Custom validators are preferred when complex validation logic is required, which can't be achieved with standard Bean Validation annotations. While standard annotations are suitable for many cases, custom validators are necessary for scenarios where specific and intricate validation rules are needed. Custom validators may increase code complexity but allow for highly tailored validation logic.

In Mockito, you can use verify along with times(callCount) to verify that a method was called a specific number of times. This is useful for testing the behavior of methods.

To validate a field based on multiple conditions or constraints in a Spring Boot application, you can combine multiple annotations like @Min, @Max, and @Pattern. These annotations allow you to define various rules for a single field. Creating a custom validator class for each condition (Option 3) would be cumbersome and is not the recommended approach.

For a mobile application that needs to access services on behalf of the user, the Authorization Code Grant is most suitable. This grant type involves a redirection-based flow where the user authenticates themselves on the authorization server, and the mobile app receives an authorization code, which can be securely exchanged for an access token. This is a more secure approach compared to the Implicit Grant, which is suitable for browser-based apps. The other grant types are not typically used for mobile apps accessing on behalf of the user.

The @PreAuthorize annotation is used to apply security constraints at the method level based on SpEL (Spring Expression Language) expressions. You can define complex conditions using SpEL to control method access. The other options are used for filtering, not method-level security.

You can manage bean lifecycle events, such as initialization and destruction, in Spring Boot by using the @Bean annotation along with @PostConstruct and @PreDestroy methods. These methods allow you to specify custom initialization and destruction logic for your beans. The other options mentioned (XML configuration, @Service with initMethod and destroyMethod, and configuring lifecycles in the main application class constructor) are not the recommended or common approaches for managing bean lifecycles in Spring Boot.

In a Spring Cloud application, you can customize Ribbon's load-balancing strategy by implementing a custom IRule and configuring it as a bean. Ribbon uses IRule to determine which instance to route a request to. By creating a custom IRule, you can define your own load-balancing logic. The other options are not used to customize Ribbon's load-balancing strategy. The @LoadBalanced annotation is used to enable client-side load balancing with RestTemplate. The ribbon.strategy and ribbon.loadBalancer properties are not used for customizing the load-balancing strategy directly.

In Spring Boot, the interface used to create custom validators for a class is Validator. You can implement this interface to define custom validation logic for your domain objects. It allows you to specify the conditions under which an object is considered valid. The other options (Validatable, ValidationInterface, and SpringValidator) are not standard Spring Boot interfaces for creating custom validators.

To create a Spring Boot project with desired configurations, the Spring Initializr website provides a user-friendly interface. Spring Initializr allows developers to select project settings, dependencies, and configurations, and it generates a project structure accordingly. This simplifies the process of setting up a Spring Boot project.

In Spring Boot, the "application.yml" file is commonly used to configure the application's settings. This YAML file allows users to define various properties that control the behavior of the Spring Boot application. While other file formats like .properties are also used, "application.yml" is the most common in modern Spring Boot projects.