//以下方法注意 byte[] bytes = EntityUtils.toByteArray(response.getEntity());

public static byte[] downloadUrl(String download_url) throws Exception {


    //请求URL
    //账单文件的下载地址的有效时间为30s
    URIBuilder uriBuilder = new URIBuilder(download_url);
    HttpGet httpGet = new HttpGet(uriBuilder.build());
    httpGet.addHeader("Accept", "application/json");

    //执行请求
    CloseableHttpResponse response = httpClient.execute(httpGet);
    try {
        int statusCode = response.getStatusLine().getStatusCode();
        if (statusCode == 200) {
            //重点在此处返回byte[]数组
            byte[] bytes = EntityUtils.toByteArray(response.getEntity());
            return bytes;
        } else if (statusCode == 204) {
            System.out.println("success");
        } else {
            System.out.println("failed,resp code = " + statusCode + ",return body = " + EntityUtils.toString(response.getEntity()));
            throw new IOException("request failed");
        }
    } finally {
        response.close();
    }
    return null;
}

/**
     * 解密二级商户账单
     * @param associatedData
     * @param nonce
     * @param ciphertext 二进制密文，需以字节数组形式处理，不需要进行Base64解码。
     * @return
     * @throws GeneralSecurityException
     * @throws IOException
     */
    public String decryptToString(byte[] associatedData, byte[] nonce, byte[] ciphertext)
            throws GeneralSecurityException, IOException {
        try {
            Cipher cipher = Cipher.getInstance("AES/GCM/NoPadding");

            SecretKeySpec key = new SecretKeySpec(aesKey, "AES");
            GCMParameterSpec spec = new GCMParameterSpec(TAG_LENGTH_BIT, nonce);

            cipher.init(Cipher.DECRYPT_MODE, key, spec);
            cipher.updateAAD(associatedData);

            return new String(cipher.doFinal(ciphertext), "utf-8");
        } catch (NoSuchAlgorithmException | NoSuchPaddingException e) {
            throw new IllegalStateException(e);
        } catch (InvalidKeyException | InvalidAlgorithmParameterException e) {
            throw new IllegalArgumentException(e);
        }
    }

}

官方给出下载API账单的详细说明，有空可以看下https://pay.weixin.qq.com/wiki/doc/apiv3/apis/chapter3_1_8.shtml