Why does a GET endpoint need a parameter in Spring Boot - spring

Question

I'm trying to learn the latest Spring Boot and am going through some of their documentation on handling form submission. I'm taking a look at the code for the controller they use for the GET that serves up the view containing the form, and which also handles capturing the information from the POST.
package hello;
import org.springframework.stereotype.Controller;
import org.springframework.ui.Model;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.ModelAttribute;
import org.springframework.web.bind.annotation.PostMapping;
#Controller
public class GreetingController {
#GetMapping("/greeting")
public String greetingForm(Model model) { // where does 'model' come from?
model.addAttribute("greeting", new Greeting());
return "greeting";
}
#PostMapping("/greeting")
public String greetingSubmit(#ModelAttribute Greeting greeting) {
return "result";
}
}
What I don't understand is how does the greetingForm(Model model) method take a parameter? The GET request surely isn't sending a whole Model over in its request, just the URI, right? Also, they don't list what the Model class code is, so I can't examine that.
This is their explanation:
The greetingForm() method uses a Model object to expose a new Greeting
to the view template. The Greeting object in the following code
contains fields such as id and content that correspond to the form
fields in the greeting view, and will be used to capture the
information from the form.
I also don't understand how just returning the string "greeting" translates into a view being served. Is there another layer which is actually calling this method?
I come from a Play! Framework background, and I'm used to my controller endpoints having to initialize and send the entire template back with the response - so this is confusing.

Answer 1

Spring does a lot of work on it's own to determine what to inject into controller handler methods and what to do with the return value.
From the Spring docs:
Handler methods annotated with this annotation can have very flexible signatures. The exact details of the supported method arguments and return values depend on the specific #Controller model supported. Both Spring Web MVC and Spring WebFlux support this annotation with some differences. More details are available in the Spring Framework reference.
Spring analyzes the arguments of the method. Model is a type that Spring understands, so it is injected into the method when called.
Handler methods can also have a variety of return types. When the return type is a String, Spring understands that to mean the name of a view to render and return to the client. This is also where the Model comes in; the values you put into the Model instance are bound to the view during rendering.