genJWK.java

import java.io.IOException;
import java.net.URISyntaxException;
import java.nio.file.Files;
import java.nio.file.Paths;
import java.security.KeyFactory;
import java.security.NoSuchAlgorithmException;
import java.security.PrivateKey;
import java.security.PublicKey;
import java.security.interfaces.RSAPublicKey;
import java.security.spec.InvalidKeySpecException;
import java.security.spec.PKCS8EncodedKeySpec;
import java.security.spec.X509EncodedKeySpec;
import java.security.cert.X509Certificate;
import java.security.cert.CertificateFactory;
import java.util.Base64;
import java.util.Base64.Encoder;
import java.util.Base64.Decoder;
import java.util.HashMap;
import java.util.Map;
import java.io.FileInputStream;
import com.google.gson.*;

public class genJWK {
  public static void main(String[] args) {
      System.out.println("JWKS Generator");
    try{
      Gson gson = new Gson();
      System.out.println( generateJWK(args[0],args[0]));
      System.out.println();
      System.out.println( gson.toJson(generateJWK(args[0],args[0])));
    } catch(Exception e){
      e.printStackTrace();
    }
  }

private static Map<String, Object> generateJWK(String env, String location){
  Map<String, Object> values = new HashMap<>();
  try{
    FileInputStream fin = new FileInputStream(location);
    CertificateFactory f = CertificateFactory.getInstance("X.509");
    X509Certificate certificate = (X509Certificate)f.generateCertificate(fin);

        System.out.println("Subject DN : " + certificate.getSubjectDN().getName());
        System.out.println("Issuer : " + certificate.getIssuerDN().getName());
        System.out.println("Not After: " + certificate.getNotAfter());
        System.out.println("Not Before: " + certificate.getNotBefore());
        System.out.println("version: " + certificate.getVersion());
        System.out.println("serial number : " + certificate.getSerialNumber());

        PublicKey pk = certificate.getPublicKey();
        //System.out.println("PublicKey : \n" + pk);


    RSAPublicKey rsa = (RSAPublicKey) pk;

    values.put("kty", rsa.getAlgorithm()); // getAlgorithm() returns kty not algorithm
    values.put("kid", env);
    
    values.put("n", Base64.getUrlEncoder().encodeToString(rsa.getModulus().toByteArray()));
    
    values.put("e", Base64.getUrlEncoder().encodeToString(rsa.getPublicExponent().toByteArray()));

    values.put("alg", "RS256");
    values.put("use", "sig");
    } catch(Exception e){
      e.printStackTrace();
    }
    return values;
  }
}