jmockit

import org.mockito.internal.util.reflection.Whitebox;

//...

@Mock
private Person mockedPerson;
private Test underTest;

// ...

@Test
public void testMethod() {
    Whitebox.setInternalState(underTest, "person", mockedPerson);
    // ...

}

ArgumentCaptor<Person> argument = ArgumentCaptor.forClass(Person.class);
verify(mock).doSomething(argument.capture());
assertEquals("John", argument.getValue().getName());

PowerMock

Whennew.