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 = num;   }    public int getNum() {     return num;   } 

I'd like to stub getNum() in the above example, but I can't figure out how to mock the actual SingletonObject class. I thought using Powermock to prepare the test would help since enums are inherently final.

//... rest of test code @Test public void test() {   PowerMockito.mock(SingletonObject.class);   when(SingletonObject.INSTANCE.getNum()).thenReturn(1); //does not work } 

This is using PowerMockMockito 1.4.10 and Mockito 1.8.5.

Answer 1

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.