import java.io.File;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.Paths;
import java.security.cert.CertificateFactory;
import javax.security.cert.X509Certificate;
 
 
public class Main {
 
    public static void main(String[] args) {
        try {
            String path = new String("d:\\temp\\testov__der.cer");
            FileInputStream is = new FileInputStream(new File(path));
            CertificateFactory cf = CertificateFactory.getInstance("X.509");
            java.security.cert.Certificate cert = cf.generateCertificate(is);
            java.security.cert.X509Certificate t = (java.security.cert.X509Certificate) cert;
            String subject  = new String(t.getSubjectDN().toString());
            System.out.println(subject);
        } catch (Exception e) {
            System.out.println("Перехвачено исключение");
        }
    }
}