Redirect to another URL and back for Certifcate base Login - spring

Question

Currently i developing an application which have two routes which are in different domain.
Example :
Route 1: https://test.apps.com
Route 2: https://test.cert-apps.com
Users uses Route 1 to access the application. In the Login page there is an option for Certificate based login . But certificate based authentication is only enabled in route 2.
So how do i do the certificate based authentication by redirecting from Route 1 to Route 2 and once client is authenticated redirect to route 1.
Following are the code that i currently using which are not working:
#Controller
#RequestMapping("/login")
public class LoginContr {
#RequestMapping("/certificate")
public String message(HttpServletRequest request, HttpServletResponse response) {
try {
sendGET();
} catch (IOException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
return "login";
}
#RequestMapping("/extractcertificate")
private String extractEmailFromCertificate(HttpServletRequest request) {
String email = "";
//Code to extract email from certificate
return email;
}
private static void sendGET() throws IOException {
URL obj = new URL("https://test.cert-apps.com/login/extractcertificate");
HttpURLConnection con = (HttpURLConnection) obj.openConnection();
con.setRequestMethod("GET");
int responseCode = con.getResponseCode();
System.out.println("GET Response Code :: " + responseCode);
if (responseCode == HttpURLConnection.HTTP_OK) { // success
BufferedReader in = new BufferedReader(new InputStreamReader(con.getInputStream()));
String inputLine;
StringBuffer response = new StringBuffer();
while ((inputLine = in.readLine()) != null) {
response.append(inputLine);
}
in.close();
// print result
System.out.println(response.toString());
} else {
System.out.println("GET request not worked");
}
}
}
The above code gives the error:
javax.net.ssl.SSLHandshakeException: sun.security.validator.ValidatorException: PKIX path building failed: sun.security.provider.certpath.SunCertPathBuilderException: unable to find valid certification path to requested target
at sun.security.ssl.Alerts.getSSLException(Alerts.java:192)
sendGet() method sends a request to Route 2 and calls /extractcertificate webservice to do certificate based authentication and once its done it will retrive the email address.
Technology using for app: Java,Spring,Angualar 4, Cloud Foundary
Is there any alternative to do this or any inbuilt implementation , any tips would be great advantage..