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:

• InstantSource

Constructor Summary

Constructors

Modifier	Constructor	Description
protected	Clock()	Constructor accessible by subclasses.

Method Summary

Modifier and Type	Method	Description
boolean	equals(Object obj)	Checks if this clock is equal to another clock.
static Clock	fixed(Instant fixedInstant, ZoneId zone)	Obtains a clock that always returns the same instant.
abstract ZoneId	getZone()	Gets the time-zone being used to create dates and times.
int	hashCode()	A hash code for this clock.
abstract Instant	instant()	Gets the current instant of the clock.
long	millis()	Gets the current millisecond instant of the clock.
static Clock	offset(Clock baseClock, Duration offsetDuration)	Obtains a clock that returns instants from the specified clock with the specified duration added.
static Clock	system(ZoneId zone)	Obtains a clock that returns the current instant using the best available system clock.
static Clock	systemDefaultZone()	Obtains a clock that returns the current instant using the best available system clock, converting to date and time using the default time-zone.
static Clock	systemUTC()	Obtains a clock that returns the current instant using the best available system clock, converting to date and time using the UTC time-zone.
static Clock	tick(Clock baseClock, Duration tickDuration)	Obtains a clock that returns instants from the specified clock truncated to the nearest occurrence of the specified duration.
static Clock	tickMillis(ZoneId zone)	Obtains a clock that returns the current instant ticking in whole milliseconds using the best available system clock.
static Clock	tickMinutes(ZoneId zone)	Obtains a clock that returns the current instant ticking in whole minutes using the best available system clock.
static Clock	tickSeconds(ZoneId zone)	Obtains a clock that returns the current instant ticking in whole seconds using the best available system clock.
abstract Clock	withZone(ZoneId zone)	Returns a copy of this clock with a different time-zone.

Methods declared in class Object

clone, finalize, getClass, notify, notifyAll, toString, wait, wait, wait

Constructor Details

Clock

protected Clock()

Constructor accessible by subclasses.

Method Details

systemUTC

public static Clock systemUTC()

Obtains a clock that returns the current instant using the best available system clock, converting to date and time using the UTC time-zone.

This clock, rather than systemDefaultZone(), should be used when you need the current instant without the date or time.

This clock is based on the best available system clock. This may use System.currentTimeMillis(), or a higher resolution clock if one is available.

Conversion from instant to date or time uses the UTC time-zone.

The returned implementation is immutable, thread-safe and Serializable. It is equivalent to system(ZoneOffset.UTC).

Returns:

a clock that uses the best available system clock in the UTC zone, not null

systemDefaultZone

public static Clock systemDefaultZone()

Obtains a clock that returns the current instant using the best available system clock, converting to date and time using the default time-zone.

This clock is based on the best available system clock. This may use System.currentTimeMillis(), or a higher resolution clock if one is available.

Using this method hard codes a dependency to the default time-zone into your application. It is recommended to avoid this and use a specific time-zone whenever possible. The UTC clock should be used when you need the current instant without the date or time. <