Tests

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Goal

Write a repository slice test with @DataJpaTest and a web layer test with @WebMvcTest. After this page ./mvnw test passes with both test classes green, and you have seen the two main Spring Boot test slices in action.

Prerequisites

• Validation and errors

Why two test slices

Spring Boot offers test slices — annotations that start only the part of the application context relevant to one layer:

Slice	Starts	Does NOT start
@DataJpaTest	JPA, H2, repositories	Web layer, controllers
@WebMvcTest	MockMvc, controllers, advice	JPA, real service beans

This keeps tests fast and focused. A @DataJpaTest confirms the repository query is correct without an HTTP request. A @WebMvcTest confirms the controller serializes correctly without a real database.

Repository slice test

Create src/test/java/com/example/itemsapi/ItemRepositoryTest.java:

package com.example.itemsapi;

import org.junit.jupiter.api.Test;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.autoconfigure.orm.jpa.DataJpaTest;

import static org.assertj.core.api.Assertions.assertThat;

@DataJpaTest
class ItemRepositoryTest {

    @Autowired
    ItemRepository repo;

    @Test
    void savesAndFinds() {
        Item it = new Item();
        it.setName("pen");
        Item saved = repo.save(it);
        assertThat(repo.findById(saved.getId())).isPresent();
    }
}

What it does:

• @DataJpaTest starts an embedded H2 instance and the JPA layer. Each test runs in a transaction that is rolled back at the end, so tests are isolated.
• repo.save(it) persists the item and returns it with the generated id populated.
• assertThat(...).isPresent() uses AssertJ (bundled with spring-boot-starter-test) to verify the saved item can be retrieved by its ID.

Web layer test

Create src/test/java/com/example/itemsapi/ItemControllerTest.java:

package com.example.itemsapi;

import org.junit.jupiter.api.Test;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.autoconfigure.web.servlet.WebMvcTest;
import org.springframework.boot.test.mock.mockito.MockBean;
import org.springframework.http.MediaType;
import org.springframework.test.web.servlet.MockMvc;

import static org.springframework.test.web.servlet.request.MockMvcRequestBuilders.post;
import static org.springframework.test.web.servlet.result.MockMvcResultMatchers.status;

@WebMvcTest(ItemController.class)
class ItemControllerTest {

    @Autowired
    MockMvc mvc;

    @MockBean
    ItemService service;

    @Test
    void rejectsBlankName() throws Exception {
        mvc.perform(post("/items")
                .contentType(MediaType.APPLICATION_JSON)
                .content("{\"name\":\"\"}"))
           .andExpect(status().isBadRequest());
    }
}

What it does:

• @WebMvcTest(ItemController.class) starts MockMvc and the ItemController, ApiExceptionHandler, and the validation infrastructure — nothing else.
• @MockBean ItemService service — Spring replaces the real ItemService bean with a Mockito mock. The test does not reach the repository at all.
• mvc.perform(post("/items")...) — sends a fake HTTP POST with an empty name field.
• .andExpect(status().isBadRequest()) — asserts the response is HTTP 400, which our ApiExceptionHandler sets when MethodArgumentNotValidException is thrown.

Verify: tests pass

./mvnw test

Expected output (last lines):

[INFO] Tests run: 2, Failures: 0, Errors: 0, Skipped: 0
[INFO] BUILD SUCCESS

If ItemControllerTest fails with a NoSuchBeanDefinitionException for ItemService, check that @MockBean is present — @WebMvcTest does not load service beans automatically.

What you have built

At the end of this tier your project has:

• An Item entity mapped to an H2 table.
• An ItemRepository with zero SQL.
• An ItemService that encapsulates list, get-by-id, and create.
• An ItemController that exposes GET /items, GET /items/{id}, and POST /items.
• An ApiExceptionHandler that returns clean JSON for 404 and 400 errors.
• Two test slices that cover the repository and the web layer independently.

The next step is to swap H2 for a persistent database (PostgreSQL) and deploy the application — that is the Advanced tier.