title: Use GZIP, JSON responses and JQuery
body: <p>However, I want to compress my responses with GZIP wheren possible. I tried using the <a href="http://www.headfirstlabs.com/books/hfsvlt/" rel="noreferrer">Compression filter code</a> available for free download in the headfirst site. It works great for html, images, css and javascript. </p>
  
  <p>I post the filter next. It checks if GZIP is an accepted encoding and it adds gzip as Content-Encoding. See: <code>wrappedResp.setHeader("Content-Encoding", "gzip");</code></p>
  
  <pre><code>public class CompressionFilter implements Filter {
  
    private ServletContext ctx;
    private FilterConfig cfg;
  
      /**
       * The init method saves the config object and a quick reference to the
       * servlet context object (for logging purposes).
       */
    public void init(FilterConfig cfg) 
       throws ServletException {
      this.cfg = cfg;
      ctx = cfg.getServletContext();
      //ctx.log(cfg.getFilterName() + " initialized.");
    }
  
          /**
           * The heart of this filter wraps the response object with a Decorator
           * that wraps the output stream with a compression I/O stream.
           * Compression of the output stream is only performed if and only if
           * the client includes an Accept-Encoding header (specifically, for gzip).
           */
        public void doFilter(ServletRequest req,
                     ServletResponse resp,
                     FilterChain fc)
           throws IOException, ServletException {
          HttpServletRequest request = (HttpServletRequest) req;
          HttpServletResponse response = (HttpServletResponse) resp;
  
          // Dose the client accept GZIP compression?
          String valid_encodings = request.getHeader("Accept-Encoding");
          if ( (valid_encodings != null) &amp;&amp; (valid_encodings.indexOf("gzip") &gt; -1) ) {
  
            // Then wrap the response object with a compression wrapper
            // We'll look at this class in a minute.
            CompressionResponseWrapper wrappedResp = new CompressionResponseWrapper(response);
  
            // Declare that the response content is being GZIP encoded.
            wrappedResp.setHeader("Content-Encoding", "gzip");
  
            // Chain to the next component (thus processing the request)
            fc.doFilter(request, wrappedResp);
  
            // A GZIP compression stream must be "finished" which also
            // flushes the GZIP stream buffer which sends all of its
            // data to the original response stream.
            GZIPOutputStream gzos = wrappedResp.getGZIPOutputStream();
            gzos.finish();
            // The container handles the rest of the work.
  
            //ctx.log(cfg.getFilterName() + ": finished the request.");
  
          } else {
            fc.doFilter(request, response);
            //ctx.log(cfg.getFilterName() + ": no encoding performed.");
          }   
        }
  
        public void destroy() {
            // nulling out my instance variables
            cfg = null;
            ctx = null;
        }
      }
  </code></pre>
  
  <p>I was using the next code to send JSON responses in Struts web application. </p>
  
  <pre><code>public ActionForward get(ActionMapping mapping,
      ActionForm     form,
      HttpServletRequest request,
      HttpServletResponse response) {
         JSONObject json = // Do some logic here
         RequestUtils.populateWithJSON(response, json);
         return null;         
  }
  
  public static void populateWithJSON(HttpServletResponse response,JSONObject json) {
      if(json!=null) {
          response.setContentType("text/x-json;charset=UTF-8");       
          response.setHeader("Cache-Control", "no-cache");
          try {
               response.getWriter().write(json.toString());
          } catch (IOException e) {
              throw new ApplicationException("IOException in populateWithJSON", e);
          }               
      }
   }
  </code></pre>
  
  <p>It works fine without compression but if I compress JSON responses, I can not see my JSON objects anymore. I handle JSON Ajax calls with JQuery with code snippets as follows:    </p>
  
  <pre><code>    $.post(url,parameters, function(json) {
      // Do some DOM manipulation with the data contained in the JSON Object
  }, "json");
  </code></pre>
  
  <p>If I see the response with Firebug it is empty. </p>
  
  <p>Should I refractor my compression filter to skip compression in JSON responses? or there is a workaround to this?</p>
  
  <p>For me, it looks like JQuery does not recognize the response as JSON because I am adding the Gzip compression. </p>