To optimize the JVM for a Spring Boot application, you should tune garbage collection settings. Garbage collection optimization is crucial because it helps manage memory efficiently. You can adjust parameters like heap size, garbage collection algorithms, and pause times to match your application's needs. Disabling garbage collection is not a practical solution as it will lead to memory issues. Minimizing the heap size without considering application requirements can result in performance problems. Using a custom class loader may have specific use cases but isn't a general JVM optimization technique.

In Spring Boot, the "@ConditionalOnProperty" annotation is used to conditionally enable a configuration based on the presence of a specific property. This annotation allows you to configure components or beans based on the values of properties, making it a powerful tool for conditional configuration in your application.

The deleteById method of a JpaRepository deletes an entity from the database by its primary key. It is a convenient method for removing specific entities based on their unique identifier. The other options do not accurately describe the behavior of this method; in particular, it does not mark the entity as deleted without removing it from the database.

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.