Spring Boot JUnit and Mockito Example - Service Layer Testing

In this tutorial, we will learn how to test the Service layer in the Spring Boot application using JUnit 5 and Mockito framework.

Create Spring Boot Project

We’ll use Spring initializr web tool to bootstrap our application. 

Go to http://start.spring.io

Select Java in the language section.

Enter Artifact as springboot-testing-demo

Add Lombok, JPA, and MySQL dependencies.

Click Generate to generate and download the project.

Once the project is generated, unzip it and import it into your favorite IDE.

Maven Dependencies

Here is a complete pom.xml file for your reference:

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
	xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 https://maven.apache.org/xsd/maven-4.0.0.xsd">
	<modelVersion>4.0.0</modelVersion>
	<parent>
		<groupId>org.springframework.boot</groupId>
		<artifactId>spring-boot-starter-parent</artifactId>
		<version>2.4.5</version>
		<relativePath/> <!-- lookup parent from repository -->
	</parent>
	<groupId>com.example</groupId>
	<artifactId>Spring-boot-tutorial</artifactId>
	<version>1.0.0</version>
	<name>Spring-boot-tutorial</name>
	<description>Demo project for Spring Boot</description>
	<properties>
		<java.version>11</java.version>
	</properties>
	<dependencies>
		<dependency>
			<groupId>mysql</groupId>
			<artifactId>mysql-connector-java</artifactId>
			<scope>runtime</scope>
		</dependency>

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

	<build>
		<plugins>
			<plugin>
				<groupId>org.springframework.boot</groupId>
				<artifactId>spring-boot-maven-plugin</artifactId>
				<configuration>
					<excludes>
						<exclude>
							<groupId>org.projectlombok</groupId>
							<artifactId>lombok</artifactId>
						</exclude>
					</excludes>
				</configuration>
			</plugin>
		</plugins>
	</build>

</project>

We have added dependencies:

• spring-boot-starter-test - main dependencies for unit and integration testing.
• spring-boot-starter-data-jpa - to connect to a database and perform DB operations
• lombok - to reduce boilerplate code

The spring-boot-maven-plugin is used to create an executable jar with the Spring Boot application.

Configure MySQL Database

We’ll need to configure MySQL database URL, username, and password so that Spring Boot can create a Data source.

Open src/main/resources/application.properties file and add the following properties to it -

spring.datasource.url = jdbc:mysql://localhost:3306/demo?autoReconnect=true&useUnicode=true&characterEncoding=UTF-8&allowMultiQueries=true&useSSL=false
spring.datasource.username = root
spring.datasource.password = root
spring.jpa.properties.hibernate.dialect = org.hibernate.dialect.MySQL5InnoDBDialect
spring.jpa.hibernate.ddl-auto = update

Change spring.datasource.username and spring.datasource.password as per your MySQL installation.

Note that, I’ve set spring.jpa.hibernate.ddl-auto property to update. This property updates the database schema whenever you create or modify the domain models in your application.

Create JPA Entity

Let's create a Department JPA entity:

import lombok.*;

import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.validation.constraints.NotBlank;

@Entity
@Data
@NoArgsConstructor
@AllArgsConstructor
@Builder
public class Department {

    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private Long departmentId;
    private String departmentName;
    private String departmentAddress;
    private String departmentCode;
}

Create Spring Data JPA Repository

Let’s now create the repository for accessing the data from the database. Let's create a DepartmentRepository interface and extend it to JpaRepository to get CRUD methods:

import com.example.Springboot.tutorial.entity.Department;
import org.springframework.data.jpa.repository.JpaRepository;
import org.springframework.stereotype.Repository;

@Repository
public interface DepartmentRepository extends JpaRepository<Department, Long> {

    public Department findByDepartmentName(String departmentName);

    public Department findByDepartmentNameIgnoreCase(String departmentName);
}

Service Layer

To keep it simple, we are going to write a JUnit test case for a single method fetchDepartmentByName():

DepartmentService

import com.example.Springboot.tutorial.entity.Department;

import java.util.List;

public interface DepartmentService {
    Department fetchDepartmentByName(String departmentName);
}

DepartmentServiceImpl

import com.example.Springboot.tutorial.entity.Department;
import com.example.Springboot.tutorial.repository.DepartmentRepository;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Service;

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

@Service
public class DepartmentServiceImpl implements DepartmentService{

    @Autowired
    private DepartmentRepository departmentRepository;


    @Override
    public Department fetchDepartmentByName(String departmentName) {
        return departmentRepository.findByDepartmentNameIgnoreCase(departmentName);
    }
}

We have implemented the server layer, next we will write a simple JUnit test for DepartmentService.

Testing Service Layer

Let's go ahead and use JUnit 5 and Mockito to write a JUnit test case. 

Note that instead of connecting to a database, we are mocking the objects and testing only the Service layer.

Let's create a DepartmentServiceTest class and annotate it with @SpringBootTest annotation like this:

import com.example.Springboot.tutorial.entity.Department;
import com.example.Springboot.tutorial.repository.DepartmentRepository;
import org.junit.jupiter.api.BeforeEach;
import org.junit.jupiter.api.DisplayName;
import org.junit.jupiter.api.Test;
import org.mockito.Mockito;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.context.SpringBootTest;
import org.springframework.boot.test.mock.mockito.MockBean;

import static org.junit.jupiter.api.Assertions.assertEquals;

@SpringBootTest
class DepartmentServiceTest {

    @Autowired
    private DepartmentService departmentService;

    @MockBean
    private DepartmentRepository departmentRepository;

    @BeforeEach
    void setUp() {
        Department department =
                Department.builder()
                .departmentName("IT")
                .departmentAddress("Ahmedabad")
                .departmentCode("IT-06")
                .departmentId(1L)
                .build();

        Mockito.when(departmentRepository.findByDepartmentNameIgnoreCase("IT"))
                .thenReturn(department);

    }

    @Test
    @DisplayName("Get Data based on Valida Department Name")
    public void whenValidDepartmentName_thenDepartmentShouldFound() {
        String departmentName = "IT";
        Department found =
                departmentService.fetchDepartmentByName(departmentName);

        assertEquals(departmentName, found.getDepartmentName());
    }
}

We are mocking the DepartmentRepository interface using @MockBean annotation:

    @MockBean
    private DepartmentRepository departmentRepository;

The @SpringBootTest annotation loads the complete Spring application context. In contrast, a test slice annotation only loads beans required to test a particular layer. And because of this, we can avoid unnecessary mocking and side effects.

We can use the @MockBean to add mock objects to the Spring application context. The mock will replace any existing bean of the same type in the application context.

@BeforeEach is used to signal that the annotated method should be executed before each @Test method in the current class:

    @BeforeEach
    void setUp() {
        Department department =
                Department.builder()
                .departmentName("IT")
                .departmentAddress("Ahmedabad")
                .departmentCode("IT-06")
                .departmentId(1L)
                .build();

        Mockito.when(departmentRepository.findByDepartmentNameIgnoreCase("IT"))
                .thenReturn(department);

    }

We wrote a JUnit test for fetchDepartmentByName() method:

    @Test
    @DisplayName("Get Data based on Valida Department Name")
    public void whenValidDepartmentName_thenDepartmentShouldFound() {
        String departmentName = "IT";
        Department found =
                departmentService.fetchDepartmentByName(departmentName);

        assertEquals(departmentName, found.getDepartmentName());
    }

Similarly, you can write JUnit test cases for all the positive and negative scenarios.