使用filter机制来GZIP压缩网页

package com.jsptw.filter;
import java.io.*;
import java.util.zip.GZIPOutputStream;
import javax.servlet.*;
import javax.servlet.http.*;
public class GZIPEncodeFilter implements Filter {
      public void init(FilterConfig filterConfig) {}
      public void doFilter( ServletRequest request, ServletResponse response, FilterChain chain)
                                      throws IOException, ServletException {        
        String transferEncoding = getGZIPEncoding((HttpServletRequest)request);        
        if (transferEncoding == null)
        {            
          chain.doFilter(request, response);        
        }
        else
        {          
          ((HttpServletResponse)response).setHeader("Content-Encoding", transferEncoding);
            GZIPEncodableResponse wrappedResponse = new GZIPEncodableResponse((HttpServletResponse)response);
            chain.doFilter(request, wrappedResponse);          
            wrappedResponse.flush();        
        }    
      }    
      public void destroy() {}    
      private static String getGZIPEncoding(HttpServletRequest request) {        
        String acceptEncoding = request.getHeader("Accept-Encoding");        
          if (acceptEncoding == null)
            return null;        
          acceptEncoding = acceptEncoding.toLowerCase();        
          if (acceptEncoding.indexOf("x-gzip") >= 0)
          {
            return "x-gzip";
          }        
          if (acceptEncoding.indexOf("gzip") >= 0)
          {            
            return "gzip";        
          }        
          return null;    
      }    
      
      private class GZIPEncodableResponse extends HttpServletResponseWrapper {        
        private GZIPServletStream wrappedOut;        
          public GZIPEncodableResponse(HttpServletResponse response) throws IOException {          
            super(response);            
              wrappedOut = new GZIPServletStream(response.getOutputStream());        
          }        
          public ServletOutputStream getOutputStream() throws IOException {            
            return wrappedOut;        
          }        
          private PrintWriter wrappedWriter;        
          public PrintWriter getWriter() throws IOException {            
            if (wrappedWriter == null) {                
                wrappedWriter = new PrintWriter( new OutputStreamWriter( getOutputStream(), getCharacterEncoding()));            }            
                 return wrappedWriter;        
              }        
          public void flush() throws IOException {            
            if (wrappedWriter != null) {              
                wrappedWriter.flush();            
              }          
              wrappedOut.finish();        
          }    
      }    
      
      private class GZIPServletStream extends ServletOutputStream {        
        private GZIPOutputStream outputStream;        
          public GZIPServletStream(OutputStream source) throws IOException {            
         outputStream = new GZIPOutputStream(source);        
       }        
       public void finish() throws IOException {
         OutputStream.finish();        
       }        
      public void write(byte[] buf) throws IOException {            
        outputStream.write(buf);        
      }        
      public void write(byte[] buf, int off, int len) throws IOException {            
        outputStream.write(buf, off, len);        
      }        
      public void write(int c) throws IOException {            
        outputStream.write(c);        
      }        
      public void flush() throws IOException {            
        outputStream.flush();        
      }      
      public void close() throws IOException {            
        outputStream.close();        
      }    
    }
}

<filter>        
<filter-name>GZIPEncoder</filter-name>        
<filter-class>com.jsptw.filter.GZIPEncodeFilter</filter-class>
</filter>

<filter-mapping>  
<filter-name> GZIPEncoder</filter-name>  
<url-pattern>/*</url-pattern>
</filter-mapping>  

if (transferEncoding == null)
{            
  chain.doFilter(request, response);  
}
else
{          
  ((HttpServletResponse)response).setHeader("Content-Encoding", transferEncoding);
  GZIPEncodableResponse wrappedResponse = new GZIPEncodableResponse((HttpServletResponse)response);
  chain.doFilter(request, wrappedResponse);          
  wrappedResponse.flush();        
}