Web-Slice-Tests mit @WebMvcTest

To test a controller you don't need a database, a real service, or a running server - you need to check that a request is routed correctly, validated, and turned into the right status and JSON. @WebMvcTest loads just the web layer and gives you MockMvc to fire requests at it.

The slice

@WebMvcTest(BookController.class) starts Spring MVC and that one controller - no services, no repositories, no database. Because the controller depends on BookService, you supply a mock with @MockitoBean:

@WebMvcTest(BookController.class)
class BookControllerWebTest {

    @Autowired MockMvc mvc;              // fires simulated HTTP requests
    @MockitoBean BookService service;    // a Mockito mock, put into the context

    @Test
    void returns404WhenBookMissing() throws Exception {
        when(service.findByIsbn("000")).thenThrow(new BookNotFoundException("000"));

        mvc.perform(get("/api/books/000"))
            .andExpect(status().isNotFound())
            .andExpect(jsonPath("$.title").value("Not found"));
    }
}

MockMvc runs the request through the real MVC machinery - routing, argument binding, validation, your exception handler, JSON serialization - without a network or server.

What it's perfect for

• Status codes — 200, 201, 404, 400.
• Validation — POST an invalid body, expect 400 and the error details.
• JSON shape — assert fields with jsonPath.
• Routing & mapping — the right method handles the right URL and verb.

@Test
void rejectsInvalidNewBook() throws Exception {
    String badBody = "{\"isbn\":\"bad\",\"title\":\"\",\"author\":\"X\",\"copies\":0}";
    mvc.perform(post("/api/books").contentType(APPLICATION_JSON).content(badBody))
        .andExpect(status().isBadRequest());
}