In Spring Boot's project structure, the application properties file should be placed in the "src/main/resources" directory with the filename "application.properties." This is the standard location for configuration properties in a Spring Boot application. Placing it elsewhere or with a different name may require custom configuration.

To validate an object graph with nested objects and associations using JSR-303 Bean Validation, you should use cascading validation by annotating the relevant fields or methods with @Valid. This ensures that the entire object graph is validated, including nested objects and associations. This is a standard and recommended approach in Spring Boot applications.

The primary role of the @RestController annotation in Spring Boot is to define a controller class. It marks a Java class as a controller and combines @Controller and @ResponseBody annotations. This annotation is used to create RESTful web services, and it simplifies the process of building REST APIs by eliminating the need to annotate individual methods with @ResponseBody. It doesn't specify the package structure or application version.

To authorize access to method-level security in Spring Security, the @PreAuthorize annotation can be used. This annotation allows you to specify expressions for controlling access to methods based on user roles or custom conditions. It's a powerful tool for fine-grained authorization control.

Enabling lazy initialization in a Spring Boot application slightly increases the startup time. Lazy initialization means that beans are only created when they are first requested, rather than at application startup. While this can save memory and improve startup times for large applications, there is a small performance penalty when the bean is first used, as it has to be created on-demand. Therefore, enabling lazy initialization can slightly increase the startup time due to the overhead of creating beans when they are needed.

The TestRestTemplate utility is primarily used for performing HTTP requests in integration tests in Spring Boot applications. It provides a simple and convenient way to send HTTP requests and receive responses in your tests.

To ensure correct handling of interactions with external services, you would use mock objects or libraries like WireMock to simulate the responses of external services. This approach allows you to control the behavior of the external services during testing without actually making real requests. Live testing against real external services is not recommended in automated testing as it introduces dependencies and can be unreliable. Skipping testing interactions or relying solely on documentation is not a robust testing strategy.

In Spring Cloud, Ribbon is a client-side load balancer that dynamically routes client requests to multiple instances of a service. This helps distribute the load evenly and provides fault tolerance by automatically rerouting requests if a service instance fails.

In JUnit, the @BeforeEach annotation is used to execute a method before each test method in the test class. This is often used for setup operations required before each test case.

To optimize the performance of a Spring Boot application, developers can use "Actuator" to profile and monitor the application in real-time. Spring Boot Actuator provides various production-ready features, including endpoints for monitoring and managing the application. These endpoints can be used to gather metrics, health information, and other runtime data, helping developers identify and address performance issues.