How to use HttpsURLConnection through proxy by setProperty?

Your URL connection is https whereas you are only setting the http proxy.

Try setting the https proxy.

//System.setProperty("https.proxySet", "true"); 
 System.setProperty("https.proxyHost",10.100.21.11);
 System.setProperty("https.proxyPort","443");

EDIT @EJP is correct. There is no https.proxySet .. I copied your original question and included in the answer.

You will need to create a Proxy object for it. Create one as below:

Proxy proxy = new Proxy(Proxy.Type.HTTP, new InetSocketAddress(proxyServer, Integer.parseInt(proxyPort)));

Now use this proxy to create the HttpURLConnection object.

HttpURLConnection connection = (HttpURLConnection) new URL(url).openConnection(proxy);

If you have to set the credentials for the proxy, set the Proxy-Authorization request property:

String uname_pwd = proxyUsername + ":" + proxyPassword
String authString = "Basic " + new sun.misc.BASE64Encoder().encode(uname_pwd.getBytes())
connection.setRequestProperty("Proxy-Authorization", authString);

And finally, you connect:

connection.connect();

thank you @divinedragon!

Same code on kotlin:

 fun testProxy(login: String, pass: String, proxyData: ProxyData): String {
    val url = URL("http://api.ipify.org")
    val proxy = Proxy(Proxy.Type.HTTP, InetSocketAddress(proxyData.ip, proxyData.port))
    val connection = url.openConnection(proxy) as HttpURLConnection

    val loginPass = "$login:$pass"
    val encodedLoginPass = Base64.getEncoder().encodeToString(loginPass.toByteArray())
    val authString = "Basic $encodedLoginPass"
    connection.setRequestProperty("Proxy-Authorization", authString);
    with(connection) {
        requestMethod = "GET"  // optional default is GET
        connectTimeout = 2000
        readTimeout = 2000
        return inputStream.bufferedReader().readText()
    }
}