The @Secured annotation in Spring Security is used to secure methods based on role-based conditions. You can specify the required roles in the annotation, and only users with those roles can access the method.

In a Spring Boot application, to secure microservices using OAuth2 and JWT, the best practice is to secure each microservice individually. Each microservice should implement OAuth2 and JWT security, and a centralized authentication server can be used for token validation. This approach ensures that each microservice has its own security context and can enforce its own authorization rules. Securing all microservices with a single token is not recommended, as it can lead to security vulnerabilities if one microservice is compromised.

The @SpringBootApplication annotation in Spring Boot is a convenience annotation that adds all of the following: @Configuration, @EnableAutoConfiguration, and @ComponentScan. Thus, it enables component scanning, allowing Spring to automatically discover and register beans. This is crucial for allowing the Spring context to be aware of all the components, services, repositories, etc. available in the project.

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.

Spring Cloud and Eureka work together for service discovery by adding the @EnableDiscoveryClient annotation to the Spring Boot application class. This annotation enables the application to register itself with the Eureka server and discover other services.

In Spring Boot, the "@Primary" annotation is used to designate a specific bean as the primary candidate for autowiring when there are multiple candidates for the same type. This is particularly useful when you have multiple beans of the same type, and you want to specify which one should be injected by default. The other options are related to autowiring but do not serve this specific purpose.