Unit test POST API in spring-boot + kotlin + Junit - spring-boot

Question

I'm pretty new to spring boot and kotlin. I've started with one basic app from net and writing unit test, but I'm getting following error:
org.springframework.web.util.NestedServletException: Request processing failed; nested exception is java.lang.IllegalStateException: articleRepository.save(article) must not be null
Let me show you the code: Entity Class
#Entity
data class Article (
#Id #GeneratedValue(strategy = GenerationType.IDENTITY)
val id: Long = 0,
#get: NotBlank
val title: String = "",
#get: NotBlank
val content: String = ""
)
controller:
#PostMapping("/articles")
fun createNewArticle(#Valid #RequestBody article: Article) : Article {
return articleRepository.save(article)
}
Repository:
#Repository
interface ArticleRepository : JpaRepository<Article, Long>
Test File:
RunWith(SpringRunner::class)
#SpringBootTest
class KotlinDemoApplicationTests {
lateinit var mvc: MockMvc
#InjectMocks
lateinit var controller: ArticleController
#Mock
lateinit var respository: ArticleRepository
#Before
fun setup() {
MockitoAnnotations.initMocks(this)
mvc = MockMvcBuilders.standaloneSetup(controller).setMessageConverters(MappingJackson2HttpMessageConverter()).build()
}
#Test
fun createBlog() {
var article = Article(1, "Test", "Test Content")
var jsonData = jacksonObjectMapper().writeValueAsString(article)
mvc.perform(MockMvcRequestBuilders.post("/api/articles/").contentType(MediaType.APPLICATION_JSON).content(jsonData))
.andExpect(MockMvcResultMatchers.status().isOk)
.andDo(MockMvcResultHandlers.print())
.andReturn()
}
}
When I'm running this test file, getting error mentioned above.
Please help me with this.

Answer 1

The problem is your ArticleRepository mock.
While you correctly inject it into your Controller, you're not specifiying what a call to save should return. It therefore returns null, which is not allowed in Kotin because you specified it as non-optional.
Either you allow your controller's createNewArticle to return null, by adding a ?, that is changing its signature to
fun createNewArticle(#Valid #RequestBody article: Article) : Article? {...}
Or you set-up the mock so that it does not return null, but an article.
#Before
fun setup() {
MockitoAnnotations.initMocks(this)
...
`when`(respository.save(any())
.thenReturn(Article()) // creates a new article
}
(Alternatively, there's also Mockito's returnsFirstArg() in case you don't want to invoke the construtor.)
Note that using any() in this case will only work if you're using mockito-kotlin
If you don't want to use it, check this answer