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, the module that enables the development of reactive applications is spring-webflux. This module provides a foundation for building reactive, non-blocking applications. It includes support for creating reactive RESTful services and interacting with reactive data sources. Reactive programming is particularly useful for handling high concurrency and low latency scenarios.

To integrate a custom authentication provider in Spring Security, you should add a custom AuthenticationProvider bean to the Spring application context and configure it in the security configuration. This allows Spring Security to use your custom logic for authentication. Options 1 and 4 are not correct; they do not represent the standard way of integrating custom authentication providers. Option 2 is also incorrect as authentication providers are typically configured programmatically, not in properties files.

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.

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.

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.

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.

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.

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.

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.