In this tutorial, we will learn how to write a Unit test for Spring Boot GET REST API using JUnit and Mockito framework.

Learn Spring Boot Unit Testing CRUD REST API with JUnit and Mockito.

Spring boot provides spring-boot-starter-test dependency for unit testing and integration testing of the Spring boot application:

		<dependency>
			<groupId>org.springframework.boot</groupId>
			<artifactId>spring-boot-starter-test</artifactId>
			<scope>test</scope>
		</dependency>

The spring-boot-starter-test dependency is a primary dependency for testing the Spring Boot Applications. It holds all the necessary elements required for the testing.

For the Unit testing Spring Boot REST APIs, we gonna use the following testing libraries:

JUnit 5 Framework
Mockito 4 (Latest)
Hamcrest framework
AssertJ Library
JsonPath Library

JUnit 5 Framework

It's the de facto standard testing framework for Java.

The current version of JUnit is 5+. The main goal of JUnit 5 is to support Java 8 and above, as well as enable many different styles of testing.

Learn JUnit at https://www.javaguides.net/p/junit-5.html

Mockito 4 (Latest)

Mockito is a mocking framework. It is a Java-based library used to create simple and basic test APIs for performing unit testing of Java applications.

The main purpose of using the Mockito framework is to simplify the development of a test by mocking external dependencies and using them in the test code.

Learn about the Mockito framework at https://site.mockito.org/

Create Spring Boot Application

Using spring initializr, create a Spring Boot project and add the following dependencies:

Spring Web
Spring Data JPA
Lombok

Generate the Spring boot project as a zip file, extract it, and import it into IntelliJ IDEA.

Make sure that you have added the below dependencies in your spring boot project:

		<dependency>
			<groupId>org.springframework.boot</groupId>
			<artifactId>spring-boot-starter-data-jpa</artifactId>
		</dependency>
		<dependency>
			<groupId>org.springframework.boot</groupId>
			<artifactId>spring-boot-starter-web</artifactId>
		</dependency>
		<dependency>
			<groupId>org.projectlombok</groupId>
			<artifactId>lombok</artifactId>
			<optional>true</optional>
		</dependency>
		<dependency>
			<groupId>org.springframework.boot</groupId>
			<artifactId>spring-boot-starter-test</artifactId>
			<scope>test</scope>
		</dependency>

Create JPA Entity

Next, let's create an Employee JPA entity with the following content:

import lombok.*;

import javax.persistence.*;

@Setter
@Getter
@AllArgsConstructor
@NoArgsConstructor
@Builder

@Entity
@Table(name = "employees")
public class Employee {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private long id;

    @Column(name = "first_name", nullable = false)
    private String firstName;

    @Column(name = "last_name", nullable = false)
    private String lastName;

    @Column(nullable = false)
    private String email;
}

Note that we are using Lombok annotations to reduce the boilerplate code.

Create Repository Layer

Let's create EmployeeRepository which extends the JpaRepository interface:

import net.javaguides.springboot.model.Employee;
import org.springframework.data.jpa.repository.JpaRepository;

public interface EmployeeRepository extends JpaRepository<Employee, Long> {

}

Create Service Layer

EmployeeService

Let's create an EmployeeService interface with CRUD methods:

import net.javaguides.springboot.model.Employee;

import java.util.List;
import java.util.Optional;

public interface EmployeeService {
    List<Employee> getAllEmployees();
    Optional<Employee> getEmployeeById(long id);
}

EmployeeServiceImpl

Let's create an EmployeeServiceImpl class that implements the EmployeeService interface:

import net.javaguides.springboot.exception.ResourceNotFoundException;
import net.javaguides.springboot.model.Employee;
import net.javaguides.springboot.repository.EmployeeRepository;
import net.javaguides.springboot.service.EmployeeService;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Service;

import java.util.List;
import java.util.Optional;

@Service
public class EmployeeServiceImpl implements EmployeeService {

    private EmployeeRepository employeeRepository;

    public EmployeeServiceImpl(EmployeeRepository employeeRepository) {
        this.employeeRepository = employeeRepository;
    }

    @Override
    public List<Employee> getAllEmployees() {
        return employeeRepository.findAll();
    }

    @Override
    public Optional<Employee> getEmployeeById(long id) {
        return employeeRepository.findById(id);
    }
}

Controller Layer - GET REST API

import net.javaguides.springboot.model.Employee;
import net.javaguides.springboot.service.EmployeeService;
import org.springframework.http.HttpStatus;
import org.springframework.http.ResponseEntity;
import org.springframework.web.bind.annotation.*;

import java.util.List;

@RestController
@RequestMapping("/api/employees")
public class EmployeeController {

    private EmployeeService employeeService;

    public EmployeeController(EmployeeService employeeService) {
        this.employeeService = employeeService;
    }

    @GetMapping
    public List<Employee> getAllEmployees(){
        return employeeService.getAllEmployees();
    }

    @GetMapping("{id}")
    public ResponseEntity<Employee> getEmployeeById(@PathVariable("id") long employeeId){
        return employeeService.getEmployeeById(employeeId)
                .map(ResponseEntity::ok)
                .orElseGet(() -> ResponseEntity.notFound().build());
    }
}

7. Writing Unit Test Spring Boot GET REST APIs

Now, let's create a Unit test for Spring boot GET REST API. We gonna use the @WebMvcTest annotation to load only EmployeeController class.

import com.fasterxml.jackson.databind.ObjectMapper;
import net.javaguides.springboot.model.Employee;
import net.javaguides.springboot.service.EmployeeService;

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 org.springframework.test.web.servlet.ResultActions;

import java.util.ArrayList;
import java.util.List;
import java.util.Optional;

import static org.hamcrest.CoreMatchers.is;
import static org.springframework.test.web.servlet.result.MockMvcResultMatchers.*;
import static org.springframework.test.web.servlet.result.MockMvcResultHandlers.*;
import static org.mockito.ArgumentMatchers.*;
import static org.mockito.BDDMockito.*;
import static org.springframework.test.web.servlet.request.MockMvcRequestBuilders.*;

@WebMvcTest
public class EmployeeControllerTests {

    @Autowired
    private MockMvc mockMvc;

    @MockBean
    private EmployeeService employeeService;

    @Autowired
    private ObjectMapper objectMapper;


    // JUnit test for Get All employees REST API
    @Test
    public void givenListOfEmployees_whenGetAllEmployees_thenReturnEmployeesList() throws Exception{
        // given - precondition or setup
        List<Employee> listOfEmployees = new ArrayList<>();
        listOfEmployees.add(Employee.builder().firstName("Ramesh").lastName("Fadatare").email("ramesh@gmail.com").build());
        listOfEmployees.add(Employee.builder().firstName("Tony").lastName("Stark").email("tony@gmail.com").build());
        given(employeeService.getAllEmployees()).willReturn(listOfEmployees);

        // when -  action or the behaviour that we are going test
        ResultActions response = mockMvc.perform(get("/api/employees"));

        // then - verify the output
        response.andExpect(status().isOk())
                .andDo(print())
                .andExpect(jsonPath("$.size()",
                        is(listOfEmployees.size())));

    }

    // JUnit test for GET employee by id REST API
    @Test
    public void givenEmployeeId_whenGetEmployeeById_thenReturnEmployeeObject() throws Exception{
        // given - precondition or setup
        long employeeId = 1L;
        Employee employee = Employee.builder()
                .firstName("Ramesh")
                .lastName("Fadatare")
                .email("ramesh@gmail.com")
                .build();
        given(employeeService.getEmployeeById(employeeId)).willReturn(Optional.of(employee));

        // when -  action or the behaviour that we are going test
        ResultActions response = mockMvc.perform(get("/api/employees/{id}", employeeId));

        // then - verify the output
        response.andExpect(status().isOk())
                .andDo(print())
                .andExpect(jsonPath("$.firstName", is(employee.getFirstName())))
                .andExpect(jsonPath("$.lastName", is(employee.getLastName())))
                .andExpect(jsonPath("$.email", is(employee.getEmail())));

    }
}

The @MockBean annotation tells Spring to create a mock instance of EmployeeService and add it to the application context so that it's injected into EmployeeController. We have a handle on it in the test so that we can define its behavior before running each test.

Note that we are using MockMvc class to make REST API calls.

We are using ResultActions class to handle the response of the REST API.

We are using Mockito to stub the method calls.

If you want to understand in detail then I highly suggest my Udemy course: Testing Spring Boot Application with JUnit and Mockito (Includes Testcontainers)

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