@SpringBootApplication
public class MainApplication {
  public static void main(String[] args) {
    ApplicationContext ctx = SpringApplication.run(MainApplication.class, args);
    AccountService accService = ctx.getBean(AccountService.class);
    accService.loadDictionary();
  }
}

@SpringBootTest(useMainMethod = SpringBootTest.UseMainMethod.ALWAYS)

@Service
public class ExtService {

    @Retryable(
        value = { HttpServerErrorException.class, ResourceAccessException.class },
        maxAttempts = 3,
        backoff = @Backoff(delay = 2000, multiplier = 2)
    )
    public ExtServiceResponse process(ExtServiceRequest request) {
        // Call external payment gateway
        return restTemplate.postForObject("https://ext-gateway/api/service", request,
            ExtServiceResponse.class);
    }

    @Recover
    public ExtServiceResponse recover(Exception e, ExtServiceRequest request) {
        // Fallback logic after retries are exhausted
        log.error("Failed after retries: {}", e.getMessage());
        return new ExtServiceResponse("FAILED");
    }
}

// С иммутабельными объектами:

record Book(String title, int price) {} // Иммутабельный

public class BookJob implements Runnable {
    private final Book book;
    .... // конструктор

    @Override
    public void run() {
        System.out.println(book.title() + " " + book.price());
    }
}

// Обмен сообщениями:

...
new Thread(() -> {
    try {
        queue.put("mess1");
    } catch (InterruptedException e) {}
}).start();

...
new Thread(() -> {
    try {
        String mess = queue.take();
    } catch (InterruptedException e) {}
}).start();

@WebMvcTest(HelloController.class)
class HelloControllerTest {

  @Autowired
  private MockMvc mockMvc;

  @MockBean
  private HelloService helloService; // замокано

  @Test
  void sayHello_returnsExpectedMessage() throws Exception {
    given(helloService.getMessage()).willReturn("Hello!");
    mockMvc.perform(get("/api/hello"))
      .andExpect(status().isOk())
      .andExpect(content().string("Hello!"));
  }
}

@Configuration
class WebMvcConfig implements WebMvcConfigurer {

  @Override
  public void configurePathMatch(PathMatchConfigurer configurer) {
    configurer.addPathPrefix("/api",
      aClass -> aClass.getPackage().getName()
        .startsWith("com.sivalabs.bookstore"));
  }
}