The key difference is that @RestController is specifically designed for RESTful APIs and returns data in JSON format by default, while @Controller is used for traditional MVC applications and returns HTML by default. Mixing them up can lead to unexpected results, so choosing the right annotation is crucial for the desired functionality.

When creating a Custom Validator in Spring Boot, the isValid method must return void to indicate whether the value meets the constraint. The isValid method is used to perform the validation logic, and it should not return a boolean value directly. Instead, it should use the provided ConstraintValidatorContext to report validation errors.

In a traditional Spring MVC application, the @Controller annotation is used to define a controller class that handles HTTP requests. It interacts with the view by forwarding control to a specific view template based on the request mapping defined in the controller method. The controller doesn't directly render HTML/JSP content or generate view templates dynamically; it simply controls the flow between the request and the view template. It can also handle AJAX requests if configured accordingly.

To ensure that security configurations are loaded in the correct order, you can use the @Order annotation (Option 1) on your security configuration classes. This allows you to specify the order in which they are loaded. The order of annotation evaluation is indeed controllable in Spring Security, and it's essential for enforcing security constraints correctly. The other options are not relevant for controlling configuration order.

In Spring Security, the method used to configure HTTP security is configure(HttpSecurity http). This method allows you to define security rules for HTTP requests, such as authentication, authorization, and access control. While the other options may sound plausible, configure(HttpSecurity http) is the standard method name for this purpose.

When the @Valid annotation is used in Spring Boot controller methods, it serves the purpose of validating incoming request data. This annotation is typically applied to method parameters, such as a @RequestBody, to trigger validation of the request body based on the validation rules defined for the associated class. It ensures that incoming data is valid according to the specified constraints. The other options are not the correct purposes of the @Valid annotation.

To optimize the garbage collection in the JVM for a Spring Boot application, developers can configure the -XX:MaxGCPauseMillis option in JVM parameters. This option allows developers to specify a target maximum pause time for garbage collection operations. By setting an appropriate value for this option, developers can fine-tune garbage collection behavior to minimize application pauses, ensuring smoother and more predictable application performance. Proper garbage collection configuration is essential for maintaining optimal application responsiveness and resource utilization.

To configure Spring Security to authenticate users using multiple providers, you would typically use AuthenticationManagerBuilder with authenticationProvider() to specify each authentication provider. This allows Spring Security to check against multiple providers for authentication. The other options are not standard practices for achieving this goal.

To configure a custom method security expression handler, you need to implement the MethodSecurityExpressionHandler interface, create a bean of it, and register it in the Spring context. This allows you to define custom security expressions for method-level security checks.

To stream data from the server to the client as soon as it's available in a Spring Boot application using reactive programming principles, you should use WebSocket or Server-Sent Events (SSE). WebSocket and SSE allow for real-time data push to the client, unlike the other options, which involve more traditional request-response mechanisms or polling, which may not be as efficient for real-time updates.