1. Overview

This document will cover the following annotations of the Mockito library: @Mock, @Spy, @Captor and @InectMocks

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2. Enable Mockito Annotations

There are different ways to enable the use of annotations with Mockito tests.

2.1. MockitoJUnitRunner

The first option we have is to annotate the JUnit test with a MockitoJUnitRunner

@ExtendWith(MockitoExtension.class)
public class MockitoAnnotationUnitTest {
    ...
}

2.2. MockitoAnnotations.openMocks()

Alternatively, we can enable Mockito annotations programmatically by invoking MockitoAnnotations.openMocks():

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

2.3. MockitoJUnit.rule()

Lastly, we can use a MockitoJUnit.rule():

public class MockitoAnnotationsInitWithMockitoJUnitRuleUnitTest {

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

    ...
}

In this case, we must remember to make our rule public.

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3. @Mock Annotation

The most widely used annotation in Mockito is @Mock. We can use @Mock to create and inject mocked instances without having to call Mockito.mock manually.
In the following example, we’ll create a mocked ArrayList manually without using the @Mock annotation:

@Test
public void whenNotUseMockAnnotation_thenCorrect() {
    List mockList = Mockito.mock(ArrayList.class);
    
    mockList.add("one");
    Mockito.verify(mockList).add("one");
    assertEquals(0, mockList.size());

    Mockito.when(mockList.size()).thenReturn(100);
    assertEquals(100, mockList.size());
}