code: public static byte[] sign(byte[] contentByteArray, PrivateKey privateKey) {
          try {
              Signature signature = Signature.getInstance(signAlgorithm);
              signature.initSign(privateKey);
              signature.update(contentByteArray);
              return signature.sign();
          } catch (NoSuchAlgorithmException e) {
              e.printStackTrace();
          } catch (SignatureException e) {
              e.printStackTrace();
          } catch (InvalidKeyException e) {
              e.printStackTrace();
          }
          return null;
      }
docs: 对内容进行加签
  <p>Function: encrypt</p>
  <p>Description: </p>
  
  @param contentByteArray 明文
  @param privateKey       私钥
  @return
  @throws NoSuchAlgorithmException
  @throws InvalidKeyException
  @throws SignatureException
  @throws UnsupportedEncodingException
  @author acexy@thankjava.com
  @date 2016年8月10日 下午5:41:51
  @version 1.0