Spring Code TIP-2: Assert expected log message in a test-case . . .

Logging is helpful to know the insights of a running application. It is also useful in investigating issues. Sometimes, we would like to log certain configuration details after the application starts up. This can be achieved by implementing functional interface CommandLineRunner. Any bean that implements this interface is a special type of bean that runs after the application context is fully loaded.

CommandLineRunner

CommandLineRunner is a functional interface with a single method run(String... args). Beans implementing this interface are executed after the application context is loaded and before the Spring Boot application starts.

For example the following application class defines a CommandLineRunner bean. The bean returns a lambda expression which defines the behavior of the CommandLineRunner. As an altervative to defining a bean, the MyApplication class can also implement this interface and provide implementation for run method. As shown below, this bean checks and if the jdbcClient is available, it executes a SQL query to get the database version and logs the result. This is run after the application context is fully loaded so that we know the version of the Database that the application is using. This is one such very useful information.

@SpringBootApplication @Slf4j public class MyApplication { public static void main(String[] args) { SpringApplication.run(MyApplication.class, args); } @Bean public CommandLineRunner initializationChecks(@Autowired(required = false) JdbcClient jdbcClient) { return args -> { if (jdbcClient != null) { log.info("Database check: {}", jdbcClient.sql("SELECT version()").query(String.class).single()); } }; } }

Now, say we want to assert this log message in a test-case. That way we ensure that the log message contains the expected version of the Database, and the Database won't get changed/upgraded without a test-case catching it.

Spring Code TIP - test log message

JUnit's @ExtendsWith annotation and Spring Boot's OutputCaptureExtension can be leveraged to  achieve this.

The following is an integration test, that does this:

@SpringBootTest(useMainMethod = SpringBootTest.UseMainMethod.ALWAYS) @ExtendWith(OutputCaptureExtension.class) class MyApplicationIntegrationTest { @Autowired ApplicationContext applicationContext; @Autowired MyService myService; @Test @DisplayName("An integration Smoke Test to ensure that the application context loads, autowiring works, and checks DB version.") void smokeTest_context_loads_and_autowiring_works(CapturedOutput output) { var otherService = applicationContext.getBean(OtherService.class); assertThat(otherService).isInstanceOf(OtherService.class); assertThat(myService).isNotNull(); assertThat(output).contains("Database version: PostgreSQL 15.3"); } }

Highlighted is the code in focus, assuming that the Database used is PostgreSQL 15.3.