How to abstract away java.time.Clock for testing purposes in Spring

Personally, I would simply add the clock in the constructor...

public MyServiceImpl(Clock clock) {
  this.clock = clock;
}

...and perhaps add a nice default constructor...

public MyServiceImpl() {
  this(Clock.systemDefaultZone());
}

This way you can get the default thing via spring and create a custom clock version manually, for example in your tests.

Of course, you could also forgo the default constructor and simply add a Clock bean in your productive configuration, for example like this...

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

...which allows you to use a mocked Clock as a bean in your test configuration, automatically allowing Spring to @Autowire it via constructor injection.