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How do I use GET method with sending json data in android?

API route in Python (Flask)
#app.route('/secret')
def secret():
if request.get_json(force=True)['key'] == 'secret key':
return jsonify(msg='Hello!')
It is working linux terminal
curl -iX GET -d '{"key":"secret key"}' localhost
Linux terminal output this
{"msg":"Hello!"}
It doesn't need to work in browser.
try{
HttpURLConnection connection = (HttpURLConnection)
new URL("http://<my local ip>/secret").openConnection();
connection.setRequestMethod("GET");
connection.setDoOutput(true);
connection.connect();
JSONObject jsonInput = new JSONObject();
jsonInput.put("key", "secret key");
OutputStream os = connection.getOutputStream();
byte[] input = jsonInput.toString().getBytes(StandardCharsets.UTF_8);
os.write(input, 0, input.length);
os.flush();
os.close();
BufferedReader br = new BufferedReader(new InputStreamReader(connection.getInputStream()));
StringBuilder response = new StringBuilder();
String responseLine = null;
while ((responseLine = br.readLine()) != null) {
response.append(responseLine.trim());
}
return response.toString();
} catch (IOException | JSONException e) {
Log.e("MainActivity", "Error: " + e.getMessage());
}
Although the GET method is set to the connection request in my codes, a POST request is being sent to the Python server.
Python Interpreter
Is it impossible to fix this?

Request Body is not recommended in HTTP GET requests. See HERE
A payload within a GET request message has no defined semantics;
sending a payload body on a GET request might cause some existing
implementations to reject the request.
When you try to write on a URL, you are implicitly POSTing on it despite you had set GET as the HTTP method. At below lines:
OutputStream os = connection.getOutputStream();
byte[] input = jsonInput.toString().getBytes(StandardCharsets.UTF_8);
os.write(input, 0, input.length);
For confirmation of my words see Writing to a URLConnection
writing to a URL is often called posting to a URL. The server
recognizes the POST request and reads the data sent from the client.

curl and Java program are treated differently by CouchDB

This question might be a little bit naive...
I have written a small Java program that should query a CouchDB instance. However, CouchDB constantly returns that my user is not authorized. Using the same URL with curl works.
The Java Code:
HttpGet request = new HttpGet("http://admin:PASSWORD#localhost:5984/ecm_ng_nonpart/_security");
request.addHeader("accept", "application/xml");
try {
System.out.println("Executing request " + request.getMethod() + " " + request.getUri());
ClassicHttpResponse resp = httpClient.execute(request);
BufferedReader br = new BufferedReader(new InputStreamReader(resp.getEntity().getContent()));
String line;
while ((line = br.readLine()) != null) {
System.out.println(line);
}
} catch (IOException e) {
e.printStackTrace();
}
The response:
{"error":"unauthorized","reason":"You are not authorized to access this db."}
The curl output:
C:\Users\joche>curl -X GET http://admin:PASSWORD#localhost:5984/ecm_ng_nonpart/_security
{"members":{"roles":["_admin"],"names":["ecmnp","admin"]},"admins":{"roles":["_admin"],"names":["admin"]}}
Any idea what I am doing wrong?

Using the same URL with curl works.
Your posted information does not support this claim. Your URLs are different in the two cases.
Java: _sessions
HttpGet request = new HttpGet("http://admin:PASSWORD#localhost:5984/ecm_ng_nonpart/_sessions");
Curl: _security.
curl -X GET http://admin:PASSWORD#localhost:5984/ecm_ng_nonpart/_security

JSON POST Data via HttpURLConnection

I am trying to make a request to my RESTful API using Android and HttpURLConnection. The data must be sent in the JSON format via POST data.
Here is my code:
JSONObject check_request = new JSONObject();
check_request.put("username", username);
JSONObject request = BuildRequest(check_request, "username_check", false);
Log.i("DEBUG", request.toString());
// DEBUG OUTPUT: {"timestamp":1526900318,"request":{"username":"blubberfucken","type":"username_check"}}
URL request_url = new URL(apiURL);
HttpURLConnection connection = (HttpURLConnection)request_url.openConnection();
connection.setRequestProperty("User-Agent", "TheGameApp");
connection.setRequestMethod("POST");
connection.setRequestProperty("Content-type", "application/json; charset=UTF-8");
connection.setDoOutput(true);
connection.setDoInput(true);
OutputStream os = connection.getOutputStream();
os.write(request.toString().getBytes("UTF-8"));
os.flush();
InputStream in = new BufferedInputStream(connection.getInputStream());
String result = "";
BufferedReader br = new BufferedReader(new InputStreamReader(in, "UTF8"));
String str;
while ((str = br.readLine()) != null)
{
result += str;
}
Log.i("DEBUG", result);
//JSONObject result_json = new JSONObject(result);
os.close();
in.close();
connection.disconnect();
You can see the Debug output as a Comment. The Problem is that the API does not receive any POST data. I have used PHPs var_dump to dump $_POST and $_REQUEST which both are empty arrays.
What am I missing here?
As the question popped up if the API work. This cURL command works fine with the correct result (it is the same JSON data as the debugger printed):
curl -d '{"timestamp":1526900318,"request":{"username":"blubberfucken","type":"username_check"}}' -H "Content-Type: application/json" -X POST http://localhost/v1/api.php

Just for the sake of completeness: The example above is working. The solution to the problem was pa part in PHP on the server side, where I checked the content type and used strpos to search for application/json in $_SERVER['CONTENT-TYPE'] and switched the needle and haystack (thus searching for application/json; charset=UTF8 in the string application/json instead of the other way around).

How to POST in Java

I have something that looks like this:
POST /o/oauth2/token HTTP/1.1
Host: accounts.google.com
Content-Type: application/x-www-form-urlencoded
grant_type=assertion&assertion_type=http%3A%2F%2Foauth.net%2Fgrant_type%2Fjwt%2F1.0%2Fbearer&assertion=eyJhbGciOiJSUzI1NiIs
How would I go about using this in Java? I have all the information already so I wouldn't need to parse it.
Basically I need to POST with 3 different data and using curl has been working for me but I need to do it in java:
curl -d 'grant_type=assertion&assertion_type=http%3A%2F%2Foauth.net%2Fgrant_type%2Fjwt%2F1.0%2Fbearer&assertion=eyJhbGciOiJSUzI1NiIsInR5i' https://accounts.google.com/o/oauth2/token
I cut off some data so its easier to read so it wont work.
So a big problem is that the curl would work while most tutorials I try for Java would give me HTTP response error 400.
Like should I be encoding the date like this:
String urlParameters = URLEncoder.encode("grant_type", "UTF-8") + "="+ URLEncoder.encode("assertion", "UTF-8") + "&" + URLEncoder.encode("assertion_type", "UTF-8") + "=" + URLEncoder.encode("http://oauth.net/grant_type/jwt/1.0/bearer", "UTF-8") + "&" + URLEncoder.encode("assertion", "UTF-8") + "=" + URLEncoder.encode(encodedMessage, "UTF-8");
or not:
String urlParameters ="grant_type=assertion&assertion_type=http://oauth.net/grant_type/jwt/1.0/bearer&assertion=" + encodedMessage;
Using this as the code:
URL url = new URL(targetURL);
URLConnection conn = url.openConnection();
conn.setDoOutput(true);
OutputStreamWriter writer = new OutputStreamWriter(conn.getOutputStream());
writer.write(urlParameters);
writer.flush();
String line;
BufferedReader reader = new BufferedReader(new InputStreamReader(conn.getInputStream()));
while ((line = reader.readLine()) != null) {
System.out.println(line);
}
writer.close();
reader.close();

Use something like HttpClient or similar.
It can post pre-URL-encoded data to a URI, although I don't know if you could just throw a complete request body at it--might need to parse it out, but there are likely libraries for that as well.

Here's a simple example of Apache HttpClient with request body from their docs (slightly modified to show how the execute works):
HttpClient client = new HttpClient();
PostMethod post = new PostMethod("http://jakarata.apache.org/");
NameValuePair[] data = {
new NameValuePair("user", "joe"),
new NameValuePair("password", "bloggs")
};
post.setRequestBody(data);
int returnCode = client.execute(post);
// check return code ...
InputStream in = post.getResponseBodyAsStream();
See the Apache HttpClient site for more info, examples and tutorials. This link might help you too.

Imgur API request using Java returns 400 status

I am trying to send a GET request to the Imgur API to upload an image.
When I use the following code I receive a 400 status response from the Imgur server - which, according to the Imgur error documentation, means I am missing or have incorrect parameters.
I know the parameters are correct as I have tested them directly in the browser URL (which successfully uploads an image) - so I must not be adding the parameters correctly within the code:
private void addImage(){
String data = URLEncoder.encode("image", "UTF-8") + "=" + URLEncoder.encode("http://www.lefthandedtoons.com/toons/justin_pooling.gif", "UTF-8");
data += "&" + URLEncoder.encode("key", "UTF-8") + "=" + URLEncoder.encode("myPublicConsumerKey", "UTF-8");
// Send data
java.net.URL url = new java.net.URL("http://api.imgur.com/2/upload.json");
URLConnection conn = url.openConnection();
conn.setDoOutput(true);
OutputStreamWriter wr = new OutputStreamWriter(conn.getOutputStream());
wr.write(data);
wr.flush();
// Get the response
BufferedReader rd = new BufferedReader(new InputStreamReader(conn.getInputStream()));
String line;
while ((line = rd.readLine()) != null) {
Logger.info( line );
}
wr.close();
rd.close();
}
This code is based on the API examples provided by Imgur.
Can anyone tell me what I am doing wrong and how I may resolve the problem?
Thanks.

In this sample, imgur service returns 400 Bad Request status response with a non-empty body because of incorrect API key. In case of non successful HTTP response you shold read the response body from an error input stream. For example:
// Get the response
InputStream is;
if (((HttpURLConnection) conn).getResponseCode() == 400)
is = ((HttpURLConnection) conn).getErrorStream();
else
is = conn.getInputStream();
BufferedReader rd = new BufferedReader(new InputStreamReader(is));
And, by the way your example is POST, not GET, because you are sending the parameters in the request body instead of the URL.