Mockito mocks locally final class but fails in Jenkins

Question

I have written some unit tests for a static method. The static method takes only one argument. The argument's type is a final class. In terms of code:

public class Utility {

   public static Optional<String> getName(Customer customer) {
       // method's body.
   }
}

public final class Customer {
   // class definition
}

So for the Utility class I have created a test class UtilityTests in which I have written tests for this method, getName. The unit testing framework is TestNG and the mocking library that is used is Mockito. So a typical test has the following structure:

public class UtilityTests {

   @Test
   public void getNameTest() {
     // Arrange
     Customer customerMock = Mockito.mock(Customer.class);
     Mockito.when(...).thenReturn(...);

     // Act
     Optional<String> name = Utility.getName(customerMock);

     // Assert
     Assert.assertTrue(...);
   }
}

What is the problem ?

Whereas the tests run successfully locally, inside IntelliJ, they fail on Jenkins (when I push my code in the remote branch, a build is triggered and unit tests run at the end). The error message is sth like the following:

org.mockito.exceptions.base.MockitoException: Cannot mock/spy class com.packagename.Customer Mockito cannot mock/spy because : - final class

What I tried ?

I searched a bit, in order to find a solution but I didn't make it. I note here that I am not allowed to change the fact that Customer is a final class. In addition to this, I would like if possible to not change it's design at all (e.g. creating an interface, that would hold the methods that I want to mock and state that the Customer class implements that interface, as correctly Jose pointed out in his comment). The thing that I tried is the second option mentioned at mockito-final. Despite the fact that this fixed the problem, it brake some other unit tests :(, that cannot be fixed in none apparent way.

Questions

So here are the two questions I have:

1. How that is possible in the first place ? Shouldn't the test fail both locally and in Jenkins ?
2. How this can be fixed based in the constraints I mentioned above ?

Thanks in advance for any help.

Answer 1

An alternative approach would be to use the 'method to class' pattern.

1. Move the methods out of the customer class into another class/classes, say CustomerSomething eg/CustomerFinances (or whatever it's responsibility is).
2. Add a constructor to Customer.
3. Now you don't need to mock Customer, just the CustomerSomething class! You may not need to mock that either if it has no external dependencies.

Here's a good blog on the topic: https://simpleprogrammer.com/back-to-basics-mock-eliminating-patterns/

Answer 2

How that is possible in the first place? Shouldn't the test fail both locally and in Jenkins ?

It's obviously a kind of env-specifics. The only question is - how to determine the cause of difference.

I'd suggest you to check org.mockito.internal.util.MockUtil#typeMockabilityOf method and compare, what mockMaker is actually used in both environments and why.

If mockMaker is the same - compare loaded classes IDE-Client vs Jenkins-Client - do they have any difference on the time of test execution.

How this can be fixed based in the constraints I mentioned above?

The following code is written in assumption of OpenJDK 12 and Mockito 2.28.2, but I believe you can adjust it to any actually used version.

public class UtilityTest {    
    @Rule
    public InlineMocksRule inlineMocksRule = new InlineMocksRule();

    @Rule
    public MockitoRule mockitoRule = MockitoJUnit.rule();

    @Test
    public void testFinalClass() {
        // Given
        String testName = "Ainz Ooal Gown";
        Client client = Mockito.mock(Client.class);
        Mockito.when(client.getName()).thenReturn(testName);

        // When
        String name = Utility.getName(client).orElseThrow();

        // Then
        assertEquals(testName, name);
    }

    static final class Client {
        final String getName() {
            return "text";
        }
    }

    static final class Utility {
        static Optional<String> getName(Client client) {
            return Optional.ofNullable(client).map(Client::getName);
        }
    }    
}

With a separate rule for inline mocks:

import org.junit.rules.TestRule;
import org.junit.runner.Description;
import org.junit.runners.model.Statement;
import org.mockito.internal.configuration.plugins.Plugins;
import org.mockito.internal.util.MockUtil;

import java.lang.invoke.MethodHandles;
import java.lang.invoke.VarHandle;
import java.lang.reflect.Field;
import java.lang.reflect.Modifier;

public class InlineMocksRule implements TestRule {
    private static Field MOCK_MAKER_FIELD;

    static {
        try {
            MethodHandles.Lookup lookup = MethodHandles.privateLookupIn(Field.class, MethodHandles.lookup());
            VarHandle modifiers = lookup.findVarHandle(Field.class, "modifiers", int.class);

            MOCK_MAKER_FIELD = MockUtil.class.getDeclaredField("mockMaker");
            MOCK_MAKER_FIELD.setAccessible(true);

            int mods = MOCK_MAKER_FIELD.getModifiers();
            if (Modifier.isFinal(mods)) {
                modifiers.set(MOCK_MAKER_FIELD, mods & ~Modifier.FINAL);
            }
        } catch (IllegalAccessException | NoSuchFieldException ex) {
            throw new RuntimeException(ex);
        }
    }

    @Override
    public Statement apply(Statement base, Description description) {
        return new Statement() {
            @Override
            public void evaluate() throws Throwable {
                Object oldMaker = MOCK_MAKER_FIELD.get(null);
                MOCK_MAKER_FIELD.set(null, Plugins.getPlugins().getInlineMockMaker());
                try {
                    base.evaluate();
                } finally {
                    MOCK_MAKER_FIELD.set(null, oldMaker);
                }
            }
        };
    }
}