public static String getSignContent(Map<String, String> sortedParams) {
    StringBuffer content = new StringBuffer();
    List<String> keys = new ArrayList<String>(sortedParams.keySet());
    Collections.sort(keys);	
    int index = 0;
    for (int i = 0; i < keys.size(); i++) {
        String key = keys.get(i);
        String value = sortedParams.get(key);
        if (StringUtils.areNotEmpty(key, value)) {
            content.append((index == 0 ? "" : "&") + key + "=" + value);
            index++;
        }
    }
    return content.toString();
}

public static String rsa256Sign(String content, String privateKey,
                                String charset) throws BaiChangApiException {
    try {       PrivateKey priKey = getPrivateKeyFromPKCS8(BaiChangOpenConstants.SIGN_TYPE_RSA,
            new ByteArrayInputStream(privateKey.getBytes()));
        java.security.Signature signature = java.security.Signature
            .getInstance(BaiChangOpenConstants.SIGN_SHA256RSA_ALGORITHMS);
        signature.initSign(priKey);
        if (StringUtils.isEmpty(charset)) {
            signature.update(content.getBytes());
        } else {
            signature.update(content.getBytes(charset));
        }
        byte[] signed = signature.sign();
        return new String(Base64.encodeBase64(signed));
    } catch (Exception e) {
        throw new BaiChangApiException("RSAcontent = " + content + "; charset = " + charset, e);
    }
}