how to run testcontainer with dynamic port for spring data elasticsearch

Newer versions of Spring provide @DynamicPropertySource for exactly this use case: https://docs.spring.io/spring-framework/docs/current/javadoc-api/org/springframework/test/context/DynamicPropertySource.html

Your code should look roughly like this:

 @SpringJUnitConfig(...)
 @Testcontainers
 class ExampleIntegrationTests {

     @Container
     static ElasticsearchContainer elastic= new ElasticsearchContainer(ELASTICSEARCH_IMAGE)
        .withEnv("discovery.type", "single-node");

     // ...

     @DynamicPropertySource
     static void elasticProperties(DynamicPropertyRegistry registry) {
         registry.add("spring.elasticsearch.rest.uris", elastic::getHttpHostAddress);
     }

 }

1. You can use context configuration initialiser to set properties during runtime, which you can later use in your RestClientConfig.

Let me show you on the example of Postgresql container setup:

@SpringBootTest(webEnvironment = SpringBootTest.WebEnvironment.RANDOM_PORT, classes = Application.class)
@ContextConfiguration(initializers = AbstractTestcontainersTest.DockerPostgreDataSourceInitializer.class)
public abstract class AbstractTestcontainersTest {

    protected static final String DB_CONTAINER_NAME = "postgres-auth-test";
    protected static PostgreSQLContainer<?> postgreDBContainer =
            new PostgreSQLContainer<>(DockerImageName.parse("public.ecr.aws/docker/library/postgres:12.10-alpine")
                    .asCompatibleSubstituteFor("postgres"))
            .withUsername("postgres")
            .withPassword("change_me")
            .withInitScript("db.sql")
            .withCreateContainerCmdModifier(cmd -> cmd.withName(DB_CONTAINER_NAME))
            .withDatabaseName("zpot_main");

    @BeforeAll
    public static void beforeAll() throws ShellExecutionException {
        postgreDBContainer.start();
    }

    @AfterAll
    public static void afterAll() {
        postgreDBContainer.stop();
    }

    public static class DockerPostgreDataSourceInitializer implements ApplicationContextInitializer<ConfigurableApplicationContext> {

        @Override
        public void initialize(ConfigurableApplicationContext applicationContext) {

            TestPropertySourceUtils.addInlinedPropertiesToEnvironment(
                    applicationContext,
                    "spring.datasource.url=" + postgreDBContainer.getJdbcUrl(),
                    "spring.datasource.username=" + postgreDBContainer.getUsername(),
                    "spring.datasource.password=" + postgreDBContainer.getPassword()
            );
        }
    }
}

All the configuration is done in DockerPostgreDataSourceInitializer, where I set all the properties I need. You also need to annotate your test class with @ContextConfiguration annotaion. You can do something similar with your ElasticSearchContainer. As I just checked the ElasticSearchContainer has a method getHttpHostAddress() which returns host+dynamic_port combination for your container. You can get that host-port pair and set in in properties to be used later in your client configuration. If you need just port you can call container.getMappedPort(9200) and again set that port in properties.

2. Regarding @Testcontainers annotation, you need it if you want testcontainers to manage your container lifecycle. In that case you also need to annotate container with @Container annotation. Your container will be started either once before all test methods in a class if your container is a static field or before each test method if it's a regular field. You can read more about that here: https://www.testcontainers.org/test_framework_integration/junit_5/#extension. Or you can start your container manually either in @BeforeAll or @BeforeEach annotated setup methods. In other words no, you don't have to use @Testcontainers annotation.