I was testing upgrading RESTeasy from v2.0.1.GA to v2.2.2.GA and discovered a problem with the @GZIP annotation in builds since 2.2-beta-1 (when CommitHeaderOutputStream.java was introduced). We have a MessageBodyWriter implemented for a custom data type, and in the writeTo() method we add a new HTTP header. When using the @GZIP annotation on the method and querying the REST API using a client which sends Accept-Encoding: gzip, we are finding that any headers added in our MessageBodyWriter are not included in the response.

I stepped through in the debugger, and found that this sequence seems to be the issue:

• In GZIPEncodingInterceptor.write, creates new EndableGZIPOutputStream wrapping the CommitHeaderOutputStream.
• EndableGZIPOutputStream calls super constructor GZIPOutputStream.
• GZIPOutputStream constructor calls GZIPOutputStream.writeHeader().
• GZIPOutputStream.writeHeader calls out.write(byte[]).
• CommitHeaderOutputStream.write(byte[]) calls the commit callback which writes the current headers to the response.
• GZIPEncodingInterceptor calls context.proceed(), which calls into our MessageBodyWriter.writeTo() method, but any headers added at this point are ignored because they have already been committed to the response.