why does the program need to check API header "Accept"

Question

I am using swagger codegen to generate the controller for me. And there is a if statement there but it seems to me it is not necessary. Thank you.

        String accept = request.getHeader("Accept");
        if (accept != null && accept.contains("application/json"))

Why do you think that it's not necessary? Its checking for the condition of `Accept` header being present and being `application/json`. This is pretty standard for json based api's. If you don't need it, then you can remove it though. — sarveshseri, Jul 21 '21 at 03:11
but there is a header ''Content-Type". why do i still need to check it? — timo, Jul 21 '21 at 03:25

chytonpide · Answer 1 · 2021-07-21T04:43:41.430

Content-Type: it is a header that tells the format of data sent in HTTP messages (both requests and responses).

Accept: it is a header that is placed in a request when requested from a browser (client) to a web server. This Accept header simply means that I(client) will only allow you this data type as a response.

If the server supports multiple data types, The server determines a data type of response using a accept header like this,

@GetMapping("/someresources")
public ResponseEntity<String> getSomeresources(@RequestHeader("accept") String accept) {
        
        List<SomeResource> someresources = someService.someresources();
        
        //for react app
        if(accept.contains("application/json")) {
            SomeresourcesRepresentation representation = new SomeresourcesRepresentation(someresources);
            String serialziedRepresentaiton = jsonSerializer.serialize(representation);
            
            ResponseEntity<String> response = ResponseEntity
                    .status(HttpStatus.OK)
                    .contentType(MediaType.APPLICATION_JSON)
                    .body(serialziedRepresentaiton);
            
            return response;
        } 

        //for web browser
        if(accept.contains("text/html")) {
            
            String html = "<!doctype html>"
                    + "<html>"
                    + "<head>"
                    + "<meta charset='UTF-8'>"
                    + "<title>summary</title>"
                    + "</head>"
                    + "<body>"
                    + "someresources size : "+someresources.size()
                    + "</body>"
                    + "</html>";
            
            ResponseEntity<String> response = ResponseEntity
                    .status(HttpStatus.OK)
                    .contentType(MediaType.TEXT_HTML)
                    .body(html);

            return response;
        }
        
        return ResponseEntity.notFound().build();
    }

Gaurav Jeswani · Accepted Answer · 2021-07-21T07:10:06.167

Your swagger document must have something like below to API method:

responses:
   "200":
     description: OK
     content:
       application/json:

As per this, the response of your API is of type application/json:. But additionally in future, if the server decides to produce some other type of response also as below:

 responses:
   "200":
     content:
       image/svg+xml:
         schema:
           type: string
       application/json:
         schema:
           $ref: "#/components/schemas/MyDto"

In this case to decide the response type in response Accept parameter would be necessary. So in my opinion there are 2 reasons for this condition to generate:

That Client and Server are in same contract for returning content.
If tomorrow new content type is added old code is not breaking.

why does the program need to check API header "Accept"

2 Answers2