Java 8's Clock class is a big step in the right direction for testability, but it's still a pain. You can test using Clock.fixed() but that's immutable so you can't test how your code responds to the passage of time. You can use a mocking library to stub the instant() and getZone() methods, but that's still awkward and indirect.

This library provides a mutable MockClock class that can be constructed as of a given date and time, then adjusted as needed. Here's an example of how to use it:

import com.statemachinesystems.mockclock.MockClock;
import java.time.ZoneId;

MockClock clock = MockClock.at(2015, 12, 10, 11, 16, ZoneId.of("UTC"));
ClassUnderTest testSubject = new ClassUnderTest(clock);

assertThat(testSubject.someMethod(), is(expectedValueAtStartTime));

clock.advanceBySeconds(30);

assertThat(testSubject.someMethod(), is(expectedValueAfter30Seconds));

This library is in the Maven Central repo, so just add the following chunk to your pom.xml (or the equivalent for Gradle/SBT/whatever):

<dependency>
    <groupId>com.statemachinesystems</groupId>
    <artifactId>mock-clock</artifactId>
    <version>1.0</version>
</dependency>

© 2015 State Machine Systems Ltd. Apache Licence, Version 2.0