import ru.CryptoPro.reprov.x509.X509CertImpl;

import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.Path;
import java.security.cert.CertificateException;

public class Main {
    public static void main(String[] args) throws IOException, CertificateException {
        byte[] file = Files.readAllBytes(Path.of(/* path to cer file */));
        X509CertImpl cert = new X509CertImpl(file);
        System.out.println(cert.getSubjectX500PrincipalInternal().toString());
    }
}