Clockpublic interface InstantSource
Instances of this interface are used to access a pluggable representation of the current instant. For example, InstantSource can be used instead of System.currentTimeMillis().
The primary purpose of this abstraction is to allow alternate instant sources 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 interface 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 is.
Best practice for applications is to pass an InstantSource into any method that requires the current instant. A dependency injection framework is one way to achieve this:
public class MyBean {
private InstantSource source; // dependency inject
...
public void process(Instant endInstant) {
if (source.instant().isAfter(endInstant) {
...
}
}
}
This approach allows an alternative source, such as fixed or offset to be used during testing. The system factory method provides a source based on the best available system clock. This may use System.currentTimeMillis(), or a higher resolution clock if one is available.
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 InstantSource work on a time-scale that ignores leap seconds, as described in 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 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.
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 system clock. Applications requiring a more accurate system clock must implement this abstract class themselves using a different external system clock, such as an NTP server.| Modifier and Type | Method | Description |
|---|---|---|
static InstantSource |
fixed |
Obtains a source that always returns the same instant. |
Instant |
instant() |
Gets the current instant of the source. |
default long |
millis() |
Gets the current millisecond instant of the source. |
static InstantSource |
offset |
Obtains a source that returns instants from the specified source with the specified duration added. |
static InstantSource |
system() |
Obtains a source that returns the current instant using the best available system clock. |
static InstantSource |
tick |
Obtains a source that returns instants from the specified source truncated to the nearest occurrence of the specified duration. |
default Clock |
withZone |
Returns a clock with the specified time-zone. |
static InstantSource system()
This source is based on the best available system clock. This may use System.currentTimeMillis(), or a higher resolution system clock if one is available.
The returned implementation is immutable, thread-safe and Serializable.
static InstantSource tick(InstantSource baseSource, Duration tickDuration)
This source will only tick as per the specified duration. Thus, if the duration is half a second, the source 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 source.
Implementations may use a caching strategy for performance reasons. As such, it is possible that the start of the requested duration observed via this source will be later than that observed directly via the underlying source.
The returned implementation is immutable, thread-safe and Serializable providing that the base source is.
baseSource - the base source to base the ticking source on, not nulltickDuration - the duration of each visible tick, not negative, not nullIllegalArgumentException - 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 nanosstatic InstantSource fixed(Instant fixedInstant)
This source simply returns the specified instant. As such, it is not a source that represents the current instant. The main use case for this is in testing, where the fixed source ensures tests are not dependent on the current source.
The returned implementation is immutable, thread-safe and Serializable.
fixedInstant - the instant to use, not nullstatic InstantSource offset(InstantSource baseSource, Duration offsetDuration)
This source wraps another source, 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 source.
The returned implementation is immutable, thread-safe and Serializable providing that the base source is.
baseSource - the base source to add the duration to, not nulloffsetDuration - the duration to add, not nullInstant instant()
This returns an instant representing the current instant as defined by the source.
DateTimeException - if the instant cannot be obtained, not thrown by most implementationsdefault long millis()
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 source in high performance use cases where the creation of an object would be unacceptable.
instant().DateTimeException - if the instant cannot be obtained, not thrown by most implementationsdefault Clock withZone(ZoneId zone)
This returns a Clock, which is an extension of this interface that combines this source and the specified time-zone.
The returned implementation is immutable, thread-safe and Serializable providing that this source is.
Serializable subclass of Clock that combines this source and the specified zone.zone - the time-zone to use, not null
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