code: public static PrivateKey decryptPrivateKey(String privateKeyStr) {
          try {
              KeyFactory keyFactory = KeyFactory.getInstance(algorithm);
              PKCS8EncodedKeySpec keySpec = new PKCS8EncodedKeySpec(Base64Util.decode(privateKeyStr));
              return keyFactory.generatePrivate(keySpec);
          } catch (NoSuchAlgorithmException e) {
              e.printStackTrace();
          } catch (InvalidKeySpecException e) {
              e.printStackTrace();
          }
          return null;
      }
docs: 从Base64字符中解出私钥
  <p>Function: decryptPrivateKey</p>
  <p>Description: </p>
  
  @param privateKeyStr
  @return
  @author acexy@thankjava.com
  @date 2016年8月11日 上午9:43:39
  @version 1.0