In Spring Boot, the @PostMapping annotation is used to map HTTP POST requests onto specific handler methods in a controller class. When you apply this annotation to a method, it tells Spring that this method should be invoked when an HTTP POST request with a matching URL is received. It's a key annotation for handling POST requests in a RESTful API.

To implement a complex request mapping strategy with custom conditions, you would typically need to create a custom RequestMappingHandlerMapping. This allows you to define intricate conditions and behaviors for mapping requests to controller methods. Using annotations or configuration properties alone may not provide the level of customization needed for complex mappings. A separate utility class may not integrate seamlessly with Spring Boot's request handling mechanism.

In Spring Boot, the primary annotation used to mark the main class of your application is @SpringBootApplication. This annotation not only marks the class as the main entry point but also enables various Spring Boot features like auto-configuration, component scanning, and more. It's the starting point for your Spring Boot application.

Application properties in Spring Boot can be overridden for different environments by using environment-specific property files. By naming these files application-{profile}.properties (e.g., application-dev.properties for the 'dev' profile), Spring Boot can load the properties specific to the active profile, allowing you to configure different settings for each environment. This is a key feature for maintaining configurations across various deployment scenarios.

To create conditional beans within custom Auto Configuration, you can use the @ConditionalOnClass annotation. This annotation allows you to specify that a particular bean should be created only if a specified class is present in the classpath. It's useful for scenarios where you want to conditionally configure beans based on the availability of certain classes.

To perform unit testing on the web layer of a Spring Boot application without loading the complete context, you should use the @WebMvcTest annotation. This annotation focuses only on the web layer and is suitable for testing controllers.

In a Spring Boot application, the @Transactional annotation is used to demarcate transaction boundaries. It is applied to methods, indicating that the method should be wrapped in a transaction, ensuring that either all operations within the method succeed or none of them do. This is crucial for maintaining data consistency in the database.

The @PreAuthorize annotation in Spring Security allows for complex SpEL expressions to define fine-grained access control conditions. This means you can use expressions involving multiple variables and logic, making it more versatile than @Secured, which primarily works with roles.

The @SpringBootApplication annotation in Spring Boot is a convenience annotation that adds all of the following: @Configuration, @EnableAutoConfiguration, and @ComponentScan. Thus, it enables component scanning, allowing Spring to automatically discover and register beans. This is crucial for allowing the Spring context to be aware of all the components, services, repositories, etc. available in the project.

To implement the client credentials grant in a Spring Boot application, the client must send a request to the token endpoint with the client_credentials grant type. This grant type is used when the client, typically a service or application, needs to authenticate itself directly with the authorization server to obtain an access token. The other options are different OAuth2 grant types used for various scenarios.