InetSocketAddress proxyInet = new InetSocketAddress("10.100.21.11",80);
Proxy proxy = new Proxy(Proxy.Type.HTTP, proxyInet);
URL httpsUrl = new URL("https://192.168.17.22:8443/test");
HttpsURLConnection httpsCon = (HttpsURLConnection) httpsUrl.openConnection(proxy);

httpsCon.setDoOutput(true);
httpsCon.setDoInput(true);
httpsCon.setRequestMethod("POST");
OutputStream out = httpsCon.getOutputStream();
OutputStreamWriter owriter = new OutputStreamWriter(out);

owriter.write("<request>test</request>");
owriter.flush();
owriter.close();
...

This method workable and I observed packets flow also met my expectation.
HttpClient ---> ProxyServer ---> HttpServer

But when I use set Property method:

2.setProperty

System.setProperty("http.proxySet", "true");
System.setProperty("http.proxyHost",10.100.21.11);
System.setProperty("http.proxyPort","80");

URL httpsUrl = new URL("https://192.168.17.22:8443/test");
HttpsURLConnection httpsCon = (HttpsURLConnection)httpsUrl.openConnection();

httpsCon.setDoOutput(true);
httpsCon.setDoInput(true);
httpsCon.setRequestMethod("POST");
OutputStream out = httpsCon.getOutputStream();
OutputStreamWriter owriter = new OutputStreamWriter(out);

owriter.write("<request>test</request>");
owriter.flush();
owriter.close();
...

I got a NoRouteToHostException: Network is unreachable.
It make me confused.I did not see any packets between HttpClient and ProxyServer.
But HttpClient can ping to ProxyServer(10.100.12.10 ping 10.100.21.11)

So I remove proxy setting(as without using proxy):
Also got NoRouteToHostException: Network is unreachable.
I thought this is reasonable.Because there is no route to extranet.

I guess it seems like to setProperty method that the inner function of httpsUrlConnection will to check this url can be reachable or not.

But it is weird. 1st method can be success.

Have any idea? Or what are different between 1st and 2nd method?

+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

Update

System.setProperty("https.proxyHost",10.100.21.11);
System.setProperty("https.proxyPort","80"); 

It can work and packets flow are correct what I expect for.
But set https.proxyPort=443 is not workable for me

System.setProperty("https.proxyPort","443");

It will thorow a exception as bellow:

java.net.SocketException: Unexpected end of file from server 
at sun.net.www.http.HttpClient.parseHTTPHeader(HttpClient.java:770)
....

So I thought Apache Proxy have also to be modified to the right configuration.

Answers

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.

Answers

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();