Spring Boot Testing Guide

This document explains Unit Tests, Controller Tests, and Integration Tests in Spring Boot, including when and how to use common testing annotations.

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Test Types Overview

Test Type	Purpose	Spring Context
Unit Test	Test business logic in isolation	❌ No Spring Context
Controller Test	Test web layer (MVC) only	⚠️ Partial Context
Integration Test	Test full application flow	✅ Full Context

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Unit Tests

When to Use

• Testing service or component logic
• No Spring container needed
• Fast and isolated
• External dependencies are mocked

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Common Annotations

• @ExtendWith(MockitoExtension.class)
• @Mock
• @InjectMocks
• @Test

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Example Unit Test

Location: src/test/java/com/example/service/OrderServiceTest.java

package com.example.service;

import org.junit.jupiter.api.Test;
import org.junit.jupiter.api.extension.ExtendWith;
import org.mockito.InjectMocks;
import org.mockito.Mock;
import org.mockito.junit.jupiter.MockitoExtension;

import static org.mockito.Mockito.when;
import static org.assertj.core.api.Assertions.assertThat;

@ExtendWith(MockitoExtension.class)
class OrderServiceTest {

  @Mock
  private PaymentClient paymentClient;

  @InjectMocks
  private OrderService orderService;

  @Test
  void shouldCreateOrderSuccessfully() {
    when(paymentClient.pay()).thenReturn(true);

    boolean result = orderService.createOrder();

    assertThat(result).isTrue();
  }
}

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Key Points

• No @SpringBootTest
• No application context startup
• Fastest type of test
• Preferred for business logic

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Controller Tests (Web Layer Tests)

When to Use

• Testing REST controllers
• Request/response validation
• HTTP status codes and JSON mapping
• Service layer is mocked

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Common Annotations

• @WebMvcTest
• @MockBean
• @Test
• MockMvc

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Example Controller Test

Location: src/test/java/com/example/controller/OrderControllerTest.java

package com.example.controller;

import com.example.service.OrderService;
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.test.web.servlet.MockMvc;

import static org.mockito.Mockito.when;
import static org.springframework.test.web.servlet.request.MockMvcRequestBuilders.get;
import static org.springframework.test.web.servlet.result.MockMvcResultMatchers.status;

@WebMvcTest(OrderController.class)
class OrderControllerTest {

  @Autowired
  private MockMvc mockMvc;

  @MockBean
  private OrderService orderService;

  @Test
  void shouldReturnOk() throws Exception {
    when(orderService.isHealthy()).thenReturn(true);

    mockMvc.perform(get("/orders/health"))
           .andExpect(status().isOk());
  }
}

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Key Points

• Loads only MVC components
• Does NOT load full context
• Faster than integration tests
• Ideal for API contract validation

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Integration Tests

When to Use

• Testing full application behavior
• Database, messaging, security, configuration
• Verifying beans wiring together correctly

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Common Annotations

• @SpringBootTest
• @ActiveProfiles
• @Test
• @AutoConfigureMockMvc

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Example Integration Test

Location: src/test/java/com/example/integration/OrderIntegrationTest.java

package com.example.integration;

import org.junit.jupiter.api.Test;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.autoconfigure.web.servlet.AutoConfigureMockMvc;
import org.springframework.boot.test.context.SpringBootTest;
import org.springframework.test.web.servlet.MockMvc;

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

@SpringBootTest
@AutoConfigureMockMvc
class OrderIntegrationTest {

  @Autowired
  private MockMvc mockMvc;

  @Test
  void shouldLoadContextAndReturnOk() throws Exception {
    mockMvc.perform(get("/orders/health"))
           .andExpect(status().isOk());
  }
}

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Key Points

• Loads full Spring context
• Slowest but most realistic
• Should be limited in number
• Best for end-to-end verification

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Annotation Summary

Annotation	Usage
@ExtendWith(MockitoExtension.class)	Unit tests with Mockito
@Mock	Mock a dependency
@InjectMocks	Inject mocks into class under test
@WebMvcTest	Controller (MVC) tests
@MockBean	Mock Spring beans
@SpringBootTest	Full integration test

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Recommended Testing Strategy

• ✅ Mostly Unit Tests
• ✅ Some Controller Tests
• ⚠️ Few Integration Tests

This approach provides fast feedback while still ensuring application reliability.