Understanding How Spring MVC's @RequestMapping POST Works

Question

I have a simple Controller that looks like this:-

@Controller
@RequestMapping(value = "/groups")
public class GroupsController {
    // mapping #1
    @RequestMapping(method = RequestMethod.GET)
    public String main(@ModelAttribute GroupForm groupForm, Model model) {
        ...
    }

    // mapping #2
    @RequestMapping(value = "/{id}", method = RequestMethod.GET)
    public String changeGroup(@PathVariable Long id, @ModelAttribute GroupForm groupForm, Model model) {
        ...
    }

    // mapping #3
    @RequestMapping(method = RequestMethod.POST)
    public String save(@Valid @ModelAttribute GroupForm groupForm, BindingResult bindingResult, Model model) {
        ...
    }
}

Basically, this page has the following functionalities:-

• User visits main page (/groups GET).
• User creates a new group (/groups POST) or selects a specific group (/groups/1 GET).
• User edits an existing group (/groups/1 POST).

I understand how both GET request mappings work here. Mapping #2 is defined, otherwise (/groups/1 GET) will cause a "No mapping found" exception.

What I'm trying to understand here is why mapping #3 handles both (/groups POST) and (/groups/1 POST)? It makes sense that it should handle (/groups POST) here since the request mapping matches the URI. Why (/groups/1 POST) isn't causing a "No mapping found" exception being thrown here? In fact, it almost seems like any POST with URI beginning with /groups (ex: /groups/bla/1 POST) will also be handled by mapping #3.

Can someone provide a clear explanation of this to me? Thanks much.

CLARIFICATION

I understand the fact that I can use more appropriate methods (like GET, POST, PUT or DELETE)... or I can create yet another request mapping to handle /groups/{id} POST.

However, what I want to really know is...

.... "Why does mapping #3 handle /groups/1 POST too?"

The "closest match" reasoning don't seem to hold true because if I remove mapping #2, then I would think mapping #1 will handle /groups/1 GET, but it doesn't and it causes a "No mapping found" exception.

I'm just a little stumped here.

Answer 1

This is complicated, I think it is better to read the code.

In Spring 3.0 The magic is done by method public Method resolveHandlerMethod(HttpServletRequest request) of the inner class ServletHandlerMethodResolver of org.springframework.web.servlet.mvc.annotation.AnnotationMethodHandlerAdapter.

An instance of this class exists for every Request Controller Class, and has a field handlerMethods that contains a list of all the request methods.

But let me summarize how I understand it

• Spring first checks if at least one handler method matches (this can contain false negatives)
• Then it creates a map of all really matching handler methods
• Then it sorts the map by request path: RequestSpecificMappingInfoComparator
• and takes the first one

The sorting works this way: the RequestSpecificMappingInfoComparator first compares the path with the help of an AntPathMatcher, if two methods are equal according to this, then other metrics (like number of parameters, number of headers, etc.) are taken into account with respect to the request.

Answer 2

Spring tries to find the mapping which matches the closest.
Hence, in your case of any POST request, the only map found for the request type is Mapping# 3. Neither of Mapping 1 or Mapping 2 matches your request type, and hence are ignored. May be you can try removing the Mapping #3, and see that Spring throws a runtime error since it does not find a match!

Answer 3

I would add a PUT mapping for /groups/{id}. I guess POST would work too but not strictly correct from a HTTP perspective.

adding @RequestMapping("/{id}", POST) should cover it?

Answer 4

add @PathVariable to the Long id parameter in mapping #2