Let's quickly write a unit test to test the preceding controller method:

    @RunWith(SpringRunner::class)
    @WebMvcTest(BasicController::class)
    class BasicControllerTest {
      @Autowired
      lateinit var mvc: MockMvc;
      @Test
      fun `GET welcome returns "Hello World"`() {
        mvc.perform(
           MockMvcRequestBuilders.get("/welcome").accept(
           MediaType.APPLICATION_JSON))
           .andExpect(status().isOk())
           .andExpect(content().string(equalTo("Hello World")));
       } 
     }

In the preceding unit test, we will launch up a Mock MVC instance with BasicController. A few quick things to note are as follows:

• The annotations @RunWith(SpringRunner.class) and @WebMvcTest(BasicController::class) are similar to Java, except for the class references.
• @Autowired lateinit var mvc: MockMvc: This autowires the MockMvc bean that can be used to make requests. Properties declared as non-null must be initialized in the constructor. For properties that are autowired through the dependency injection, we can avoid null checks by adding lateinit to the variable declaration.
• fun `GET welcome returns "Hello World"`(): This is a unique feature of Kotlin. Instead of giving the test method a name, we are giving a description for the test. This is awesome because, ideally, the test method will not be called from another method.
• mvc.perform(MockMvcRequestBuilders.get("/welcome").accept(MediaType.APPLICATION_JSON)): This performs a request to /welcome with the Accept header value, application/json, which is similar to the Java code.
• andExpect(status().isOk()): This expects that the status of the response is 200 (success).
• andExpect(content().string(equalTo("Hello World"))): This expects that the content of the response is equal to "Hello World".