To conditionally apply caching logic in Spring Boot, developers use the @Cacheable annotation. This annotation allows them to specify conditions under which the caching logic should be applied, typically by providing a SpEL (Spring Expression Language) expression. It's a powerful tool for selectively caching method results.

In Spring Boot, the @ConditionalOnProperty annotation is used to conditionally enable or disable certain parts of auto-configuration based on the presence or absence of specific properties in the application.properties file. This provides fine-grained control over which auto-configuration options are activated based on the application's configuration. The other annotations serve different purposes in Spring Boot.

In Spring, the process of creating an instance of a bean, wiring it up, and making it available for use is called "Dependency Injection." This core concept of Spring allows for the automatic injection of dependencies into a class, making it more flexible and easier to manage. Inversion of Control (IoC) is a broader concept that encompasses Dependency Injection. Aspect-Oriented Programming (AOP) and Bean Configuration are related but not the exact terms used for this specific process.

The @Bean annotation is used to define a bean in the Spring context. When you annotate a method with @Bean, it tells Spring that the method should be used to create and configure a bean. This is commonly used for defining custom beans in Java-based Spring configurations. The other annotations (@Inject, @Component, and @Service) have different purposes and are not used for defining beans in the same way as @Bean.

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 create efficient custom auto-configurations in Spring Boot, it's essential to minimize the use of conditionals, as excessive conditionals can increase complexity and impact startup time negatively. Additionally, you can leverage lazy initialization to defer the creation of beans until they're needed, optimizing startup. Breaking auto-configuration into smaller, modular configurations also helps improve maintainability and startup time. Avoiding unnecessary dependencies is crucial, as adding more dependencies can increase the startup time.