Mock singleton class with void method using PowerMockito

Question

I keep on getting WanterButNotInvoked exception on my UT with the below code:

@RunWith( PowerMockRunner.class )
@PrepareForTest( { CounterHandler.class, 
TrHandlerProtocolsCounterHandler.class, IProtocolCounterHandler.class } )

@Test
public void testInitialize() 
{
    IProtocolCounterHandler counter = new TrHandlerProtocolsCounterHandler();
    CounterHandler mockCounter = PowerMockito.spy( CounterHandler.getInstance() );
    PowerMockito.doNothing().when( mockCounter ).initialize();

    counter.initialize();
    Mockito.verify( mockCounter ).initialize();

} ======================
CounterHandler class

public void initialize() {
    //do something
}

======================
TrHandlerProtocolsCounterHandler class

@Override
public void initialize()  {
    CounterHandler.getInstance().initialize();
}

I have debugged that Counterhandler.getInstance.initialize was executed but when im trying to verify it, it says that there were no interactions with the mock. Thanks!

Answer 1

The problem here is that you have created a new instance of CounterHandler that is the spy of the singleton instance. However, you have done nothing to replace the singleton instance help by the CounterHandler class with this spy. Therefore when your class calls getInstance it gets the actual instance, not the spy.

What you need to do is mock the constructor call so that the singleton instance is a mock. Read this article which shows how to do it and provides a discussion of alternatives to using Singleton.

Mocking a singleton with EasyMock and PowerMock