Module java.base
Package java.time

Class Clock

java.lang.Object
java.time.Clock
All Implemented Interfaces:
InstantSource
public abstract class Clock extends Object implements InstantSource
A clock providing access to the current instant, date and time using a time-zone.

Instances of this abstract class are used to access a pluggable representation of the current instant, which can be interpreted using the stored time-zone to find the current date and time. For example, 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.

As such, this abstract class 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 and time-zone are.

Best practice for applications is to pass a Clock into any method that requires the current instant and time-zone. 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 alternative 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.

Implementation Requirements:
This abstract class 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 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.

Since:
1.8
See Also: