Is a mocked bean (@MockBean) mocked before the Spring Context loads the actual Spring Bean?

Question

Let's use the following as an example.

@Autowired
@MockBean
private Foo foobar;

Does the Spring Context load the class Foo first, and then apply the mock? Or does the @Mockbean get detected somehow, and Spring creates and applies the mock instead of loading the class Foo into the Spring Context. I have a suspicion that it is the latter, but I would like confirmation.

Answer 1

Spring will throw an exception.

Let's define the class Foo.

@Component
public class Foo {
    public Foo() {
        System.out.println("I am not a mock");
    }
}

Whenever testing using @Autowired, spring injects an instance of Foo and the constructor will print "I am not a mock", as in the code below.

@SpringBootTest(classes = Main.class)
@RunWith(SpringRunner.class)
public class FooTest {

    @Autowired
    Foo foo;

    @Test
    public void test() {
        System.out.println(foo);
    }
}

On the other hand, using @MockBean, spring will not create the real bean and the message in the constructor will not be printed. This scenario is represented by the following code.

@SpringBootTest(classes = Main.class)
@RunWith(SpringRunner.class)
public class FooTest {

    @MockBean
    Foo foo;
    @Test
    public void test() {
        System.out.println(foo);
    }
}

However, when you try to use both annotations together, spring will throw a BeanCreationException caused by an IllegalStateException. It means that the field foo cannot have an existing value. This scenario will happen in executing the code below:

@SpringBootTest(classes = Main.class)
@RunWith(SpringRunner.class)
public class FooTest {
   // this will not work
    @Autowired
    @MockBean
    Foo foo;

    @Test
    public void test() {
        System.out.println(foo);
    }
}

And the stack trace will be something like:

org.springframework.beans.factory.BeanCreationException: Could not inject field: com.tbp.Foo com.FooTest.foo; nested exception is java.lang.IllegalStateException: The field com.tbp.Foo com.FooTest.foo cannot have an existing value
    at org.springframework.boot.test.mock.mockito.MockitoPostProcessor.inject(MockitoPostProcessor.java:413) ~[spring-boot-test-1.5.2.RELEASE.jar:1.5.2.RELEASE]