Spring MVC - Why not able to use @RequestBody and @RequestParam together

Question

Using HTTP dev client with Post request and Content-Type application/x-www- form-urlencoded

1) Only @RequestBody

URL: localhost:8080/SpringMVC/welcome
Body: name=abc

@RequestMapping(method = RequestMethod.POST)
public String printWelcome(@RequestBody String body, Model model) {
    model.addAttribute("message", body);
    return "hello";
}
// Gives body as 'name=abc' as expected

2) Only @RequestParam

URL: localhost:8080/SpringMVC/welcome
In Body - name=abc

@RequestMapping(method = RequestMethod.POST)
public String printWelcome(@RequestParam String name, Model model) {
    model.addAttribute("name", name);
    return "hello";
}
// Gives name as 'abc' as expected

3) Both together

URL: localhost:8080/SpringMVC/welcome
Body: name=abc

@RequestMapping(method = RequestMethod.POST)
public String printWelcome(
    @RequestBody String body, 
    @RequestParam String name, Model model) 
{
    model.addAttribute("name", name);
    model.addAttribute("message", body);
    return "hello";
}
// HTTP Error Code 400 - The request sent by the client was syntactically incorrect.

4) Above with params position changed

URL: localhost:8080/SpringMVC/welcome
Body: name=abc

@RequestMapping(method = RequestMethod.POST)
public String printWelcome(
    @RequestParam String name, 
    @RequestBody String body, Model model) 
{
    model.addAttribute("name", name);
    model.addAttribute("message", body);
    return "hello";
}
// No Error. Name  is 'abc'. body is empty

5) Together but get type url parameters

URL: localhost:8080/SpringMVC/welcome?name=xyz
Body: name=abc

@RequestMapping(method = RequestMethod.POST)
public String printWelcome(
    @RequestBody String body, 
    @RequestParam String name, Model model) 
{
    model.addAttribute("name", name);
    model.addAttribute("message", body);
    return "hello";
}
// name is 'xyz' and body is 'name=abc'

6) Same as 5) but with parameters position changed

@RequestMapping(method = RequestMethod.POST)
public String printWelcome(
    @RequestParam String name, 
    @RequestBody String body, Model model) 
{
    model.addAttribute("name", name);
    model.addAttribute("message", body);
    return "hello";
}
// name = 'xyz,abc' body is empty

Can someone explain this behaviour?

Answer

The @RequestBody javadoc states

Annotation indicating a method parameter should be bound to the body of the web request.

It uses registered instances of HttpMessageConverter to deserialize the request body into an object of the annotated parameter type.

And the @RequestParam javadoc states

Annotation which indicates that a method parameter should be bound to a web request parameter.

1. Spring binds the body of the request to the parameter annotated with @RequestBody.

2. Spring binds request parameters from the request body (url-encoded parameters) to your method parameter. Spring will use the name of the parameter, ie. name, to map the parameter.

3. Parameters are resolved in order. The @RequestBody is processed first. Spring will consume all the HttpServletRequest InputStream. When it then tries to resolve the @RequestParam, which is by default required, there is no request parameter in the query string or what remains of the request body, ie. nothing. So it fails with 400 because the request can't be correctly handled by the handler method.

4. The handler for @RequestParam acts first, reading what it can of the HttpServletRequest InputStream to map the request parameter, ie. the whole query string/url-encoded parameters. It does so and gets the value abc mapped to the parameter name. When the handler for @RequestBody runs, there's nothing left in the request body, so the argument used is the empty string.

5. The handler for @RequestBody reads the body and binds it to the parameter. The handler for @RequestParam can then get the request parameter from the URL query string.

6. The handler for @RequestParam reads from both the body and the URL query String. It would usually put them in a Map, but since the parameter is of type String, Spring will serialize the Map as comma separated values. The handler for @RequestBody then, again, has nothing left to read from the body.