Parsing Facebook signed_request using Java returns malformed JSON

Question

I'm trying to parse Facebook signed_request inside Java Servlet's doPost. And I decode the signed request using commons-codec-1.3's Base64. Here is the code which I used to do it inside servlet's doPost

String signedRequest = (String) req.getParameter("signed_request");
String payload = signedRequest.split("[.]", 2)[1];
payload = payload.replace("-", "+").replace("_", "/").trim();
String jsonString = new String(Base64.decodeBase64(payload.getBytes()));

when I System.out the jsonString it's malformed. Sometime's it misses the ending } of JSON sometime it misses "} in the end of the string.

How can I get the proper JSON response from Facebook?

Answer 1

facebook is using Base64 for URLs and you are probably trying to decode the text using the standard Base64 algorithm. among other things, the URL variant doesn't required padding with "=".

1. you could add the required characters in code (padding, etc)
2. you can use commons-codec 1.5 ( new Base64(true)), where they added support for this encoding.