How to write a unit test for a Spring Boot Controller endpoint

I have a sample Spring Boot app with the following

Boot main class

@SpringBootApplication
public class DemoApplication {

    public static void main(String[] args) {
        SpringApplication.run(DemoApplication.class, args);
    }

Controller

@RestController
@EnableAutoConfiguration
public class HelloWorld {
    @RequestMapping("/")
    String gethelloWorld() {
        return "Hello World!";
    }

}

What’s the easiest way to write a unit test for the controller? I tried the following but it complains about failing to autowire WebApplicationContext

@RunWith(SpringJUnit4ClassRunner.class)
@SpringApplicationConfiguration(classes = DemoApplication.class)
public class DemoApplicationTests {

    final String BASE_URL = "http://localhost:8080/";

    @Autowired
    private WebApplicationContext wac;

    private MockMvc mockMvc;

    @Before
    public void setup() {
        this.mockMvc = MockMvcBuilders.webAppContextSetup(this.wac).build();
    }

    @Test
    public void testSayHelloWorld() throws Exception{

         this.mockMvc.perform(get("/")
                 .accept(MediaType.parseMediaType("application/json;charset=UTF-8")))
                 .andExpect(status().isOk())
                 .andExpect(content().contentType("application/json"));
    }

    @Test
    public void contextLoads() {
    }

}

Answer

Spring MVC offers a standaloneSetup that supports testing relatively simple controllers, without the need of context.

Build a MockMvc by registering one or more @Controller’s instances and configuring Spring MVC infrastructure programmatically. This allows full control over the instantiation and initialization of controllers, and their dependencies, similar to plain unit tests while also making it possible to test one controller at a time.

An example test for your controller can be something as simple as

public class DemoApplicationTests {

    private MockMvc mockMvc;

    @Before
    public void setup() {
        this.mockMvc = MockMvcBuilders.standaloneSetup(new HelloWorld()).build();
    }

    @Test
    public void testSayHelloWorld() throws Exception {
        this.mockMvc.perform(get("/")
           .accept(MediaType.parseMediaType("application/json;charset=UTF-8")))
           .andExpect(status().isOk())
           .andExpect(content().contentType("application/json"));

    }
}