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package java.time;
import java.time.Clock.SourceClock;
import java.time.Clock.SystemInstantSource;
import java.util.Objects;
/**
* Provides access to the current instant.
*
* Instances of this interface are used to access a pluggable representation of the current instant.
* For example, {@code InstantSource} can be used instead of {@link 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 {@code 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 {@link #fixed(Instant) fixed}
* or {@link #offset(InstantSource, Duration) offset} to be used during testing.
*
* The {@code system} factory method provides a source based on the best available
* system clock. This may use {@link System#currentTimeMillis()}, or a higher
* resolution clock if one is available.
*
* @implSpec
* This interface 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 {@code InstantSource} 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 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 {@code Serializable} wherever possible and must
* document whether or not they do support serialization.
*
* @implNote
* The implementation provided here is based on the same underlying system 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 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.
*
* @since 17
*/
public interface InstantSource {
/**
* Obtains a source that returns the current instant using the best available
* system clock.
*
* This source is based on the best available system clock. This may use
* {@link System#currentTimeMillis()}, or a higher resolution system clock if
* one is available.
*
* The returned implementation is immutable, thread-safe and
* {@code Serializable}.
*
* @return a source that uses the best available system clock, not null
*/
static InstantSource system() {
return SystemInstantSource.INSTANCE;
}
//-------------------------------------------------------------------------
/**
* Obtains a source that returns instants from the specified source truncated to
* the nearest occurrence of the specified duration.
*
* 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
* {@code Serializable} providing that the base source is.
*
* @param baseSource the base source to base the ticking source on, not null
* @param tickDuration the duration of each visible tick, not negative, not null
* @return a source 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
*/
static InstantSource tick(InstantSource baseSource, Duration tickDuration) {
Objects.requireNonNull(baseSource, "baseSource");
return Clock.tick(baseSource.withZone(ZoneOffset.UTC), tickDuration);
}
//-----------------------------------------------------------------------
/**
* Obtains a source that always returns the same instant.
*
* 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 {@code Serializable}.
*
* @param fixedInstant the instant to use, not null
* @return a source that always returns the same instant, not null
*/
static InstantSource fixed(Instant fixedInstant) {
return Clock.fixed(fixedInstant, ZoneOffset.UTC);
}
//-------------------------------------------------------------------------
/**
* Obtains a source that returns instants from the specified source with the
* specified duration added.
*
* 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 {@code Serializable}
* providing that the base source is.
*
* @param baseSource the base source to add the duration to, not null
* @param offsetDuration the duration to add, not null
* @return a source based on the base source with the duration added, not null
*/
static InstantSource offset(InstantSource baseSource, Duration offsetDuration) {
Objects.requireNonNull(baseSource, "baseSource");
return Clock.offset(baseSource.withZone(ZoneOffset.UTC), offsetDuration);
}
//-----------------------------------------------------------------------
/**
* Gets the current instant of the source.
*
* This returns an instant representing the current instant as defined by the source.
*
* @return the current instant from this source, not null
* @throws DateTimeException if the instant cannot be obtained, not thrown by most implementations
*/
Instant instant();
//-------------------------------------------------------------------------
/**
* Gets the current millisecond instant of the source.
*
* This returns the millisecond-based instant, measured from 1970-01-01T00:00Z (UTC).
* This is equivalent to the definition of {@link System#currentTimeMillis()}.
*
* 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 source in high performance use cases
* where the creation of an object would be unacceptable.
*
* @implSpec
* The default implementation calls {@link #instant()}.
*
* @return the current millisecond instant from this source, 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
*/
default long millis() {
return instant().toEpochMilli();
}
//-----------------------------------------------------------------------
/**
* Returns a clock with the specified time-zone.
*
* This returns a {@link 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 {@code Serializable}
* providing that this source is.
*
* @implSpec
* The default implementation returns an immutable, thread-safe and
* {@code Serializable} subclass of {@link Clock} that combines this
* source and the specified zone.
*
* @param zone the time-zone to use, not null
* @return a clock based on this source with the specified time-zone, not null
*/
default Clock withZone(ZoneId zone) {
return new SourceClock(this, zone);
}
}