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public class ProxyServlet extends HttpServlet {
        
   private final HttpClient httpClient = new HttpClient();
   private String forwardHost;
   private String forwardPort;
        
        
   @Override
   public void init(ServletConfig config) throws ServletException {
      forwardHost = config.getInitParameter("forward.host");
      forwardPort = config.getInitParameter("forward.port");
   }
        
   @Override
   protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
                
      
      try 
      // compute the new url
      String uri = req.getRequestURI();
      if (req.getQueryString() != null) {
         uri += "?" + req.getQueryString();
      }
      uri = req.getScheme() + "://" + forwardHost + ":" + forwardPort + uri;
                 
      // handle proxy issues (hop-by-hop headers, cache, via header, ...)
      // ...
                 
      // create the forward request 
      HttpRequest forwardRequest = new HttpRequest(req.getMethod(), uri);
        
      // copy the request headers
      for (Enumeration<String> en = req.getHeaderNames(); en.hasMoreElements(); ) {
         String headername = en.nextElement();
         for (Enumeration<String> en2 = req.getHeaders(headername); en2.hasMoreElements(); ) {
            forwardRequest.addHeader(headername, en2.nextElement());
         }
      }
      forwardRequest.setHost(forwardHost + ":" + forwardPort);
                
      // forward the request in a synchronous manner
      HttpResponse response = httpClient.call(forwardRequest);
                
      // handle proxy issues (hop-by-hop headers, ...)
      // ...
                
      // copy the response headers
      response.removeHeader("Server");
      for (String headername : response.getHeaderNameSet()) {
         for (String headervalue : response.getHeaderList(headername)) {
            resp.addHeader(headername, headervalue);
         }
      }
      // copy the body (if exists)
      if (response.hasBody()) {
         byte[] body = response.getBlockingBody().readBytes();
         resp.getOutputStream().write(body);
      }
   }
}

  class ForwardHandler implements IHttpRequestHandler {
   private final HttpClient httpClient = new HttpClient();
   private String forwardHost;
   private int forwardPort;

   public ForwardHandler(String forwardHost, int forwardPort) {
      this.forwardHost = forwardHost;
      this. forwardPort = forwardPort;
   }

   public void onRequest(HttpRequest req, final IHttpResponseContext respCtx) throws IOException {
    
      // handle proxy issues (hop-by-hop headers, cache, via header, ...)
      // ...

      // update the target UTI (Host header will be update automatically)
      req.updateTargetURI(forwardHost, forwardPort);
  
      
      // create the response handler (timeout is not handled here)
      IHttpResponseHandler responseHandler = new IHttpResponseHandler() {

         @Execution(Execution.NONTHREADED)   // performance optimization
         public void onResponse(HttpResponse resp) throws IOException {
            // handle proxy issues (hop-by-hop headers, ...)
            // ...

            // return the response 
            respCtx.send(resp);
         }

         // ...
      };
      
      // .. and forward the request
      try {
         httpClient.send(req, responseHandler);
      } catch (ConnectException ce) {
         respCtx.sendError(502);
      }
   }
}


IServer proxy = new HttpServer(8080, new ForwardHandler("localhost", 80));
proxy.run();