public abstract class Clock

A clock providing access to the current instant, date and time using a time-zone.

Instances of this class are used to find the current instant, which can be interpreted using the stored time-zone to find the current date and time. As such, a 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.

Best practice for applications is to pass a Clock into any method that requires the current instant. 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 alternate 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.

Since:  1.8

@implSpec This abstract class must be implemented with care to ensure other classes operate correctly. All implementations that can be instantiated must be final, immutable and thread-safe.

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 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 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.
@implNote The clock implementation provided here is based on the same underlying 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 clock. Applications requiring a more accurate clock must implement this abstract class themselves using a different external clock, such as an NTP server.