GZipOutputStream和Appengine

Question

I'm writing a java servlet on AppEngine. This servlet generates png images. I would like to "gzip" the response. I do it this way:

resp.setHeader("Content-Encoding","gzip");
resp.setContentType("image/png");

// ... png generation ...

GZIPOutputStream gzos = new GZIPOutputStream(resp.getOutputStream());
gzos.write(myPNGdata); 
gzos.close();

But: in development server, it's ok, the png displays fine and the response is well gzipped. Then I test on production server (AppEngine) and all I get is a "broken" image...

What could be wrong with my code? Is it related to dev/prod environment?

Off course, If I don't gzip the output, it's ok in both environments.

Thanks for any help.

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Edit: I tried this too:

GZIPOutputStream gzos = new GZIPOutputStream(resp.getOutputStream());
gzos.write(ImagesServiceFactory.makeImage(readImage("somePicture.png")).getImageData());
gzos.flush();
gzos.finish();
gzos.close();

and it doesn't work either.

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Edit 2: in fact, the response is gzip. I fetched the servlet with "curl theUrl > tmp.gz", then I gunzip "tmp.gz", and the image is fine. But no browser can display it correctly :( What's wrong with my gzip?

Answer 1

The App Engine infrastructure will take care of gzipping content for you when appropriate. You shouldn't do it yourself.

Answer 2

Check the size of your downloaded image. If it is smaller then you expecting, most likely you need to flush the stream before closing.