Why Mockito is Returning Null in Spring Boot JUnit Tests: Common Errors and Solutions

Introduction

In the world of Java and Spring Boot, Mockito is an indispensable tool for testing. It allows developers to easily mock objects and simulate complex behavior. However, it's not uncommon to encounter a pesky issue where Mockito returns null in your SpringBoot JUnit tests. In this article, we'll explore the most common reasons for this issue and what you can do to resolve it.

The Most Common Reasons for Mockito Null Return

1. Forgetting the @RunWith Annotation or the call to initMocks

One of the most common mistakes in Spring Boot JUnit tests is forgetting the @RunWith() annotation or neglecting to call (this). These are essential for initializing the Mockito framework to start creating mock objects.

Solution: Ensure you include the @RunWith() annotation at the top of your test class. Alternatively, call (this) in the setUp() method or the constructor of your test class.

@RunWith()
public class MyServiceTest {
    @InjectMocks
    private MyService myService;
    @Mock
    private MyRepository myRepository;
    @Before
    public void setUp() {
        (this);
    }
}

2. Mismatch in Matcher Input Parameters

2.1 Matcher Mismatch Issue

Another reason Mockito might return null is due to a matcher mismatch. For example, if the actual method call in your test method doesn't match the matcher you've provided, then Mockito might return null for the mock object's call.

Solution: Make sure that the matchers you use in your when() calls actually match the parameters passed by your SUT (System Under Test).

@Test
public void testMethod() {
    // Given
    MyEntity myEntity  new MyEntity();
    when((())).thenReturn(myEntity);
    // When
    MyServiceResult result  ();
    // Then
    assertNotNull(result);
}

2.2 Example of Mismatch in Matcher

Let's illustrate this with a more concrete example:

Example: Finding by ID

Suppose you have a method in your service that calls your repository to find an entity by ID:

public MyEntity findEntityById(Long id) {
    return (id);
}

And your test method looks like this:

@Test
public void testFindEntityById() {
    // Given
    MyEntity myEntity  new MyEntity();
    when((1L)).thenReturn(myEntity);
    // When
    MyEntity result  (1);
    // Then
    assertNotNull(result);
}

Note that in the when() call, the matcher is 1L, but in your test method, you are calling (1) without the L suffix. This mismatch will result in Mockito returning null.

2.3 Testing with Multiple Arguments

Mismatches can also occur when you are using multiple arguments in your matchers. Ensure that all the arguments match exactly in the when() call and the actual method call.

@Test
public void testMethod() {
    // Given
    MyEntity myEntity  new MyEntity();
    when(((), 1L)).thenReturn(myEntity);
    // When
    MyServiceResult result  ();
    // Then
    assertNotNull(result);
}

Conclusion

Mockito is a powerful tool but requires careful setup and matching. By ensuring that you initialize Mockito correctly and perform your matchers accurately, you can avoid the common pitfalls of null returns and make your tests more reliable.

Related Keywords

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