Test containers for database testing in Spring Boot - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to start a PostgreSQL test container.

Spring Boot

PostgreSQLContainer<?> postgres = new PostgreSQLContainer<>("[1]:latest");
postgres.start();

Drag options to blanks, or click blank then click option'

Apostgres

Bpostgresql:latest

Cpostgresql

Dpostgres:latest

Attempts:

3 left

💡 Hint

Common Mistakes

Using just 'postgres' without tag

Using incorrect image name like 'postgresql' instead of 'postgres'

The correct Docker image name for PostgreSQL test container is postgres:latest.

2fill in blank

medium

Complete the code to get the JDBC URL from the running container.

Spring Boot

String jdbcUrl = postgres.[1]();

Drag options to blanks, or click blank then click option'

AgetJdbcUrl

BjdbcUrl

CgetJdbcURL

DgetUrl

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect method name 'getJdbcURL' (case sensitive)

Using 'getUrl' which returns container URL, not JDBC URL

The method to get the JDBC URL from a PostgreSQLContainer is getJdbcUrl().

3fill in blank

hard

Fix the error in the annotation to use Testcontainers with JUnit 5.

Spring Boot

@[1]
public class MyDatabaseTest {
    // test code
}

Drag options to blanks, or click blank then click option'

AEnableTestcontainers

BTestcontainers

CExtendWith(TestcontainersExtension.class)

DRunWith(Testcontainers.class)

Attempts:

3 left

💡 Hint

Common Mistakes

Using JUnit 4 annotation @RunWith

Using @Testcontainers without extension

For JUnit 5, use @ExtendWith(TestcontainersExtension.class) to enable Testcontainers support.

4fill in blank

hard

Fill both blanks to define a Spring Boot test configuration that uses the container's JDBC URL and credentials.

Spring Boot

@DynamicPropertySource
static void datasourceProperties(DynamicPropertyRegistry registry) {
    registry.add("spring.datasource.url", () -> [1]);
    registry.add("spring.datasource.username", () -> [2]);
}

Drag options to blanks, or click blank then click option'

Apostgres.getJdbcUrl()

Bpostgres.getUsername()

Cpostgres.getPassword()

Dpostgres.getUrl()

Attempts:

3 left

💡 Hint

Common Mistakes

Using getUrl() instead of getJdbcUrl()

Using getPassword() for username

Use getJdbcUrl() for the datasource URL and getUsername() for the username from the container.

5fill in blank

hard

Fill all three blanks to create a Map of properties for Spring Boot datasource configuration using the container.

Spring Boot

Map<String, String> props = Map.of(
    "spring.datasource.url", [1],
    "spring.datasource.username", [2],
    "spring.datasource.password", [3]
);

Drag options to blanks, or click blank then click option'

Apostgres.getJdbcUrl()

Bpostgres.getUsername()

Cpostgres.getPassword()

Dpostgres.getUrl()

Attempts:

3 left

💡 Hint

Common Mistakes

Using getUrl() instead of getJdbcUrl()

Mixing up username and password methods

Use getJdbcUrl(), getUsername(), and getPassword() from the container to configure Spring Boot datasource properties.

Practice

1. What is the main purpose of using Testcontainers in Spring Boot database testing?

easy

A. To run real database instances in temporary containers for reliable tests

B. To mock database calls without any real database

C. To speed up unit tests by skipping database interactions

D. To deploy databases permanently on production servers

Solution

1. Step 1: Understand Testcontainers role

   Testcontainers run real database instances inside temporary containers during tests.

2. Step 2: Compare with other options

   Mocking or skipping databases is not what Testcontainers do; they provide real environments temporarily.

3. Final Answer:

   To run real database instances in temporary containers for reliable tests -> Option A

4. Quick Check:

   Testcontainers purpose = run real temporary databases [OK]

Hint: Testcontainers run real temporary databases, not mocks [OK]

Common Mistakes:

• Confusing Testcontainers with mocking frameworks
• Thinking Testcontainers deploy permanent databases
• Assuming Testcontainers skip database usage

2. Which of the following is the correct way to start a PostgreSQL Testcontainer in a Spring Boot test class?

easy

A. static PostgreSQLContainer postgres = new PostgreSQLContainer<>("postgres:latest").start();

B. PostgreSQLContainer postgres = new PostgreSQLContainer("postgres:latest").run();

C. static PostgreSQLContainer postgres = new PostgreSQLContainer("postgres:latest").begin();

D. PostgreSQLContainer postgres = new PostgreSQLContainer().start();

Solution

1. Step 1: Check correct instantiation and start method

   The correct way is to create a static container with generic type and call start().

2. Step 2: Validate options

   static PostgreSQLContainer postgres = new PostgreSQLContainer<>("postgres:latest").start(); uses correct syntax with generic type and start(). Others use wrong method names or miss generics.

3. Final Answer:

   static PostgreSQLContainer postgres = new PostgreSQLContainer<>("postgres:latest").start(); -> Option A

4. Quick Check:

   Use start() with generic type for Testcontainers [OK]

Hint: Use start() method and generic type for containers [OK]

Common Mistakes:

• Using .run() or .begin() instead of .start()
• Omitting generic type
• Not declaring container as static when needed

3. Given this Spring Boot test snippet using Testcontainers:

 @Testcontainers
 class MyRepositoryTest {
  @Container
  static PostgreSQLContainer postgres = new PostgreSQLContainer<>("postgres:13").withDatabaseName("testdb").withUsername("user").withPassword("pass");

  @Autowired
  MyRepository repo;

  @Test
  void testCount() {
    long count = repo.count();
    System.out.println(count);
  }
}

What will System.out.println(count); print if the database is empty?

medium

A. null

B. 0

C. Throws NullPointerException

D. 1

Solution

1. Step 1: Understand repo.count() behavior

   The count() method returns the number of records in the repository's table.

2. Step 2: Database is empty so count is zero

   Since the database is fresh and empty, count() returns 0, not null or exception.

3. Final Answer:

   0 -> Option B

4. Quick Check:

   Empty DB count() = 0 [OK]

Hint: Empty database count returns zero, not null or error [OK]

Common Mistakes:

• Expecting null instead of zero
• Assuming count() throws exception on empty DB
• Confusing count() with find methods

4. You wrote this test using Testcontainers but it fails with org.testcontainers.containers.ContainerLaunchException. What is the likely cause?

 @Container
 static PostgreSQLContainer postgres = new PostgreSQLContainer<>("postgres:latest");

 @Test
 void testDb() {
   // test code
 }

medium

A. The PostgreSQL image name is incorrect

B. The container must be started manually with .start() in the test

C. Docker is not running or accessible on the test machine

D. Testcontainers do not support PostgreSQL

Solution

1. Step 1: Understand ContainerLaunchException meaning

   This exception usually means Docker cannot start the container.

2. Step 2: Check common causes

   Most common cause is Docker daemon not running or inaccessible to Testcontainers.

3. Final Answer:

   Docker is not running or accessible on the test machine -> Option C

4. Quick Check:

   ContainerLaunchException = Docker not running [OK]

Hint: Check Docker daemon is running if container launch fails [OK]

Common Mistakes:

• Thinking image name is wrong when it is correct
• Trying to start container manually when @Container does it
• Believing Testcontainers don't support PostgreSQL

5. You want to write a Spring Boot integration test using Testcontainers with a MySQL database. You want the container to start once before all tests and stop after all tests. Which setup is correct?

hard

A. Use @SpringBootTest without any container setup; rely on embedded database

B. Create a new MySQLContainer instance inside each test method and call start() manually

C. Use @BeforeEach to start a MySQLContainer and @AfterEach to stop it

D. Declare a static MySQLContainer with @Container annotation in the test class and use @Testcontainers on the class

Solution

1. Step 1: Understand lifecycle annotations

   Using static container with @Container and @Testcontainers on class starts container once before all tests and stops after all.

2. Step 2: Compare other options

   Creating container per test or starting/stopping each test is inefficient. Embedded DB is different approach.

3. Final Answer:

   Declare a static MySQLContainer with @Container annotation in the test class and use @Testcontainers on the class -> Option D

4. Quick Check:

   Static @Container + @Testcontainers = start once before all tests [OK]

Hint: Use static @Container field with @Testcontainers class annotation [OK]

Common Mistakes:

• Starting container inside each test method
• Using @BeforeEach/@AfterEach for container lifecycle
• Assuming embedded DB replaces Testcontainers