How to Mock a Non-Static Method from Another Class in Java Unit Testing?

How can I mock a non-static method of a different class while unit testing Java code?

Mocking non-static methods in Java unit testing can be challenging, especially when working with frameworks like Mockito. In this guide, we'll explore how to efficiently mock a non-static method from a different class using Mockito, enabling you to isolate the class under test and facilitate effective unit tests.

import static org.mockito.Mockito.*;
import org.junit.jupiter.api.*;
import org.mockito.InjectMocks;
import org.mockito.Mock;

class MyServiceTest {
    @Mock
    private ExternalService externalService;

    @InjectMocks
    private MyService myService;

    @BeforeEach
    void setUp() {
        MockitoAnnotations.openMocks(this);
    }

    @Test
    void testMyServiceMethod() {
        // Arrange
        when(externalService.nonStaticMethod(anyString())).thenReturn("Mocked Response");

        // Act
        String result = myService.myMethod();

        // Assert
        assertEquals("Expected Result", result);
    }
}

Causes

• Non-static methods are bound to instances and are generally more complex to mock compared to static methods.
• When testing, you often need to isolate the unit of work without merging dependencies.

Solutions

• Use a mocking framework like Mockito which allows you to create mock objects and specify behavior for non-static methods.
• Annotate the test class with @RunWith(MockitoJUnitRunner.class) to enable Mockito annotations if you're using JUnit 4, or @ExtendWith(MockitoExtension.class) for JUnit 5.
• Create mock instances of the class containing the non-static method and define behavior using when() and thenReturn() methods.

Mistake: Forgetting to initialize mocks with MockitoAnnotations.openMocks(this) or using the correct runner.

Solution: Ensure you call MockitoAnnotations.openMocks() in the @BeforeEach setup method or use the appropriate test runner.

Mistake: Not verifying that the mocked method was called as expected.

Solution: Always include verification logic in your tests to confirm that the mocked methods were invoked with the right parameters.