To create conditional beans within custom Auto Configuration, you can use the @ConditionalOnClass annotation. This annotation allows you to specify that a particular bean should be created only if a specified class is present in the classpath. It's useful for scenarios where you want to conditionally configure beans based on the availability of certain classes.

To perform unit testing on the web layer of a Spring Boot application without loading the complete context, you should use the @WebMvcTest annotation. This annotation focuses only on the web layer and is suitable for testing controllers.

In a Spring Boot application, the @Transactional annotation is used to demarcate transaction boundaries. It is applied to methods, indicating that the method should be wrapped in a transaction, ensuring that either all operations within the method succeed or none of them do. This is crucial for maintaining data consistency in the database.

The @PreAuthorize annotation in Spring Security allows for complex SpEL expressions to define fine-grained access control conditions. This means you can use expressions involving multiple variables and logic, making it more versatile than @Secured, which primarily works with roles.

To implement the client credentials grant in a Spring Boot application, the client must send a request to the token endpoint with the client_credentials grant type. This grant type is used when the client, typically a service or application, needs to authenticate itself directly with the authorization server to obtain an access token. The other options are different OAuth2 grant types used for various scenarios.

To ensure fault tolerance and resilience in Spring Cloud microservices, you should implement Circuit Breaker patterns using tools like Hystrix. This helps prevent cascading failures and allows graceful degradation when a service is experiencing issues.

The primary role of a Resource Server in OAuth2 is to protect and serve protected resources. It validates access tokens presented by clients and enforces access control to ensure that only authorized clients can access protected resources. It does not generate access tokens (which is the responsibility of the Authorization Server), authenticate users, or store user credentials or data.

In OAuth2, the "Resource server" is responsible for serving protected resources to the client after successful authentication and authorization. It validates access tokens and ensures that the client has the necessary permissions to access the requested resources.

In a Spring Boot e-commerce application, the best approach to handle exceptions during inventory updates (Option 4) is to implement a centralized exception handling strategy with custom exception classes. This approach ensures proper rollback mechanisms and data integrity. Using try-catch blocks (Option 1) for local handling is not recommended for such critical operations. Logging failures and returning generic messages (Option 2) is insufficient for maintaining data integrity. Propagating exceptions (Option 3) without proper handling is also not ideal.

To implement custom exception handling in a Spring Boot application with different response bodies for each exception type while ensuring unhandled exceptions are addressed, you can use the @ExceptionHandler annotation on controller methods. This approach allows you to handle specific exceptions with custom logic while ensuring unhandled exceptions are still processed. Using a global exception handler may not address specific exception types adequately.