Spy an already initialized Java object Mockito

Question

I am using the Mocktio library to write some test cases, since I have an elaborate inhertance structure, I have a few objects which are instantiated in the parent class, and I would like to mock one of its function call. Does Mockito library provide any way to spy on a already initialized object?

Also, the object is not directly instantiable.

Similar to the following -

     public class A {
        protected static MyObject a;
        public static void someMethod() {
          a = myObjectBuilder.createObj(); 
        }
     }

And another class B looks something similar to

     class B  extends A {
       @BeforeClass
       public static void setUpBeforeClass() {
          someMethod();
       }
       @Test
       public void mockTest() { 
         // now mock behavior  of some method of MyObject a
         // Missing line to spy object a.
         Mockito.doReturn(false).when(a).xyz();
         /* Now call some method that triggers a.xyz() 
            again, it is not a direct call,
            there are multiple layer of abstraction 
         */
       }
     }

Edit: I have tried the following and it does not work

   MyObject mock_object = Mockito.spy(a);
   Mockito.doReturn(false).when(mock_object).xyz();

Answer 1

Basically, don't do initialisation in BeforeClass, it runs only once but you need to have new spy in each test, or you must "reinitialise" spy object before each test. Please examine this code:

import org.junit.Before;
import org.junit.BeforeClass;
import org.junit.Test;
import org.mockito.Mockito;
import static org.assertj.core.api.Assertions.assertThat;

class MyObject{
    public String cos;

    public MyObject(String cos) {
        this.cos = cos;
    }

    public boolean xyz() {
        return true;
    }
}

class A {
    protected static MyObject a;
    public void someMethod() {
        a = new MyObject("cccc");
    }
}

public class B  extends A {
    @Before
    public void setUpBeforeTest() {
        someMethod();
    }
    @Test
    public void mockTest() {
        MyObject mock_object = Mockito.spy(a);
        Mockito.doReturn(false).when(mock_object).xyz();
        assertThat(mock_object.xyz()).isFalse();
    }
    @Test
    public void mockTest2() {
        MyObject mock_object = Mockito.spy(a);
        Mockito.doReturn(true).when(mock_object).xyz();
        assertThat(mock_object.xyz()).isTrue();
    }

}

If you want it your way, please change:

public void someMethod() {
      a = myObjectBuilder.createObj(); 
}

into:

public static void someMethod() {
      a = myObjectBuilder.createObj(); 
}

You can't call non static method from static initialiser @BeforeClass:

class A {
    protected static MyObject a;
    public static void someMethod() {
        a = new MyObject("cccc");
    }
}

public class B  extends A {
    @BeforeClass
    public static void setUpBeforeClass() {
        someMethod();
    }
    @Test
    public void mockTest() {
        MyObject mock_object = Mockito.spy(a);
        Mockito.doReturn(false).when(mock_object).xyz();
        assertThat(mock_object.xyz()).isFalse();
    }

@Test
public void mockTest2() {
    MyObject mock_object = Mockito.spy(a);
    // Here we replace original object with our spy
    A.a = mock_object;
    Mockito.doReturn(false).when(mock_object).xyz();
    assertThat(a.xyz()).isFalse();
}
}

Another example (in this case we replace object a with mock (spy is not needed):

class MyObject{
    public String cos;

    public MyObject(String cos) {
        this.cos = cos;
    }

    public boolean xyz() {
        return true;
    }
}

class A {
    protected MyObject a;

    public A() {
        a = new MyObject("ggggg");
    }

    public String doSomethingWithA(){
        if(a.xyz()){
            return a.cos;
        }
        else{
            return "aaaa";
        }
    }
}

@RunWith(MockitoJUnitRunner.class)
public class B  {

    @Mock
    MyObject mock_object;

    @InjectMocks
    A systemUnderTest = new A();

    @Test
    public void mockTest1() {
        Mockito.doReturn(false).when(mock_object).xyz();
        assertThat(systemUnderTest.doSomethingWithA()).isEqualTo("aaaa");
    }
    @Test
    public void mockTest2() {
        Mockito.doReturn(true).when(mock_object).xyz();
        assertThat(systemUnderTest.doSomethingWithA()).isNull();
    }

}