Mocking a singleton with mockito
What you are asking is not possible because your legacy code relies on a static method getInstance()
and Mockito does not allow to mock static methods, so the following line won't work
when(FormatterService.getInstance()).thenReturn(formatter);
There are 2 ways around this problem:
Use a different mocking tool, such as PowerMock, that allows to mock static methods.
Refactor your code, so that you don't rely on the static method. The least invasive way I can think of to achieve this is by adding a constructor to
DriverSnapshotHandler
that injects aFormatterService
dependency. This constructor will be only used in tests and you production code will continue to use the real singleton instance.public static class DriverSnapshotHandler { private final FormatterService formatter; //used in production code public DriverSnapshotHandler() { this(FormatterService.getInstance()); } //used for tests DriverSnapshotHandler(FormatterService formatter) { this.formatter = formatter; } public String getImageURL() { return formatter.formatTachoIcon(); } }
Then, your test should look like this :
FormatterService formatter = mock(FormatterService.class);
when(formatter.formatTachoIcon()).thenReturn("MockedURL");
DriverSnapshotHandler handler = new DriverSnapshotHandler(formatter);
handler.getImageURL();
verify(formatter, atLeastOnce()).formatTachoIcon();
I think it is possible. See an example how to test a singleton
Before a test:
@Before
public void setUp() {
formatter = mock(FormatterService.class);
setMock(formatter);
when(formatter.formatTachoIcon()).thenReturn(MOCKED_URL);
}
private void setMock(FormatterService mock) {
try {
Field instance = FormatterService.class.getDeclaredField("instance");
instance.setAccessible(true);
instance.set(instance, mock);
} catch (Exception e) {
throw new RuntimeException(e);
}
}
After the test - it is important to clean up the class, because other tests will be confused with the mocked instance.
@After
public void resetSingleton() throws Exception {
Field instance = FormatterService.class.getDeclaredField("instance");
instance.setAccessible(true);
instance.set(null, null);
}
The test:
@Test
public void testFormatterServiceIsCalled() {
DriverSnapshotHandler handler = new DriverSnapshotHandler();
String url = handler.getImageURL();
verify(formatter, atLeastOnce()).formatTachoIcon();
assertEquals(MOCKED_URL, url);
}