Introduction to @WebMvcTest

Testing in Spring Boot

Spring Boot offers various utilities to start an application for testing, either fully or partially. We can use @SpringBootTest as a class annotation for a JUnit test which will start our complete application, including an embedded Tomcat to serve the HTML pages.

However, for tests that will only test a controller for example, we don’t need to start the full application.

Spring Boot has test slices that allows us to start, say, only the database layer (@DataJpaTest), the web layer (@WebMvcTest), or the JSON converters (@JsonTest), etc.

Using @WebMvcTest

We will use @WebMvcTest since it is the test slice we want for testing our UserController. The @WebMvcTest annotation will automatically configure a mock servlet environment. Using MockMvc, we can interact with our controller and validate requests and responses.

The following things will be created when using @WebMvcTest:

• All @Controller beans (or only a single one when specifying the class name on the @WebMvcTest annotation).

• All @ControllerAdvice beans.

• All @JsonComponent beans, so custom JSON serializers and deserializers will be active.

• org.springframework.core.convert.converter.Converter beans.

• Filter beans.

• WebMvcConfigurer beans.

• HandlerMethodArgumentResolver beans.

• Spring Security configuration beans.

What is not created?

• @Component

• @Service

• @Repository

• @Configuration

If our controller needs any of the above four, we should:

• add them by using @Import.

• add mock implementations by using @MockBean.

• use @TestConfiguration with @Bean.

Why are we not doing plain unit tests for controllers?

You might wonder why we are immediately doing an integration test and not a plain unit test. In the case of a controller, there is quite a lot going on in terms of annotations interpreted by the Spring Framework and the security configuration of the application. Moreover, as a good rule of thumb, a controller should not contain much logic but should instead delegate to helper classes (Services and/or repositories). Given all that, the value of a plain unit test would be minimal.