Difference between @Mock, @MockBean and Mockito.mock()

Plain Mockito library

import org.mockito.Mock;
...
@Mock
MyService myservice;

and

import org.mockito.Mockito;
...
MyService myservice = Mockito.mock(MyService.class);

come from the Mockito library and are functionally equivalent. They allow to mock a class or an interface and to record and verify behaviors on it.

The way using annotation is shorter, so preferable and often preferred.

Note that to enable Mockito annotations during test executions, the MockitoAnnotations.initMocks(this) static method has to be called.
To avoid side effect between tests, it is advised to do it before each test execution :

@Before 
public void initMocks() {
    MockitoAnnotations.initMocks(this);
}

Another way to enable Mockito annotations is annotating the test class with @RunWith by specifying the MockitoJUnitRunner that does this task and also other useful things :

@RunWith(org.mockito.runners.MockitoJUnitRunner.class)
public MyClassTest{...}

Spring Boot library wrapping Mockito library

This is indeed a Spring Boot class:

import org.springframework.boot.test.mock.mockito.MockBean;
...
@MockBean
MyService myservice;

The class is included in the spring-boot-test library.

It allows to add Mockito mocks in a Spring ApplicationContext.
If a bean, compatible with the declared class exists in the context, it replaces it by the mock.
If it is not the case, it adds the mock in the context as a bean.

Javadoc reference :

Annotation that can be used to add mocks to a Spring ApplicationContext. ... If any existing single bean of the same type defined in the context will be replaced by the mock, if no existing bean is defined a new one will be added.

When use classic/plain Mockito and when use @MockBean from Spring Boot ?

Unit tests are designed to test a component in isolation from other components and unit tests have also a requirement : being as fast as possible in terms of execution time as these tests may be executed each day dozen times on the developer machines.

Consequently, here is a simple guideline :

As you write a test that doesn't need any dependencies from the Spring Boot container, the classic/plain Mockito is the way to follow : it is fast and favors the isolation of the tested component.
If your test needs to rely on the Spring Boot container and you want also to add or mock one of the container beans : @MockBean from Spring Boot is the way.

Typical usage of Spring Boot @MockBean

As we write a test class annotated with @WebMvcTest (web test slice).

The Spring Boot documentation summarizes that very well :

Often @WebMvcTest will be limited to a single controller and used in combination with @MockBean to provide mock implementations for required collaborators.

Here is an example :

import org.junit.Test;
import org.junit.runner.RunWith;
import org.mockito.Mockito;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.autoconfigure.web.servlet.WebMvcTest;
import org.springframework.boot.test.mock.mockito.MockBean;
import org.springframework.http.MediaType;
import org.springframework.test.context.junit4.SpringRunner;
import org.springframework.test.web.servlet.MockMvc;

import static org.springframework.test.web.servlet.request.MockMvcRequestBuilders.*;
import static org.springframework.test.web.servlet.result.MockMvcResultMatchers.*;

@RunWith(SpringRunner.class)
@WebMvcTest(FooController.class)
public class FooControllerTest {

    @Autowired
    private MockMvc mvc;

    @MockBean
    private FooService fooServiceMock;

    @Test
    public void testExample() throws Exception {
         Foo mockedFoo = new Foo("one", "two");

         Mockito.when(fooServiceMock.get(1))
                .thenReturn(mockedFoo);

         mvc.perform(get("foos/1")
            .accept(MediaType.TEXT_PLAIN))
            .andExpect(status().isOk())
            .andExpect(content().string("one two"));
    }

}

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At the end its easy to explain. If you just look into the javadocs of the annotations you will see the differences:

@Mock: (org.mockito.Mock)

Mark a field as a mock.

Allows shorthand mock creation. Minimizes repetitive mock creation code. Makes the test class more readable. Makes the verification error easier to read because the field name is used to identify the mock.

@MockBean: (org.springframework.boot.test.mock.mockito.MockBean)

Annotation that can be used to add mocks to a Spring ApplicationContext. Can be used as a class level annotation or on fields in either @Configuration classes, or test classes that are @RunWith the SpringRunner.

Mocks can be registered by type or by bean name. Any existing single bean of the same type defined in the context will be replaced by the mock, if no existing bean is defined a new one will be added.

When @MockBean is used on a field, as well as being registered in the application context, the mock will also be injected into the field.

Mockito.mock()

Its just the representation of a @Mock.

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Mocktio.mock() :-

Will create a mock object of either class or interface. We can use this mock to stub the return values and verify if they are called. We must use the when(..) and thenReturn(..) methods for mock objects whose class methods will be invoked during test case execution.

@Mock :-

This is shorthand for mock() method so it is preferable and often used. Both mock() and @Mock are functionally equivalent. Easier to identify the problem in mock failure as name of the field appears in the error message.

To enable the Mockito annotation during test execution we need to call the MockitoAnnotations.initMocks(this) method but this method is deprecated instead we can call - MockitoAnnotations.openMocks(this). In order to avoid the side effects it is advised to call this method before test case executions.

Another way to enable the Mockito annotations is by annotating the test class with @RunWith by specifying the MockitoJUnitRunner that does this task and also other useful things.

@MockBean :- It is used to add the mock objects into spring application context. This mock will replace the existing bean of same type in application context. If no bean is available, then new bean will be added. This is useful in integration test case.

When we write a test case that doesn't need any dependencies from the Spring Boot container, the classic/plain Mockito is used and it is fast and favors the isolation of the tested component.

If our test case needs to rely on the Spring Boot container and want to add or mock one of the container beans then @MockBean is used.

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