How to mock an enum singleton class using Mockito/Powermock?

Question

I am unsure on how to mock an enum singleton class.

public enum SingletonObject{
  INSTANCE;
  private int num;

  protected setNum(int num) {
    this.num =         


        

Answer 1

In addition to above Matt Lachman answer, create a object factory for power mock in SingleTonConsumerTest.class

@ObjectFactory
public IObjectFactory getObjectFactory() {
    return new org.powermock.modules.testng.PowerMockObjectFactory();
}

This will remove Mockito cannot mock/spy following: - final classes - anonymous classes - primitive types error.

Answer 2

Have an interface with the methods you intend to mock

public interface SingletonInterface {
  int getNum();
}

Let the enum implement the interface

public enum SingletonObject implements SingletonInterface {
    INSTANCE;
    private int num;

    protected void setNum(int num) {
        this.num = num;
    }

    @Override
    public int getNum() {
        return num;
    }
}

Mock the interface

@Test
public void test() {
  SingletonInterface singleton = Mockito.mock(SingletonInterface.class);
  when(singleton.getNum()).thenReturn(1); //does work
}

Answer 3

If you want to stub out what INSTANCE returns, you can do it but it's kind of nasty (using reflection & bytecode manipulation). I created & tested a simple project with three classes using the PowerMock 1.4.12 / Mockito 1.9.0. All classes were in the same package.

SingletonObject.java

public enum SingletonObject {
    INSTANCE;
    private int num;

    protected void setNum(int num) {
        this.num = num;
    }

    public int getNum() {
        return num;
    }
}

SingletonConsumer.java

public class SingletonConsumer {
    public String consumeSingletonObject() {
        return String.valueOf(SingletonObject.INSTANCE.getNum());
    }
}

SingletonConsumerTest.java

import static org.junit.Assert.*;
import static org.powermock.api.mockito.PowerMockito.*;

import org.junit.Test;
import org.junit.runner.RunWith;
import org.powermock.core.classloader.annotations.PrepareForTest;
import org.powermock.modules.junit4.PowerMockRunner;
import org.powermock.reflect.Whitebox;

@RunWith(PowerMockRunner.class)
@PrepareForTest({SingletonObject.class})
public class SingletonConsumerTest {
    @Test
    public void testConsumeSingletonObject() throws Exception {
        SingletonObject mockInstance = mock(SingletonObject.class);
        Whitebox.setInternalState(SingletonObject.class, "INSTANCE", mockInstance);

        when(mockInstance.getNum()).thenReturn(42);

        assertEquals("42", new SingletonConsumer().consumeSingletonObject());
    }
}

The call to the Whitebox.setInternalState replaces INSTANCE with a mock object that you can manipulate within your test.