You can customize the build steps in a Spring Boot project by modifying the build.gradle file (for Gradle) or the pom.xml file (for Maven) in the project directory. These build files allow you to define additional dependencies, plugins, and build tasks to tailor the project's build process to your specific needs. The other options are not the correct ways to customize build steps.

The primary role of an OAuth2 Authorization Server is to issue access tokens to authorized clients. These access tokens are used to authenticate and authorize requests made by clients to protected resources on behalf of the resource owner. While user authentication is a part of the OAuth2 flow, the primary function of the Authorization Server is to issue tokens.

To optimize a reactive Spring Boot application for handling a large number of simultaneous connections, it's crucial to reduce blocking operations and optimize database queries. This is because reactive applications excel at handling non-blocking, asynchronous tasks, and database interactions can be a common bottleneck. While increasing server resources may help, it won't address the underlying application inefficiencies. Using a reactive database driver can be beneficial but may not solve all performance issues. Decreasing the number of threads would likely worsen performance.

The @Transactional annotation in Spring Data JPA marks a method as transactional, ensuring that it runs within a database transaction. This annotation is crucial for maintaining data consistency and integrity in JPA-based applications. It helps manage transactions, including starting, committing, or rolling back when exceptions occur. The other options do not accurately represent the role of the @Transactional annotation in Spring Data JPA.

In Spring Boot, the @Repository annotation is used to indicate a component whose role is to represent a data repository. It is typically applied to classes that interact with a database, providing data access operations. @Component is a more general-purpose annotation, and @Entity is used to represent persistent entities in JPA. @Controller is used for defining controllers in Spring MVC.

In Spring Boot, to handle exceptions globally across multiple controllers, you can define a custom exception handler using the @ControllerAdvice annotation. This allows you to centralize exception handling logic and apply it across multiple controllers, promoting code reusability and maintainability. The other options do not provide a scalable and organized approach to handle exceptions globally.

In JUnit tests in Spring Boot, the Assert.assertEquals method is commonly used to assert that the actual result of a test meets the expected result. It is used to perform assertions and verify the correctness of your code.

In Spring Boot, the @ControllerAdvice annotation is used to define a class as global advice for all controllers. This class can contain methods annotated with @ExceptionHandler, @InitBinder, or @ModelAttribute, which are applied globally to controllers. It's a crucial mechanism for adding cross-cutting concerns, such as exception handling, to your Spring Boot application. The other options are not used for this purpose.

In JUnit, the @Test annotation is used to denote a test method. It marks a method as a test case that should be run by the JUnit test runner.

In a microservices architecture, managing configurations and secrets centrally is essential. Spring Cloud provides Spring Cloud Config Server, which allows you to store configurations in a centralized location. Options 1, 3, and 4 are not recommended practices and can lead to maintenance challenges.