Could you explain what Testcontainers are and how they are used in application testing?

Testcontainers is a library for Java (and other languages) that allows running Docker containers during application testing. It is used to create isolated, reproducible, and manageable environments, such as databases, message brokers, or other services needed for integration tests.

The main idea is that instead of relying on external services or mocking them, tests run real services in containers that automatically start before the test and are destroyed afterward.

Example of usage with PostgreSQL database:

import org.testcontainers.containers.PostgreSQLContainer;

public class MyTest {
    static PostgreSQLContainer<?> postgres = new PostgreSQLContainer<>("postgres:13")
        .withDatabaseName("testdb")
        .withUsername("user")
        .withPassword("pass");

    @BeforeAll
    public static void setUp() {
        postgres.start();
        // Configure database connection via postgres.getJdbcUrl(), getUsername(), getPassword()
    }

    @AfterAll
    public static void tearDown() {
        postgres.stop();
    }

    @Test
    public void testDatabase() {
        // Tests using the real database in the container
    }
}

Thus, Testcontainers simplify writing integration tests, improve their reliability, and make the testing environment more controllable.