How to properly post url in android with java using HttpURLConnection

Question

I try to use HttpURLConnection to send a post request to my local (xampp) server with an url like this http://xxx.xxx.0.3/Company/index.php/booking/c200/p-205/2025-02-09 8:2 , the server php file take param in url and send data to mysql database .

The url works fine on postman agent , and even another get method request works smooth in the android application .

Yet when i try the post method with following code :

public void postOrder() {
        TextView tv = findViewById(R.id.tv1);
        Thread t = new Thread( new Runnable() {
            @Override
            public void run() {
                HttpURLConnection conn = null;
                try {
                    String link = "http://xxx.xxx.0.3/Company/index.php/booking/c200/p-205/2025-02-09 8:2";
                    URL url = new URL(link);
                    conn = (HttpURLConnection) url.openConnection();
                    conn.setRequestMethod("POST");
                    conn.setReadTimeout(10000 /*ms*/);
                    conn.setConnectTimeout(15000 /*ms*/);
                    conn.connect();
                }
                catch (IOException e) {
                    Log.d("HTTP error: ", e.toString());
                }
                finally {
                    conn.disconnect();
                }
            }
        } );
        t.start();
}

It never sent the url and thus no data is stored to database .

And with 6 hours of trial and error , google and searching , i added this line of code :

InputStream is = conn.getInputStream();

And it finally works . Please answer me why it only works after adding this line of code , what does it do ? I thought the url is triggered right after conn.connect();