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HTTPURLConnection write command works only with read

I am writing a file into AWS S3 bucket using HTTPURLConnection. This connection's outputstream-write command works only if it is followed by getInputStream or getResponseCode. Please help me to understand this. Is it required to read responseCode?
URL endpointUrl = new URL("muURL");
HttpURLConnection connection = (HttpURLConnection) endpointUrl.openConnection();
connection.setRequestMethod("PUT");
connection.setRequestProperty("Authorization", authorization);
connection.setRequestProperty("x-amz-content-sha256", hashPayload);
connection.setRequestProperty("X-Amz-Date", myDate);
connection.setRequestProperty("Content-Type","application/octet-stream");
connection.setRequestProperty("Content-Length",contentLen);
//connection.setRequestProperty("Accept","*/*");
connection.setUseCaches(false);
//connection.setDoInput(true);
connection.setDoOutput(true);
DataOutputStream wr = new DataOutputStream(connection.getOutputStream());
wr.writeBytes(payload);
StringBuilder builder = new StringBuilder();
// If I comment this line which gets responseCode , then file is not written in S3.
builder.append(connection.getResponseCode());
wr.flush();
wr.close();
connection.disconnect();
I am expecting this code to work without getResponseCode line.

Java doesn't send HTTP POST Request

I'm implementing some simple java class in order to send an HTTP Request with POST method and also another java class in order to receive it.
The server works fine when I make a POST request by means of my browser(Chrome), or an application(I have used Postman in this case) but it ends up with problem when I send HTTP Request with java!
My sending HTTP class is "Sender.java", containing the following snippet:
String url = "http://localhost:8082/";
URL obj = new URL(url);
HttpURLConnection con = (HttpURLConnection) obj.openConnection();
// Setting basic post request
con.setRequestMethod("POST");
//con.setRequestProperty("Accept-Language", "en-US,en;q=0.5");
//con.setRequestProperty("Content-Type","text/plain");
// Send post request
con.setDoOutput(true);
OutputStream os = con.getOutputStream();
os.write("Just Some Text".getBytes("UTF-8"));
os.flush();
os.close();
//connect to the Server(resides at Server.java)
con.connect();
I have commented some lines of code setting Headers like "Accept-Language" and "Content-Type" because I don't know whether or not are these headers required for the java program to work out?
The server is another java program named "Server.java". Here is the snippet related to reading HTTP Request made by the Sender.java(if need be).
int servPort = 8082;
// Create a server socket to accept HTTP client connection requests
HttpServer server = HttpServer.create(new InetSocketAddress(servPort), 0);
System.out.println("server started at " + servPort);
server.createContext("/", new PostHandler());//PostHandler implements HttpHandler
server.setExecutor(null);
server.start();
All I want is to send a plaintext as the body of my HTTP Request with the Post method. I have read plenty of sites and even related questions at this site. But it still doesn't work out. In other words, whenever I create an HTTP Request from "Sender.java", nothing appears at "Server.java". I just want to know what's wrong with my snippets and how should I fix that?

I tested this and it's working:
//Sender.java
String url = "http://localhost:8082/";
URL obj = new URL(url);
HttpURLConnection con = (HttpURLConnection) obj.openConnection();
con.setRequestMethod("POST");
con.setDoOutput(true);
OutputStream os = con.getOutputStream();
os.write("Just Some Text".getBytes("UTF-8"));
os.flush();
int httpResult = con.getResponseCode();
con.disconnect();
As you can see, connect is not necessary. The key line is
int httpResult = con.getResponseCode();

When you send a POST form using the browser, it sends the form in a certain format, defined in RFC1866, you have to recreate this on Java when making a post request.
With this format, its important you set the Content-Type header to application/x-www-form-urlencoded, and pass the body as you would do in a url with a get request.
Borrowing some code of my previous answer to POST in Java:
URL obj = new URL(url);
HttpURLConnection con = (HttpURLConnection) obj.openConnection();
// Setting basic post request
con.setRequestMethod("POST");
Map<String,String> form = new HashMap<>();
// Define the fields
form.put("username", "root");
form.put("password", "sjh76HSn!"); // This is a fake password obviously
// Build the body
StringJoiner sj = new StringJoiner("&");
for(Map.Entry<String,String> entry : arguments.entrySet())
sj.add(URLEncoder.encode(entry.getKey(), "UTF-8") + "="
+ URLEncoder.encode(entry.getValue(), "UTF-8"));
byte[] out = sj.toString().getBytes(StandardCharsets.UTF_8);
int length = out.length;
// Prepare our `con` object
con.setFixedLengthStreamingMode(length);
con.setRequestProperty("Content-Type", "application/json; charset=UTF-8");
con.connect();
try (OutputStream os = con.getOutputStream()) {
os.write(out);
}

Maybe “localhost” in the sender url does not resolve to the same ip that the server binds to? Try changing to 127.0.0.1 or your actual IP address.

try with PrintStream
String url = "http://localhost:8082/";
URL obj = new URL(url);
HttpURLConnection con = (HttpURLConnection) obj.openConnection();
// Setting basic post request
con.setRequestMethod("POST");
//con.setRequestProperty("Accept-Language", "en-US,en;q=0.5");
//con.setRequestProperty("Content-Type","text/plain");
// Send post request
con.setDoOutput(true);
OutputStream os = con.getOutputStream();
java.io.PrintStream printStream = new java.io.PrintStream(os);
printStream.println("Just Some Text");
con.getInputStream();//Send request
os.flush();
os.close();

Send Multiple POST Requests Through a DataOutputStream in Java

I am trying to use a for loop to send multiple POST requests through a DataOutputStream and then close it. At the moment, only the first index of the "trades" array list is sent to the website. Any other indexes are ignored and I'm assuming they are not being sent. I wonder if I am properly flushing the stream? Thank you!!!
Examples of trades values: "101841599", "101841801"
Example of code value: 85e4c22
Snippet of my code:
private ArrayList<String> trades = new ArrayList<String>();
private String code;
String url = "http://www.dota2lounge.com/ajax/bumpTrade.php";
URL obj = new URL(url);
HttpURLConnection con = (HttpURLConnection) obj.openConnection();
con.setRequestMethod("POST");
con.setRequestProperty("Accept-Language", "en-US,en;q=0.8");
con.setRequestProperty("Cookie", cookie);
con.setDoOutput(true);
DataOutputStream wr = new DataOutputStream(con.getOutputStream());
for(int i=0; i<trades.size(); i++){
wr = new DataOutputStream(con.getOutputStream());
wr.writeBytes("trade=" + trades.get(i) + "&code=" + code);
wr.flush();
System.out.println("again");
}
wr.flush();
wr.close();

It turns out I had to actually get the response for it to properly close the connection before I started a new one. Appending these lines to the end of the for loop fixed the issue:
int nothing = con.getResponseCode();
String morenothing = con.getResponseMessage();

From the HttpURLConnection javadoc:
"Each HttpURLConnection instance is used to make a single request but the underlying network connection to the HTTP server may be transparently shared by other instances."
So if you want to send multiple requests, then for each request call obj.openConnection(), set the connection settings, open the OutputStream, and write the data. Your Java runtime is permitted to keep the actual connection open to save time and bandwidth.

Send file from Action to Servlet

My goal is to upload a file from an action to a servlet.
Till now, I thought I had it working in this way:
Action: reads file as bytearray, converts it into String and puts String on request
HttpURLConnection conn =null;
String url = "http://myServlet");
URL obj = new URL(url);
conn = (HttpURLConnection) obj.openConnection();
conn.setDoOutput(true);
conn.setRequestMethod("GET");
StringBuffer requestParams = new StringBuffer();
requestParams.append("fileString");
requestParams.append("=").append(URLEncoder.encode(fileString, "ISO-8859-1"));
//Append more params
OutputStreamWriter wr = new OutputStreamWriter(conn.getOutputStream());
wr.write(requestParams.toString());
wr.flush();
conn.getContentLength();
Servlet: get String parameter, converts it back to bytearray and re-creates the file
receivedString = request.getParameter("fileString");
//Convert to bytearray and create file
But I guess this isn't a good solution, cause sometimes the call just fails because of the string (length maybe?)
Which is the right way to send my file? I can't find a way to send the file AND additional informations putting them on the request.

HttpURLConnection to Tomcat

I am trying to connect from a java desktop application to a jsp Servlet to send a file.
Clientcoding:
HttpURLConnection urlConnection = null;
URL url = null;
url = new URL("http://127.0.0.1:8080/emobile/AddTripMobile");
urlConnection = (HttpURLConnection) url.openConnection();
urlConnection.setDoOutput(true);
OutputStream out = new BufferedOutputStream(
urlConnection.getOutputStream());
out.write(12); //The data to send
out.flush();
If I connect with the desktop application to the server nothing happens.
(I set a breakpoint in the doGet and doPost)
Any suggestions?

You need to add the following :
InputStream is = urlConnection.getInputStream();
out.write(12); //The data to send
out.flush();

Try closing the output stream.