How to Write Unit Tests for File Upload APIs in Spring Boot

In response to a question about how to write unit tests for file upload endpoints, the article explains why backend developers should master unit testing and demonstrates a practical example using Spring Boot and MockMvc.

First, a simple test for a regular GET endpoint is shown, highlighting the use of @SpringBootTest, MockMvc, and assertions on the response.

@SpringBootTest
public class Chapter11ApplicationTests {
    private MockMvc mvc;

    @Before
    public void setUp() {
        mvc = MockMvcBuilders.standaloneSetup(new HelloController()).build();
    }

    @Test
    public void getHello() throws Exception {
        mvc.perform(MockMvcRequestBuilders.get("/hello")
                .accept(MediaType.APPLICATION_JSON))
                .andExpect(status().isOk())
                .andExpect(content().string(equalTo("Hello World")));
    }
}

For a file‑upload API the same MockMvc framework is used, but the request type changes to multipart and a MockMultipartFile is created to simulate the uploaded file.

@SpringBootTest(classes = Chapter43Application.class)
public class FileTest {

    @Autowired
    protected WebApplicationContext context;
    protected MockMvc mvc;

    @BeforeEach
    public void setUp() {
        mvc = MockMvcBuilders.webAppContextSetup(context).build();
    }

    @Test
    public void uploadFile() throws Exception {
        MockMultipartFile file = new MockMultipartFile(
                "file",
                "hello.txt",
                MediaType.TEXT_PLAIN_VALUE,
                "Hello, World!".getBytes()
        );

        MvcResult result = mvc.perform(
                MockMvcRequestBuilders.multipart("/upload")
                        .file(file))
                .andDo(print())
                .andExpect(status().isOk())
                .andReturn();
    }
}

The article emphasizes that the core testing logic remains the same; only the request type and payload differ. It encourages developers to consider testability when designing APIs and even adopt test‑driven development.

Code Example

Full examples are available in the GitHub repository at https://github.com/dyc87112/SpringBoot-Learning/ under the chapter4-3 directory.