# Interface InstantSource

All Known Implementing Classes:
  `Clock`

public interface InstantSource
Provides access to the current instant.

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.

Implementation Requirements:
  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 `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.

Implementation Note:
  The implementation provided here is based on the same underlying system clock as `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

- ## Method Summary

  Modifier and Type
  Method
  Description
  `static InstantSource`
  `fixed(Instant fixedInstant)`
  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(InstantSource baseSource, Duration offsetDuration)`
  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(InstantSource baseSource, Duration tickDuration)`
  Obtains a source that returns instants from the specified source truncated to the nearest occurrence of the specified duration.
  `default Clock`
  `withZone(ZoneId zone)`
  Returns a clock with the specified time-zone.

- ## Method Details

  - ### system

    static InstantSource system()
    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 `System.currentTimeMillis()`, or a higher resolution system clock if one is available.

    The returned implementation is immutable, thread-safe and `Serializable`.

    Returns:
      a source that uses the best available system clock, not null

  - ### tick

    static InstantSource tick(InstantSource baseSource, Duration tickDuration)
    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 `Serializable` providing that the base source is.

    Parameters:
      `baseSource` - the base source to base the ticking source on, not null
      `tickDuration` - the duration of each visible tick, not negative, not null
    Returns:
      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
      `ArithmeticException` - if the duration is too large to be represented as nanos

  - ### fixed

    static InstantSource fixed(Instant fixedInstant)
    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 `Serializable`.

    Parameters:
      `fixedInstant` - the instant to use, not null
    Returns:
      a source that always returns the same instant, not null

  - ### offset

    static InstantSource offset(InstantSource baseSource, Duration offsetDuration)
    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 `Serializable` providing that the base source is.

    Parameters:
      `baseSource` - the base source to add the duration to, not null
      `offsetDuration` - the duration to add, not null
    Returns:
      a source based on the base source with the duration added, not null

  - ### instant

    Instant instant()
    Gets the current instant of the source.

    This returns an instant representing the current instant as defined by the source.

    Returns:
      the current instant from this source, not null
    Throws:
      `DateTimeException` - if the instant cannot be obtained, not thrown by most implementations

  - ### millis

    default long millis()
    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 `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.

    Implementation Requirements:
      The default implementation calls `instant()`.
    Returns:
      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

  - ### withZone

    default Clock withZone(ZoneId zone)
    Returns a clock with the specified time-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.

    Implementation Requirements:
      The default implementation returns an immutable, thread-safe and `Serializable` subclass of `Clock` that combines this source and the specified zone.
    Parameters:
      `zone` - the time-zone to use, not null
    Returns:
      a clock based on this source with the specified time-zone, not null