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In the previous tutorial, we have seen Spring Boot Unit Testing CRUD REST API with JUnit and Mockito. In this tutorial, we will learn how to perform unit testing Spring boot service layer unit testing using JUnit 5 and the Mockito framework.

In order to test Service layer components, we have to mock the Repository layer components using the Mockito framework. We don't have to use a database for Unit testing.

Check out my Spring boot testing Udemy course: Testing Spring Boot Application with JUnit and Mockito (Includes Testcontainers)

Spring boot provides spring-boot-starter-test dependency for unit testing and integration testing of 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 service layer, we are going to use the following testing libraries:

JUnit 5 Framework
Mockito 4 (Latest)
AssertJ 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/

AssertJ Library

AssertJ is a Java library that provides a rich set of assertions and truly helpful error messages, improves test code readability, and is designed to be super easy to use within your favorite IDE.

Spring boot starter test dependency internally provides assertj-core dependency so we don’t have to add assertj-core dependency manually in our Spring boot project.

Read more about AssertJ at https://assertj.github.io/doc/

Tools and technologies used

Java 11+
Spring Boot
Lombok
JUnit 5 Framework
Hamcrest
AssertJ
JsonPath
Mockito
IntelliJ IDEA
Docker
Maven

1. Create Spring Boot Application

Using spring initialize, 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.

2. Maven Dependencies

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>

3. 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.

@Entity annotation is used to mark the class as a persistent Java class.

@Table annotation is used to provide the details of the table that this entity will be mapped to.

@Id annotation is used to define the primary key.

@GeneratedValue annotation is used to define the primary key generation strategy. In the above case, we have declared the primary key to be an Auto Increment field.

@Column annotation is used to define the properties of the column that will be mapped to the annotated field. You can define several properties like name, length, nullable, updateable, etc.

4. 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> {

}

5. 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 {
    Employee saveEmployee(Employee employee);
    List<Employee> getAllEmployees();
    Optional<Employee> getEmployeeById(long id);
    Employee updateEmployee(Employee updatedEmployee);
    void deleteEmployee(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 Employee saveEmployee(Employee employee) {

        Optional<Employee> savedEmployee = employeeRepository.findByEmail(employee.getEmail());
        if(savedEmployee.isPresent()){
            throw new ResourceNotFoundException("Employee already exist with given email:" + employee.getEmail());
        }
        return employeeRepository.save(employee);
    }

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

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

    @Override
    public Employee updateEmployee(Employee updatedEmployee) {
        return employeeRepository.save(updatedEmployee);
    }

    @Override
    public void deleteEmployee(long id) {
        employeeRepository.deleteById(id);
    }
}

6. Unit Testing Service Layer using JUnit 5 and Mockito

Let us start writing unit tests for EmployeeService. We should be able to write unit tests for EmployeeService WITHOUT using any Spring features.

We are going to create a mock of EmployeeRepository using @Mock and create an EmployeeServiceImpl instance using the mock EmployeeRepository instance.

import net.javaguides.springboot.exception.ResourceNotFoundException;
import net.javaguides.springboot.model.Employee;
import net.javaguides.springboot.repository.EmployeeRepository;
import net.javaguides.springboot.service.impl.EmployeeServiceImpl;
import static org.assertj.core.api.Assertions.assertThat;
import org.junit.jupiter.api.BeforeEach;
import org.junit.jupiter.api.DisplayName;
import org.junit.jupiter.api.Test;
import org.junit.jupiter.api.extension.ExtendWith;

import static org.mockito.ArgumentMatchers.any;
import static org.mockito.BDDMockito.given;
import static org.mockito.BDDMockito.willDoNothing;
import static org.mockito.Mockito.*;

import org.mockito.InjectMocks;
import org.mockito.Mock;
import org.mockito.junit.jupiter.MockitoExtension;

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

@ExtendWith(MockitoExtension.class)
public class EmployeeServiceTests {

    @Mock
    private EmployeeRepository employeeRepository;

    @InjectMocks
    private EmployeeServiceImpl employeeService;

    private Employee employee;

    @BeforeEach
    public void setup(){
        //employeeRepository = Mockito.mock(EmployeeRepository.class);
        //employeeService = new EmployeeServiceImpl(employeeRepository);
        employee = Employee.builder()
                .id(1L)
                .firstName("Ramesh")
                .lastName("Fadatare")
                .email("ramesh@gmail.com")
                .build();
    }

    // JUnit test for saveEmployee method
    @DisplayName("JUnit test for saveEmployee method")
    @Test
    public void givenEmployeeObject_whenSaveEmployee_thenReturnEmployeeObject(){
        // given - precondition or setup
        given(employeeRepository.findByEmail(employee.getEmail()))
                .willReturn(Optional.empty());

        given(employeeRepository.save(employee)).willReturn(employee);

        System.out.println(employeeRepository);
        System.out.println(employeeService);

        // when -  action or the behaviour that we are going test
        Employee savedEmployee = employeeService.saveEmployee(employee);

        System.out.println(savedEmployee);
        // then - verify the output
        assertThat(savedEmployee).isNotNull();
    }

    // JUnit test for saveEmployee method
    @DisplayName("JUnit test for saveEmployee method which throws exception")
    @Test
    public void givenExistingEmail_whenSaveEmployee_thenThrowsException(){
        // given - precondition or setup
        given(employeeRepository.findByEmail(employee.getEmail()))
                .willReturn(Optional.of(employee));

        System.out.println(employeeRepository);
        System.out.println(employeeService);

        // when -  action or the behaviour that we are going test
        org.junit.jupiter.api.Assertions.assertThrows(ResourceNotFoundException.class, () -> {
            employeeService.saveEmployee(employee);
        });

        // then
        verify(employeeRepository, never()).save(any(Employee.class));
    }

    // JUnit test for getAllEmployees method
    @DisplayName("JUnit test for getAllEmployees method")
    @Test
    public void givenEmployeesList_whenGetAllEmployees_thenReturnEmployeesList(){
        // given - precondition or setup

        Employee employee1 = Employee.builder()
                .id(2L)
                .firstName("Tony")
                .lastName("Stark")
                .email("tony@gmail.com")
                .build();

        given(employeeRepository.findAll()).willReturn(List.of(employee,employee1));

        // when -  action or the behaviour that we are going test
        List<Employee> employeeList = employeeService.getAllEmployees();

        // then - verify the output
        assertThat(employeeList).isNotNull();
        assertThat(employeeList.size()).isEqualTo(2);
    }

    // JUnit test for getAllEmployees method
    @DisplayName("JUnit test for getAllEmployees method (negative scenario)")
    @Test
    public void givenEmptyEmployeesList_whenGetAllEmployees_thenReturnEmptyEmployeesList(){
        // given - precondition or setup

        Employee employee1 = Employee.builder()
                .id(2L)
                .firstName("Tony")
                .lastName("Stark")
                .email("tony@gmail.com")
                .build();

        given(employeeRepository.findAll()).willReturn(Collections.emptyList());

        // when -  action or the behaviour that we are going test
        List<Employee> employeeList = employeeService.getAllEmployees();

        // then - verify the output
        assertThat(employeeList).isEmpty();
        assertThat(employeeList.size()).isEqualTo(0);
    }

    // JUnit test for getEmployeeById method
    @DisplayName("JUnit test for getEmployeeById method")
    @Test
    public void givenEmployeeId_whenGetEmployeeById_thenReturnEmployeeObject(){
        // given
        given(employeeRepository.findById(1L)).willReturn(Optional.of(employee));

        // when
        Employee savedEmployee = employeeService.getEmployeeById(employee.getId()).get();

        // then
        assertThat(savedEmployee).isNotNull();

    }

    // JUnit test for updateEmployee method
    @DisplayName("JUnit test for updateEmployee method")
    @Test
    public void givenEmployeeObject_whenUpdateEmployee_thenReturnUpdatedEmployee(){
        // given - precondition or setup
        given(employeeRepository.save(employee)).willReturn(employee);
        employee.setEmail("ram@gmail.com");
        employee.setFirstName("Ram");
        // when -  action or the behaviour that we are going test
        Employee updatedEmployee = employeeService.updateEmployee(employee);

        // then - verify the output
        assertThat(updatedEmployee.getEmail()).isEqualTo("ram@gmail.com");
        assertThat(updatedEmployee.getFirstName()).isEqualTo("Ram");
    }

    // JUnit test for deleteEmployee method
    @DisplayName("JUnit test for deleteEmployee method")
    @Test
    public void givenEmployeeId_whenDeleteEmployee_thenNothing(){
        // given - precondition or setup
        long employeeId = 1L;

        willDoNothing().given(employeeRepository).deleteById(employeeId);

        // when -  action or the behaviour that we are going test
        employeeService.deleteEmployee(employeeId);

        // then - verify the output
        verify(employeeRepository, times(1)).deleteById(employeeId);
    }
}

Notice that we are using the assertThat() method to assert the conditions using the AssertJ library.

Mockito @Mock annotation is useful when we want to use the mocked object at multiple places.

When we want to inject a mocked object into another mocked object, we can use @InjectMocks annotation. @InjectMock creates the mock object of the class and injects the mocks that are marked with the annotations @Mock into it.

JUnit test for saveEmployee method:

    // JUnit test for saveEmployee method
    @DisplayName("JUnit test for saveEmployee method")
    @Test
    public void givenEmployeeObject_whenSaveEmployee_thenReturnEmployeeObject(){
        // given - precondition or setup
        given(employeeRepository.findByEmail(employee.getEmail()))
                .willReturn(Optional.empty());

        given(employeeRepository.save(employee)).willReturn(employee);

        System.out.println(employeeRepository);
        System.out.println(employeeService);

        // when -  action or the behaviour that we are going test
        Employee savedEmployee = employeeService.saveEmployee(employee);

        System.out.println(savedEmployee);
        // then - verify the output
        assertThat(savedEmployee).isNotNull();
    }

JUnit test for saveEmployee method which throws Exception:

  // JUnit test for saveEmployee method
    @DisplayName("JUnit test for saveEmployee method which throws exception")
    @Test
    public void givenExistingEmail_whenSaveEmployee_thenThrowsException(){
        // given - precondition or setup
        given(employeeRepository.findByEmail(employee.getEmail()))
                .willReturn(Optional.of(employee));

        System.out.println(employeeRepository);
        System.out.println(employeeService);

        // when -  action or the behaviour that we are going test
        org.junit.jupiter.api.Assertions.assertThrows(ResourceNotFoundException.class, () -> {
            employeeService.saveEmployee(employee);
        });

        // then
        verify(employeeRepository, never()).save(any(Employee.class));
    }

JUnit test for getAllEmployees method:

 @DisplayName("JUnit test for getAllEmployees method")
    @Test
    public void givenEmployeesList_whenGetAllEmployees_thenReturnEmployeesList(){
        // given - precondition or setup

        Employee employee1 = Employee.builder()
                .id(2L)
                .firstName("Tony")
                .lastName("Stark")
                .email("tony@gmail.com")
                .build();

        given(employeeRepository.findAll()).willReturn(List.of(employee,employee1));

        // when -  action or the behaviour that we are going test
        List<Employee> employeeList = employeeService.getAllEmployees();

        // then - verify the output
        assertThat(employeeList).isNotNull();
        assertThat(employeeList.size()).isEqualTo(2);
    }

JUnit test for getEmployeeById method

    // JUnit test for getEmployeeById method
    @DisplayName("JUnit test for getEmployeeById method")
    @Test
    public void givenEmployeeId_whenGetEmployeeById_thenReturnEmployeeObject(){
        // given
        given(employeeRepository.findById(1L)).willReturn(Optional.of(employee));

        // when
        Employee savedEmployee = employeeService.getEmployeeById(employee.getId()).get();

        // then
        assertThat(savedEmployee).isNotNull();

    }

JUnit test for updateEmployee method:

    // JUnit test for updateEmployee method
    @DisplayName("JUnit test for updateEmployee method")
    @Test
    public void givenEmployeeObject_whenUpdateEmployee_thenReturnUpdatedEmployee(){
        // given - precondition or setup
        given(employeeRepository.save(employee)).willReturn(employee);
        employee.setEmail("ram@gmail.com");
        employee.setFirstName("Ram");
        // when -  action or the behaviour that we are going test
        Employee updatedEmployee = employeeService.updateEmployee(employee);

        // then - verify the output
        assertThat(updatedEmployee.getEmail()).isEqualTo("ram@gmail.com");
        assertThat(updatedEmployee.getFirstName()).isEqualTo("Ram");
    }

JUnit test for deleteEmployee method

    // JUnit test for deleteEmployee method
    @DisplayName("JUnit test for deleteEmployee method")
    @Test
    public void givenEmployeeId_whenDeleteEmployee_thenNothing(){
        // given - precondition or setup
        long employeeId = 1L;

        willDoNothing().given(employeeRepository).deleteById(employeeId);

        // when -  action or the behaviour that we are going test
        employeeService.deleteEmployee(employeeId);

        // then - verify the output
        verify(employeeRepository, times(1)).deleteById(employeeId);
    }

7. Demo - Run JUnit Tests

8. Conclusion

In this tutorial, we have discussed how to perform Spring Boot unit testing CRUD REST APIs using JUnit and Mockito frameworks.

If you want to learn more about Spring boot testing then highly suggest my Udemy course: Testing Spring Boot Application with JUnit and Mockito (Includes Testcontainers)

9. Spring Boot Testing Tutorials and Guides