code: public static String encryptByPrivateKey(String data, String privateKey) throws Exception {
  		PKCS8EncodedKeySpec priPKCS8 = new PKCS8EncodedKeySpec(base64.decode(privateKey));
  		KeyFactory keyf = KeyFactory.getInstance("RSA");
  		PrivateKey priKey = keyf.generatePrivate(priPKCS8);
  		java.security.Signature signature = java.security.Signature.getInstance("SHA256WithRSA");
  
  		signature.initSign(priKey);
  		signature.update(data.getBytes("UTf-8"));
  		byte[] signed = signature.sign();
  		return base64.encodeToString(signed);
  	}
docs: 私钥签名
  
  @param data
  @param privateKey
  @return
  @throws Exception