Testing with H2 Database in Spring Boot

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Using H2 Database in JUnit Tests with Spring Boot

H2 is a lightweight, in-memory database ideal for unit and integration testing in Java applications, especially with Spring Boot. It allows you to run tests quickly without needing an external database setup. Below is a step-by-step guide assuming a Spring Boot project with JPA/Hibernate. If you’re not using Spring, you can use H2 via plain JDBC (see notes at the end).

Step 1: Add H2 Dependency

Include the H2 dependency in your pom.xml (Maven) or build.gradle (Gradle). Scope it to test to avoid production inclusion.

Maven (pom.xml):

<dependency>
    <groupId>com.h2database</groupId>
    <artifactId>h2</artifactId>
    <scope>test</scope>
</dependency>

Gradle (build.gradle):

testImplementation 'com.h2database:h2'

This pulls in the H2 JAR for test execution only.

Step 2: Configure H2 in Test Properties

Create or update src/test/resources/application.properties (or application-test.yml) to point to H2. This overrides production DB settings.

application.properties:

# H2 Database Configuration
spring.datasource.url=jdbc:h2:mem:testdb
spring.datasource.driverClassName=org.h2.Driver
spring.datasource.username=sa
spring.datasource.password=

# H2 Console (optional, for debugging)
spring.h2.console.enabled=true

# JPA/Hibernate Settings
spring.jpa.database-platform=org.hibernate.dialect.H2Dialect
spring.jpa.hibernate.ddl-auto=create-drop
spring.jpa.show-sql=true

mem:testdb: In-memory database named “testdb”.
create-drop: Auto-creates tables on startup and drops them on shutdown.
Enable the H2 console at http://localhost:8080/h2-console during tests for inspection (use JDBC URL: jdbc:h2:mem:testdb).

If using profiles, activate with @ActiveProfiles("test") in your test class.

Step 3: Write a JUnit Test

Use @SpringBootTest for full context or @DataJpaTest for repository-focused tests. Annotate with @Test and use JUnit 5 (@ExtendWith(SpringExtension.class)).

Example: Testing a JPA Repository Assume you have an Entity like User and a UserRepository extending JpaRepository.

import org.junit.jupiter.api.Test;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.autoconfigure.orm.jpa.DataJpaTest;
import org.springframework.test.context.ActiveProfiles;

import static org.assertj.core.api.Assertions.assertThat;

@DataJpaTest  // Loads only JPA-related beans for faster tests
@ActiveProfiles("test")  // Activates test profile
public class UserRepositoryTest {

    @Autowired
    private UserRepository userRepository;

    @Test
    public void shouldSaveAndFindUser() {
        // Given
        User user = new User("John Doe", "john@example.com");
        userRepository.save(user);

        // When
        User foundUser = userRepository.findByEmail("john@example.com").orElse(null);

        // Then
        assertThat(foundUser).isNotNull();
        assertThat(foundUser.getName()).isEqualTo("John Doe");
    }
}

@DataJpaTest: Auto-configures H2 and rolls back transactions after each test.
Use AssertJ or JUnit assertions for verification.
For service-layer tests, use @SpringBootTest and inject services/repositories.

Run with mvn test or your IDE’s test runner. Each test starts with a fresh DB.

Step 4: Advanced Tips

Flyway/Liquibase Migration: If using schema migrations, add them to src/test/resources and H2 will apply them.
Realistic Testing: H2 mimics most SQL dialects but may differ from production (e.g., PostgreSQL). For closer matches, consider Testcontainers with a real DB container.

Exclude Production Config: In pom.xml, use profiles to exclude real DB drivers in tests:

<profile>
    <id>test</id>
    <dependencies>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-data-jpa</artifactId>
            <exclusions>
                <exclusion>
                    <groupId>mysql</groupId>  <!-- Or your prod DB -->
                    <artifactId>mysql-connector-java</artifactId>
                </exclusion>
            </exclusions>
        </dependency>
    </dependencies>
</profile>

Troubleshooting: If tests use the production DB, ensure spring.profiles.active=test or use @TestPropertySource.

Standalone JUnit (No Spring)

For plain JDBC without Spring:

Add H2 dependency (as above).
In your test: ```java import org.h2.jdbcx.JdbcDataSource; import org.junit.jupiter.api.Test;

public class StandaloneH2Test { @Test public void testH2Connection() throws Exception { JdbcDataSource ds = new JdbcDataSource(); ds.setURL(“jdbc:h2:mem:test;DB_CLOSE_DELAY=-1”); ds.setUser(“sa”); ds.setPassword(“”);

    try (Connection conn = ds.getConnection();
         Statement stmt = conn.createStatement()) {
        stmt.execute("CREATE TABLE users (id INT PRIMARY KEY, name VARCHAR(255))");
        stmt.execute("INSERT INTO users VALUES (1, 'Test User')");
        ResultSet rs = stmt.executeQuery("SELECT * FROM users");
        assertTrue(rs.next());
    }
} } ``` This creates an in-memory DB per test.

For more details, see the references below.

Setting Up H2 for Testing in Spring Boot
Self-Contained Testing Using an In-Memory Database | Baeldung
Setting Up H2 for Testing in Spring Boot application
Test Spring Data Repositories with H2 in-memory database

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