How do I mock a private field using PowerMockito?

Question

The solution proposed as a duplicate is not a PowerMockito solution and as a consequence does not answer this question. Further, this question IS answered legitimately below.

IDK if this is a duplicate or not but I sure can't find the item in question if it is. I keep expecting this to be really simple, since it is pretty simple with reflection but I'd rather do it using the right tools.

Clarification: Legacy code. No getters / setters.

Is it correct to use Whitebox for this? I thought it was "Off Limits" i.e. part of the internal API? ...or was that strictly Mockito?

Answer 1

See Whitebox.setInternalState(...).

For example - given class A which needs to be tested:

public class A {
    private B b;

    public A() {
        b = new B();
    }

    public void doSomething() {
        b.doSomething();
    }

}

which has a private instance of B:

public class B {

    public void doSomething() {
        // some long running, resource intensive process...
        System.out.println("Real B.doSomething() was invoked.");
    }

}

then Whitebox can be used to set the private state of A so it can be tested:

import static org.mockito.Mockito.doNothing;
import static org.mockito.Mockito.verify;
import org.junit.Before;
import org.junit.Test;
import org.junit.runner.RunWith;
import org.mockito.Mock;
import org.mockito.runners.MockitoJUnitRunner;
import org.powermock.reflect.Whitebox;

@RunWith(MockitoJUnitRunner.class)
public class ATest {

    @Mock
    private B b;

    private A a;

    @Before
    public void prepareTest() {
        doNothing().when(b).doSomething();

        a = new A();
        Whitebox.setInternalState(a, B.class, b);
    }

    @Test
    public void doSomething() {
        a.doSomething();
        verify(b).doSomething();
    }

}