Mocking a singleton with mockito
<p>I need to test some legacy code, which uses a singleton in a a method call. The purpose of the test is to ensure that the clas sunder test makes a call to singletons method. 
I have seen similar questions on SO, but all the answers require other dependencies (different test frameworks) - I'm unfortunately limited to using Mockito and JUnit, but this should be perfectly possible with such popular framework.</p>

<p>The singleton:</p>

<pre><code>public class FormatterService {

    private static FormatterService INSTANCE;

    private FormatterService() {
    }

    public static FormatterService getInstance() {
        if (INSTANCE == null) {
            INSTANCE = new FormatterService();
        }
        return INSTANCE;
    }

    public String formatTachoIcon() {
        return "URL";
    }

}
</code></pre>

<p>The class under test:</p>

<pre><code>public class DriverSnapshotHandler {

    public String getImageURL() {
        return FormatterService.getInstance().formatTachoIcon();
    }

}
</code></pre>

<p>The unit test:</p>

<pre><code>public class TestDriverSnapshotHandler {

    private FormatterService formatter;

    @Before
    public void setUp() {

        formatter = mock(FormatterService.class);

        when(FormatterService.getInstance()).thenReturn(formatter);

        when(formatter.formatTachoIcon()).thenReturn("MockedURL");

    }

    @Test
    public void testFormatterServiceIsCalled() {

        DriverSnapshotHandler handler = new DriverSnapshotHandler();
        handler.getImageURL();

        verify(formatter, atLeastOnce()).formatTachoIcon();

    }

}
</code></pre>

<p>The idea was to configure the expected behaviour of the dreaded singleton, since the class under test will call it's getInstance and then formatTachoIcon methods. Unfortunately this fails with an error message:</p>

<pre><code>when() requires an argument which has to be 'a method call on a mock'.
</code></pre>