Mockito is primarily used to create mock objects, which simulate the behavior of real objects in a controlled way. Mock objects are helpful for isolating the code being tested and verifying interactions between objects.

Bean Validation can be applied to method parameters in Spring Boot by annotating the method parameters with validation annotations such as @Valid. This allows you to validate the input parameters of a method and apply validation rules to them.

The primary advantage of using Spring Cloud Config Server in a microservices environment is centralized configuration management. It allows you to store configuration properties externally and manage them in a centralized manner, making it easier to update and maintain configurations across multiple services.

The primary role of the @Transactional annotation in Spring Data JPA is to manage database transactions. It marks a method, class, or even an interface to indicate that a transaction should be created and managed around the annotated method or methods. This ensures data consistency by committing changes if everything succeeds or rolling back if an exception occurs during the annotated operation. It is essential for maintaining data integrity in a Spring Data JPA application.

Converting a traditional blocking application to a non-blocking reactive application using WebFlux in Spring Boot comes with several challenges. These challenges include managing backpressure to prevent data overflow, understanding reactive operators to compose and transform data streams effectively, and adapting to the asynchronous nature of reactive programming. It is not necessary or practical to eliminate all database calls and replace them with external REST API calls when transitioning to reactive programming. Ensuring strict blocking behavior contradicts the non-blocking nature of WebFlux. Replacing all reactive components with traditional blocking components would defeat the purpose of adopting reactive programming.

Maven is the primary build tool used for Spring Boot projects by default when generating a project using start.spring.io. Spring Boot favors Maven as the build tool due to its wide adoption and robust capabilities for managing dependencies and building projects. Other build tools like Gradle can be used but are not the default choice.

The @DataJpaTest annotation is used for testing the data access layer of a Spring Boot application. It typically scans and loads data access components such as repositories and entity classes, enabling database-related testing.

In a Spring Boot application, handling different databases and schemas among microservices can be achieved using Spring Boot's multi-datasource support. This allows you to configure multiple datasources and associate them with specific microservices. Creating separate applications for each microservice would lead to unnecessary complexity. Sharing a single database and schema can cause conflicts and scalability issues. Using a NoSQL database is an option but might not always be suitable depending on the application's requirements.

In Spring Boot, the @Bean annotation is used to define a bean that holds business logic. When you use @Bean, you can configure and customize the creation of the bean, making it suitable for holding the application's business logic. The other annotations (@Component and @Service) are used for different purposes like component scanning and service layer, respectively.

In Spring Boot, "Reactor" allows developing reactive applications by providing an alternative to the traditional, servlet-based, blocking architecture. Reactor is a foundational framework for reactive programming in Java and is used extensively in Spring's reactive stack. It provides the building blocks for creating non-blocking, event-driven applications.