Duration.java

/*
 * Copyright (c) 2012, 2018, Oracle and/or its affiliates. All rights reserved.
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.  Oracle designates this
 * particular file as subject to the "Classpath" exception as provided
 * by Oracle in the LICENSE file that accompanied this code.
 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
 * or visit www.oracle.com if you need additional information or have any
 * questions.
 */

/*
 * This file is available under and governed by the GNU General Public
 * License version 2 only, as published by the Free Software Foundation.
 * However, the following notice accompanied the original version of this
 * file:
 *
 * Copyright (c) 2007-2012, Stephen Colebourne & Michael Nascimento Santos
 *
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions are met:
 *
 *  * Redistributions of source code must retain the above copyright notice,
 *    this list of conditions and the following disclaimer.
 *
 *  * Redistributions in binary form must reproduce the above copyright notice,
 *    this list of conditions and the following disclaimer in the documentation
 *    and/or other materials provided with the distribution.
 *
 *  * Neither the name of JSR-310 nor the names of its contributors
 *    may be used to endorse or promote products derived from this software
 *    without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
 * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
 * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
 * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR
 * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
 * EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
 * PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
 * PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
 * LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
 * NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
 * SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */
package java.time;

import java.io.DataInput;
import java.io.DataOutput;
import java.io.IOException;
import java.io.InvalidObjectException;
import java.io.ObjectInputStream;
import java.io.Serializable;
import java.math.BigDecimal;
import java.math.BigInteger;
import java.math.RoundingMode;
import java.time.format.DateTimeParseException;
import java.time.temporal.ChronoField;
import java.time.temporal.ChronoUnit;
import java.time.temporal.Temporal;
import java.time.temporal.TemporalAmount;
import java.time.temporal.TemporalUnit;
import java.time.temporal.UnsupportedTemporalTypeException;
import java.util.List;
import java.util.Objects;
import java.util.regex.Matcher;
import java.util.regex.Pattern;

import static java.time.LocalTime.MINUTES_PER_HOUR;
import static java.time.LocalTime.NANOS_PER_MILLI;
import static java.time.LocalTime.NANOS_PER_SECOND;
import static java.time.LocalTime.SECONDS_PER_DAY;
import static java.time.LocalTime.SECONDS_PER_HOUR;
import static java.time.LocalTime.SECONDS_PER_MINUTE;
import static java.time.temporal.ChronoField.NANO_OF_SECOND;
import static java.time.temporal.ChronoUnit.DAYS;
import static java.time.temporal.ChronoUnit.NANOS;
import static java.time.temporal.ChronoUnit.SECONDS;

/**
 * A time-based amount of time, such as '34.5 seconds'.
 * <p>
 * This class models a quantity or amount of time in terms of seconds and nanoseconds.
 * It can be accessed using other duration-based units, such as minutes and hours.
 * In addition, the {@link ChronoUnit#DAYS DAYS} unit can be used and is treated as
 * exactly equal to 24 hours, thus ignoring daylight savings effects.
 * See {@link Period} for the date-based equivalent to this class.
 * <p>
 * A physical duration could be of infinite length.
 * For practicality, the duration is stored with constraints similar to {@link Instant}.
 * The duration uses nanosecond resolution with a maximum value of the seconds that can
 * be held in a {@code long}. This is greater than the current estimated age of the universe.
 * <p>
 * The range of a duration requires the storage of a number larger than a {@code long}.
 * To achieve this, the class stores a {@code long} representing seconds and an {@code int}
 * representing nanosecond-of-second, which will always be between 0 and 999,999,999.
 * The model is of a directed duration, meaning that the duration may be negative.
 * <p>
 * The duration is measured in "seconds", but these are not necessarily identical to
 * the scientific "SI second" definition based on atomic clocks.
 * This difference only impacts durations measured near a leap-second and should not affect
 * most applications.
 * See {@link Instant} for a discussion as to the meaning of the second and time-scales.
 *
 * <p>
 * This is a <a href="{@docRoot}/java.base/java/lang/doc-files/ValueBased.html">value-based</a>
 * class; use of identity-sensitive operations (including reference equality
 * ({@code ==}), identity hash code, or synchronization) on instances of
 * {@code Duration} may have unpredictable results and should be avoided.
 * The {@code equals} method should be used for comparisons.
 *
 * @implSpec This class is immutable and thread-safe.
 * @since 1.8
 */
// 时间段，包含秒/纳秒部件，精确到纳秒
public final class Duration implements TemporalAmount, Comparable<Duration>, Serializable {
    
    /**
     * Constant for nanos per second.
     */
    private static final BigInteger BI_NANOS_PER_SECOND = BigInteger.valueOf(NANOS_PER_SECOND);
    
    /**
     * Constant for a duration of zero.
     */
    public static final Duration ZERO = new Duration(0, 0); // 各项时间量为0的时间段
    
    /**
     * The number of seconds in the duration.
     */
    private final long seconds; // 秒部件
    
    /**
     * The number of nanoseconds in the duration, expressed as a fraction of the
     * number of seconds. This is always positive, and never exceeds 999,999,999.
     */
    private final int nanos;    // 纳秒部件
    
    
    
    /*▼ 构造器 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Constructs an instance of {@code Duration} using seconds and nanoseconds.
     *
     * @param seconds the length of the duration in seconds, positive or negative
     * @param nanos   the nanoseconds within the second, from 0 to 999,999,999
     */
    private Duration(long seconds, int nanos) {
        super();
        this.seconds = seconds;
        this.nanos = nanos;
    }
    
    /*▲ 构造器 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 工厂方法 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Obtains a {@code Duration} representing a number of standard 24 hour days.
     * <p>
     * The seconds are calculated based on the standard definition of a day,
     * where each day is 86400 seconds which implies a 24 hour day.
     * The nanosecond in second field is set to zero.
     *
     * @param days the number of days, positive or negative
     *
     * @return a {@code Duration}, not null
     *
     * @throws ArithmeticException if the input days exceeds the capacity of {@code Duration}
     */
    // 返回由"天"构造的时间段
    public static Duration ofDays(long days) {
        // 获取days天包含的秒数
        long seconds = Math.multiplyExact(days, SECONDS_PER_DAY);
        return create(seconds, 0);
    }
    
    /**
     * Obtains a {@code Duration} representing a number of standard hours.
     * <p>
     * The seconds are calculated based on the standard definition of an hour,
     * where each hour is 3600 seconds.
     * The nanosecond in second field is set to zero.
     *
     * @param hours the number of hours, positive or negative
     *
     * @return a {@code Duration}, not null
     *
     * @throws ArithmeticException if the input hours exceeds the capacity of {@code Duration}
     */
    // 返回由"小时"构造的时间段
    public static Duration ofHours(long hours) {
        // 获取hours小时包含的秒数
        long seconds = Math.multiplyExact(hours, SECONDS_PER_HOUR);
        return create(seconds, 0);
    }
    
    /**
     * Obtains a {@code Duration} representing a number of standard minutes.
     * <p>
     * The seconds are calculated based on the standard definition of a minute,
     * where each minute is 60 seconds.
     * The nanosecond in second field is set to zero.
     *
     * @param minutes the number of minutes, positive or negative
     *
     * @return a {@code Duration}, not null
     *
     * @throws ArithmeticException if the input minutes exceeds the capacity of {@code Duration}
     */
    // 返回由"分钟"构造的时间段
    public static Duration ofMinutes(long minutes) {
        // 获取minutes分包含的秒数
        long seconds = Math.multiplyExact(minutes, SECONDS_PER_MINUTE);
        return create(seconds, 0);
    }
    
    /**
     * Obtains a {@code Duration} representing a number of seconds.
     * <p>
     * The nanosecond in second field is set to zero.
     *
     * @param seconds the number of seconds, positive or negative
     *
     * @return a {@code Duration}, not null
     */
    // 返回由"秒"构造的时间段
    public static Duration ofSeconds(long seconds) {
        return create(seconds, 0);
    }
    
    /**
     * Obtains a {@code Duration} representing a number of seconds and an
     * adjustment in nanoseconds.
     * <p>
     * This method allows an arbitrary number of nanoseconds to be passed in.
     * The factory will alter the values of the second and nanosecond in order
     * to ensure that the stored nanosecond is in the range 0 to 999,999,999.
     * For example, the following will result in exactly the same duration:
     * <pre>
     *  Duration.ofSeconds(3, 1);
     *  Duration.ofSeconds(4, -999_999_999);
     *  Duration.ofSeconds(2, 1000_000_001);
     * </pre>
     *
     * @param seconds        the number of seconds, positive or negative
     * @param nanoAdjustment the nanosecond adjustment to the number of seconds, positive or negative
     *
     * @return a {@code Duration}, not null
     *
     * @throws ArithmeticException if the adjustment causes the seconds to exceed the capacity of {@code Duration}
     */
    // 返回由"秒"和"纳秒"构造的时间段
    public static Duration ofSeconds(long seconds, long nanoAdjustment) {
        long secs = Math.addExact(seconds, Math.floorDiv(nanoAdjustment, NANOS_PER_SECOND));
        int nos = (int) Math.floorMod(nanoAdjustment, NANOS_PER_SECOND);
        return create(secs, nos);
    }
    
    /**
     * Obtains a {@code Duration} representing a number of milliseconds.
     * <p>
     * The seconds and nanoseconds are extracted from the specified milliseconds.
     *
     * @param millis the number of milliseconds, positive or negative
     *
     * @return a {@code Duration}, not null
     */
    // 返回由"毫秒"构造的时间段
    public static Duration ofMillis(long millis) {
        long secs = millis / 1000;
        int mos = (int) (millis % 1000);
        if(mos<0) {
            mos += 1000;
            secs--;
        }
        return create(secs, mos * 1000_000);
    }
    
    /**
     * Obtains a {@code Duration} representing a number of nanoseconds.
     * <p>
     * The seconds and nanoseconds are extracted from the specified nanoseconds.
     *
     * @param nanos the number of nanoseconds, positive or negative
     *
     * @return a {@code Duration}, not null
     */
    // 返回由"纳秒"构造的时间段
    public static Duration ofNanos(long nanos) {
        long secs = nanos / NANOS_PER_SECOND;
        int nos = (int) (nanos % NANOS_PER_SECOND);
        if(nos<0) {
            nos += NANOS_PER_SECOND;
            secs--;
        }
        return create(secs, nos);
    }
    
    /**
     * Obtains a {@code Duration} representing an amount in the specified unit.
     * <p>
     * The parameters represent the two parts of a phrase like '6 Hours'. For example:
     * <pre>
     *  Duration.of(3, SECONDS);
     *  Duration.of(465, HOURS);
     * </pre>
     * Only a subset of units are accepted by this method.
     * The unit must either have an {@linkplain TemporalUnit#isDurationEstimated() exact duration} or
     * be {@link ChronoUnit#DAYS} which is treated as 24 hours. Other units throw an exception.
     *
     * @param amount the amount of the duration, measured in terms of the unit, positive or negative
     * @param unit   the unit that the duration is measured in, must have an exact duration, not null
     *
     * @return a {@code Duration}, not null
     *
     * @throws DateTimeException   if the period unit has an estimated duration
     * @throws ArithmeticException if a numeric overflow occurs
     */
    // 返回由amount个unit单位的时间量构造的时间段
    public static Duration of(long amount, TemporalUnit unit) {
        return ZERO.plus(amount, unit);
    }
    
    /**
     * Obtains an instance of {@code Duration} from a temporal amount.
     * <p>
     * This obtains a duration based on the specified amount.
     * A {@code TemporalAmount} represents an  amount of time, which may be
     * date-based or time-based, which this factory extracts to a duration.
     * <p>
     * The conversion loops around the set of units from the amount and uses
     * the {@linkplain TemporalUnit#getDuration() duration} of the unit to
     * calculate the total {@code Duration}.
     * Only a subset of units are accepted by this method. The unit must either
     * have an {@linkplain TemporalUnit#isDurationEstimated() exact duration}
     * or be {@link ChronoUnit#DAYS} which is treated as 24 hours.
     * If any other units are found then an exception is thrown.
     *
     * @param amount the temporal amount to convert, not null
     *
     * @return the equivalent duration, not null
     *
     * @throws DateTimeException   if unable to convert to a {@code Duration}
     * @throws ArithmeticException if numeric overflow occurs
     */
    // 将指定的"时间段"转换为Duration
    public static Duration from(TemporalAmount amount) {
        Objects.requireNonNull(amount, "amount");
    
        Duration duration = ZERO;
    
        // 获取"时间段"amount内包含的时间量单位
        List<TemporalUnit> temporalUnits = amount.getUnits();
    
        // 遍历所有时间量单位
        for(TemporalUnit unit : temporalUnits) {
            // 获取"时间段"amount中指定的时间量单位unit对应的时间量数值
            long amountToAdd = amount.get(unit);
        
            // 将该时间量累加到ZERO上
            duration = duration.plus(amountToAdd, unit);
        }
    
        return duration;
    }
    
    /**
     * Obtains a {@code Duration} from a text string such as {@code PnDTnHnMn.nS}.
     * <p>
     * This will parse a textual representation of a duration, including the
     * string produced by {@code toString()}. The formats accepted are based
     * on the ISO-8601 duration format {@code PnDTnHnMn.nS} with days
     * considered to be exactly 24 hours.
     * <p>
     * The string starts with an optional sign, denoted by the ASCII negative
     * or positive symbol. If negative, the whole period is negated.
     * The ASCII letter "P" is next in upper or lower case.
     * There are then four sections, each consisting of a number and a suffix.
     * The sections have suffixes in ASCII of "D", "H", "M" and "S" for
     * days, hours, minutes and seconds, accepted in upper or lower case.
     * The suffixes must occur in order. The ASCII letter "T" must occur before
     * the first occurrence, if any, of an hour, minute or second section.
     * At least one of the four sections must be present, and if "T" is present
     * there must be at least one section after the "T".
     * The number part of each section must consist of one or more ASCII digits.
     * The number may be prefixed by the ASCII negative or positive symbol.
     * The number of days, hours and minutes must parse to a {@code long}.
     * The number of seconds must parse to a {@code long} with optional fraction.
     * The decimal point may be either a dot or a comma.
     * The fractional part may have from zero to 9 digits.
     * <p>
     * The leading plus/minus sign, and negative values for other units are
     * not part of the ISO-8601 standard.
     * <p>
     * Examples:
     * <pre>
     *    "PT20.345S" -- parses as "20.345 seconds"
     *    "PT15M"     -- parses as "15 minutes" (where a minute is 60 seconds)
     *    "PT10H"     -- parses as "10 hours" (where an hour is 3600 seconds)
     *    "P2D"       -- parses as "2 days" (where a day is 24 hours or 86400 seconds)
     *    "P2DT3H4M"  -- parses as "2 days, 3 hours and 4 minutes"
     *    "PT-6H3M"    -- parses as "-6 hours and +3 minutes"
     *    "-PT6H3M"    -- parses as "-6 hours and -3 minutes"
     *    "-PT-6H+3M"  -- parses as "+6 hours and -3 minutes"
     * </pre>
     *
     * @param text the text to parse, not null
     *
     * @return the parsed duration, not null
     *
     * @throws DateTimeParseException if the text cannot be parsed to a duration
     */
    /*
     * 从给定的文本中解析出一段时间信息，文本格式为"PnDTnHnMn.nS"。
     *
     * "PT20.345S"  ->  20.345秒
     * "PT15M"      ->  15分
     * "PT10H"      ->  10小时
     * "P2D"        ->  2天
     * "P2DT3H4M"   ->  2天3小时4分
     * "PT-6H3M"    -> -6小时+3分
     * "-PT6H3M"    -> -6小时-3分
     * "-PT-6H+3M"  -> +6小时-3分
     */
    public static Duration parse(CharSequence text) {
        Objects.requireNonNull(text, "text");
        
        Matcher matcher = Lazy.PATTERN.matcher(text);
        if(matcher.matches()) {
            // check for letter T but no time sections
            if(!charMatch(text, matcher.start(3), matcher.end(3), 'T')) {
                boolean negate = charMatch(text, matcher.start(1), matcher.end(1), '-');
                
                int dayStart = matcher.start(2), dayEnd = matcher.end(2);
                int hourStart = matcher.start(4), hourEnd = matcher.end(4);
                int minuteStart = matcher.start(5), minuteEnd = matcher.end(5);
                int secondStart = matcher.start(6), secondEnd = matcher.end(6);
                int fractionStart = matcher.start(7), fractionEnd = matcher.end(7);
                
                if(dayStart >= 0 || hourStart >= 0 || minuteStart >= 0 || secondStart >= 0) {
                    long daysAsSecs = parseNumber(text, dayStart, dayEnd, SECONDS_PER_DAY, "days");
                    long hoursAsSecs = parseNumber(text, hourStart, hourEnd, SECONDS_PER_HOUR, "hours");
                    long minsAsSecs = parseNumber(text, minuteStart, minuteEnd, SECONDS_PER_MINUTE, "minutes");
                    long seconds = parseNumber(text, secondStart, secondEnd, 1, "seconds");
                    boolean negativeSecs = secondStart >= 0 && text.charAt(secondStart) == '-';
                    int nanos = parseFraction(text, fractionStart, fractionEnd, negativeSecs ? -1 : 1);
                    try {
                        return create(negate, daysAsSecs, hoursAsSecs, minsAsSecs, seconds, nanos);
                    } catch(ArithmeticException ex) {
                        throw new DateTimeParseException("Text cannot be parsed to a Duration: overflow", text, 0, ex);
                    }
                }
            }
        }
        
        throw new DateTimeParseException("Text cannot be parsed to a Duration", text, 0);
    }
    
    /*▲ 工厂方法 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 转换 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Gets the number of days in this duration.
     * <p>
     * This returns the total number of days in the duration by dividing the
     * number of seconds by 86400.
     * This is based on the standard definition of a day as 24 hours.
     * <p>
     * This instance is immutable and unaffected by this method call.
     *
     * @return the number of days in the duration, may be negative
     */
    // 返回当前"时间段"包含的天数
    public long toDays() {
        return seconds / SECONDS_PER_DAY;
    }
    
    /**
     * Gets the number of hours in this duration.
     * <p>
     * This returns the total number of hours in the duration by dividing the
     * number of seconds by 3600.
     * <p>
     * This instance is immutable and unaffected by this method call.
     *
     * @return the number of hours in the duration, may be negative
     */
    // 返回当前"时间段"包含的小时数
    public long toHours() {
        return seconds / SECONDS_PER_HOUR;
    }
    
    /**
     * Gets the number of minutes in this duration.
     * <p>
     * This returns the total number of minutes in the duration by dividing the
     * number of seconds by 60.
     * <p>
     * This instance is immutable and unaffected by this method call.
     *
     * @return the number of minutes in the duration, may be negative
     */
    // 返回当前"时间段"包含的分钟数
    public long toMinutes() {
        return seconds / SECONDS_PER_MINUTE;
    }
    
    /**
     * Gets the number of seconds in this duration.
     * <p>
     * This returns the total number of whole seconds in the duration.
     * <p>
     * This instance is immutable and unaffected by this method call.
     *
     * @return the whole seconds part of the length of the duration, positive or negative
     *
     * @since 9
     */
    // 返回当前"时间段"包含的秒数
    public long toSeconds() {
        return seconds;
    }
    
    /**
     * Converts this duration to the total length in milliseconds.
     * <p>
     * If this duration is too large to fit in a {@code long} milliseconds, then an
     * exception is thrown.
     * <p>
     * If this duration has greater than millisecond precision, then the conversion
     * will drop any excess precision information as though the amount in nanoseconds
     * was subject to integer division by one million.
     *
     * @return the total length of the duration in milliseconds
     *
     * @throws ArithmeticException if numeric overflow occurs
     */
    // 返回当前"时间段"包含的毫秒数
    public long toMillis() {
        long tempSeconds = seconds;
        long tempNanos = nanos;
        if(tempSeconds<0) {
            // change the seconds and nano value to
            // handle Long.MIN_VALUE case
            tempSeconds = tempSeconds + 1;
            tempNanos = tempNanos - NANOS_PER_SECOND;
        }
        long millis = Math.multiplyExact(tempSeconds, 1000);
        millis = Math.addExact(millis, tempNanos / NANOS_PER_MILLI);
        return millis;
    }
    
    /**
     * Converts this duration to the total length in nanoseconds expressed as a {@code long}.
     * <p>
     * If this duration is too large to fit in a {@code long} nanoseconds, then an
     * exception is thrown.
     *
     * @return the total length of the duration in nanoseconds
     *
     * @throws ArithmeticException if numeric overflow occurs
     */
    // 返回当前"时间段"包含的纳秒数
    public long toNanos() {
        long tempSeconds = seconds;
        long tempNanos = nanos;
        if(tempSeconds<0) {
            // change the seconds and nano value to
            // handle Long.MIN_VALUE case
            tempSeconds = tempSeconds + 1;
            tempNanos = tempNanos - NANOS_PER_SECOND;
        }
        long totalNanos = Math.multiplyExact(tempSeconds, NANOS_PER_SECOND);
        totalNanos = Math.addExact(totalNanos, tempNanos);
        return totalNanos;
    }
    
    
    
    /*
     * // 2天3小时4分5秒6毫秒
     * Duration duration = Duration.parse("P2DT3H4M5.006S");
     * System.out.println(duration);
     *
     * System.out.println(duration.toDaysPart());    // 2
     * System.out.println(duration.toHoursPart());   // 3
     * System.out.println(duration.toMinutesPart()); // 4
     * System.out.println(duration.toSecondsPart()); // 5
     * System.out.println(duration.toMillisPart());  // 6
     */
    
    /**
     * Extracts the number of days in the duration.
     * <p>
     * This returns the total number of days in the duration by dividing the
     * number of seconds by 86400.
     * This is based on the standard definition of a day as 24 hours.
     * <p>
     * This instance is immutable and unaffected by this method call.
     *
     * @return the number of days in the duration, may be negative
     *
     * @since 9
     */
    // 将当前"时间段"转换为"天"部件
    public long toDaysPart() {
        return seconds / SECONDS_PER_DAY;
    }
    
    /**
     * Extracts the number of hours part in the duration.
     * <p>
     * This returns the number of remaining hours when dividing {@link #toHours}
     * by hours in a day.
     * This is based on the standard definition of a day as 24 hours.
     * <p>
     * This instance is immutable and unaffected by this method call.
     *
     * @return the number of hours part in the duration, may be negative
     *
     * @since 9
     */
    // 将当前"时间段"转换为"天"部件
    public int toHoursPart() {
        return (int) (toHours() % 24);
    }
    
    /**
     * Extracts the number of minutes part in the duration.
     * <p>
     * This returns the number of remaining minutes when dividing {@link #toMinutes}
     * by minutes in an hour.
     * This is based on the standard definition of an hour as 60 minutes.
     * <p>
     * This instance is immutable and unaffected by this method call.
     *
     * @return the number of minutes parts in the duration, may be negative
     *
     * @since 9
     */
    // 将当前"时间段"转换为"分钟"部件
    public int toMinutesPart() {
        return (int) (toMinutes() % MINUTES_PER_HOUR);
    }
    
    /**
     * Extracts the number of seconds part in the duration.
     * <p>
     * This returns the remaining seconds when dividing {@link #toSeconds}
     * by seconds in a minute.
     * This is based on the standard definition of a minute as 60 seconds.
     * <p>
     * This instance is immutable and unaffected by this method call.
     *
     * @return the number of seconds parts in the duration, may be negative
     *
     * @since 9
     */
    // 将当前"时间段"转换为"秒"部件
    public int toSecondsPart() {
        return (int) (seconds % SECONDS_PER_MINUTE);
    }
    
    /**
     * Extracts the number of milliseconds part of the duration.
     * <p>
     * This returns the milliseconds part by dividing the number of nanoseconds by 1,000,000.
     * The length of the duration is stored using two fields - seconds and nanoseconds.
     * The nanoseconds part is a value from 0 to 999,999,999 that is an adjustment to
     * the length in seconds.
     * The total duration is defined by calling {@link #getNano()} and {@link #getSeconds()}.
     * <p>
     * This instance is immutable and unaffected by this method call.
     *
     * @return the number of milliseconds part of the duration.
     *
     * @since 9
     */
    // 将当前"时间段"转换为"毫秒"部件（注："毫秒"部件与"纳秒"部件通常不一起使用）
    public int toMillisPart() {
        return nanos / 1000_000;
    }
    
    /**
     * Get the nanoseconds part within seconds of the duration.
     * <p>
     * The length of the duration is stored using two fields - seconds and nanoseconds.
     * The nanoseconds part is a value from 0 to 999,999,999 that is an adjustment to
     * the length in seconds.
     * The total duration is defined by calling {@link #getNano()} and {@link #getSeconds()}.
     * <p>
     * This instance is immutable and unaffected by this method call.
     *
     * @return the nanoseconds within the second part of the length of the duration, from 0 to 999,999,999
     *
     * @since 9
     */
    // 将当前"时间段"转换为"纳秒"部件（注："毫秒"部件与"纳秒"部件通常不一起使用）
    public int toNanosPart() {
        return nanos;
    }
    
    /*▲ 转换 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 部件 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Gets the number of seconds in this duration.
     * <p>
     * The length of the duration is stored using two fields - seconds and nanoseconds.
     * The nanoseconds part is a value from 0 to 999,999,999 that is an adjustment to
     * the length in seconds.
     * The total duration is defined by calling this method and {@link #getNano()}.
     * <p>
     * A {@code Duration} represents a directed distance between two points on the time-line.
     * A negative duration is expressed by the negative sign of the seconds part.
     * A duration of -1 nanosecond is stored as -1 seconds plus 999,999,999 nanoseconds.
     *
     * @return the whole seconds part of the length of the duration, positive or negative
     */
    // 返回秒部件的值
    public long getSeconds() {
        return seconds;
    }
    
    /**
     * Gets the number of nanoseconds within the second in this duration.
     * <p>
     * The length of the duration is stored using two fields - seconds and nanoseconds.
     * The nanoseconds part is a value from 0 to 999,999,999 that is an adjustment to
     * the length in seconds.
     * The total duration is defined by calling this method and {@link #getSeconds()}.
     * <p>
     * A {@code Duration} represents a directed distance between two points on the time-line.
     * A negative duration is expressed by the negative sign of the seconds part.
     * A duration of -1 nanosecond is stored as -1 seconds plus 999,999,999 nanoseconds.
     *
     * @return the nanoseconds within the second part of the length of the duration, from 0 to 999,999,999
     */
    // 返回纳秒部件的值
    public int getNano() {
        return nanos;
    }
    
    /*▲ 部件 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 加法 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Returns a copy of this duration with the specified duration added.
     * <p>
     * This instance is immutable and unaffected by this method call.
     *
     * @param duration the duration to add, positive or negative, not null
     *
     * @return a {@code Duration} based on this duration with the specified duration added, not null
     *
     * @throws ArithmeticException if numeric overflow occurs
     */
    /*
     * 对当前"时间段"的值与参数中的"时间段"求和
     *
     * 如果求和后的值与当前"时间段"的值相等，则直接返回当前"时间段"对象。
     * 否则，需要构造"求和"后的新对象再返回。
     */
    public Duration plus(Duration duration) {
        return plus(duration.getSeconds(), duration.getNano());
    }
    
    /**
     * Returns a copy of this duration with the specified duration added.
     * <p>
     * The duration amount is measured in terms of the specified unit.
     * Only a subset of units are accepted by this method.
     * The unit must either have an {@linkplain TemporalUnit#isDurationEstimated() exact duration} or
     * be {@link ChronoUnit#DAYS} which is treated as 24 hours. Other units throw an exception.
     * <p>
     * This instance is immutable and unaffected by this method call.
     *
     * @param amountToAdd the amount to add, measured in terms of the unit, positive or negative
     * @param unit        the unit that the amount is measured in, must have an exact duration, not null
     *
     * @return a {@code Duration} based on this duration with the specified duration added, not null
     *
     * @throws UnsupportedTemporalTypeException if the unit is not supported
     * @throws ArithmeticException              if numeric overflow occurs
     */
    /*
     * 对当前"时间段"的值累加amountToAdd个unit单位的时间量
     *
     * 如果累加后的值与当前"时间段"的值相等，则直接返回当前"时间段"对象。
     * 否则，需要构造"累加"操作后的新对象再返回。
     */
    public Duration plus(long amountToAdd, TemporalUnit unit) {
        Objects.requireNonNull(unit, "unit");
        
        if(unit == DAYS) {
            return plus(Math.multiplyExact(amountToAdd, SECONDS_PER_DAY), 0);
        }
        
        // 要求unit具备精确的秒数，即必须为"时间"单位，而不能是"日期"单位
        if(unit.isDurationEstimated()) {
            throw new UnsupportedTemporalTypeException("Unit must not have an estimated duration");
        }
        
        if(amountToAdd == 0) {
            return this;
        }
        
        // 单位换算
        if(unit instanceof ChronoUnit) {
            switch((ChronoUnit) unit) {
                case NANOS:
                    return plusNanos(amountToAdd);
                case MICROS:
                    return plusSeconds((amountToAdd / (1000_000L * 1000)) * 1000).plusNanos((amountToAdd % (1000_000L * 1000)) * 1000);
                case MILLIS:
                    return plusMillis(amountToAdd);
                case SECONDS:
                    return plusSeconds(amountToAdd);
            }
            
            // 如果是MINUTES、HOURS、HALF_DAYS，则需要计算amountToAdd个该单位代表多少秒
            long duration = Math.multiplyExact(unit.getDuration().seconds, amountToAdd);
            return plusSeconds(duration);
        }
        
        // 计算需要累加的Duration值：每个unit代表的Duration * unit的数量
        Duration duration = unit.getDuration().multipliedBy(amountToAdd);
        
        // 先加秒再加纳秒
        return plusSeconds(duration.getSeconds()).plusNanos(duration.getNano());
    }
    
    /**
     * Returns a copy of this duration with the specified duration in standard 24 hour days added.
     * <p>
     * The number of days is multiplied by 86400 to obtain the number of seconds to add.
     * This is based on the standard definition of a day as 24 hours.
     * <p>
     * This instance is immutable and unaffected by this method call.
     *
     * @param daysToAdd the days to add, positive or negative
     *
     * @return a {@code Duration} based on this duration with the specified days added, not null
     *
     * @throws ArithmeticException if numeric overflow occurs
     */
    /*
     * 在当前"时间段"的值上累加daysToAdd天
     *
     * 如果累加后的值与当前"时间段"的值相等，则直接返回当前"时间段"对象。
     * 否则，需要构造"累加"操作后的新对象再返回。
     */
    public Duration plusDays(long daysToAdd) {
        return plus(Math.multiplyExact(daysToAdd, SECONDS_PER_DAY), 0);
    }
    
    /**
     * Returns a copy of this duration with the specified duration in hours added.
     * <p>
     * This instance is immutable and unaffected by this method call.
     *
     * @param hoursToAdd the hours to add, positive or negative
     *
     * @return a {@code Duration} based on this duration with the specified hours added, not null
     *
     * @throws ArithmeticException if numeric overflow occurs
     */
    /*
     * 在当前"时间段"的值上累加hoursToAdd小时
     *
     * 如果累加后的值与当前"时间段"的值相等，则直接返回当前"时间段"对象。
     * 否则，需要构造"累加"操作后的新对象再返回。
     */
    public Duration plusHours(long hoursToAdd) {
        return plus(Math.multiplyExact(hoursToAdd, SECONDS_PER_HOUR), 0);
    }
    
    /**
     * Returns a copy of this duration with the specified duration in minutes added.
     * <p>
     * This instance is immutable and unaffected by this method call.
     *
     * @param minutesToAdd the minutes to add, positive or negative
     *
     * @return a {@code Duration} based on this duration with the specified minutes added, not null
     *
     * @throws ArithmeticException if numeric overflow occurs
     */
    /*
     * 在当前"时间段"的值上累加minutesToAdd分钟
     *
     * 如果累加后的值与当前"时间段"的值相等，则直接返回当前"时间段"对象。
     * 否则，需要构造"累加"操作后的新对象再返回。
     */
    public Duration plusMinutes(long minutesToAdd) {
        return plus(Math.multiplyExact(minutesToAdd, SECONDS_PER_MINUTE), 0);
    }
    
    /**
     * Returns a copy of this duration with the specified duration in seconds added.
     * <p>
     * This instance is immutable and unaffected by this method call.
     *
     * @param secondsToAdd the seconds to add, positive or negative
     *
     * @return a {@code Duration} based on this duration with the specified seconds added, not null
     *
     * @throws ArithmeticException if numeric overflow occurs
     */
    /*
     * 在当前"时间段"的值上累加secondsToAdd秒
     *
     * 如果累加后的值与当前"时间段"的值相等，则直接返回当前"时间段"对象。
     * 否则，需要构造"累加"操作后的新对象再返回。
     */
    public Duration plusSeconds(long secondsToAdd) {
        return plus(secondsToAdd, 0);
    }
    
    /**
     * Returns a copy of this duration with the specified duration in milliseconds added.
     * <p>
     * This instance is immutable and unaffected by this method call.
     *
     * @param millisToAdd the milliseconds to add, positive or negative
     *
     * @return a {@code Duration} based on this duration with the specified milliseconds added, not null
     *
     * @throws ArithmeticException if numeric overflow occurs
     */
    /*
     * 在当前"时间段"的值上累加millisToAdd毫秒
     *
     * 如果累加后的值与当前"时间段"的值相等，则直接返回当前"时间段"对象。
     * 否则，需要构造"累加"操作后的新对象再返回。
     */
    public Duration plusMillis(long millisToAdd) {
        return plus(millisToAdd / 1000, (millisToAdd % 1000) * 1000_000);
    }
    
    /**
     * Returns a copy of this duration with the specified duration in nanoseconds added.
     * <p>
     * This instance is immutable and unaffected by this method call.
     *
     * @param nanosToAdd the nanoseconds to add, positive or negative
     *
     * @return a {@code Duration} based on this duration with the specified nanoseconds added, not null
     *
     * @throws ArithmeticException if numeric overflow occurs
     */
    /*
     * 在当前"时间段"的值上累加nanosToAdd纳秒
     *
     * 如果累加后的值与当前"时间段"的值相等，则直接返回当前"时间段"对象。
     * 否则，需要构造"累加"操作后的新对象再返回。
     */
    public Duration plusNanos(long nanosToAdd) {
        return plus(0, nanosToAdd);
    }
    
    /*▲ 加法 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 减法 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Returns a copy of this duration with the specified duration subtracted.
     * <p>
     * This instance is immutable and unaffected by this method call.
     *
     * @param duration the duration to subtract, positive or negative, not null
     *
     * @return a {@code Duration} based on this duration with the specified duration subtracted, not null
     *
     * @throws ArithmeticException if numeric overflow occurs
     */
    /*
     * 对当前"时间段"与参数中的"时间段"的值求差
     *
     * 如果求差后的值与当前"时间段"的值相等，则直接返回当前"时间段"对象。
     * 否则，需要构造"求差"后的新对象再返回。
     */
    public Duration minus(Duration duration) {
        long secsToSubtract = duration.getSeconds();
        int nanosToSubtract = duration.getNano();
        
        if(secsToSubtract == Long.MIN_VALUE) {
            return plus(Long.MAX_VALUE, -nanosToSubtract).plus(1, 0);
        }
        
        return plus(-secsToSubtract, -nanosToSubtract);
    }
    
    /**
     * Returns a copy of this duration with the specified duration subtracted.
     * <p>
     * The duration amount is measured in terms of the specified unit.
     * Only a subset of units are accepted by this method.
     * The unit must either have an {@linkplain TemporalUnit#isDurationEstimated() exact duration} or
     * be {@link ChronoUnit#DAYS} which is treated as 24 hours. Other units throw an exception.
     * <p>
     * This instance is immutable and unaffected by this method call.
     *
     * @param amountToSubtract the amount to subtract, measured in terms of the unit, positive or negative
     * @param unit             the unit that the amount is measured in, must have an exact duration, not null
     *
     * @return a {@code Duration} based on this duration with the specified duration subtracted, not null
     *
     * @throws ArithmeticException if numeric overflow occurs
     */
    /*
     * 对当前"时间段"的值减去amountToAdd个unit单位的时间量
     *
     * 如果减去后的值与当前"时间段"的值相等，则直接返回当前"时间段"对象。
     * 否则，需要构造"减去"操作后的新对象再返回。
     */
    public Duration minus(long amountToSubtract, TemporalUnit unit) {
        if(amountToSubtract == Long.MIN_VALUE) {
            return plus(Long.MAX_VALUE, unit).plus(1, unit);
        }
        
        return plus(-amountToSubtract, unit);
    }
    
    /**
     * Returns a copy of this duration with the specified duration in standard 24 hour days subtracted.
     * <p>
     * The number of days is multiplied by 86400 to obtain the number of seconds to subtract.
     * This is based on the standard definition of a day as 24 hours.
     * <p>
     * This instance is immutable and unaffected by this method call.
     *
     * @param daysToSubtract the days to subtract, positive or negative
     *
     * @return a {@code Duration} based on this duration with the specified days subtracted, not null
     *
     * @throws ArithmeticException if numeric overflow occurs
     */
    /*
     * 在当前"时间段"的值上减去daysToSubtract天
     *
     * 如果减去后的值与当前"时间段"的值相等，则直接返回当前"时间段"对象。
     * 否则，需要构造"减去"操作后的新对象再返回。
     */
    public Duration minusDays(long daysToSubtract) {
        if(daysToSubtract == Long.MIN_VALUE) {
            return plusDays(Long.MAX_VALUE).plusDays(1);
        }
        
        return plusDays(-daysToSubtract);
    }
    
    /**
     * Returns a copy of this duration with the specified duration in hours subtracted.
     * <p>
     * The number of hours is multiplied by 3600 to obtain the number of seconds to subtract.
     * <p>
     * This instance is immutable and unaffected by this method call.
     *
     * @param hoursToSubtract the hours to subtract, positive or negative
     *
     * @return a {@code Duration} based on this duration with the specified hours subtracted, not null
     *
     * @throws ArithmeticException if numeric overflow occurs
     */
    /*
     * 在当前"时间段"的值上减去hoursToSubtract小时
     *
     * 如果减去后的值与当前"时间段"的值相等，则直接返回当前"时间段"对象。
     * 否则，需要构造"减去"操作后的新对象再返回。
     */
    public Duration minusHours(long hoursToSubtract) {
        if(hoursToSubtract == Long.MIN_VALUE) {
            return plusHours(Long.MAX_VALUE).plusHours(1);
        }
        
        return plusHours(-hoursToSubtract);
    }
    
    /**
     * Returns a copy of this duration with the specified duration in minutes subtracted.
     * <p>
     * The number of hours is multiplied by 60 to obtain the number of seconds to subtract.
     * <p>
     * This instance is immutable and unaffected by this method call.
     *
     * @param minutesToSubtract the minutes to subtract, positive or negative
     *
     * @return a {@code Duration} based on this duration with the specified minutes subtracted, not null
     *
     * @throws ArithmeticException if numeric overflow occurs
     */
    /*
     * 在当前"时间段"的值上减去minutesToSubtract分钟
     *
     * 如果减去后的值与当前"时间段"的值相等，则直接返回当前"时间段"对象。
     * 否则，需要构造"减去"操作后的新对象再返回。
     */
    public Duration minusMinutes(long minutesToSubtract) {
        if(minutesToSubtract == Long.MIN_VALUE) {
            return plusMinutes(Long.MAX_VALUE).plusMinutes(1);
        }
        
        return plusMinutes(-minutesToSubtract);
    }
    
    /**
     * Returns a copy of this duration with the specified duration in seconds subtracted.
     * <p>
     * This instance is immutable and unaffected by this method call.
     *
     * @param secondsToSubtract the seconds to subtract, positive or negative
     *
     * @return a {@code Duration} based on this duration with the specified seconds subtracted, not null
     *
     * @throws ArithmeticException if numeric overflow occurs
     */
    /*
     * 在当前"时间段"的值上减去secondsToSubtract秒
     *
     * 如果减去后的值与当前"时间段"的值相等，则直接返回当前"时间段"对象。
     * 否则，需要构造"减去"操作后的新对象再返回。
     */
    public Duration minusSeconds(long secondsToSubtract) {
        if(secondsToSubtract == Long.MIN_VALUE) {
            return plusSeconds(Long.MAX_VALUE).plusSeconds(1);
        }
        
        return plusSeconds(-secondsToSubtract);
    }
    
    /**
     * Returns a copy of this duration with the specified duration in milliseconds subtracted.
     * <p>
     * This instance is immutable and unaffected by this method call.
     *
     * @param millisToSubtract the milliseconds to subtract, positive or negative
     *
     * @return a {@code Duration} based on this duration with the specified milliseconds subtracted, not null
     *
     * @throws ArithmeticException if numeric overflow occurs
     */
    /*
     * 在当前"时间段"的值上减去millisToSubtract毫秒
     *
     * 如果减去后的值与当前"时间段"的值相等，则直接返回当前"时间段"对象。
     * 否则，需要构造"减去"操作后的新对象再返回。
     */
    public Duration minusMillis(long millisToSubtract) {
        if(millisToSubtract == Long.MIN_VALUE) {
            return plusMillis(Long.MAX_VALUE).plusMillis(1);
        }
        
        return plusMillis(-millisToSubtract);
    }
    
    /**
     * Returns a copy of this duration with the specified duration in nanoseconds subtracted.
     * <p>
     * This instance is immutable and unaffected by this method call.
     *
     * @param nanosToSubtract the nanoseconds to subtract, positive or negative
     *
     * @return a {@code Duration} based on this duration with the specified nanoseconds subtracted, not null
     *
     * @throws ArithmeticException if numeric overflow occurs
     */
    /*
     * 在当前"时间段"的值上减去nanosToSubtract纳秒
     *
     * 如果减去后的值与当前"时间段"的值相等，则直接返回当前"时间段"对象。
     * 否则，需要构造"减去"操作后的新对象再返回。
     */
    public Duration minusNanos(long nanosToSubtract) {
        if(nanosToSubtract == Long.MIN_VALUE) {
            return plusNanos(Long.MAX_VALUE).plusNanos(1);
        }
        
        return plusNanos(-nanosToSubtract);
    }
    
    /*▲ 减法 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 乘法 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Returns a copy of this duration multiplied by the scalar.
     * <p>
     * This instance is immutable and unaffected by this method call.
     *
     * @param multiplicand the value to multiply the duration by, positive or negative
     *
     * @return a {@code Duration} based on this duration multiplied by the specified scalar, not null
     *
     * @throws ArithmeticException if numeric overflow occurs
     */
    /*
     * 在当前"时间段"的值上乘以multiplicand(即放大multiplicand倍)
     *
     * 如果乘以后的值与当前"时间段"的值相等，则直接返回当前"时间段"对象。
     * 否则，需要构造"乘以"操作后的新对象再返回。
     */
    public Duration multipliedBy(long multiplicand) {
        if(multiplicand == 0) {
            return ZERO;
        }
        
        if(multiplicand == 1) {
            return this;
        }
        
        // 返回当前Duration包含的秒；这里其实也包含了纳秒，所以返回值应当用小数表示
        BigDecimal seconds = toBigDecimalSeconds();
        // 缩放倍数
        BigDecimal scale = BigDecimal.valueOf(multiplicand);
        
        // 使用乘法放大
        return create(seconds.multiply(scale));
    }
    
    /*▲ 乘法 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 除法 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Returns a copy of this duration divided by the specified value.
     * <p>
     * This instance is immutable and unaffected by this method call.
     *
     * @param divisor the value to divide the duration by, positive or negative, not zero
     *
     * @return a {@code Duration} based on this duration divided by the specified divisor, not null
     *
     * @throws ArithmeticException if the divisor is zero or if numeric overflow occurs
     */
    /*
     * 在当前"时间段"的值上除以divisor(即缩小divisor倍)
     *
     * 如果divisor后的值与当前"时间段"的值相等，则直接返回当前"时间段"对象。
     * 否则，需要构造"divisor"操作后的新对象再返回。
     */
    public Duration dividedBy(long divisor) {
        if(divisor == 0) {
            throw new ArithmeticException("Cannot divide by zero");
        }
        
        if(divisor == 1) {
            return this;
        }
        
        // 返回当前Duration包含的秒；这里其实也包含了纳秒，所以返回值应当用小数表示
        BigDecimal seconds = toBigDecimalSeconds();
        // 缩放倍数
        BigDecimal scale = seconds.divide(BigDecimal.valueOf(divisor), RoundingMode.DOWN);
        
        // 使用除法缩小
        return create(scale);
    }
    
    /*▲ 除法 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 其他运算 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Returns a copy of this duration with the length negated.
     * <p>
     * This method swaps the sign of the total length of this duration.
     * For example, {@code PT1.3S} will be returned as {@code PT-1.3S}.
     * <p>
     * This instance is immutable and unaffected by this method call.
     *
     * @return a {@code Duration} based on this duration with the amount negated, not null
     *
     * @throws ArithmeticException if numeric overflow occurs
     */
    // 取相反数
    public Duration negated() {
        return multipliedBy(-1);
    }
    
    /**
     * Returns number of whole times a specified Duration occurs within this Duration.
     * <p>
     * This instance is immutable and unaffected by this method call.
     *
     * @param divisor the value to divide the duration by, positive or negative, not null
     *
     * @return number of whole times, rounded toward zero, a specified
     * {@code Duration} occurs within this Duration, may be negative
     *
     * @throws ArithmeticException if the divisor is zero, or if numeric overflow occurs
     * @since 9
     */
    // 计算当前"时间段"是指定"时间段"divisor的几倍
    public long dividedBy(Duration divisor) {
        Objects.requireNonNull(divisor, "divisor");
    
        // 返回当前Duration包含的秒；这里其实也包含了纳秒，所以返回值应当用小数表示
        BigDecimal dividendBigD = toBigDecimalSeconds();
        // 返回当前缩放因子divisor包含的秒
        BigDecimal divisorBigD = divisor.toBigDecimalSeconds();
    
        // 使用除法缩小
        BigDecimal value = dividendBigD.divideToIntegralValue(divisorBigD);
    
        // 将当前数值转换为long
        return value.longValueExact();
    }
    
    /**
     * Returns a copy of this duration with a positive length.
     * <p>
     * This method returns a positive duration by effectively removing the sign from any negative total length.
     * For example, {@code PT-1.3S} will be returned as {@code PT1.3S}.
     * <p>
     * This instance is immutable and unaffected by this method call.
     *
     * @return a {@code Duration} based on this duration with an absolute length, not null
     *
     * @throws ArithmeticException if numeric overflow occurs
     */
    // 取绝对值
    public Duration abs() {
        return isNegative() ? negated() : this;
    }
    
    /**
     * Checks if this duration is negative, excluding zero.
     * <p>
     * A {@code Duration} represents a directed distance between two points on
     * the time-line and can therefore be positive, zero or negative.
     * This method checks whether the length is less than zero.
     *
     * @return true if this duration has a total length less than zero
     */
    // 判断当前"时间段"是否为负
    public boolean isNegative() {
        return seconds<0;
    }
    
    /*▲ 其他运算 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 增加/减少 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Adds this duration to the specified temporal object.
     * <p>
     * This returns a temporal object of the same observable type as the input
     * with this duration added.
     * <p>
     * In most cases, it is clearer to reverse the calling pattern by using
     * {@link Temporal#plus(TemporalAmount)}.
     * <pre>
     *   // these two lines are equivalent, but the second approach is recommended
     *   dateTime = thisDuration.addTo(dateTime);
     *   dateTime = dateTime.plus(thisDuration);
     * </pre>
     * <p>
     * The calculation will add the seconds, then nanos.
     * Only non-zero amounts will be added.
     * <p>
     * This instance is immutable and unaffected by this method call.
     *
     * @param temporal the temporal object to adjust, not null
     *
     * @return an object of the same type with the adjustment made, not null
     *
     * @throws DateTimeException   if unable to add
     * @throws ArithmeticException if numeric overflow occurs
     */
    /*
     * 增加目标时间量temporal
     *
     * 尝试将当前"时间段"累加到指定的时间量temporal上，
     * 如果累加后的值与原值相同，则返回temporal自身；否则，会构造一个新对象再返回。
     */
    @Override
    public Temporal addTo(Temporal temporal) {
        
        // 先尝试添加秒部件中的值
        if(seconds != 0) {
            temporal = temporal.plus(seconds, SECONDS);
        }
        
        // 再尝试添加纳秒部件中的值
        if(nanos != 0) {
            temporal = temporal.plus(nanos, NANOS);
        }
        
        return temporal;
    }
    
    /**
     * Subtracts this duration from the specified temporal object.
     * <p>
     * This returns a temporal object of the same observable type as the input
     * with this duration subtracted.
     * <p>
     * In most cases, it is clearer to reverse the calling pattern by using
     * {@link Temporal#minus(TemporalAmount)}.
     * <pre>
     *   // these two lines are equivalent, but the second approach is recommended
     *   dateTime = thisDuration.subtractFrom(dateTime);
     *   dateTime = dateTime.minus(thisDuration);
     * </pre>
     * <p>
     * The calculation will subtract the seconds, then nanos.
     * Only non-zero amounts will be added.
     * <p>
     * This instance is immutable and unaffected by this method call.
     *
     * @param temporal the temporal object to adjust, not null
     *
     * @return an object of the same type with the adjustment made, not null
     *
     * @throws DateTimeException   if unable to subtract
     * @throws ArithmeticException if numeric overflow occurs
     */
    /*
     * 减少目标时间量temporal
     *
     * 尝试从指定的时间量temporal上减去当前"时间段"，
     * 如果减少后的值与原值相同，则返回temporal自身；否则，会构造一个新对象再返回。
     */
    @Override
    public Temporal subtractFrom(Temporal temporal) {
        if(seconds != 0) {
            temporal = temporal.minus(seconds, SECONDS);
        }
        
        if(nanos != 0) {
            temporal = temporal.minus(nanos, NANOS);
        }
        
        return temporal;
    }
    
    /*▲ 增加/减少 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 时间量单位 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Gets the set of units supported by this duration.
     * <p>
     * The supported units are {@link ChronoUnit#SECONDS SECONDS},
     * and {@link ChronoUnit#NANOS NANOS}.
     * They are returned in the order seconds, nanos.
     * <p>
     * This set can be used in conjunction with {@link #get(TemporalUnit)}
     * to access the entire state of the duration.
     *
     * @return a list containing the seconds and nanos units, not null
     */
    // 返回当前"时间段"上可用的时间量单位，这其实是该"时间段"的组成部件
    @Override
    public List<TemporalUnit> getUnits() {
        return DurationUnits.UNITS;
    }
    
    /**
     * Gets the value of the requested unit.
     * <p>
     * This returns a value for each of the two supported units,
     * {@link ChronoUnit#SECONDS SECONDS} and {@link ChronoUnit#NANOS NANOS}.
     * All other units throw an exception.
     *
     * @param unit the {@code TemporalUnit} for which to return the value
     *
     * @return the long value of the unit
     *
     * @throws DateTimeException                if the unit is not supported
     * @throws UnsupportedTemporalTypeException if the unit is not supported
     */
    // 返回当前"时间段"中指定的时间量单位unit对应的时间量数值
    @Override
    public long get(TemporalUnit unit) {
        if(unit == SECONDS) {
            return seconds;
        } else if(unit == NANOS) {
            return nanos;
        } else {
            throw new UnsupportedTemporalTypeException("Unsupported unit: " + unit);
        }
    }
    
    /*▲ 时间量单位 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 整合 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Returns a copy of this duration with the specified amount of seconds.
     * <p>
     * This returns a duration with the specified seconds, retaining the
     * nano-of-second part of this duration.
     * <p>
     * This instance is immutable and unaffected by this method call.
     *
     * @param seconds the seconds to represent, may be negative
     *
     * @return a {@code Duration} based on this period with the requested seconds, not null
     */
    /*
     * 将指定的"秒"整合到当前时间量中以构造时间量对象。
     *
     * 如果整合后的值与当前时间量中的值相等，则直接返回当前时间量对象。
     * 否则，需要构造"整合"后的新对象再返回。
     *
     * 注：整合过程，通常是时间量部件的替换/覆盖过程。
     * 　　至于是替换/覆盖一个部件还是多个部件，则需要根据参数的意义而定。
     *
     * 影响部件：秒
     */
    public Duration withSeconds(long seconds) {
        return create(seconds, nanos);
    }
    
    /**
     * Returns a copy of this duration with the specified nano-of-second.
     * <p>
     * This returns a duration with the specified nano-of-second, retaining the
     * seconds part of this duration.
     * <p>
     * This instance is immutable and unaffected by this method call.
     *
     * @param nanoOfSecond the nano-of-second to represent, from 0 to 999,999,999
     *
     * @return a {@code Duration} based on this period with the requested nano-of-second, not null
     *
     * @throws DateTimeException if the nano-of-second is invalid
     */
    /*
     * 将指定的"纳秒"整合到当前时间量中以构造时间量对象。
     *
     * 如果整合后的值与当前时间量中的值相等，则直接返回当前时间量对象。
     * 否则，需要构造"整合"后的新对象再返回。
     *
     * 注：整合过程，通常是时间量部件的替换/覆盖过程。
     * 　　至于是替换/覆盖一个部件还是多个部件，则需要根据参数的意义而定。
     *
     * 影响部件：纳秒
     */
    public Duration withNanos(int nanoOfSecond) {
        NANO_OF_SECOND.checkValidIntValue(nanoOfSecond);
        return create(seconds, nanoOfSecond);
    }
    
    /*▲ 整合 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 杂项 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Obtains a {@code Duration} representing the duration between two temporal objects.
     * <p>
     * This calculates the duration between two temporal objects. If the objects
     * are of different types, then the duration is calculated based on the type
     * of the first object. For example, if the first argument is a {@code LocalTime}
     * then the second argument is converted to a {@code LocalTime}.
     * <p>
     * The specified temporal objects must support the {@link ChronoUnit#SECONDS SECONDS} unit.
     * For full accuracy, either the {@link ChronoUnit#NANOS NANOS} unit or the
     * {@link ChronoField#NANO_OF_SECOND NANO_OF_SECOND} field should be supported.
     * <p>
     * The result of this method can be a negative period if the end is before the start.
     * To guarantee to obtain a positive duration call {@link #abs()} on the result.
     *
     * @param startInclusive the start instant, inclusive, not null
     * @param endExclusive   the end instant, exclusive, not null
     *
     * @return a {@code Duration}, not null
     *
     * @throws DateTimeException   if the seconds between the temporals cannot be obtained
     * @throws ArithmeticException if the calculation exceeds the capacity of {@code Duration}
     */
    // 计算两个时间量之间相差多少个"时间段"
    public static Duration between(Temporal startInclusive, Temporal endExclusive) {
        try {
            // 计算startInclusive到endExclusive之间相差多少纳秒
            long diff = startInclusive.until(endExclusive, NANOS);
            
            // 返回由"纳秒"构造的时间段
            return ofNanos(diff);
        } catch(DateTimeException | ArithmeticException ex) {
            long secs = startInclusive.until(endExclusive, SECONDS);
            long nanos;
            try {
                nanos = endExclusive.getLong(NANO_OF_SECOND) - startInclusive.getLong(NANO_OF_SECOND);
                if(secs>0 && nanos<0) {
                    secs++;
                } else if(secs<0 && nanos>0) {
                    secs--;
                }
            } catch(DateTimeException ex2) {
                nanos = 0;
            }
            return ofSeconds(secs, nanos);
        }
    }
    
    /**
     * Returns a copy of this {@code Duration} truncated to the specified unit.
     * <p>
     * Truncating the duration returns a copy of the original with conceptual fields
     * smaller than the specified unit set to zero.
     * For example, truncating with the {@link ChronoUnit#MINUTES MINUTES} unit will
     * round down towards zero to the nearest minute, setting the seconds and
     * nanoseconds to zero.
     * <p>
     * The unit must have a {@linkplain TemporalUnit#getDuration() duration}
     * that divides into the length of a standard day without remainder.
     * This includes all
     * {@linkplain ChronoUnit#isTimeBased() time-based units on {@code ChronoUnit}}
     * and {@link ChronoUnit#DAYS DAYS}. Other ChronoUnits throw an exception.
     * <p>
     * This instance is immutable and unaffected by this method call.
     *
     * @param unit the unit to truncate to, not null
     *
     * @return a {@code Duration} based on this duration with the time truncated, not null
     *
     * @throws DateTimeException                if the unit is invalid for truncation
     * @throws UnsupportedTemporalTypeException if the unit is not supported
     * @since 9
     */
    /*
     * 截断(对齐)
     *
     * 将当前Duration按照unit单位进行截断(对齐)，返回截断(对齐)后的新对象。
     */
    public Duration truncatedTo(TemporalUnit unit) {
        Objects.requireNonNull(unit, "unit");
        
        // 截断(对齐)到纳秒的话，可以直接返回，因为最小的计数单位就是纳秒
        if(unit == ChronoUnit.NANOS) {
            return this;
        }
        
        // 截断(对齐)到秒也可以直接转换
        if(unit == ChronoUnit.SECONDS && (seconds >= 0 || nanos == 0)) {
            return new Duration(seconds, 0);
        }
        
        // 将时间量单位unit转换为对应的"时间段"后返回
        Duration unitDur = unit.getDuration();
        // 如果该"时间段"超过了一天，抛异常
        if(unitDur.getSeconds()>LocalTime.SECONDS_PER_DAY) {
            throw new UnsupportedTemporalTypeException("Unit is too large to be used for truncation");
        }
        
        // 获取"时间段"unitDur包含的纳秒数
        long dur = unitDur.toNanos();
        /*
         * 要求该"时间段"反映一天的中某个时间单位，比如毫秒、小时等，不能是随便的一个值...
         * 当然，系统内置的单位都是有效的单位...
         */
        if((LocalTime.NANOS_PER_DAY % dur) != 0) {
            throw new UnsupportedTemporalTypeException("Unit must divide into a standard day without remainder");
        }
        
        // 计算截断，将超出不足一个时间量单位unit的部分舍去
        long nod = (seconds % LocalTime.SECONDS_PER_DAY) * LocalTime.NANOS_PER_SECOND + nanos;
        long result = (nod / dur) * dur;
        
        /*
         * 构造一个新的Duration，该Duration在当前Duration的基础上累加了给定的纳秒偏移
         *
         * 注：这里的result - nod往往是负数，所以相当于减去的效果
         */
        return plusNanos(result - nod);
    }
    
    /**
     * Checks if this duration is zero length.
     * <p>
     * A {@code Duration} represents a directed distance between two points on
     * the time-line and can therefore be positive, zero or negative.
     * This method checks whether the length is zero.
     *
     * @return true if this duration has a total length equal to zero
     */
    // 判断当前"时间段"的值是否为0，即该"时间段"内所有计时部件的值为0
    public boolean isZero() {
        return (seconds | nanos) == 0;
    }
    
    /*▲ 杂项 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    private static boolean charMatch(CharSequence text, int start, int end, char c) {
        return (start >= 0 && end == start + 1 && text.charAt(start) == c);
    }
    
    private static long parseNumber(CharSequence text, int start, int end, int multiplier, String errorText) {
        // regex limits to [-+]?[0-9]+
        if(start<0 || end<0) {
            return 0;
        }
        
        try {
            long val = Long.parseLong(text, start, end, 10);
            return Math.multiplyExact(val, multiplier);
        } catch(NumberFormatException | ArithmeticException ex) {
            throw (DateTimeParseException) new DateTimeParseException("Text cannot be parsed to a Duration: " + errorText, text, 0).initCause(ex);
        }
    }
    
    private static int parseFraction(CharSequence text, int start, int end, int negate) {
        // regex limits to [0-9]{0,9}
        if(start<0 || end<0 || end - start == 0) {
            return 0;
        }
        
        try {
            int fraction = Integer.parseInt(text, start, end, 10);
            
            // for number strings smaller than 9 digits, interpret as if there were trailing zeros
            for(int i = end - start; i<9; i++) {
                fraction *= 10;
            }
            return fraction * negate;
        } catch(NumberFormatException | ArithmeticException ex) {
            throw (DateTimeParseException) new DateTimeParseException("Text cannot be parsed to a Duration: fraction", text, 0).initCause(ex);
        }
    }
    
    private static Duration create(boolean negate, long daysAsSecs, long hoursAsSecs, long minsAsSecs, long secs, int nanos) {
        long seconds = Math.addExact(daysAsSecs, Math.addExact(hoursAsSecs, Math.addExact(minsAsSecs, secs)));
        if(negate) {
            return ofSeconds(seconds, nanos).negated();
        }
        
        return ofSeconds(seconds, nanos);
    }
    
    /**
     * Obtains an instance of {@code Duration} using seconds and nanoseconds.
     *
     * @param seconds        the length of the duration in seconds, positive or negative
     * @param nanoAdjustment the nanosecond adjustment within the second, from 0 to 999,999,999
     */
    // 根据指定的秒和纳秒创建一个Duration
    private static Duration create(long seconds, int nanoAdjustment) {
        if((seconds | nanoAdjustment) == 0) {
            return ZERO;
        }
        
        return new Duration(seconds, nanoAdjustment);
    }
    
    /**
     * Creates an instance of {@code Duration} from a number of seconds.
     *
     * @param seconds the number of seconds, up to scale 9, positive or negative
     *
     * @return a {@code Duration}, not null
     *
     * @throws ArithmeticException if numeric overflow occurs
     */
    // 根据指定的秒创建一个Duration
    private static Duration create(BigDecimal seconds) {
        BigInteger nanos = seconds.movePointRight(9).toBigIntegerExact();
        BigInteger[] divRem = nanos.divideAndRemainder(BI_NANOS_PER_SECOND);
        if(divRem[0].bitLength()>63) {
            throw new ArithmeticException("Exceeds capacity of Duration: " + nanos);
        }
        return ofSeconds(divRem[0].longValue(), divRem[1].intValue());
    }
    
    /**
     * Returns a copy of this duration with the specified duration added.
     * <p>
     * This instance is immutable and unaffected by this method call.
     *
     * @param secondsToAdd the seconds to add, positive or negative
     * @param nanosToAdd   the nanos to add, positive or negative
     *
     * @return a {@code Duration} based on this duration with the specified seconds added, not null
     *
     * @throws ArithmeticException if numeric overflow occurs
     */
    // 在当前Duration值的基础上加上指定的秒和纳秒，并构造为一个新对象后返回
    private Duration plus(long secondsToAdd, long nanosToAdd) {
        if((secondsToAdd | nanosToAdd) == 0) {
            return this;
        }
        
        long epochSec = Math.addExact(seconds, secondsToAdd);
        epochSec = Math.addExact(epochSec, nanosToAdd / NANOS_PER_SECOND);
        nanosToAdd = nanosToAdd % NANOS_PER_SECOND;
        long nanoAdjustment = nanos + nanosToAdd;  // safe int+NANOS_PER_SECOND
        
        return ofSeconds(epochSec, nanoAdjustment);
    }
    
    /**
     * Converts this duration to the total length in seconds and
     * fractional nanoseconds expressed as a {@code BigDecimal}.
     *
     * @return the total length of the duration in seconds, with a scale of 9, not null
     */
    // 返回当前Duration包含的秒；这里其实也包含了纳秒，所以返回值应当用小数表示
    private BigDecimal toBigDecimalSeconds() {
        // 纳秒转秒
        BigDecimal bigDecimal = BigDecimal.valueOf(nanos, 9);
        // 秒部件的值加上纳秒部件的值
        return BigDecimal.valueOf(seconds).add(bigDecimal);
    }
    
    
    /**
     * Compares this duration to the specified {@code Duration}.
     * <p>
     * The comparison is based on the total length of the durations.
     * It is "consistent with equals", as defined by {@link Comparable}.
     *
     * @param otherDuration the other duration to compare to, not null
     *
     * @return the comparator value, negative if less, positive if greater
     */
    @Override
    public int compareTo(Duration otherDuration) {
        int cmp = Long.compare(seconds, otherDuration.seconds);
        if(cmp != 0) {
            return cmp;
        }
        return nanos - otherDuration.nanos;
    }
    
    /**
     * A string representation of this duration using ISO-8601 seconds
     * based representation, such as {@code PT8H6M12.345S}.
     * <p>
     * The format of the returned string will be {@code PTnHnMnS}, where n is
     * the relevant hours, minutes or seconds part of the duration.
     * Any fractional seconds are placed after a decimal point in the seconds section.
     * If a section has a zero value, it is omitted.
     * The hours, minutes and seconds will all have the same sign.
     * <p>
     * Examples:
     * <pre>
     *    "20.345 seconds"                 -- "PT20.345S
     *    "15 minutes" (15 * 60 seconds)   -- "PT15M"
     *    "10 hours" (10 * 3600 seconds)   -- "PT10H"
     *    "2 days" (2 * 86400 seconds)     -- "PT48H"
     * </pre>
     * Note that multiples of 24 hours are not output as days to avoid confusion
     * with {@code Period}.
     *
     * @return an ISO-8601 representation of this duration, not null
     */
    @Override
    public String toString() {
        if(this == ZERO) {
            return "PT0S";
        }
    
        long effectiveTotalSecs = seconds;
        if(seconds<0 && nanos>0) {
            effectiveTotalSecs++;
        }
        long hours = effectiveTotalSecs / SECONDS_PER_HOUR;
        int minutes = (int) ((effectiveTotalSecs % SECONDS_PER_HOUR) / SECONDS_PER_MINUTE);
        int secs = (int) (effectiveTotalSecs % SECONDS_PER_MINUTE);
        StringBuilder buf = new StringBuilder(24);
        buf.append("PT");
        if(hours != 0) {
            buf.append(hours).append('H');
        }
        if(minutes != 0) {
            buf.append(minutes).append('M');
        }
        if(secs == 0 && nanos == 0 && buf.length()>2) {
            return buf.toString();
        }
        if(seconds<0 && nanos>0) {
            if(secs == 0) {
                buf.append("-0");
            } else {
                buf.append(secs);
            }
        } else {
            buf.append(secs);
        }
        if(nanos>0) {
            int pos = buf.length();
            if(seconds<0) {
                buf.append(2 * NANOS_PER_SECOND - nanos);
            } else {
                buf.append(nanos + NANOS_PER_SECOND);
            }
            while(buf.charAt(buf.length() - 1) == '0') {
                buf.setLength(buf.length() - 1);
            }
            buf.setCharAt(pos, '.');
        }
        buf.append('S');
        return buf.toString();
    }
    
    /**
     * Checks if this duration is equal to the specified {@code Duration}.
     * <p>
     * The comparison is based on the total length of the durations.
     *
     * @param otherDuration the other duration, null returns false
     *
     * @return true if the other duration is equal to this one
     */
    @Override
    public boolean equals(Object otherDuration) {
        if(this == otherDuration) {
            return true;
        }
        if(otherDuration instanceof Duration) {
            Duration other = (Duration) otherDuration;
            return this.seconds == other.seconds && this.nanos == other.nanos;
        }
        return false;
    }
    
    /**
     * A hash code for this duration.
     *
     * @return a suitable hash code
     */
    @Override
    public int hashCode() {
        return ((int) (seconds ^ (seconds >>> 32))) + (51 * nanos);
    }
    
    
    
    /*▼ 序列化 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Serialization version.
     */
    private static final long serialVersionUID = 3078945930695997490L;
    
    /**
     * Writes the object using a
     * <a href="../../serialized-form.html#java.time.Ser">dedicated serialized form</a>.
     *
     * @return the instance of {@code Ser}, not null
     *
     * @serialData <pre>
     *  out.writeByte(1);  // identifies a Duration
     *  out.writeLong(seconds);
     *  out.writeInt(nanos);
     * </pre>
     */
    private Object writeReplace() {
        return new Ser(Ser.DURATION_TYPE, this);
    }
    
    /**
     * Defend against malicious streams.
     *
     * @param s the stream to read
     *
     * @throws InvalidObjectException always
     */
    private void readObject(ObjectInputStream s) throws InvalidObjectException {
        throw new InvalidObjectException("Deserialization via serialization delegate");
    }
    
    static Duration readExternal(DataInput in) throws IOException {
        long seconds = in.readLong();
        int nanos = in.readInt();
        return Duration.ofSeconds(seconds, nanos);
    }
    
    void writeExternal(DataOutput out) throws IOException {
        out.writeLong(seconds);
        out.writeInt(nanos);
    }
    
    /*▲ 序列化 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    /**
     * The pattern for parsing.
     */
    private static class Lazy {
        static final Pattern PATTERN = Pattern.compile("([-+]?)P(?:([-+]?[0-9]+)D)?" + "(T(?:([-+]?[0-9]+)H)?(?:([-+]?[0-9]+)M)?(?:([-+]?[0-9]+)(?:[.,]([0-9]{0,9}))?S)?)?", Pattern.CASE_INSENSITIVE);
    }
    
    /**
     * Private class to delay initialization of this list until needed.
     * The circular dependency between Duration and ChronoUnit prevents
     * the simple initialization in Duration.
     */
    private static class DurationUnits {
        // 当前"时间段"内包含的时间量单位
        static final List<TemporalUnit> UNITS = List.of(SECONDS, NANOS);
    }
    
}