Spring Boot Tests with JUnit5 Jupiter and Shared Containers

Currently, a typical test setup with Spring Boot tests and shared containers does not work anymore with JUnit 5: Using static testcontainers with Spring Boot's ApplicationContextInitializer to configure the server's ports in the Spring context.

The problem is that the Spring application context gets initialized during Jupiter extension's post processing while testcontainers are started during before all callbacks. The latter happens after post processing.

See
https://github.com/junit-team/junit5/blob/c9ae6e261550481362f17d21e88137a8c71fc7c8/junit-jupiter-engine/src/main/java/org/junit/jupiter/engine/descriptor/ClassBasedTestDescriptor.java#L184
vs.
https://github.com/junit-team/junit5/blob/c9ae6e261550481362f17d21e88137a8c71fc7c8/junit-jupiter-engine/src/main/java/org/junit/jupiter/engine/descriptor/ClassBasedTestDescriptor.java#L189

One approach to solve this could be to start the testcontainers also during post processing (TestInstancePostProcesso#postProcessTestInstancer)

Then the only remaining problem would be to ensure that the TestcontainersExtension always gets created before the SpringExtension.

Problematic Example

@SpringBootTest
@ContextConfiguration(initializers = {Initializer.class})
@Testcontainers
@TestInstance(TestInstance.Lifecycle.PER_CLASS)
public class JupiterSharedTestcontainersTests {

  @Container
  public static JdbcDatabaseContainer postgreSQLContainer = new PostgreSQLContainer();

  @Test
  public void testTomatoes() {
    assertTrue("tomatoes".equals("tomatoes"));
  }
  
  public static class Initializer implements ApplicationContextInitializer<ConfigurableApplicationContext> {
    @Override
    public void initialize(ConfigurableApplicationContext configurableApplicationContext) {
      TestPropertyValues.of(
          "spring.datasource.url=jdbc:postgresql://"
              + postgreSQLContainer.getContainerIpAddress()
              + ":" + postgreSQLContainer.getMappedPort(PostgreSQLContainer.POSTGRESQL_PORT)
              + "/db_name"
      ).applyTo(configurableApplicationContext.getEnvironment());
    }
  }

}

What do you think?

Start with the problematic Spring Boot/Jupiter example in the issue and compare the JUnit Jupiter ClassBasedTestDescriptor.java ordering around the referenced lines 184 and 189. Reproduce the interaction between ApplicationContextInitializer, Testcontainers, and Jupiter callbacks, then determine whether a supported ordering can make the shared-container setup work without breaking other extensions.