Class Clock
- All Implemented Interfaces:
InstantSource
Instances of this abstract class are used to access a pluggable representation of the
current instant, which can be interpreted using the stored time-zone to find the
current date and time.
For example, Clock
can be used instead of System.currentTimeMillis()
and TimeZone.getDefault()
.
Use of a Clock
is optional. All key date-time classes also have a
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.
As such, this abstract class does not guarantee the result actually represents the current instant on the time-line. Instead, it allows the application to provide a controlled view as to what the current instant and time-zone are.
Best practice for applications is to pass a Clock
into any method
that requires the current instant and time-zone. A dependency injection framework
is one way to achieve this:
public class MyBean { private Clock clock; // dependency inject ... public void process(LocalDate eventDate) { if (eventDate.isBefore(LocalDate.now(clock)) { ... } } }This approach allows an alternative clock, such as
fixed
or offset
to be used during testing.
The system
factory methods provide clocks based on the best available
system clock. This may use System.currentTimeMillis()
, or a higher
resolution clock if one is available.
- Implementation Requirements:
- This abstract class must be implemented with care to ensure other classes operate correctly.
All implementations must be thread-safe - a single instance must be capable of be invoked
from multiple threads without negative consequences such as race conditions.
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.
The returned instants from
Clock
work on a time-scale that ignores leap seconds, as described inInstant
. 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.Implementations should implement
Serializable
wherever possible and must document whether or not they do support serialization. - Since:
- 1.8
- See Also:
-
Constructor Summary
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Method Summary
Modifier and TypeMethodDescriptionboolean
Checks if this clock is equal to another clock.static Clock
Obtains a clock that always returns the same instant.abstract ZoneId
getZone()
Gets the time-zone being used to create dates and times.int
hashCode()
A hash code for this clock.abstract Instant
instant()
Gets the current instant of the clock.long
millis()
Gets the current millisecond instant of the clock.static Clock
Obtains a clock that returns instants from the specified clock with the specified duration added.static Clock
Obtains a clock that returns the current instant using the best available system clock.static Clock
Obtains a clock that returns the current instant using the best available system clock, converting to date and time using the default time-zone.static 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.static Clock
Obtains a clock that returns instants from the specified clock truncated to the nearest occurrence of the specified duration.static Clock
tickMillis
(ZoneId zone) Obtains a clock that returns the current instant ticking in whole milliseconds using the best available system clock.static Clock
tickMinutes
(ZoneId zone) Obtains a clock that returns the current instant ticking in whole minutes using the best available system clock.static Clock
tickSeconds
(ZoneId zone) Obtains a clock that returns the current instant ticking in whole seconds using the best available system clock.abstract Clock
Returns a copy of this clock with a different time-zone.
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Constructor Details
-
Clock
protected Clock()Constructor accessible by subclasses.
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Method Details
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systemUTC
Obtains a clock that returns the current instant using the best available system clock, converting to date and time using the UTC time-zone.This clock, rather than
systemDefaultZone()
, should be used when you need the current instant without the date or time.This clock is based on the best available system clock. This may use
System.currentTimeMillis()
, or a higher resolution clock if one is available.Conversion from instant to date or time uses the UTC time-zone.
The returned implementation is immutable, thread-safe and
Serializable
. It is equivalent tosystem(ZoneOffset.UTC)
.- Returns:
- a clock that uses the best available system clock in the UTC zone, not null
-
systemDefaultZone
Obtains a clock that returns the current instant using the best available system clock, converting to date and time using the default time-zone.This clock is based on the best available system clock. This may use
System.currentTimeMillis()
, or a higher resolution clock if one is available.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
UTC clock
should be used when you need the current instant without the date or time.The returned implementation is immutable, thread-safe and
Serializable
. It is equivalent tosystem(ZoneId.systemDefault())
.- Returns:
- a clock that uses the best available system clock in the default zone, not null
- See Also:
-
system
Obtains a clock that returns the current instant using the best available system clock.This clock is based on the best available system clock. This may use
System.currentTimeMillis()
, or a higher resolution clock if one is available.Conversion from instant to date or time uses the specified time-zone.
The returned implementation is immutable, thread-safe and
Serializable
.- Parameters:
zone
- the time-zone to use to convert the instant to date-time, not null- Returns:
- a clock that uses the best available system clock in the specified zone, not null
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tickMillis
Obtains a clock that returns the current instant ticking in whole milliseconds using the best available system clock.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
system(ZoneId)
.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.
The returned implementation is immutable, thread-safe and
Serializable
. It is equivalent totick(system(zone), Duration.ofMillis(1))
.- Parameters:
zone
- the time-zone to use to convert the instant to date-time, not null- Returns:
- a clock that ticks in whole milliseconds using the specified zone, not null
- Since:
- 9
-
tickSeconds
Obtains a clock that returns the current instant ticking in whole seconds using the best available system clock.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
system(ZoneId)
.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.
The returned implementation is immutable, thread-safe and
Serializable
. It is equivalent totick(system(zone), Duration.ofSeconds(1))
.- Parameters:
zone
- the time-zone to use to convert the instant to date-time, not null- Returns:
- a clock that ticks in whole seconds using the specified zone, not null
-
tickMinutes
Obtains a clock that returns the current instant ticking in whole minutes using the best available system clock.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
system(ZoneId)
.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.
The returned implementation is immutable, thread-safe and
Serializable
. It is equivalent totick(system(zone), Duration.ofMinutes(1))
.- Parameters:
zone
- the time-zone to use to convert the instant to date-time, not null- Returns:
- a clock that ticks in whole minutes using the specified zone, not null
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tick
Obtains a clock that returns instants from the specified clock truncated to the nearest occurrence of the specified duration.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.
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.
A duration of zero or one nanosecond would have no truncation effect. Passing one of these will return the underlying clock.
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.
The returned implementation is immutable, thread-safe and
Serializable
providing that the base clock is.- Parameters:
baseClock
- the base clock to base the ticking clock on, not nulltickDuration
- the duration of each visible tick, not negative, not null- Returns:
- 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 secondArithmeticException
- if the duration is too large to be represented as nanos
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fixed
Obtains a clock that always returns the same instant.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.
The returned implementation is immutable, thread-safe and
Serializable
.- Parameters:
fixedInstant
- the instant to use as the clock, not nullzone
- the time-zone to use to convert the instant to date-time, not null- Returns:
- a clock that always returns the same instant, not null
-
offset
Obtains a clock that returns instants from the specified clock with the specified duration added.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.
A duration of zero would have no offsetting effect. Passing zero will return the underlying clock.
The returned implementation is immutable, thread-safe and
Serializable
providing that the base clock is.- Parameters:
baseClock
- the base clock to add the duration to, not nulloffsetDuration
- the duration to add, not null- Returns:
- a clock based on the base clock with the duration added, not null
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getZone
Gets the time-zone being used to create dates and times.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.
- Returns:
- the time-zone being used to interpret instants, not null
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withZone
Returns a copy of this clock with a different time-zone.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.
- Specified by:
withZone
in interfaceInstantSource
- Parameters:
zone
- the time-zone to change to, not null- Returns:
- a clock based on this clock with the specified time-zone, not null
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millis
public long millis()Gets the current millisecond instant of the clock.This returns the millisecond-based instant, measured from 1970-01-01T00:00Z (UTC). This is equivalent to the definition of
System.currentTimeMillis()
.Most applications should avoid this method and use
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.The default implementation currently calls
instant()
.- Specified by:
millis
in interfaceInstantSource
- Returns:
- 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
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instant
Gets the current instant of the clock.This returns an instant representing the current instant as defined by the clock.
- Specified by:
instant
in interfaceInstantSource
- Returns:
- the current instant from this clock, not null
- Throws:
DateTimeException
- if the instant cannot be obtained, not thrown by most implementations
-
equals
Checks if this clock is equal to another clock.Clocks should override this method to compare equals based on their state and to meet the contract of
Object.equals(java.lang.Object)
. If not overridden, the behavior is defined byObject.equals(java.lang.Object)
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hashCode
public int hashCode()A hash code for this clock.Clocks should override this method based on their state and to meet the contract of
Object.hashCode()
. If not overridden, the behavior is defined byObject.hashCode()
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