Integration Testing with Testcontainers

Testcontainers is a Java library that supports JUnit tests by providing lightweight, throwaway instances of common databases, Selenium web browsers, or anything that can run in a Docker container.

Lifecycle Management

You can configure container lifecycles in two ways:

• @Container Annotation: Tied to the JUnit test lifecycle. If the container field is static, it starts once for the whole class. If non-static, a fresh container starts for every individual test (very slow).
• Singleton Pattern (Manual Start): Start the containers manually in a base abstract class. All test classes extend this base class and share the same container instances, greatly speeding up local suite execution.

Code Walkthrough

This class demonstrates using Testcontainers to run a real PostgreSQL instance and dynamic port mapping integration.

import org.junit.jupiter.api.Test;
import org.testcontainers.containers.PostgreSQLContainer;
import org.testcontainers.junit.jupiter.Container;
import org.testcontainers.junit.jupiter.Testcontainers;
import java.sql.*;
import static org.junit.jupiter.api.Assertions.assertTrue;
@Testcontainers
class OrderRepositoryTest {
@Container
    private static final PostgreSQLContainer<?> postgres = 
        new PostgreSQLContainer<>("postgres:15-alpine")
            .withDatabaseName("testdb")
            .withUsername("testuser")
            .withPassword("testpass");
@Test
    void testConnectionAndQuery() throws SQLException {
        // Testcontainers maps host ports dynamically to avoid port conflicts
        String jdbcUrl = postgres.getJdbcUrl();
try (Connection conn = DriverManager.getConnection(
                jdbcUrl, postgres.getUsername(), postgres.getPassword())) {
Statement stmt = conn.createStatement();
            stmt.execute("CREATE TABLE orders (id INT PRIMARY KEY, amount NUMERIC);");
            stmt.execute("INSERT INTO orders VALUES (1, 99.99);");
ResultSet rs = stmt.executeQuery("SELECT count(*) FROM orders;");
            assertTrue(rs.next());
            assertEquals(1, rs.getInt(1));
        }
    }
}

Interview-Relevant Information