// 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"));
  }
}