src/java.base/share/classes/java/time/Clock.java
changeset 47216 71c04702a3d5
parent 45329 9a5dc512845a
child 50473 bef0ed237504
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/src/java.base/share/classes/java/time/Clock.java	Tue Sep 12 19:03:39 2017 +0200
@@ -0,0 +1,742 @@
+/*
+ * Copyright (c) 2012, 2015, 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.IOException;
+import java.io.ObjectInputStream;
+import static java.time.LocalTime.NANOS_PER_MINUTE;
+import static java.time.LocalTime.NANOS_PER_SECOND;
+import static java.time.LocalTime.NANOS_PER_MILLI;
+import java.io.Serializable;
+import java.util.Objects;
+import java.util.TimeZone;
+import jdk.internal.misc.VM;
+
+/**
+ * A clock providing access to the current instant, date and time using a time-zone.
+ * <p>
+ * Instances of this class are used to find the current instant, which can be
+ * interpreted using the stored time-zone to find the current date and time.
+ * As such, a clock can be used instead of {@link System#currentTimeMillis()}
+ * and {@link TimeZone#getDefault()}.
+ * <p>
+ * Use of a {@code Clock} is optional. All key date-time classes also have a
+ * {@code now()} factory method that uses the system clock in the default time zone.
+ * The primary purpose of this abstraction is to allow alternate clocks to be
+ * plugged in as and when required. Applications use an object to obtain the
+ * current time rather than a static method. This can simplify testing.
+ * <p>
+ * Best practice for applications is to pass a {@code Clock} into any method
+ * that requires the current instant. A dependency injection framework is one
+ * way to achieve this:
+ * <pre>
+ *  public class MyBean {
+ *    private Clock clock;  // dependency inject
+ *    ...
+ *    public void process(LocalDate eventDate) {
+ *      if (eventDate.isBefore(LocalDate.now(clock)) {
+ *        ...
+ *      }
+ *    }
+ *  }
+ * </pre>
+ * This approach allows an alternate clock, such as {@link #fixed(Instant, ZoneId) fixed}
+ * or {@link #offset(Clock, Duration) offset} to be used during testing.
+ * <p>
+ * The {@code system} factory methods provide clocks based on the best available
+ * system clock This may use {@link System#currentTimeMillis()}, or a higher
+ * resolution clock if one is available.
+ *
+ * @implSpec
+ * This abstract class must be implemented with care to ensure other classes operate correctly.
+ * All implementations that can be instantiated must be final, immutable and thread-safe.
+ * <p>
+ * The principal methods are defined to allow the throwing of an exception.
+ * In normal use, no exceptions will be thrown, however one possible implementation would be to
+ * obtain the time from a central time server across the network. Obviously, in this case the
+ * lookup could fail, and so the method is permitted to throw an exception.
+ * <p>
+ * The returned instants from {@code Clock} work on a time-scale that ignores leap seconds,
+ * as described in {@link Instant}. If the implementation wraps a source that provides leap
+ * second information, then a mechanism should be used to "smooth" the leap second.
+ * The Java Time-Scale mandates the use of UTC-SLS, however clock implementations may choose
+ * how accurate they are with the time-scale so long as they document how they work.
+ * Implementations are therefore not required to actually perform the UTC-SLS slew or to
+ * otherwise be aware of leap seconds.
+ * <p>
+ * Implementations should implement {@code Serializable} wherever possible and must
+ * document whether or not they do support serialization.
+ *
+ * @implNote
+ * The clock implementation provided here is based on the same underlying clock
+ * as {@link System#currentTimeMillis()}, but may have a precision finer than
+ * milliseconds if available.
+ * However, little to no guarantee is provided about the accuracy of the
+ * underlying clock. Applications requiring a more accurate clock must implement
+ * this abstract class themselves using a different external clock, such as an
+ * NTP server.
+ *
+ * @since 1.8
+ */
+public abstract class Clock {
+
+    /**
+     * Obtains a clock that returns the current instant using the best available
+     * system clock, converting to date and time using the UTC time-zone.
+     * <p>
+     * This clock, rather than {@link #systemDefaultZone()}, should be used when
+     * you need the current instant without the date or time.
+     * <p>
+     * This clock is based on the best available system clock.
+     * This may use {@link System#currentTimeMillis()}, or a higher resolution
+     * clock if one is available.
+     * <p>
+     * Conversion from instant to date or time uses the {@linkplain ZoneOffset#UTC UTC time-zone}.
+     * <p>
+     * The returned implementation is immutable, thread-safe and {@code Serializable}.
+     * It is equivalent to {@code system(ZoneOffset.UTC)}.
+     *
+     * @return a clock that uses the best available system clock in the UTC zone, not null
+     */
+    public static Clock systemUTC() {
+        return SystemClock.UTC;
+    }
+
+    /**
+     * Obtains a clock that returns the current instant using the best available
+     * system clock, converting to date and time using the default time-zone.
+     * <p>
+     * This clock is based on the best available system clock.
+     * This may use {@link System#currentTimeMillis()}, or a higher resolution
+     * clock if one is available.
+     * <p>
+     * Using this method hard codes a dependency to the default time-zone into your application.
+     * It is recommended to avoid this and use a specific time-zone whenever possible.
+     * The {@link #systemUTC() UTC clock} should be used when you need the current instant
+     * without the date or time.
+     * <p>
+     * The returned implementation is immutable, thread-safe and {@code Serializable}.
+     * It is equivalent to {@code system(ZoneId.systemDefault())}.
+     *
+     * @return a clock that uses the best available system clock in the default zone, not null
+     * @see ZoneId#systemDefault()
+     */
+    public static Clock systemDefaultZone() {
+        return new SystemClock(ZoneId.systemDefault());
+    }
+
+    /**
+     * Obtains a clock that returns the current instant using the best available
+     * system clock.
+     * <p>
+     * This clock is based on the best available system clock.
+     * This may use {@link System#currentTimeMillis()}, or a higher resolution
+     * clock if one is available.
+     * <p>
+     * Conversion from instant to date or time uses the specified time-zone.
+     * <p>
+     * The returned implementation is immutable, thread-safe and {@code Serializable}.
+     *
+     * @param zone  the time-zone to use to convert the instant to date-time, not null
+     * @return a clock that uses the best available system clock in the specified zone, not null
+     */
+    public static Clock system(ZoneId zone) {
+        Objects.requireNonNull(zone, "zone");
+        if (zone == ZoneOffset.UTC) {
+            return SystemClock.UTC;
+        }
+        return new SystemClock(zone);
+    }
+
+    //-------------------------------------------------------------------------
+    /**
+     * Obtains a clock that returns the current instant ticking in whole milliseconds
+     * using the best available system clock.
+     * <p>
+     * This clock will always have the nano-of-second field truncated to milliseconds.
+     * This ensures that the visible time ticks in whole milliseconds.
+     * The underlying clock is the best available system clock, equivalent to
+     * using {@link #system(ZoneId)}.
+     * <p>
+     * Implementations may use a caching strategy for performance reasons.
+     * As such, it is possible that the start of the millisecond observed via this
+     * clock will be later than that observed directly via the underlying clock.
+     * <p>
+     * The returned implementation is immutable, thread-safe and {@code Serializable}.
+     * It is equivalent to {@code tick(system(zone), Duration.ofMillis(1))}.
+     *
+     * @param zone  the time-zone to use to convert the instant to date-time, not null
+     * @return a clock that ticks in whole milliseconds using the specified zone, not null
+     * @since 9
+     */
+    public static Clock tickMillis(ZoneId zone) {
+        return new TickClock(system(zone), NANOS_PER_MILLI);
+    }
+
+    //-------------------------------------------------------------------------
+    /**
+     * Obtains a clock that returns the current instant ticking in whole seconds
+     * using the best available system clock.
+     * <p>
+     * This clock will always have the nano-of-second field set to zero.
+     * This ensures that the visible time ticks in whole seconds.
+     * The underlying clock is the best available system clock, equivalent to
+     * using {@link #system(ZoneId)}.
+     * <p>
+     * Implementations may use a caching strategy for performance reasons.
+     * As such, it is possible that the start of the second observed via this
+     * clock will be later than that observed directly via the underlying clock.
+     * <p>
+     * The returned implementation is immutable, thread-safe and {@code Serializable}.
+     * It is equivalent to {@code tick(system(zone), Duration.ofSeconds(1))}.
+     *
+     * @param zone  the time-zone to use to convert the instant to date-time, not null
+     * @return a clock that ticks in whole seconds using the specified zone, not null
+     */
+    public static Clock tickSeconds(ZoneId zone) {
+        return new TickClock(system(zone), NANOS_PER_SECOND);
+    }
+
+    /**
+     * Obtains a clock that returns the current instant ticking in whole minutes
+     * using the best available system clock.
+     * <p>
+     * This clock will always have the nano-of-second and second-of-minute fields set to zero.
+     * This ensures that the visible time ticks in whole minutes.
+     * The underlying clock is the best available system clock, equivalent to
+     * using {@link #system(ZoneId)}.
+     * <p>
+     * Implementations may use a caching strategy for performance reasons.
+     * As such, it is possible that the start of the minute observed via this
+     * clock will be later than that observed directly via the underlying clock.
+     * <p>
+     * The returned implementation is immutable, thread-safe and {@code Serializable}.
+     * It is equivalent to {@code tick(system(zone), Duration.ofMinutes(1))}.
+     *
+     * @param zone  the time-zone to use to convert the instant to date-time, not null
+     * @return a clock that ticks in whole minutes using the specified zone, not null
+     */
+    public static Clock tickMinutes(ZoneId zone) {
+        return new TickClock(system(zone), NANOS_PER_MINUTE);
+    }
+
+    /**
+     * Obtains a clock that returns instants from the specified clock truncated
+     * to the nearest occurrence of the specified duration.
+     * <p>
+     * This clock will only tick as per the specified duration. Thus, if the duration
+     * is half a second, the clock will return instants truncated to the half second.
+     * <p>
+     * The tick duration must be positive. If it has a part smaller than a whole
+     * millisecond, then the whole duration must divide into one second without
+     * leaving a remainder. All normal tick durations will match these criteria,
+     * including any multiple of hours, minutes, seconds and milliseconds, and
+     * sensible nanosecond durations, such as 20ns, 250,000ns and 500,000ns.
+     * <p>
+     * A duration of zero or one nanosecond would have no truncation effect.
+     * Passing one of these will return the underlying clock.
+     * <p>
+     * Implementations may use a caching strategy for performance reasons.
+     * As such, it is possible that the start of the requested duration observed
+     * via this clock will be later than that observed directly via the underlying clock.
+     * <p>
+     * The returned implementation is immutable, thread-safe and {@code Serializable}
+     * providing that the base clock is.
+     *
+     * @param baseClock  the base clock to base the ticking clock on, not null
+     * @param tickDuration  the duration of each visible tick, not negative, not null
+     * @return a clock that ticks in whole units of the duration, not null
+     * @throws IllegalArgumentException if the duration is negative, or has a
+     *  part smaller than a whole millisecond such that the whole duration is not
+     *  divisible into one second
+     * @throws ArithmeticException if the duration is too large to be represented as nanos
+     */
+    public static Clock tick(Clock baseClock, Duration tickDuration) {
+        Objects.requireNonNull(baseClock, "baseClock");
+        Objects.requireNonNull(tickDuration, "tickDuration");
+        if (tickDuration.isNegative()) {
+            throw new IllegalArgumentException("Tick duration must not be negative");
+        }
+        long tickNanos = tickDuration.toNanos();
+        if (tickNanos % 1000_000 == 0) {
+            // ok, no fraction of millisecond
+        } else if (1000_000_000 % tickNanos == 0) {
+            // ok, divides into one second without remainder
+        } else {
+            throw new IllegalArgumentException("Invalid tick duration");
+        }
+        if (tickNanos <= 1) {
+            return baseClock;
+        }
+        return new TickClock(baseClock, tickNanos);
+    }
+
+    //-----------------------------------------------------------------------
+    /**
+     * Obtains a clock that always returns the same instant.
+     * <p>
+     * This clock simply returns the specified instant.
+     * As such, it is not a clock in the conventional sense.
+     * The main use case for this is in testing, where the fixed clock ensures
+     * tests are not dependent on the current clock.
+     * <p>
+     * The returned implementation is immutable, thread-safe and {@code Serializable}.
+     *
+     * @param fixedInstant  the instant to use as the clock, not null
+     * @param zone  the time-zone to use to convert the instant to date-time, not null
+     * @return a clock that always returns the same instant, not null
+     */
+    public static Clock fixed(Instant fixedInstant, ZoneId zone) {
+        Objects.requireNonNull(fixedInstant, "fixedInstant");
+        Objects.requireNonNull(zone, "zone");
+        return new FixedClock(fixedInstant, zone);
+    }
+
+    //-------------------------------------------------------------------------
+    /**
+     * Obtains a clock that returns instants from the specified clock with the
+     * specified duration added
+     * <p>
+     * This clock wraps another clock, returning instants that are later by the
+     * specified duration. If the duration is negative, the instants will be
+     * earlier than the current date and time.
+     * The main use case for this is to simulate running in the future or in the past.
+     * <p>
+     * A duration of zero would have no offsetting effect.
+     * Passing zero will return the underlying clock.
+     * <p>
+     * The returned implementation is immutable, thread-safe and {@code Serializable}
+     * providing that the base clock is.
+     *
+     * @param baseClock  the base clock to add the duration to, not null
+     * @param offsetDuration  the duration to add, not null
+     * @return a clock based on the base clock with the duration added, not null
+     */
+    public static Clock offset(Clock baseClock, Duration offsetDuration) {
+        Objects.requireNonNull(baseClock, "baseClock");
+        Objects.requireNonNull(offsetDuration, "offsetDuration");
+        if (offsetDuration.equals(Duration.ZERO)) {
+            return baseClock;
+        }
+        return new OffsetClock(baseClock, offsetDuration);
+    }
+
+    //-----------------------------------------------------------------------
+    /**
+     * Constructor accessible by subclasses.
+     */
+    protected Clock() {
+    }
+
+    //-----------------------------------------------------------------------
+    /**
+     * Gets the time-zone being used to create dates and times.
+     * <p>
+     * A clock will typically obtain the current instant and then convert that
+     * to a date or time using a time-zone. This method returns the time-zone used.
+     *
+     * @return the time-zone being used to interpret instants, not null
+     */
+    public abstract ZoneId getZone();
+
+    /**
+     * Returns a copy of this clock with a different time-zone.
+     * <p>
+     * A clock will typically obtain the current instant and then convert that
+     * to a date or time using a time-zone. This method returns a clock with
+     * similar properties but using a different time-zone.
+     *
+     * @param zone  the time-zone to change to, not null
+     * @return a clock based on this clock with the specified time-zone, not null
+     */
+    public abstract Clock withZone(ZoneId zone);
+
+    //-------------------------------------------------------------------------
+    /**
+     * Gets the current millisecond instant of the clock.
+     * <p>
+     * This returns the millisecond-based instant, measured from 1970-01-01T00:00Z (UTC).
+     * This is equivalent to the definition of {@link System#currentTimeMillis()}.
+     * <p>
+     * Most applications should avoid this method and use {@link Instant} to represent
+     * an instant on the time-line rather than a raw millisecond value.
+     * This method is provided to allow the use of the clock in high performance use cases
+     * where the creation of an object would be unacceptable.
+     * <p>
+     * The default implementation currently calls {@link #instant}.
+     *
+     * @return the current millisecond instant from this clock, measured from
+     *  the Java epoch of 1970-01-01T00:00Z (UTC), not null
+     * @throws DateTimeException if the instant cannot be obtained, not thrown by most implementations
+     */
+    public long millis() {
+        return instant().toEpochMilli();
+    }
+
+    //-----------------------------------------------------------------------
+    /**
+     * Gets the current instant of the clock.
+     * <p>
+     * This returns an instant representing the current instant as defined by the clock.
+     *
+     * @return the current instant from this clock, not null
+     * @throws DateTimeException if the instant cannot be obtained, not thrown by most implementations
+     */
+    public abstract Instant instant();
+
+    //-----------------------------------------------------------------------
+    /**
+     * Checks if this clock is equal to another clock.
+     * <p>
+     * Clocks should override this method to compare equals based on
+     * their state and to meet the contract of {@link Object#equals}.
+     * If not overridden, the behavior is defined by {@link Object#equals}
+     *
+     * @param obj  the object to check, null returns false
+     * @return true if this is equal to the other clock
+     */
+    @Override
+    public boolean equals(Object obj) {
+        return super.equals(obj);
+    }
+
+    /**
+     * A hash code for this clock.
+     * <p>
+     * Clocks should override this method based on
+     * their state and to meet the contract of {@link Object#hashCode}.
+     * If not overridden, the behavior is defined by {@link Object#hashCode}
+     *
+     * @return a suitable hash code
+     */
+    @Override
+    public  int hashCode() {
+        return super.hashCode();
+    }
+
+    //-----------------------------------------------------------------------
+    /**
+     * Implementation of a clock that always returns the latest time from
+     * {@link System#currentTimeMillis()}.
+     */
+    static final class SystemClock extends Clock implements Serializable {
+        private static final long serialVersionUID = 6740630888130243051L;
+        private static final long OFFSET_SEED =
+                System.currentTimeMillis()/1000 - 1024; // initial offest
+        static final SystemClock UTC = new SystemClock(ZoneOffset.UTC);
+
+        private final ZoneId zone;
+        // We don't actually need a volatile here.
+        // We don't care if offset is set or read concurrently by multiple
+        // threads - we just need a value which is 'recent enough' - in other
+        // words something that has been updated at least once in the last
+        // 2^32 secs (~136 years). And even if we by chance see an invalid
+        // offset, the worst that can happen is that we will get a -1 value
+        // from getNanoTimeAdjustment, forcing us to update the offset
+        // once again.
+        private transient long offset;
+
+        SystemClock(ZoneId zone) {
+            this.zone = zone;
+            this.offset = OFFSET_SEED;
+        }
+        @Override
+        public ZoneId getZone() {
+            return zone;
+        }
+        @Override
+        public Clock withZone(ZoneId zone) {
+            if (zone.equals(this.zone)) {  // intentional NPE
+                return this;
+            }
+            return new SystemClock(zone);
+        }
+        @Override
+        public long millis() {
+            // System.currentTimeMillis() and VM.getNanoTimeAdjustment(offset)
+            // use the same time source - System.currentTimeMillis() simply
+            // limits the resolution to milliseconds.
+            // So we take the faster path and call System.currentTimeMillis()
+            // directly - in order to avoid the performance penalty of
+            // VM.getNanoTimeAdjustment(offset) which is less efficient.
+            return System.currentTimeMillis();
+        }
+        @Override
+        public Instant instant() {
+            // Take a local copy of offset. offset can be updated concurrently
+            // by other threads (even if we haven't made it volatile) so we will
+            // work with a local copy.
+            long localOffset = offset;
+            long adjustment = VM.getNanoTimeAdjustment(localOffset);
+
+            if (adjustment == -1) {
+                // -1 is a sentinel value returned by VM.getNanoTimeAdjustment
+                // when the offset it is given is too far off the current UTC
+                // time. In principle, this should not happen unless the
+                // JVM has run for more than ~136 years (not likely) or
+                // someone is fiddling with the system time, or the offset is
+                // by chance at 1ns in the future (very unlikely).
+                // We can easily recover from all these conditions by bringing
+                // back the offset in range and retry.
+
+                // bring back the offset in range. We use -1024 to make
+                // it more unlikely to hit the 1ns in the future condition.
+                localOffset = System.currentTimeMillis()/1000 - 1024;
+
+                // retry
+                adjustment = VM.getNanoTimeAdjustment(localOffset);
+
+                if (adjustment == -1) {
+                    // Should not happen: we just recomputed a new offset.
+                    // It should have fixed the issue.
+                    throw new InternalError("Offset " + localOffset + " is not in range");
+                } else {
+                    // OK - recovery succeeded. Update the offset for the
+                    // next call...
+                    offset = localOffset;
+                }
+            }
+            return Instant.ofEpochSecond(localOffset, adjustment);
+        }
+        @Override
+        public boolean equals(Object obj) {
+            if (obj instanceof SystemClock) {
+                return zone.equals(((SystemClock) obj).zone);
+            }
+            return false;
+        }
+        @Override
+        public int hashCode() {
+            return zone.hashCode() + 1;
+        }
+        @Override
+        public String toString() {
+            return "SystemClock[" + zone + "]";
+        }
+        private void readObject(ObjectInputStream is)
+                throws IOException, ClassNotFoundException {
+            // ensure that offset is initialized
+            is.defaultReadObject();
+            offset = OFFSET_SEED;
+        }
+    }
+
+    //-----------------------------------------------------------------------
+    /**
+     * Implementation of a clock that always returns the same instant.
+     * This is typically used for testing.
+     */
+    static final class FixedClock extends Clock implements Serializable {
+       private static final long serialVersionUID = 7430389292664866958L;
+        private final Instant instant;
+        private final ZoneId zone;
+
+        FixedClock(Instant fixedInstant, ZoneId zone) {
+            this.instant = fixedInstant;
+            this.zone = zone;
+        }
+        @Override
+        public ZoneId getZone() {
+            return zone;
+        }
+        @Override
+        public Clock withZone(ZoneId zone) {
+            if (zone.equals(this.zone)) {  // intentional NPE
+                return this;
+            }
+            return new FixedClock(instant, zone);
+        }
+        @Override
+        public long millis() {
+            return instant.toEpochMilli();
+        }
+        @Override
+        public Instant instant() {
+            return instant;
+        }
+        @Override
+        public boolean equals(Object obj) {
+            if (obj instanceof FixedClock) {
+                FixedClock other = (FixedClock) obj;
+                return instant.equals(other.instant) && zone.equals(other.zone);
+            }
+            return false;
+        }
+        @Override
+        public int hashCode() {
+            return instant.hashCode() ^ zone.hashCode();
+        }
+        @Override
+        public String toString() {
+            return "FixedClock[" + instant + "," + zone + "]";
+        }
+    }
+
+    //-----------------------------------------------------------------------
+    /**
+     * Implementation of a clock that adds an offset to an underlying clock.
+     */
+    static final class OffsetClock extends Clock implements Serializable {
+       private static final long serialVersionUID = 2007484719125426256L;
+        private final Clock baseClock;
+        private final Duration offset;
+
+        OffsetClock(Clock baseClock, Duration offset) {
+            this.baseClock = baseClock;
+            this.offset = offset;
+        }
+        @Override
+        public ZoneId getZone() {
+            return baseClock.getZone();
+        }
+        @Override
+        public Clock withZone(ZoneId zone) {
+            if (zone.equals(baseClock.getZone())) {  // intentional NPE
+                return this;
+            }
+            return new OffsetClock(baseClock.withZone(zone), offset);
+        }
+        @Override
+        public long millis() {
+            return Math.addExact(baseClock.millis(), offset.toMillis());
+        }
+        @Override
+        public Instant instant() {
+            return baseClock.instant().plus(offset);
+        }
+        @Override
+        public boolean equals(Object obj) {
+            if (obj instanceof OffsetClock) {
+                OffsetClock other = (OffsetClock) obj;
+                return baseClock.equals(other.baseClock) && offset.equals(other.offset);
+            }
+            return false;
+        }
+        @Override
+        public int hashCode() {
+            return baseClock.hashCode() ^ offset.hashCode();
+        }
+        @Override
+        public String toString() {
+            return "OffsetClock[" + baseClock + "," + offset + "]";
+        }
+    }
+
+    //-----------------------------------------------------------------------
+    /**
+     * Implementation of a clock that adds an offset to an underlying clock.
+     */
+    static final class TickClock extends Clock implements Serializable {
+        private static final long serialVersionUID = 6504659149906368850L;
+        private final Clock baseClock;
+        private final long tickNanos;
+
+        TickClock(Clock baseClock, long tickNanos) {
+            this.baseClock = baseClock;
+            this.tickNanos = tickNanos;
+        }
+        @Override
+        public ZoneId getZone() {
+            return baseClock.getZone();
+        }
+        @Override
+        public Clock withZone(ZoneId zone) {
+            if (zone.equals(baseClock.getZone())) {  // intentional NPE
+                return this;
+            }
+            return new TickClock(baseClock.withZone(zone), tickNanos);
+        }
+        @Override
+        public long millis() {
+            long millis = baseClock.millis();
+            return millis - Math.floorMod(millis, tickNanos / 1000_000L);
+        }
+        @Override
+        public Instant instant() {
+            if ((tickNanos % 1000_000) == 0) {
+                long millis = baseClock.millis();
+                return Instant.ofEpochMilli(millis - Math.floorMod(millis, tickNanos / 1000_000L));
+            }
+            Instant instant = baseClock.instant();
+            long nanos = instant.getNano();
+            long adjust = Math.floorMod(nanos, tickNanos);
+            return instant.minusNanos(adjust);
+        }
+        @Override
+        public boolean equals(Object obj) {
+            if (obj instanceof TickClock) {
+                TickClock other = (TickClock) obj;
+                return baseClock.equals(other.baseClock) && tickNanos == other.tickNanos;
+            }
+            return false;
+        }
+        @Override
+        public int hashCode() {
+            return baseClock.hashCode() ^ ((int) (tickNanos ^ (tickNanos >>> 32)));
+        }
+        @Override
+        public String toString() {
+            return "TickClock[" + baseClock + "," + Duration.ofNanos(tickNanos) + "]";
+        }
+    }
+
+}