Module java.base
Package java.time

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