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.

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.

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.

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.

Garbage Collection optimization is significant in Spring Boot because it can reduce memory usage. Inefficient garbage collection can lead to increased memory consumption, longer pauses, and application slowdowns. By optimizing garbage collection settings and strategies, you can reduce memory overhead, minimize pause times, and improve overall application performance. Garbage Collection does not directly impact database performance or network communication speed in Spring Boot applications.

In Spring Boot's reactive streams, back pressure can be handled by using operators like onBackpressureBuffer. Back pressure is a mechanism that allows consumers to signal producers to slow down when they are overwhelmed with data. The onBackpressureBuffer operator is used to buffer excess items when the downstream subscriber can't keep up, preventing data loss and allowing the system to handle the flow of data efficiently.

To ensure that a Spring Boot application does not interact with external systems during integration testing, you can use the @SpringBootTest annotation with the webEnvironment set to SpringBootTest.WebEnvironment.NONE. This avoids starting a real HTTP server, preventing external communication. The other options do not directly address this issue.

To reduce the memory footprint of a Spring Boot application, you should minimize the use of Spring Boot starters. While starters are convenient, they often include many dependencies that may not be required for your specific application. By selectively including only the dependencies you need, you can reduce the memory overhead and improve the startup time of your application. This approach is especially valuable in microservices architectures where memory efficiency is critical.

The Spring Cloud component Zuul provides a simple, scalable, and flexible way to route API requests to microservices. Zuul is an API Gateway that can be used for routing and filtering requests to microservices.

In Spring Boot, the @CacheRefresh annotation is used to update the cache whenever the underlying data changes. It's used to refresh the cache entries for a specific method or cache name. The other options are related to cache eviction, clearing, or invalidation but not specifically refreshing the cache upon data changes.

When defining a global exception handler in Spring Boot, you can use the Throwable argument in the exception handling method to access the details of the occurred exception. This argument allows you to inspect and respond to exceptions in a generic way. The other options do not represent the correct argument type for accessing exception details.

Method-level security annotations in Spring Security are implemented by using @EnableMethodSecurity in a @Configuration class and applying @PreAuthorize and @PostAuthorize annotations on methods. Options 1 and 4 are incorrect, while Option 2 is partially correct but not the recommended approach.