In Spring Boot, to bind the method return value as the response body, you can use the @ResponseBody annotation. This annotation indicates that the return value of the method should be converted to JSON or another format and included in the HTTP response body. It's commonly used when you want to return data from a controller method in a RESTful web service.

The @Service annotation in Spring is used to define a class as a service bean, typically used for business logic. It has the same functionality as @Component but adds semantic value, indicating that the class is a service component. While @Component is a generic stereotype, @Service is specific to services, making the code more expressive and helping developers understand the role of the class. It doesn't provide security features or make the class a database entity.

In a microservices architecture, service discovery is crucial. It allows microservices to locate and communicate with each other independently, promoting autonomy and independence.

The @SpringBootApplication annotation in Spring Boot includes several other annotations, such as @Configuration, @EnableAutoConfiguration, and @ComponentScan. It is the primary annotation to enable a Spring Boot application and combines various configuration and component scanning annotations. While other options may exist as individual annotations, @SpringBootApplication is the one that encompasses them all in the context of a Spring Boot application.

In Spring Boot, the application.properties file is commonly used to define database connection properties. This file allows you to configure various aspects of your Spring Boot application, including database-related settings such as connection URLs, usernames, and passwords. The other options are not typically used for defining database connection properties.

In Spring Boot, to display specific error messages for validation errors, you can configure a custom message source and associate it with the validation framework. This allows you to define your custom error messages for validation constraints, providing better user feedback. The other options either rely on defaults, which may not meet specific requirements, or involve non-standard practices.

To customize error messages in JSR-303 Bean Validation, you use the @Message attribute of the constraint annotation. This allows you to provide a custom error message when a validation constraint is violated. The other options (@ErrorMsg, @MessageCode, and @MessageSource) do not exist as standard attributes for customizing error messages in JSR-303.

To make a method in a @Controller annotated class in Spring Boot write directly to the response body, you should use the @ResponseBody annotation. This annotation is used to indicate that the return value of the method should be serialized directly to the HTTP response body. The other options have different purposes, such as defining request mappings, specifying controller types, or handling path variables.

To optimize the performance of Spring Data JPA repositories with large datasets, it's essential to write custom, optimized SQL queries using the @Query annotation. Custom queries can be tailored for specific retrieval needs, often improving performance over auto-generated queries. While the other options can contribute to performance, writing optimized queries is the most direct way to address large dataset performance concerns.

In Spring Boot, the @EnableAuthorizationServer annotation is used to enable OAuth2 Authorization Server capabilities. It allows the application to act as an OAuth2 authorization server, handling client registration, token issuance, and other authorization-related tasks.

In Spring Boot reactive programming, the DeferredResult class is used to represent an asynchronous computation result. It allows a controller to start processing a request and return a DeferredResult immediately, deferring the actual result processing to a later time. This is useful for handling long-running or asynchronous tasks in a non-blocking manner.

Option 1 is the most common approach. Mockito is a popular Java mocking framework that allows you to mock external dependencies, isolating the microservice being tested.