PowerMockito mocking static method fails when calling method on parameter

Question

I'm trying to test a class which uses a calculator class with a number of static methods. I've successfully mocked another class in a similar way, but this one is proving more stubborn.

It seems that if the mocked method contains a method call on one of the passed in arguments the static method is not mocked (and the test breaks). Removing the internal call is clearly not an option. Is there something obvious I'm missing here?

Here's a condensed version which behaves the same way...

public class SmallCalculator {

    public static int getLength(String string){

        int length = 0;

        //length = string.length(); // Uncomment this line and the mocking no longer works... 

        return length;
    }
}

And here's the test...

import static org.junit.Assert.assertEquals;
import static org.mockito.BDDMockito.given;
import static org.mockito.Matchers.any;

import org.junit.Test;
import org.junit.runner.RunWith;
import org.powermock.api.mockito.PowerMockito;
import org.powermock.core.classloader.annotations.PrepareForTest;
import org.powermock.modules.junit4.PowerMockRunner;

import com.solveit.aps.transport.model.impl.SmallCalculator;

@RunWith(PowerMockRunner.class)
@PrepareForTest({ SmallCalculator.class})
public class SmallTester {

    @Test
    public void smallTest(){

        PowerMockito.spy(SmallCalculator.class);
        given(SmallCalculator.getLength(any(String.class))).willReturn(5);

        assertEquals(5, SmallCalculator.getLength(""));
    }
}

It seems there's some confusion about the question, so I've contrived a more 'realistic' example. This one adds a level of indirection, so that it doesn't appear that I'm testing the mocked method directly. The SmallCalculator class is unchanged:

public class BigCalculator {

    public int getLength(){

        int length  = SmallCalculator.getLength("random string");

        // ... other logic

        return length;
    }

    public static void main(String... args){

        new BigCalculator();
    }
}

And here's the new test class...

import static org.junit.Assert.assertEquals;
import static org.mockito.BDDMockito.given;
import static org.mockito.Matchers.any;

import org.junit.Test;
import org.junit.runner.RunWith;
import org.powermock.api.mockito.PowerMockito;
import org.powermock.core.classloader.annotations.PrepareForTest;
import org.powermock.modules.junit4.PowerMockRunner;

import com.solveit.aps.transport.model.impl.BigCalculator;
import com.solveit.aps.transport.model.impl.SmallCalculator;

@RunWith(PowerMockRunner.class)
@PrepareForTest({ SmallCalculator.class})
public class BigTester {

    @Test
    public void bigTest(){

        PowerMockito.spy(SmallCalculator.class);
        given(SmallCalculator.getLength(any(String.class))).willReturn(5);

        BigCalculator bigCalculator = new BigCalculator();
        assertEquals(5, bigCalculator.getLength());
    }
}

Answer 1

I've found the answer here https://blog.codecentric.de/en/2011/11/testing-and-mocking-of-static-methods-in-java/

Here's the final code which works. I've tested this approach in the original code (as well as the contrived example) and it works great. Simples...

import static org.junit.Assert.assertEquals;
import static org.mockito.Matchers.any;

import org.junit.Test;
import org.junit.runner.RunWith;
import org.powermock.api.mockito.PowerMockito;
import org.powermock.core.classloader.annotations.PrepareForTest;
import org.powermock.modules.junit4.PowerMockRunner;

@RunWith(PowerMockRunner.class)
@PrepareForTest({ SmallCalculator.class})
public class BigTester {

    @Test
    public void bigTest(){

        PowerMockito.mockStatic(SmallCalculator.class);
        PowerMockito.when(SmallCalculator.getLength(any(String.class))).thenReturn(5);

        BigCalculator bigCalculator = new BigCalculator();
        assertEquals(5, bigCalculator.getLength());
    }
}

Answer 2

First of all, remove that line:

given(SmallCalculator.getLength(any(String.class))).willReturn(5);

Since you are testing the same method. You don't want to mock the method that you're testing.

Secondly, modify your annotation to:

@PrepareForTest({ SmallCalculator.class, String.class})

Lastly, add the mock for the length(); like this:

given(String.length()).willReturn(5);

I think it will do ;)

Answer 3

Use anyString() instead of any(String.class).

When using any(String.class) the passed argument is null, as Mockito will return the default value for the reference type, which is null. As a result you get an exception.

When using the anyString(), the passed argument will be empty string.

Note, this explains why you get an exception, however you need to review the way you test your method, as explained in other comments and answers.