Solved: HTTP POST with Json on Body - Flutter/Dart


Asked by Cláudio A on May 10, 2018 (source).

This is my code to make a request to an API:

import 'dart:async';
import 'dart:convert';
import 'dart:io';
import 'package:http/http.dart' as http;

Future<http.Response> postRequest () async {
  var url ='';
  var body = jsonEncode({ 'data': { 'apikey': '12345678901234567890' } });

  print("Body: " + body);,
      headers: {"Content-Type": "application/json"},
      body: body
  ).then((http.Response response) {
    print("Response status: ${response.statusCode}");
    print("Response body: ${response.contentLength}");


I have a problem with the response from the request, where its suppose to have a body with json, but something went wrong and i think is with the json that i send on the body request, because it is a nested json object, and the value of the key is a json object. i would love to know how i can parse the json right and insert into body of the request.

this is the header response:

 {set-cookie: JSESSIONID=DA65FBCBA2796D173F8C8D78AD87F9AD;path=/testes2/;HttpOnly, last-modified: Thu, 10 May 2018 17:15:13 GMT, cache-control: no-store, no-cache, must-revalidate, max-age=0, post-check=0, pre-check=0, date: Thu, 10 May 2018 17:15:13 GMT, content-length: 0, pragma: no-cache, content-type: text/html, server: Apache-Coyote/1.1, expires: Tue, 03 Jul 2001 06:00:00 GMT}

and this is how is suppose to be:

Server: Apache-Coyote/1.1
Expires: Tue, 03 Jul 2001 06:00:00 GMT
Last-Modified: Thu, 10 May 2018 17:17:07 GMT
Cache-Control: no-store, no-cache, must-revalidate, max-age=0, post-check=0, pre-check=0
Pragma: no-cache
Content-Type: application/json;charset=UTF-8
Vary: Accept-Encoding
Set-Cookie: JSESSIONID=84813CC68E0E8EA6021CB0B4C2F245BC;path=/testes2/;HttpOnly
Keep-Alive: timeout=5, max=100
Connection: Keep-Alive
Transfer-Encoding: chunked

the body response came empty and i think its because the body i sent on the request, can anyone help me with the nested json object in value??



Question answered by Richard H (source).

OK, finally we have an answer...

You are correctly specifying headers: {"Content-Type": "application/json"}, to set your content type. Under the hood either the package http or the lower level dart:io HttpClient is changing this to application/json; charset=utf-8. However, your server web application obviously isn't expecting the suffix.

To prove this I tried it in Java, with the two versions

conn.setRequestProperty("content-type", "application/json; charset=utf-8"); // fails
conn.setRequestProperty("content-type", "application/json"); // works

Are you able to contact the web application owner to explain their bug? I can't see where Dart is adding the suffix, but I'll look later.

EDIT Later investigation shows that it's the http package that, while doing a lot of the grunt work for you, is adding the suffix that your server dislikes. If you can't get them to fix the server then you can by-pass http and use the dart:io HttpClient directly. You end up with a bit of boilerplate which is normally handled for you by http.

Working example below:

import 'dart:convert';
import 'dart:io';
import 'dart:async';

main() async {
  String url =
  Map map = {
    'data': {'apikey': '12345678901234567890'},

  print(await apiRequest(url, map));

Future<String> apiRequest(String url, Map jsonMap) async {
  HttpClient httpClient = new HttpClient();
  HttpClientRequest request = await httpClient.postUrl(Uri.parse(url));
  request.headers.set('content-type', 'application/json');
  HttpClientResponse response = await request.close();
  // todo - you should check the response.statusCode
  String reply = await response.transform(utf8.decoder).join();
  return reply;

Depending on your use case, it may be more efficient to re-use the HttpClient, rather than keep creating a new one for each request. Todo - add some error handling ;-)