Explain the mechanism of using annotations in the Retrofit library for defining network requests.

In the Retrofit library, annotations are used to define HTTP request parameters directly in the API interface. Each interface method corresponds to a network request, and annotations specify the request type (GET, POST, etc.), path, parameters, and request body.

For example:

public interface ApiService {
    @GET("users/{id}")
    Call<User> getUser(@Path("id") int userId);

    @POST("users/new")
    Call<User> createUser(@Body User user);
}

Here, @GET and @POST define the request type and path, @Path inserts a parameter into the URL, and @Body indicates that the object will be serialized into the request body. Retrofit automatically constructs and executes HTTP requests based on these annotations, simplifying network operations.