Launch full fledged Spring Boot application in separate test. This is only for integration testing with HTTP protocol, normally you should use Spring Test MVC framework to test controllers and Spring less unit testing of separate classes.

SpringBootExternalTest.java

public class SpringBootExternalTest {
    static CompletableFuture<Throwable> initializationException;

    @BeforeClass
    public static void startSpringContext() {
        initializationException = new CompletableFuture<>();
        Thread appThread = new Thread(() -> {
            SpringApplication springApplication = new SpringApplication(
                    SpringBootApplication.class);
            springApplication.addListeners((ApplicationListener<ApplicationReadyEvent>) event -> initializationException.complete(null));
            springApplication.addListeners((ApplicationListener<ApplicationFailedEvent>) event -> initializationException.complete(event.getException()));
            springApplication.run();
        });
        appThread.start();
        try {
            Throwable exception = initializationException.get(2, TimeUnit.MINUTES);
            if (exception != null) {
                throw new IllegalStateException("Failed to start Spring Boot application", exception);
            }
        } catch (InterruptedException | ExecutionException e) {
            throw new IllegalStateException(e);
        } catch (TimeoutException e) {
            throw new IllegalStateException("Timeout expired for Spring Boot application launch", e);
        }
    }
}