Introduction
The @DataMongoTest annotation in Spring Boot is designed for isolated testing of MongoDB repositories. It automatically configures an embedded MongoDB instance using Flapdoodle, which eliminates the need for a real database.
By default, @DataMongoTest:
- Automatically configures an in-memory MongoDB instance (Flapdoodle).
- Loads only MongoDB-related components (documents, repositories, etc.).
- Rolls back changes after each test, keeping the database clean.
- Excludes unnecessary beans (@Service, @Controller), making tests lightweight.
This approach allows fast and isolated repository testing without setting up an external MongoDB instance.
Project Setup
To get started, create a Spring Boot project with the following dependencies:
- Spring Boot Starter Data MongoDB – For working with MongoDB.
- Spring Boot Starter Test – Includes JUnit, AssertJ, and Mockito.
- Flapdoodle Embedded MongoDB – Provides an in-memory MongoDB instance for testing.
Spring Boot Testing Libraries
The spring-boot-starter-test dependency includes:
- JUnit – Core testing framework.
- Spring Boot Test – Utilities for integration testing.
- AssertJ – Fluent assertion library.
- Mockito – A powerful mocking framework.
Implementation
1. pom.xml Configuration
<?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>3.4.3</version> <relativePath/> <!-- lookup parent from repository --> </parent> <groupId>tech.devblueprint</groupId> <artifactId>datamongodb-spring-boot-testing-example</artifactId> <version>0.0.1-SNAPSHOT</version> <name>datamongodb-spring-boot-testing-example</name> <description>Demo project for Spring Boot</description> <properties> <java.version>17</java.version> </properties> <dependencies> <dependency> <groupId>org.springframework.boot</groupId> <artifactId>spring-boot-starter-data-mongodb</artifactId> </dependency> <dependency> <groupId>org.springframework.boot</groupId> <artifactId>spring-boot-starter-test</artifactId> <scope>test</scope> </dependency> <dependency> <groupId>de.flapdoodle.embed</groupId> <artifactId>de.flapdoodle.embed.mongo.spring30x</artifactId> <version>4.6.2</version> <scope>test</scope> </dependency> </dependencies> <build> <plugins> <plugin> <groupId>org.springframework.boot</groupId> <artifactId>spring-boot-maven-plugin</artifactId> </plugin> </plugins> </build> </project>
Key Dependencies Explained:
- spring-boot-starter-data-mongodb – Enables MongoDB support.
- spring-boot-starter-test – Provides all necessary testing libraries.
- de.flapdoodle.embed.mongo.spring30x – Automatically sets up an embedded MongoDB instance for tests.
2. Embedded MongoDB Configuration
Since @DataMongoTest automatically configures Flapdoodle, no explicit database configuration is needed.
3. Document Definition
File: src/main/java/.../document/Device.java
package tech.devblueprint.datamongodb_spring_boot_testing_example.document; import org.springframework.data.annotation.Id; import org.springframework.data.mongodb.core.mapping.Document; @Document(collection = "devices") public class Device { @Id private String id; private String name; private String type; private String manufacturer; // Default constructor public Device() { } // Parameterized constructor public Device(String id, String name, String type, String manufacturer) { this.id = id; this.name = name; this.type = type; this.manufacturer = manufacturer; } // Getter and Setter for id public String getId() { return id; } public void setId(String id) { this.id = id; } // Getter and Setter for name public String getName() { return name; } public void setName(String name) { this.name = name; } // Getter and Setter for type public String getType() { return type; } public void setType(String type) { this.type = type; } // Getter and Setter for manufacturer public String getManufacturer() { return manufacturer; } public void setManufacturer(String manufacturer) { this.manufacturer = manufacturer; } }
4. Repository Interface
File: src/main/java/.../repository/DeviceRepository.java
package tech.devblueprint.datamongodb_spring_boot_testing_example.repository; import tech.devblueprint.datamongodb_spring_boot_testing_example.document.Device; import org.springframework.data.mongodb.repository.MongoRepository; import java.util.Optional; public interface DeviceRepository extends MongoRepository<Device, String> { // Find a device by its name Optional<Device> findByName(String name);
Writing Tests with Embedded MongoDB
Using Flapdoodle for MongoDB Repository Testing
Spring Boot automatically configures Flapdoodle Embedded MongoDB for @DataMongoTest
, so we don’t need to start an external database.
File: src/test/java/.../repository/DeviceRepositoryTest.java
package tech.devblueprint.datamongodb_spring_boot_testing_example.repository; import org.junit.jupiter.api.BeforeAll; import org.junit.jupiter.api.Test; import org.junit.jupiter.api.TestInstance; import org.springframework.beans.factory.annotation.Autowired; import org.springframework.boot.test.autoconfigure.data.mongo.DataMongoTest; import tech.devblueprint.datamongodb_spring_boot_testing_example.document.Device; import java.util.Arrays; import static org.assertj.core.api.Assertions.assertThat; @DataMongoTest @TestInstance(TestInstance.Lifecycle.PER_CLASS) // Allows non-static @BeforeAll method public class DeviceRepositoryTest { @Autowired private DeviceRepository deviceRepository; @BeforeAll void setUp() { // Save initial devices into the embedded MongoDB database Device device1 = new Device("1", "SmartphoneX", "Smartphone", "ManufacturerA"); Device device2 = new Device("2", "LaptopY", "Laptop", "ManufacturerB"); Device device3 = new Device("3", "TabletZ", "Tablet", "ManufacturerA"); deviceRepository.saveAll(Arrays.asList(device1, device2, device3)); } @Test void findByName_ReturnsDevice() { // Retrieve device by name Device device = deviceRepository.findByName("SmartphoneX").orElse(null); assertThat(device).isNotNull(); assertThat(device.getName()).isEqualTo("SmartphoneX"); assertThat(device.getType()).isEqualTo("Smartphone"); assertThat(device.getManufacturer()).isEqualTo("ManufacturerA"); } }
Key Annotations Explained:
- @DataMongoTest – Loads only MongoDB-related components for testing.
- @TestInstance(TestInstance.Lifecycle.PER_CLASS) – Allows using a non-static @BeforeAll method to initialize test data.
- @BeforeAll – Inserts initial test data into the embedded MongoDB before running tests.
- @Test – Defines a test case that retrieves a device by name.
Understanding Transactions in @DataMongoTest
Unlike relational databases, MongoDB does not support transactions in the same way as SQL databases. Therefore, @DataMongoTest does not roll back changes automatically.
If needed, you should manually clean up the database before each test:
@BeforeEach void setUp() { deviceRepository.deleteAll(); }
Conclusion
In this guide, we explored how to use @DataMongoTest with an embedded MongoDB instance (Flapdoodle) instead of a real MongoDB database.