Mockito: Trying to spy on method is calling the original method

Let me quote the official documentation:

Important gotcha on spying real objects!

Sometimes it's impossible to use when(Object) for stubbing spies. Example:

List list = new LinkedList();
List spy = spy(list);

// Impossible: real method is called so spy.get(0) throws IndexOutOfBoundsException (the list is yet empty)
when(spy.get(0)).thenReturn("foo");

// You have to use doReturn() for stubbing
doReturn("foo").when(spy).get(0);

In your case it goes something like:

doReturn(resultsIWant).when(myClassSpy).method1();

My case was different from the accepted answer. I was trying to mock a package-private method for an instance that did not live in that package

package common;

public class Animal {
  void packageProtected();
}

package instances;

class Dog extends Animal { }

and the test classes

package common;

public abstract class AnimalTest<T extends Animal> {
  @Before
  setup(){
    doNothing().when(getInstance()).packageProtected();
  }

  abstract T getInstance();
}

package instances;

class DogTest extends AnimalTest<Dog> {
  Dog getInstance(){
    return spy(new Dog());
  }

  @Test
  public void myTest(){}
}

The compilation is correct, but when it tries to setup the test, it invokes the real method instead.

Declaring the method protected or public fixes the issue, tho it's not a clean solution.

In my case, using Mockito 2.0, I had to change all the any() parameters to nullable() in order to stub the real call.