code: private static PrivateKey loadPrivateKey(byte[] keyBytes) {
          PKCS8EncodedKeySpec pkcs8EncodedKeySpec = new PKCS8EncodedKeySpec(
                  keyBytes);
          try {
              KeyFactory factory = KeyFactory.getInstance(KEY_ALGORITHM);
              PrivateKey privateKey = factory
                      .generatePrivate(pkcs8EncodedKeySpec);
              return privateKey;
          } catch (NoSuchAlgorithmException | InvalidKeySpecException e) {
              // TODO Auto-generated catch block
              e.printStackTrace();
          }
          return null;
      }
docs: 还原私钥，PKCS8EncodedKeySpec 用于构建私钥的规范
  
  @param keyBytes
  @return