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:

1. Use a different mocking tool, such as PowerMock, that allows to mock static methods.

2. 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 a FormatterService 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();