javax.time
Class TimeSource

java.lang.Object
  javax.time.TimeSource

public abstract class TimeSource
extends java.lang.Object

A source providing access to the current instant.

The Time Framework for Java abstracts the concept of the "current time" into two interfaces - TimeSource and Clock. This class, provides access to the current Instant which is independent of local factors such as time-zone and cannot be queried for human-scale fields. By comparison, Clock provides access to the current date and time, via human-scale fields, but requires a time-zone.

The purpose of this abstraction is to allow alternate time-sources to be plugged in as and when required. Applications use an object to obtain the current time rather than a static method. This simplifies testing.

Best practice

The recommended best practice for applications is to avoid using the static methods. Instead, the main application should obtain the current time from a TimeSource instance that is passed to the object or method. This approach is typically implemented using a dependency injection framework.

 public class MyBean {
   final TimeSource timeSource;
   @Inject MyBean(TimeSource ts) {
       this.timeSource = ts;
   }
   public void process(Instant eventTime) {
     if (eventTime.isBefore(timeSource.instant()) {
       ...
     }
   }
 }
 

This approach allows alternate time-source implementations, such as fixed or offsetSystem to be used during testing.

 public void test_process() {
   MyBean bean = new MyBean(TimeSource.fixed(Instant.EPOCH)) {
   assert ...
 }
 

Accuracy

The main method used by applications is instant(). This returns the best value available for the current instant ignoring leap-seconds. To achieve this, the instant may not be fully accurate around a leap-second.

Two alternative methods are available and may return a more accurate instant depending on the implementation. The instant in UTC, taking into account leap seconds, can be obtained using utcInstant(). The instant in TAI, which is a simple incrementing number ignoring all human time concepts, can be obtained using taiInstant(). See the relevant classes for more detail.

Implementation notes

This is an abstract class and must be implemented with care to ensure other classes in the framework operate correctly. All implementations must be final, immutable and thread-safe. Subclasses should be Serializable wherever possible. They should also implement equals(), hashCode() and toString() based on their state.

Author:

Michael Nascimento Santos, Stephen Colebourne

Constructor Summary

protected

TimeSource() Constructor accessible by subclasses.

Method Summary

static TimeSource	fixed(InstantProvider fixedInstantProvider) Gets a time-source that always returns the same Instant.
abstract Instant	instant() Gets the current Instant.
long	millis() Gets the current millisecond instant.
static TimeSource	offsetSystem(Duration offset) Gets a time-source that obtains the current instant using the system millisecond clock and adjusts by a fixed offset.
static TimeSource	system() Gets a time-source that obtains the current instant using the system millisecond clock.
TAIInstant	taiInstant() Gets the current TAIInstant.
UTCInstant	utcInstant() Gets the current UTCInstant.

Methods inherited from class java.lang.Object

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

Constructor Detail

TimeSource

protected TimeSource()

Constructor accessible by subclasses.

Method Detail

system

public static TimeSource system()

Gets a time-source that obtains the current instant using the system millisecond clock.

The time-source wraps System.currentTimeMillis(), thus it has at best millisecond resolution.

The returned implementation is Serializable

Returns:

a TimeSource that uses the system millisecond clock, not null

fixed

public static TimeSource fixed(InstantProvider fixedInstantProvider)

Gets a time-source that always returns the same Instant.

This method converts the InstantProvider to an Instant which it then returns from the TimeSource.

The returned implementation is Serializable

Parameters:

fixedInstantProvider - the instant to return from each call to the time-source

Returns:

a TimeSource that always returns the same instant, not null

offsetSystem

public static TimeSource offsetSystem(Duration offset)

Gets a time-source that obtains the current instant using the system millisecond clock and adjusts by a fixed offset.

The time-source wraps System.currentTimeMillis(), thus it has at best millisecond resolution.

The final instant is adjusted by adding the offset. This is useful for simulating an application running at a later or earlier point in time.

The returned implementation is Serializable

Parameters:

offset - the duration by which this time-source is offset from the system millisecond clock

Returns:

a TimeSource that is offset from the system millisecond clock, not null

instant

public abstract Instant instant()

Gets the current Instant.

The instant returned by this method will vary according to the implementation. For example, the time-source returned by system() will return an instant based on System.currentTimeMillis().

Normally, this method will not throw an exception. 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.

Returns:

the current Instant from this time-source, not null

Throws:

CalendricalException - if the instant cannot be obtained, not thrown by most implementations

utcInstant

public UTCInstant utcInstant()

Gets the current UTCInstant.

The UTC time-scale differs from that used by Instant because it includes leap-seconds. An accurate implementation of this abstract class will return a UTC value that includes leap-second information.

The default implementation of this method converts the value from instant() and thus is no more accurate than that method.

Normally, this method will not throw an exception. 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.

Returns:

the current UTCInstant from this time-source, not null

Throws:

CalendricalException - if the instant cannot be obtained, not thrown by most implementations

taiInstant

public TAIInstant taiInstant()

Gets the current TAIInstant.

The TAI time-scale is a simple incrementing number of seconds from the TAI epoch of 1958-01-01(TAI). It ignores all human concepts of time such as days. An accurate implementation of this abstract class will return a TAI value in accordance with international standards.

The default implementation of this method converts the value from instant() and thus is no more accurate than that method.

Normally, this method will not throw an exception. 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.

millis

public long millis()

Gets the current millisecond instant.

The instant returned by this method will vary according to the implementation. For example, the time-source returned by system() will return System.currentTimeMillis().

This method is provided for backwards compatibility. New code should use classes such as Instant to represent an instant rather than a raw millisecond value.

Normally, this method will not throw an exception. 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.

Returns:

the current millisecond instant from this time-source, measured from the Java epoch of 1970-01-01T00:00 UTC, not null

Throws:

CalendricalException - if the instant cannot be obtained, not thrown by most implementations