Test containers for database testing in JUnit - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of JUnit test using Testcontainers PostgreSQL

What is the output of the following JUnit test when run with Testcontainers managing a PostgreSQL database?

JUnit

import org.junit.jupiter.api.Test;
import org.testcontainers.containers.PostgreSQLContainer;
import static org.junit.jupiter.api.Assertions.*;

public class DbTest {
    static PostgreSQLContainer<?> postgres = new PostgreSQLContainer<>("postgres:15.3")
        .withDatabaseName("testdb")
        .withUsername("user")
        .withPassword("pass");

    static {
        postgres.start();
    }

    @Test
    void testConnection() {
        String url = postgres.getJdbcUrl();
        assertTrue(url.contains("jdbc:postgresql://"));
        assertEquals("user", postgres.getUsername());
        assertEquals("pass", postgres.getPassword());
    }
}

A Test fails because getJdbcUrl() returns null

B Test fails with container startup timeout exception

C Test fails due to username mismatch assertion error

D Test passes successfully with all assertions true

💡 Hint

Testcontainers starts the container before tests run and provides valid connection info.

The PostgreSQLContainer starts correctly and provides valid JDBC URL, username, and password matching the configured values. All assertions pass.

❓ assertion

intermediate

Correct assertion to verify Testcontainers MySQL container is running

Which assertion correctly verifies that a MySQL Testcontainer is running during a JUnit test?

JUnit

import org.testcontainers.containers.MySQLContainer;
MySQLContainer<?> mysql = new MySQLContainer<>("mysql:8.0");
mysql.start();

A assertTrue(mysql.isRunning());

B assertEquals(true, mysql.isCreated());

C assertFalse(mysql.isRunning());

D assertNull(mysql.getJdbcUrl());

💡 Hint

Check the container's running state with the right method.

The isRunning() method returns true if the container is started and running. Other options check wrong methods or states.

🔧 Debug

advanced

Identify the cause of container startup failure in JUnit test

Given the following JUnit test code using Testcontainers, what is the most likely cause of the test failing with a container startup timeout?

JUnit

import org.junit.jupiter.api.Test;
import org.testcontainers.containers.PostgreSQLContainer;

public class DbTest {
    @Test
    void testDb() {
        PostgreSQLContainer<?> postgres = new PostgreSQLContainer<>("postgres:nonexistent-tag")
            .withDatabaseName("db")
            .withUsername("user")
            .withPassword("pass");
        postgres.start();
        assertTrue(postgres.isRunning());
    }
}

A The database name 'db' is invalid and prevents container startup

B The Docker image tag 'postgres:nonexistent-tag' does not exist, causing startup failure

C The username and password are missing and cause authentication failure

D The test method is missing @BeforeAll annotation to start the container

💡 Hint

Check the Docker image tag correctness.

Using a non-existent Docker image tag causes the container to fail to start, leading to a timeout error.

🧠 Conceptual

advanced

Why use Testcontainers for database testing in CI pipelines?

Which is the best reason to use Testcontainers for database testing in continuous integration (CI) pipelines?

A It slows down tests by requiring full OS virtualization

B It requires manual installation of databases on CI servers

C It provides a lightweight, disposable database instance isolated per test run

D It uses in-memory databases that do not support real SQL features

💡 Hint

Think about test isolation and environment consistency.

Testcontainers spins up real database containers that are isolated and disposable, ensuring consistent and clean test environments in CI.

❓ framework

expert

Correct JUnit 5 lifecycle usage with Testcontainers PostgreSQL

Which code snippet correctly manages a PostgreSQL Testcontainer lifecycle using JUnit 5 annotations to start once before all tests and stop after all tests?

A

import org.junit.jupiter.api.*;
import org.testcontainers.containers.PostgreSQLContainer;

public class DbTest {
    static PostgreSQLContainer<?> postgres = new PostgreSQLContainer<>("postgres:15.3");

    @BeforeAll
    static void startContainer() {
        postgres.start();
    }

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

    @Test
    void test() {
        Assertions.assertTrue(postgres.isRunning());
    }
}

B

import org.junit.jupiter.api.*;
import org.testcontainers.containers.PostgreSQLContainer;

public class DbTest {
    static PostgreSQLContainer<?> postgres = new PostgreSQLContainer<>("postgres:15.3");

    @BeforeEach
    void startContainer() {
        postgres.start();
    }

    @AfterEach
    void stopContainer() {
        postgres.stop();
    }

    @Test
    void test() {
        Assertions.assertTrue(postgres.isRunning());
    }
}

C

import org.junit.jupiter.api.*;
import org.testcontainers.containers.PostgreSQLContainer;

@TestInstance(TestInstance.Lifecycle.PER_METHOD)
public class DbTest {
    PostgreSQLContainer<?> postgres = new PostgreSQLContainer<>("postgres:15.3");

    @BeforeAll
    void startContainer() {
        postgres.start();
    }

    @AfterAll
    void stopContainer() {
        postgres.stop();
    }

    @Test
    void test() {
        Assertions.assertTrue(postgres.isRunning());
    }
}

D

import org.junit.jupiter.api.*;
import org.testcontainers.containers.PostgreSQLContainer;

@TestInstance(TestInstance.Lifecycle.PER_CLASS)
public class DbTest {
    PostgreSQLContainer<?> postgres = new PostgreSQLContainer<>("postgres:15.3");

    @BeforeAll
    void startContainer() {
        postgres.start();
    }

    @AfterAll
    void stopContainer() {
        postgres.stop();
    }

    @Test
    void test() {
        Assertions.assertTrue(postgres.isRunning());
    }
}

💡 Hint

Static methods with @BeforeAll/@AfterAll require static container fields unless using PER_CLASS lifecycle.

JUnit 5 requires @BeforeAll and @AfterAll methods to be static unless the test class uses PER_CLASS lifecycle. Option A correctly uses static container and static lifecycle methods.