //获取纪年,返回CE(Common Era)公历纪元,耶稣生年开始。BCE(Common Era)为公元前。
    @Override // override for Javadoc
    public Era getEra() {
        return ChronoLocalDate.super.getEra();
    }
    public int getYear() {
        return year;
    }
    public int getMonthValue() {
        return month;
    }
    public Month getMonth() {
        return Month.of(month);
    }
    public int getDayOfMonth() {
        return day;
    }
    public int getDayOfYear() {
        return getMonth().firstDayOfYear(isLeapYear()) + day - 1;
    }
    public DayOfWeek getDayOfWeek() {
        int dow0 = (int)Math.floorMod(toEpochDay() + 3, 7);
        return DayOfWeek.of(dow0 + 1);
    }
    //判断是否为闰年
    @Override // override for Javadoc and performance
    public boolean isLeapYear() {
        return IsoChronology.INSTANCE.isLeapYear(year);
    }
    @Override
    public int lengthOfMonth() {
        switch (month) {
            case 2:
                return (isLeapYear() ? 29 : 28);
            case 4:
            case 6:
            case 9:
            case 11:
                return 30;
            default:
                return 31;
        }
    }
    @Override // override for Javadoc and performance
    public int lengthOfYear() {
        return (isLeapYear() ? 366 : 365);
    }
    //返回指定的日期，传入java.time.temporal.TemporalAdjusters的实现类
    @Override
    public LocalDate with(TemporalAdjuster adjuster) {
        // optimizations
        if (adjuster instanceof LocalDate) {
            return (LocalDate) adjuster;
        }
        return (LocalDate) adjuster.adjustInto(this);
    }
    //返回指定修改字段后的日期
    @Override
    public LocalDate with(TemporalField field, long newValue) {
        if (field instanceof ChronoField) {
            ChronoField f = (ChronoField) field;
            f.checkValidValue(newValue);
            switch (f) {
                case DAY_OF_WEEK: return plusDays(newValue - getDayOfWeek().getValue());
                case ALIGNED_DAY_OF_WEEK_IN_MONTH: return plusDays(newValue - getLong(ALIGNED_DAY_OF_WEEK_IN_MONTH));
                case ALIGNED_DAY_OF_WEEK_IN_YEAR: return plusDays(newValue - getLong(ALIGNED_DAY_OF_WEEK_IN_YEAR));
                case DAY_OF_MONTH: return withDayOfMonth((int) newValue);
                case DAY_OF_YEAR: return withDayOfYear((int) newValue);
                case EPOCH_DAY: return LocalDate.ofEpochDay(newValue);
                case ALIGNED_WEEK_OF_MONTH: return plusWeeks(newValue - getLong(ALIGNED_WEEK_OF_MONTH));
                case ALIGNED_WEEK_OF_YEAR: return plusWeeks(newValue - getLong(ALIGNED_WEEK_OF_YEAR));
                case MONTH_OF_YEAR: return withMonth((int) newValue);
                case PROLEPTIC_MONTH: return plusMonths(newValue - getProlepticMonth());
                case YEAR_OF_ERA: return withYear((int) (year >= 1 ? newValue : 1 - newValue));
                case YEAR: return withYear((int) newValue);
                case ERA: return (getLong(ERA) == newValue ? this : withYear(1 - year));
            }
            throw new UnsupportedTemporalTypeException("Unsupported field: " + field);
        }
        return field.adjustInto(this, newValue);
    }
    //返回修改年后的日期
    public LocalDate withYear(int year) {
        if (this.year == year) {
            return this;
        }
        YEAR.checkValidValue(year);
        return resolvePreviousValid(year, month, day);
    }
    //返回修改月后的日期
    public LocalDate withMonth(int month) {
        if (this.month == month) {
            return this;
        }
        MONTH_OF_YEAR.checkValidValue(month);
        return resolvePreviousValid(year, month, day);
    }
    //返回修改日后的日期
    public LocalDate withDayOfMonth(int dayOfMonth) {
        if (this.day == dayOfMonth) {
            return this;
        }
        return of(year, month, dayOfMonth);
    }
    //返回修改dayOfYear后的日期
    public LocalDate withDayOfYear(int dayOfYear) {
        if (this.getDayOfYear() == dayOfYear) {
            return this;
        }
        return ofYearDay(year, dayOfYear);
    }
    //时间加上某字段后的日期，比如加上两个月localDate.plus(Period.ofMonths(2))
    @Override
    public LocalDate plus(TemporalAmount amountToAdd) {
        if (amountToAdd instanceof Period) {
            Period periodToAdd = (Period) amountToAdd;
            return plusMonths(periodToAdd.toTotalMonths()).plusDays(periodToAdd.getDays());
        }
        Objects.requireNonNull(amountToAdd, "amountToAdd");
        return (LocalDate) amountToAdd.addTo(this);
    }
    //时间加上某字段后的日期，比如加上两年localDate.plus(2, ChronoUnit.YEARS)
    @Override
    public LocalDate plus(long amountToAdd, TemporalUnit unit) {
        if (unit instanceof ChronoUnit) {
            ChronoUnit f = (ChronoUnit) unit;
            switch (f) {
                case DAYS: return plusDays(amountToAdd);
                case WEEKS: return plusWeeks(amountToAdd);
                case MONTHS: return plusMonths(amountToAdd);
                case YEARS: return plusYears(amountToAdd);
                case DECADES: return plusYears(Math.multiplyExact(amountToAdd, 10));
                case CENTURIES: return plusYears(Math.multiplyExact(amountToAdd, 100));
                case MILLENNIA: return plusYears(Math.multiplyExact(amountToAdd, 1000));
                case ERAS: return with(ERA, Math.addExact(getLong(ERA), amountToAdd));
            }
            throw new UnsupportedTemporalTypeException("Unsupported unit: " + unit);
        }
        return unit.addTo(this, amountToAdd);
    }
    //加上年
    public LocalDate plusYears(long yearsToAdd) {
        if (yearsToAdd == 0) {
            return this;
        }
        int newYear = YEAR.checkValidIntValue(year + yearsToAdd);  // safe overflow
        return resolvePreviousValid(newYear, month, day);
    }
    //加上月
    public LocalDate plusMonths(long monthsToAdd) {
        if (monthsToAdd == 0) {
            return this;
        }
        long monthCount = year * 12L + (month - 1);
        long calcMonths = monthCount + monthsToAdd;  // safe overflow
        int newYear = YEAR.checkValidIntValue(Math.floorDiv(calcMonths, 12));
        int newMonth = (int)Math.floorMod(calcMonths, 12) + 1;
        return resolvePreviousValid(newYear, newMonth, day);
    }
    //加上周
    public LocalDate plusWeeks(long weeksToAdd) {
        return plusDays(Math.multiplyExact(weeksToAdd, 7));
    }
    //加上日
    public LocalDate plusDays(long daysToAdd) {
        if (daysToAdd == 0) {
            return this;
        }
        long mjDay = Math.addExact(toEpochDay(), daysToAdd);
        return LocalDate.ofEpochDay(mjDay);
    }
    //下面同理都是减
    @Override
    public LocalDate minus(TemporalAmount amountToSubtract) {
        if (amountToSubtract instanceof Period) {
            Period periodToSubtract = (Period) amountToSubtract;
            return minusMonths(periodToSubtract.toTotalMonths()).minusDays(periodToSubtract.getDays());
        }
        Objects.requireNonNull(amountToSubtract, "amountToSubtract");
        return (LocalDate) amountToSubtract.subtractFrom(this);
    }
    @Override
    public LocalDate minus(long amountToSubtract, TemporalUnit unit) {
        return (amountToSubtract == Long.MIN_VALUE ? plus(Long.MAX_VALUE, unit).plus(1, unit) : plus(-amountToSubtract, unit));
    }
    public LocalDate minusYears(long yearsToSubtract) {
        return (yearsToSubtract == Long.MIN_VALUE ? plusYears(Long.MAX_VALUE).plusYears(1) : plusYears(-yearsToSubtract));
    }
    public LocalDate minusMonths(long monthsToSubtract) {
        return (monthsToSubtract == Long.MIN_VALUE ? plusMonths(Long.MAX_VALUE).plusMonths(1) : plusMonths(-monthsToSubtract));
    }
    public LocalDate minusWeeks(long weeksToSubtract) {
        return (weeksToSubtract == Long.MIN_VALUE ? plusWeeks(Long.MAX_VALUE).plusWeeks(1) : plusWeeks(-weeksToSubtract));
    }
    public LocalDate minusDays(long daysToSubtract) {
        return (daysToSubtract == Long.MIN_VALUE ? plusDays(Long.MAX_VALUE).plusDays(1) : plusDays(-daysToSubtract));
    }
    //使用指定的查询查询此瞬间
    @SuppressWarnings("unchecked")
    @Override
    public <R> R query(TemporalQuery<R> query) {
        if (query == TemporalQueries.localDate()) {
            return (R) this;
        }
        return ChronoLocalDate.super.query(query);
    }

    //调整指定的时间对象，使其具有与此对象相同的日期。
    @Override  // override for Javadoc
    public Temporal adjustInto(Temporal temporal) {
        return ChronoLocalDate.super.adjustInto(temporal);
    }

    //根据指定的单位计算到另一个日期的时间量。
    @Override
    public long until(Temporal endExclusive, TemporalUnit unit) {
        LocalDate end = LocalDate.from(endExclusive);
        if (unit instanceof ChronoUnit) {
            switch ((ChronoUnit) unit) {
                case DAYS: return daysUntil(end);
                case WEEKS: return daysUntil(end) / 7;
                case MONTHS: return monthsUntil(end);
                case YEARS: return monthsUntil(end) / 12;
                case DECADES: return monthsUntil(end) / 120;
                case CENTURIES: return monthsUntil(end) / 1200;
                case MILLENNIA: return monthsUntil(end) / 12000;
                case ERAS: return end.getLong(ERA) - getLong(ERA);
            }
            throw new UnsupportedTemporalTypeException("Unsupported unit: " + unit);
        }
        return unit.between(this, end);
    }

    long daysUntil(LocalDate end) {
        return end.toEpochDay() - toEpochDay();  // no overflow
    }

    private long monthsUntil(LocalDate end) {
        long packed1 = getProlepticMonth() * 32L + getDayOfMonth();  // no overflow
        long packed2 = end.getProlepticMonth() * 32L + end.getDayOfMonth();  // no overflow
        return (packed2 - packed1) / 32;
    }

    //计算此日期与另一日期之间的期间作为Period
    @Override
    public Period until(ChronoLocalDate endDateExclusive) {
        LocalDate end = LocalDate.from(endDateExclusive);
        long totalMonths = end.getProlepticMonth() - this.getProlepticMonth();  // safe
        int days = end.day - this.day;
        if (totalMonths > 0 && days < 0) {
            totalMonths--;
            LocalDate calcDate = this.plusMonths(totalMonths);
            days = (int) (end.toEpochDay() - calcDate.toEpochDay());  // safe
        } else if (totalMonths < 0 && days > 0) {
            totalMonths++;
            days -= end.lengthOfMonth();
        }
        long years = totalMonths / 12;  // safe
        int months = (int) (totalMonths % 12);  // safe
        return Period.of(Math.toIntExact(years), months, days);
    }

    //使用特定的字符串格式化时间
    @Override  // override for Javadoc and performance
    public String format(DateTimeFormatter formatter) {
        Objects.requireNonNull(formatter, "formatter");
        return formatter.format(this);
    }

    //将此日期与时间组合以创建LocalDateTime
    @Override
    public LocalDateTime atTime(LocalTime time) {
        return LocalDateTime.of(this, time);
    }
    public LocalDateTime atTime(int hour, int minute) {
        return atTime(LocalTime.of(hour, minute));
    }
    public LocalDateTime atTime(int hour, int minute, int second) {
        return atTime(LocalTime.of(hour, minute, second));
    }
    public LocalDateTime atTime(int hour, int minute, int second, int nanoOfSecond) {
        return atTime(LocalTime.of(hour, minute, second, nanoOfSecond));
    }
    public OffsetDateTime atTime(OffsetTime time) {
        return OffsetDateTime.of(LocalDateTime.of(this, time.toLocalTime()), time.getOffset());
    }

    //将此日期与午夜时间相结合以创建LocalDateTime
    public LocalDateTime atStartOfDay() {
        return LocalDateTime.of(this, LocalTime.MIDNIGHT);
    }

    //根据时区中的规则，在最早的有效时间返回从该日期开始的分区日期时间。
    public ZonedDateTime atStartOfDay(ZoneId zone) {
        Objects.requireNonNull(zone, "zone");
        // need to handle case where there is a gap from 11:30 to 00:30
        // standard ZDT factory would result in 01:00 rather than 00:30
        LocalDateTime ldt = atTime(LocalTime.MIDNIGHT);
        if (zone instanceof ZoneOffset == false) {
            ZoneRules rules = zone.getRules();
            ZoneOffsetTransition trans = rules.getTransition(ldt);
            if (trans != null && trans.isGap()) {
                ldt = trans.getDateTimeAfter();
            }
        }
        return ZonedDateTime.of(ldt, zone);
    }
    //转换为公元日(从1970年1月1日到现在经过了多少天)
    @Override
    public long toEpochDay() {
        long y = year;
        long m = month;
        long total = 0;
        total += 365 * y;
        if (y >= 0) {
            total += (y + 3) / 4 - (y + 99) / 100 + (y + 399) / 400;
        } else {
            total -= y / -4 - y / -100 + y / -400;
        }
        total += ((367 * m - 362) / 12);
        total += day - 1;
        if (m > 2) {
            total--;
            if (isLeapYear() == false) {
                total--;
            }
        }
        return total - DAYS_0000_TO_1970;
    }
    //与指定时间比较，调用compareTo0()
    @Override  // override for Javadoc and performance
    public int compareTo(ChronoLocalDate other) {
        if (other instanceof LocalDate) {
            return compareTo0((LocalDate) other);
        }
        return ChronoLocalDate.super.compareTo(other);
    }
    //与指定时间比较
    int compareTo0(LocalDate otherDate) {
        int cmp = (year - otherDate.year);
        if (cmp == 0) {
            cmp = (month - otherDate.month);
            if (cmp == 0) {
                cmp = (day - otherDate.day);
            }
        }
        return cmp;
    }
    //判断时间的先后，判断时间是在指定时间的前面
    @Override
    public boolean isAfter(ChronoLocalDate other) {
        if (other instanceof LocalDate) {
            return compareTo0((LocalDate) other) > 0;
        }
        return ChronoLocalDate.super.isAfter(other);
    }
    //判断时间的先后，判断时间是在指定时间的后面
    @Override
    public boolean isBefore(ChronoLocalDate other) {
        if (other instanceof LocalDate) {
            return compareTo0((LocalDate) other) < 0;
        }
        return ChronoLocalDate.super.isBefore(other);
    }
    //判断时间是否相等
    @Override  // override for Javadoc and performance
    public boolean isEqual(ChronoLocalDate other) {
        if (other instanceof LocalDate) {
            return compareTo0((LocalDate) other) == 0;
        }
        return ChronoLocalDate.super.isEqual(other);
    }
    @Override
    public boolean equals(Object obj) {
        if (this == obj) {
            return true;
        }
        if (obj instanceof LocalDate) {
            return compareTo0((LocalDate) obj) == 0;
        }
        return false;
    }
    //重写时间的hashCode
    @Override
    public int hashCode() {
        int yearValue = year;
        int monthValue = month;
        int dayValue = day;
        return (yearValue & 0xFFFFF800) ^ ((yearValue << 11) + (monthValue << 6) + (dayValue));
    }

    //以{@code 2007-12-03}的形式输出字符串
    @Override
    public String toString() {
        int yearValue = year;
        int monthValue = month;
        int dayValue = day;
        int absYear = Math.abs(yearValue);
        StringBuilder buf = new StringBuilder(10);
        if (absYear < 1000) {
            if (yearValue < 0) {
                buf.append(yearValue - 10000).deleteCharAt(1);
            } else {
                buf.append(yearValue + 10000).deleteCharAt(0);
            }
        } else {
            if (yearValue > 9999) {
                buf.append('+');
            }
            buf.append(yearValue);
        }
        return buf.append(monthValue < 10 ? "-0" : "-")
            .append(monthValue)
            .append(dayValue < 10 ? "-0" : "-")
            .append(dayValue)
            .toString();
    }

    //使用java.time.Ser序列化时间对象
    private Object writeReplace() {
        return new Ser(Ser.LOCAL_DATE_TYPE, this);
    }

    //防止恶意的Stream,直接抛异常
    private void readObject(ObjectInputStream s) throws InvalidObjectException {
        throw new InvalidObjectException("Deserialization via serialization delegate");
    }
    //把时间写到输出流里面
    void writeExternal(DataOutput out) throws IOException {
        out.writeInt(year);
        out.writeByte(month);
        out.writeByte(day);
    }
    //从输出流里面读取时间创建对象
    static LocalDate readExternal(DataInput in) throws IOException {
        int year = in.readInt();
        int month = in.readByte();
        int dayOfMonth = in.readByte();
        return LocalDate.of(year, month, dayOfMonth);
    }

}

后记

1. 源码是真的长，一度想放弃了。
2. 以后源码计划，只挑重点，只挑核心。不做API翻译…
3. 后面可能会有看一些 Spring redis nginx 等等…
4. 新挖一个坑：数据库
5. 尽量做到至少月更一次 (Maybe)
6. 半入坑C#。里面的委托有点意思 类似C++函数指针。