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.

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 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.

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.

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.

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.

In Spring Boot, the @Cacheable annotation is used to indicate that a method's return value should be stored in the cache. This annotation is applied to methods that you want to cache, and it allows you to specify caching parameters such as the cache name and the key. It is a fundamental annotation for caching in Spring Boot.

To secure RESTful web services in Spring Security using OAuth2, you should configure OAuth2 client and resource server details in the Spring Security configuration. Option 1 and Option 4 do not represent the correct way to secure RESTful services with OAuth2. Option 2 suggests configuring OAuth2 properties in the wrong place, and it is not a standard approach.

The UserDetailsService interface in Spring Security is used to load user-specific data and plays a crucial role in authentication mechanisms. It's responsible for fetching user details from a data source, such as a database, and returning them as a UserDetails object. This interface is a key component in the authentication process, allowing Spring Security to validate user credentials and create an Authentication object.

To implement custom authentication logic in Spring Security, developers can create a bean of type AuthenticationProvider. This allows developers to define their own logic for authenticating users, such as checking credentials against a database or external system. The AuthenticationProvider interface is a key component for custom authentication.

Option 1 is correct. Enabling garbage collection logging with JVM flags allows you to analyze memory usage patterns and identify memory leaks or excessive memory consumption, which are common causes of OutOfMemoryErrors. Increasing the number of application instances may exacerbate the issue if it's related to memory consumption. Disabling Spring beans or reducing threads may not be suitable solutions for addressing OutOfMemoryErrors.

When integrating with external APIs in a Spring Boot application, it's crucial to implement a global exception handler (Option 4) to manage failures. This handler should return standardized error responses with clear error messages and appropriate HTTP status codes, ensuring informative messages are relayed back to the user. Implementing a retry mechanism (Option 1) is a good practice, but it should be combined with proper error handling. Options 2 and 3 are not recommended as they either log errors internally or provide generic and uninformative error messages to users.