Javascript fetch is calling backend twice

Question

I have backend app written in java ee and jersey. When I posting data to my rest endpoint, using JavaScript Fetch API I see that the filter is fired twice. Once it haven't authorization header, second it have. When I try open my site from web browser then this filter is called only once. Why this happened. Maybe it's because of CORS?

below my logs from single post both printed from the same filter.

 http://localhost:8080/BlogRest/controller/endpoint/|#]
  Key=host, value=localhost:8080|#]
  Key=origin, value=http://localhost:3000|#]
  Key=access-control-request-method, value=POST|#]
  Key=content-length, value=0|#]
  Key=access-control-request-headers, value=authorization,content-type|#]
  Key=connection, value=keep-alive|#]
  Key=accept, value=*/*|#]
  Key=user-agent, value=user agent data|#]
  Key=referer, value=http://localhost:3000/|#]
  Key=accept-language, value=pl-pl|#]
  Key=accept-encoding, value=gzip, deflate|#]

second call

  http://localhost:8080/BlogRest/controller/endpoint/|#]
  Key=host, value=localhost:8080|#]
  Key=origin, value=http://localhost:3000|#]
  Key=content-type, value=application/json|#]
  Key=accept-language, value=pl-pl|#]
  Key=accept-encoding, value=gzip, deflate|#]
  Key=connection, value=keep-alive|#]
  Key=accept, value=*/*|#]
  Key=user-agent, value=user agent data|#]
  Key=authorization, value=Bearer token|#]
  Key=referer, value=http://localhost:3000/origin|#]
  Key=content-length, value=15|#]

Because of CORS, `fetch()` first runs `HEAD /path` to get headers, then `[GET/POST/PUT/DELETE] /path` to actually perform the request — 0xLogN, Mar 14 '21 at 03:04
is there way to set filter to not be called when fetch is requesting for headers? I mean this first call. — Bogus, Mar 14 '21 at 03:11

score 0 · Accepted Answer · answered Mar 14 '21 at 12:55

There are two requests because of CORS. One request method is OPTIONS, and when it is preformed then starts expected POST request. OPTIONS request doesn't call the endpoint, it just get te server configuration. So I filtered in my Java filter request by method using HttpServletRequest getMethod().

    @Override
    public void doFilter(ServletRequest request, ServletResponse response,
            FilterChain chain)
            throws IOException, ServletException {

        HttpServletRequest requ = (HttpServletRequest) request;

        if (requ.getMethod().toLowerCase().equals(HttpMethod.OPTIONS)) {
            chain.doFilter(request, response);
            return;
        }

        //do something on not options method.            

        chain.doFilter(request, response);

    }

Javascript fetch is calling backend twice

1 Answers1