To ensure the correctness of interactions between beans and the expected outcomes of complex business logic, you should use unit tests to verify the behavior of individual beans and integration tests to verify interactions between beans. This approach allows you to isolate and test individual components as well as their interactions. Skipping interactions or testing only the final outcome is not sufficient for comprehensive testing. Manual testing for bean interactions is not a scalable or reliable approach for complex applications.

The @Configuration annotation is used to mark a class as a source of bean definitions. This means that any method annotated with @Bean within a class annotated with @Configuration will define a bean in the Spring context, allowing it to be injected into other components, services, etc. This is fundamental for creating the beans that make up the application context in a Spring application.

Option 3 is the best approach. Spring Security Test provides utilities for simulating authenticated users and roles, allowing you to test security constraints and access controls effectively.

WebFlux is a good choice when you need to handle a large number of concurrent connections with high responsiveness. It excels in scenarios where non-blocking, asynchronous processing is crucial to avoid thread blocking and efficiently utilize system resources. In contrast, the traditional blocking architecture is suitable for applications with lower concurrency and when synchronous processing is sufficient.

In JUnit, the @AfterClass annotation is used to indicate that a method should be executed after all tests in the current test class have been run. This is often used for cleanup tasks after running a suite of tests.

With @WebMvcTest, only the controllers and related components are typically loaded into the application context. This is useful for testing the web layer of your application in isolation without loading the entire context.

In Spring Boot, you can use the @BeforeEach annotation to specify actions that should be executed before testing each method. This is often used for setup actions before individual test cases.

In Spring Boot, to load only the web layer of an application during integration tests, you can use the @WebMvcTest annotation. It narrows down the scope of the context loaded for testing to just the web-related components.

In Spring Boot, the @ExceptionHandler annotation is used to handle exceptions at the controller level. This annotation allows you to define methods within a controller that can handle specific exceptions thrown by that controller. The other options, such as @ControllerAdvice, @RestController, and @RequestMapping, have different roles and are not used for handling exceptions directly at the controller level.

In Spring Boot tests, you can override properties defined in application.properties by using the @TestPropertySource annotation to load custom properties. Option (1) is incorrect as it's not a standard practice. Option (3) is incorrect because modifying application.properties directly in test code is not recommended. Option (4) is incorrect; properties can be overridden.