Mock a private void method using PowerMockito

Question

public class Abcd {
    
    public String one() {
        System.out.println("Inside method one");
        StringBuilder sb = new StringBuilder();
        two(sb);
        return "Done";
    }
    
    private void two(StringBuilder sb) {
        System.out.println("Inside method two");
    }
}

Here is the test class

@RunWith(PowerMockRunner.class)
@PrepareForTest(Abcd.class)
public class TestAbcd {
    
    @Test
    public void testMethod1() throws Exception {
        Abcd abcd = PowerMockito.spy(new Abcd());
        StringBuilder sb = new StringBuilder();
        PowerMockito.doNothing().when(abcd, "two", sb);
        abcd.one();
    }
}

Console Output:

Inside method one
Inside method two

No failure trace in edited part: Failure trace:

Please let me know what mistake i am making, and how can i make it work.

Answer 1

You need @PrepareForTest annotaion to gain such a control on private methods with PowerMockito.

See this article: What does @PrepareForTest in PowerMock really mean?

In summary the test case should look like this:

@RunWith(PowerMockRunner.class)
@PrepareForTest(Abcd.class)
public class TestAbcd {
    
    @Test
    public void testMethod1() throws Exception {
        Abcd abcd = PowerMockito.spy(new Abcd());
        PowerMockito.doNothing().when(abcd, method(Abcd.class, "two", StringBuilder.class))
                .withArguments(any(StringBuilder.class));               
        abcd.one();
    }
}