Java client certificates over HTTPS/SSL

Finally solved it ;). Got a strong hint here (Gandalfs answer touched a bit on it as well). The missing links was (mostly) the first of the parameters below, and to some extent that I overlooked the difference between keystores and truststores.

The self-signed server certificate must be imported into a truststore:

keytool -import -alias gridserver -file gridserver.crt -storepass $PASS -keystore gridserver.keystore

These properties need to be set (either on the commandline, or in code):

-Djavax.net.ssl.keyStoreType=pkcs12
-Djavax.net.ssl.trustStoreType=jks
-Djavax.net.ssl.keyStore=clientcertificate.p12
-Djavax.net.ssl.trustStore=gridserver.keystore
-Djavax.net.debug=ssl # very verbose debug
-Djavax.net.ssl.keyStorePassword=$PASS
-Djavax.net.ssl.trustStorePassword=$PASS

Working example code:

SSLSocketFactory sslsocketfactory = (SSLSocketFactory) SSLSocketFactory.getDefault();
URL url = new URL("https://gridserver:3049/cgi-bin/ls.py");
HttpsURLConnection conn = (HttpsURLConnection)url.openConnection();
conn.setSSLSocketFactory(sslsocketfactory);
InputStream inputstream = conn.getInputStream();
InputStreamReader inputstreamreader = new InputStreamReader(inputstream);
BufferedReader bufferedreader = new BufferedReader(inputstreamreader);

String string = null;
while ((string = bufferedreader.readLine()) != null) {
    System.out.println("Received " + string);
}

While not recommended, you can also disable SSL cert validation alltogether:

import javax.net.ssl.*;
import java.security.SecureRandom;
import java.security.cert.X509Certificate;

public class SSLTool {

  public static void disableCertificateValidation() {
    // Create a trust manager that does not validate certificate chains
    TrustManager[] trustAllCerts = new TrustManager[] { 
      new X509TrustManager() {
        public X509Certificate[] getAcceptedIssuers() { 
          return new X509Certificate[0]; 
        }
        public void checkClientTrusted(X509Certificate[] certs, String authType) {}
        public void checkServerTrusted(X509Certificate[] certs, String authType) {}
    }};

    // Ignore differences between given hostname and certificate hostname
    HostnameVerifier hv = new HostnameVerifier() {
      public boolean verify(String hostname, SSLSession session) { return true; }
    };

    // Install the all-trusting trust manager
    try {
      SSLContext sc = SSLContext.getInstance("SSL");
      sc.init(null, trustAllCerts, new SecureRandom());
      HttpsURLConnection.setDefaultSSLSocketFactory(sc.getSocketFactory());
      HttpsURLConnection.setDefaultHostnameVerifier(hv);
    } catch (Exception e) {}
  }
}