Here it is:

package filtertest;

import javax.servlet.http.*;
import javax.servlet.*;
import java.io.*;
import java.net.*;
import java.util.*;

public class PrePostFilterMain extends GenericFilter {

  public void doFilter(final ServletRequest request,
                       final ServletResponse response,
                       FilterChain chain)
              throws IOException, ServletException {
    OutputStream out = response.getOutputStream();
    GenericResponseWrapper wrapper =
      new GenericResponseWrapper((HttpServletResponse) response);

    wrapper.setBufferSize(16384);
    chain.doFilter(request, wrapper);

    // look up file name that matches the original request URL
    String filename = null;

    String requestedPage = ((HttpServletRequest) request).getServletPath();
    requestedPage =
requestedPage.substring(requestedPage.lastIndexOf('/') + 1);
    ServletContext sc = getFilterConfig().getServletContext();
    if (sc == null)
      throw new javax.servlet.ServletException("Servlet context missing");

    filename = sc.getInitParameter(requestedPage + ".file");
    if (filename == null)
      throw new javax.servlet.ServletException("lookup of property \"" +
                                               requestedPage + ".file\"
failed");

    byte[] data = wrapper.getData();
    createFile(data, filename);
    out.write(("<hr>PrePostFilterMain output sent to file: " + filename
+ ": " +
            data.length + "/" + wrapper.getContentLength() + "
bytes<hr>").getBytes());
    out.write(data);
    out.write("<hr>FilterTest Post<hr>".getBytes());
    out.flush();
    out.close();
  }

  public void createFile(byte[] data, String filename)
            throws IOException {
    ServletContext sc = getFilterConfig().getServletContext();
    String path = sc.getRealPath(filename);
    FileOutputStream fos = new FileOutputStream(path);
    fos.write(data);
    fos.flush();
    fos.close();
  }

}
//------------------------------------------------------------

package filtertest;

import javax.servlet.http.*;
import javax.servlet.*;
import java.io.*;
import java.net.*;
import java.util.*;

public class GenericFilter implements javax.servlet.Filter {

  private FilterConfig fc;

  public GenericFilter() { }

  public void doFilter(final ServletRequest request,
                       final ServletResponse response,
                       FilterChain chain)
         throws java.io.IOException, javax.servlet.ServletException {

    chain.doFilter(request, response);
  }

  public FilterConfig getFilterConfig() {
    return fc;
  }

  public void init(FilterConfig fc) {
    this.fc = fc;
  }

  public void destroy() {
    this.fc = null;
  }
}

//--------------------------------------------------------------------

package filtertest;

import javax.servlet.http.*;
import javax.servlet.*;
import java.io.*;
import java.net.*;
import java.util.*;

public class GenericResponseWrapper extends HttpServletResponseWrapper {

  private ByteArrayOutputStream output;
  private int contentLength;
  private String contentType;

  public GenericResponseWrapper(HttpServletResponse response) {
    super(response);
    output = new ByteArrayOutputStream();
  }

  public ServletOutputStream getOutputStream() {
    return new FilterServletOutputStream(output);
  }

  public byte[] getData() {
    return output.toByteArray();
  }

  public PrintWriter getWriter() {
    return new PrintWriter(getOutputStream(), true);
  }

  public void setContentType(String type) {
    this.contentType = type;
    super.setContentType(type);
  }

  public String getContentType() {
    return this.contentType;
  }

  public int getContentLength()    {
    return contentLength;
  }

  public void setContentLength(int length) {
    this.contentLength = length;
    super.setContentLength(length);
  }

}