Adding Integration Tests in Spring Boot with Testcontainers

Good software always includes automated tests to detect errors during the creation and modification of the code. In this tutorial, integration tests will be added to a Spring Boot application, connecting to the 'real' database using the Testcontainers library.

Backgrounds

An integration test is meant to test the interaction of multiple parts of the application. So for a Spring Boot application, it would be great to examine the running app including the database (e.g. MySQL or PostgreSQL). An embedded database would not be optimal for this, as there can be significant differences to the actual database, and therefore certain features may not work or errors may be missed.

For this purpose, there is the Testcontainers library to load a Docker image of exactly the database that is used by the developer and later in production. To write an integration test for our application, we first add the dependencies.

Besides spring-boot-starter-test we need org.testcontainers:junit-jupiter as well as an org.testcontainers reference to the database we have in use. In our case we choose MySQL.

Starting the application context

To centralize the recurring logic of our test classes, we first create an abstract base class from which our IT classes then inherit. The first version of this class looks like this:

Spring Boot provides very good support for integration testing by using the @SpringBootTest annotation to load the full application context. With @ActiveProfiles we define that the application is started with the profile "it". If needed, we can use this profile to activate special settings in our code.

By setting the variable webEnvironment to RANDOM_PORT the application will be started under a random (and available) port. This will be picked up automatically by the TestRestTemplate, when using it to send calls to our application's REST API - more on that later.

Mocking the database

Now we want to configure a database that is exclusively available for our tests. For this we extend BaseIT with the following code snippet:

In the static section, the MySQLContainer is configured and started. The string "mysql:8.0" defines the Docker image to be used. This should ideally exactly match the version that is also used in production. With the parameter withReuse(true), the Docker container is not automatically terminated at the end of the tests, but available for reusage without a longer waiting period. Occasionally, the container must be terminated manually to fully reset the database.

To enable reuse, the file /<usersdir>/.testcontainers.properties must be extended by the entry testcontainers.reuse.enable=true.

In the section after @DynamicPropertySource we set the properties to link our application context to the Docker containers database. Depending on how we initialize our database schema - for example with Flyway or Liquibase - this will be automatically applied during context startup.

Running the tests

With this done, we can write our first test class. Let's assume that the following @RestController already exists. The referenced service returns all entries that exist in the "Test" table.

We can now create a class TestControllerIT that extends our abstract base class. The test method sends a GET request to the existing endpoint using our TestRestTemplate.

Even though our database schema is already initialized, we still lack explicit test data that we need for our test. For this we use the Spring annotation @Sql, which executes two scripts and thus puts our database into a known state. In our case, there is now exactly one entry in the Test table that we expect as a result.

With restTemplate.exchange(...) we send a GET request to /api/tests. The functionality is basically the same as we already know from Spring's RestTemplate class. Our defined request does not send any data (null). The response is deserialized to type List<TestDTO> and in the assertions we check that the HttpStatus and the list match our expectation.

When we run the test for the first time, we have to wait a bit for the Docker container to start, depending on our environment. It may also be useful to already cache the image beforehand using docker pull mysql:8.0 to avoid issues with the download. Our test should now go through without errors.

Conclusion

With the setup described, we have created a way to check the behavior of our application including the database from a high level. With this we have a very good addition to our unit tests. Testcontainers also offers support for other services such as RabbitMQ, so we can flexibly extend BaseIT as needed.

In the professional plan, Bootify offers the option to activate Testcontainers. This initializes the Spring Boot application including the described setup, depending on the selected database. It also generates the IT classes and scripts according to the tables and controllers created.

Further readings

Testcontainers Homepage
Official guide on testing Spring Boot applications
Docker Hub for finding images
@Sql and @SqlMergeMode explanation
Where to look for the .testcontainers.properties

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