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Unit testing a class with a Java 8 Clock

Java 8 introduced java.time.Clock which can be used as an argument to many other java.time objects, allowing you to inject a real or fake clock into them. For example, I know you can create a Clock.fixed() and then call and it will return the fixed Instant you provided. This sounds perfect for unit testing!

However, I’m having trouble figuring out how best to use this. I have a class, similar to the following:

public class MyClass {
    private Clock clock = Clock.systemUTC();

    public void method1() {
        Instant now =;
        // Do something with 'now'

Now, I want to unit test this code. I need to be able to set clock to produce fixed times so that I can test method() at different times. Clearly, I could use reflection to set the clock member to specific values, but it would be nice if I didn’t have to resort to reflection. I could create a public setClock() method, but that feels wrong. I don’t want to add a Clock argument to the method because the real code shouldn’t be concerned with passing in a clock.

What is the best approach for handling this? This is new code so I could reorganize the class.

Edit: To clarify, I need to be able to construct a single MyClass object but be able to have that one object see two different clock values (as if it were a regular system clock ticking along). As such, I cannot pass a fixed clock into the constructor.



Let me put Jon Skeet’s answer and the comments into code:

class under test:

public class Foo {
    private final Clock clock;
    public Foo(Clock clock) {
        this.clock = clock;

    public void someMethod() {
        Instant now = clock.instant();   // this is changed to make test easier
        System.out.println(now);   // Do something with 'now'

unit test:

public class FooTest() {

    private Foo foo;
    private Clock mock;

    public void setUp() {
        mock = mock(Clock.class);
        foo = new Foo(mock);

    public void ensureDifferentValuesWhenMockIsCalled() {
        Instant first =;                  // e.g. 12:00:00
        Instant second = first.plusSeconds(1);          // 12:00:01
        Instant thirdAndAfter = second.plusSeconds(1);  // 12:00:02

        when(mock.instant()).thenReturn(first, second, thirdAndAfter);

        foo.someMethod();   // string of first
        foo.someMethod();   // string of second
        foo.someMethod();   // string of thirdAndAfter 
        foo.someMethod();   // string of thirdAndAfter 
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