http://localhost/index.php?option=com_jbackend&action=post&module=user&resource=login&username=<username>&password=<password>

conn.setRequestProperty ("Authorization", session_id);

conn = (HttpURLConnection) url.openConnection();
conn.setRequestProperty("Authorization",session_id);
conn.setDoOutput(true);
wr = new DataOutputStream(conn.getOutputStream());

wr.flush();
wr.close();

int responseCode = conn.getResponseCode();

in = new BufferedReader(new InputStreamReader(conn.getInputStream()));
inputLine ="";
response = new StringBuffer();

while ((inputLine = in.readLine()) != null) {
    response.append(inputLine);
}
in.close();

// print result
System.out.println(response.toString());