GAE JSP and Facebook-api-java iframe fb_sig_* param

I am building facebook app using iFrame (using Google App Engine Java,JSP and Facebook-api-java), and when Facebook calls back to 开发者_Go百科my website, I expect to get the fb_sig_* request params for the iFrame, but I get those params in HTTP referer header instead.

When I use the php client, i get the fb_sig_* in the $[_GET] as expected.

What have I done wrong?

Thanks

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If you get them in the referrer, then it means that a redirect has taken place. If this is not the expected behaviour by Facebook, then you need to ensure that you don't have some Servlet or Filter in the request chain which redirects requests using HttpServletResponse#sendRedirect() for some reason.