To externalize error messages for validation constraints in Spring Boot, you can use the message property in the constraint annotation, and then reference externalized messages using a message source, often defined in a properties file or through Spring's message source mechanisms. This approach makes it easier to manage and internationalize error messages.

To include additional information in the OAuth2 access token, you can implement a custom token enhancer by extending DefaultTokenServices. This allows you to manipulate the token content and add the desired information. The other options are not typically used for token enhancement.

The @Primary annotation in Spring is used to give a preference to a bean when multiple beans of the same type exist. It tells Spring which bean should be considered as the primary candidate for autowiring when there are multiple candidates of the same type. The other options, such as @Qualifier, @PreferencedBean, and @Priority, do not serve the same purpose as @Primary.

The @Repository annotation in Spring is used to mark a class as a repository, especially in the context of Spring Data JPA. It helps in automatic generation of database queries based on method names. It does not generate exceptions or specify the database schema. Its primary role is to enable Spring to manage database operations and perform exception translation when necessary. It is not related to creating RESTful web services.

In Spring Boot, you can handle database connection failures and retries by implementing retry mechanisms with libraries like Spring Retry. This allows your application to automatically retry failed database operations, enhancing resilience. Using a backup database or increasing the database timeout can be part of a broader strategy, but they don't directly address retries. Manually restarting the application is not a recommended approach for handling failures.

To handle exceptions locally within a controller in Spring Boot, you can use the @ExceptionHandler annotation on a method within a @Controller or @RestController. This annotation allows you to define methods that will handle specific exceptions thrown by the controller's methods.

Option 1 is correct. Profiling production Spring Boot applications can be done using tools like VisualVM or Java Flight Recorder without significant performance impact. These tools provide insights into application behavior and resource usage. Disabling all logging (Option 2) is not a recommended approach as logging is essential for debugging and monitoring. Increasing memory allocation (Option 3) may not help with profiling. Running load tests concurrently (Option 4) can further impact performance and is not ideal for profiling.

To create a custom Auto Configuration that provides beans conditionally based on the presence of a specific library on the classpath, you should use the @ConditionalOnClass annotation. Specify the library's class in the annotation's value attribute. This approach ensures that the beans are only created when the specified class is available on the classpath, ensuring the required conditions are met.

To implement database sharding, you need to analyze the data schema and partition data into smaller chunks that can be distributed across multiple database instances. This approach improves performance and scalability. Using a single database instance is not sharding and doesn't improve scalability. Implementing a load balancer helps distribute requests but isn't the core of sharding. Using a NoSQL database is an alternative approach, not sharding itself.

Using the @Import annotation to modularize and group related auto-configurations is a recommended approach to manage and optimize a large Spring Boot application with extensive custom configurations. This reduces complexity while maintaining the flexibility of customizations. The other options are not practical or advisable approaches to handling auto-configurations in a large Spring Boot application.