When a POST request is made with an entity, the entity content may be written to a temporary file. This is done via Files.newOutputStream(tempFile). If you post larger entities and have a small -XX:DirectByteBuffer size set, the underlying DirectByteBuffer can run out of off heap memory resulting in aborted requests. With requests up to 30-50 mbs and it seems RESTEasy is writing that in full at once to the underlying stream and thus allocating such a large buffer to match through the resulting sun.nio.ch.Util.getTemporaryDirectBuffer call.