In Spring Boot, you can handle exceptions thrown by a controller method using the @ExceptionHandler annotation. This annotation allows you to define methods that can handle specific exceptions or exception types. Using System.out.println() statements is not the recommended way to handle exceptions, and not handling exceptions is not a valid approach. Defining a separate error controller is a possible strategy but less commonly used.

The @Valid annotation in a method signature within a Controller enables request parameter validation for all parameters of the annotated method. This ensures that incoming request parameters are validated against defined constraints before the method is executed. Option 2 is not accurate as the @Valid annotation itself doesn't control whether the method is called or not. Option 3 is incorrect as @Valid is related to input validation, not response validation. Option 4 is also incorrect.

In a Spring Boot application, the @EnableOAuth2AuthorizationServer annotation is used to enable OAuth2 Authorization Server features. This annotation is crucial when you want your Spring Boot application to act as an OAuth2 Authorization Server. It allows you to configure and provide OAuth2 tokens to clients securely.

You can create a custom query method in a Spring Data JPA repository by using the @Query annotation with JPQL (Java Persistence Query Language). This allows you to define custom queries in your repository interface. Extending the CrudRepository interface provides basic CRUD operations but doesn't allow you to create custom queries directly. The @NamedQuery annotation is used for predefined queries in the entity class, and there's no @CustomQuery annotation in Spring Data JPA.

Reactive Streams in Spring Boot offer an "asynchronous approach" to handle the flow of data between the producer and consumer. Reactive Streams are designed for asynchronous, non-blocking processing of data and provide a standardized way to deal with data streams, making it easier to handle data in a reactive and efficient manner.

When using @WebMvcTest, the security configurations from the main application apply by default. You should be aware that the security settings of the application being tested will be active, and you may need to adjust your tests accordingly. Options 1, 3, and 4 do not accurately describe how security configurations work in @WebMvcTest.

To enable method-level security in Spring Security, you need to use the @EnableGlobalMethodSecurity annotation on a configuration class. It's used to activate annotations like @Secured, @PreAuthorize, etc., for method-level security.

In Spring Boot, to define a global default exception handler for unhandled exceptions, you use the @ControllerAdvice annotation. This annotation allows you to define a class that can be applied globally to handle exceptions across all controllers. It provides a way to centralize exception handling in your application.

You can change the default port number of the embedded web server in a Spring Boot application by modifying the server.port property in the application.properties file. This property allows you to specify the desired port number. Spring Boot automatically configures the embedded web server (e.g., Tomcat) based on this property, making it a straightforward way to control the server's port.

To address the requirement of providing custom error responses from individual controllers while maintaining the functionality of the global exception handler in a complex Spring Boot application, you can implement individual @ExceptionHandler methods in each controller. This approach allows for controller-specific exception handling while still benefiting from the global exception handler for unhandled cases. The other options may not meet the requirement effectively.