certGenerator.setNotBefore(validityBeginDate);
certGenerator.setNotAfter(validityExpireDate);
certGenerator.setPublicKey(keyPair.getPublic());
certGenerator.setSignatureAlgorithm("SHA256WithRSAEncryption");
certificate = certGenerator.generate(keyPair.getPrivate(), "BC");
File file = new File(System.getProperty("catalina.home") + "\\logs\\UserCertificate"+username+".cert");
FileOutputStream fos = new FileOutputStream(file);
fos.write(certificate.getEncoded());
fos.close();