Auto-configure java.time.Clock

Consider autoconfiguring a default java.time.Clock instance.

I was very surprised to see that there is currently is no autoconfigured Clock instance. I know it’s only four lines of code in our application, and yet - every modern application will have them, and therefore Spring Boot can come in rescue:

@Bean
public Clock clock() {
    return Clock.systemDefaultZone();
}

I suppose this was not done because there may be people who want Clock.systemUTC() instead? Indeed, I see that, but with the exception of JEP 400 the JDK is very consistent in using the default timezone/locale etc., e.g. in ZonedDateTime.now() and all its friends.

I strongly believe the above autoconfiguration would positively please and spoil developers without taking risks. It’s a small convenience, that’s it.

[wilkinsona]

Thanks for sharing your thoughts, @estigma88.

There’s no usage of java.time.Clock in Spring Boot’s public API. There’s a little bit of usage in Spring Framework related to task scheduling and the like. Providing a mutable clock for testing in Spring Framework was proposed and declined. In declining the issue, the Framework team suggested using profiles or a @Primary bean to override the Clock in tests. This is equivalent to the one benefit of auto-configuration that I described above.

Thanks for the suggestion, @JanecekPetr. For the reasons described above, we don’t think that Spring Boot should auto-configure a Clock. If you need one in your application, a minimal amount of code is required to define one. To replace the Clock in tests, use profiles or a @Primary bean.

[wilkinsona]

I’ve written a few apps recently that have used the java.time APIs including the various now() methods and haven’t needed a Clock for them to be sufficiently testable. As such, I don’t agree that every modern application will need a Clock bean.

The one benefit that I can think of to auto-configuring a Clock is that it could be @ConditionalOnMissingBean and this would then make it easy to replace it with a FixedClock for testing.